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Title:
TRANSMISSION SYSTEM
Document Type and Number:
WIPO Patent Application WO/2009/068853
Kind Code:
A1
Abstract:
A transmission system including a first shaft (3), a first gear element (13) rotatably mounted on the first shaft (3) and a first gear selector assembly (29) including first and second sets of engagement members (35, 36) that are arranged to selectively lock the first gear element for rotation with the first shaft (3) from operational modes that include the following operational modes: lock the first gear element (13) for rotation with the first shaft (3) in the forward and reverse torque directions; lock the first gear element (13) for rotation with the first shaft (3) in the forward torque direction and not lock in the reverse torque direction; lock the first gear element (13) for rotation with the first shaft (3) in the reverse torque direction and not lock in the forward torque direction; and wherein the first gear selector assembly (29) is arranged to adjust the radial positions of the first and second sets of engagement members (35, 36) relative to the first shaft (3) to selectively engage the first gear element (13) to select the operational modes.

Inventors:
MARTIN WILLIAM WESLEY (GB)
CHILD ANTHONY JOSEPH (GB)
Application Number:
PCT/GB2008/003914
Publication Date:
June 04, 2009
Filing Date:
November 24, 2008
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
ZEROSHIFT LTD (GB)
MARTIN WILLIAM WESLEY (GB)
CHILD ANTHONY JOSEPH (GB)
International Classes:
F16D21/04; F16D41/16
Domestic Patent References:
WO2000008356A12000-02-17
Foreign References:
EP0945637A11999-09-29
US5109722A1992-05-05
US2763350A1956-09-18
Attorney, Agent or Firm:
JONATHAN, Paul, Morris et al. (Midsummer House413 Midsumer Boulvard, Central Milton Keynes MK9 3BN, GB)
Download PDF:
Claims:
Claims

1. A transmission system including a first shaft, a first gear element rotatably mounted on the first shaft and a first gear selector assembly including first and second sets of engagement members that are arranged to selectively lock the first gear element for rotation with the first shaft from operational modes that include the following operational modes: lock the first gear element for rotation with the first shaft in the forward and reverse torque directions; lock the first gear element for rotation with the first shaft in the forward torque direction and not lock in the reverse torque direction; lock the first gear element for rotation with the first shaft in the reverse torque direction and not lock in the forward torque direction; and wherein the first gear selector assembly is arranged to adjust the radial positions of the first and second sets of engagement members relative to the first shaft to selectively engage the first gear element to select the operational modes.

2. A transmission system according to claim 1, wherein the first shaft includes a hollow portion for housing the first and second sets of engagement members and a plurality of apertures, and wherein the first gear selector assembly is arranged to adjust the radial positions of the first and second sets of engagement members independently of each other, from a first operational condition wherein the first and/or second set of engagement members is substantially retracted into the hollow portion to a second operational condition wherein the first and/or second set of engagement members protrude through the apertures to engage the first gear wheel.

3. A transmission system according to claim 2, wherein the hollow portion comprises and axial cavity and the first and second sets of engagement members are arranged to move axially within the axial cavity.

4. A transmission system according to any one of the preceding claims, wherein the first and second sets of engagement members are arranged to move along substantially radial paths when adjusting their radial positions.

5. A transmission system according to any one of the preceding claims, including a second gear element rotatably mounted on the first shaft and second gear selector assembly including first and second sets of engagement members that are arranged to selectively lock the second gear element for rotation with the first shaft from operational modes that include the following operational modes: lock the second gear element for rotation with the first shaft in the forward and reverse torque directions; lock the second gear element for rotation with the first shaft in the forward torque direction and not lock in the reverse torque direction; lock the second gear element for rotation with the first shaft in the reverse torque direction and not lock in the forward torque direction, and wherein the second gear selector assembly is arranged to adjust the radial positions of the first and second sets of engagement members relative to the first shaft to selectively engage the second gear element to select the operational modes.

6. A transmission system according to claim 5, wherein the first gear selector assembly is arranged to selectively engage the second gear element and the second gear selector assembly is arranged to selectively engage the first gear element.

7. A transmission system according to claim 5 or 6, wherein, when one of the first and second gear elements is locked for rotation with the first shaft in the forward and reverse torque directions by one of the first and second gear selector assemblies, the other of the first and second gear selector assemblies is arranged to select the other of the first and second gear elements before the one of the first and second gear selector assemblies fully disengages the one of the first and second gear elements for at least some shift types.

8. A transmission system according to any one of the preceding claims, wherein each selector assembly is arranged such that when a gear element is locked for rotation with the first shaft in the forward and reverse torque directions and a driving force is transmitted, one of the first and second sets of engagement members drivingly engages the engaged gear element, and the other set of engagement members is then in an unloaded condition.

B2008/003914

26

9. A transmission system according to any one of the preceding claims, wherein each selector assembly is arranged such that when a gear element is locked for rotation with the first shaft in the forward and reverse torque directions, and a braking force is transmitted, the first set of engagement members drivingly engages the engaged gear element and the second set of engagement members is in an unloaded condition, and when a driving force is transmitted, the second set of engagement members drivingly engages the engaged gear element and the first set of engagement members is then in an unloaded condition.

10. A transmission system according to claim 7 or 8 when dependent on claim 9, wherein when one of the first and second gear elements is locked for rotation with the first shaft in the forward and reverse torque directions by one of the first and second selector assemblies, and the other of the first and second selector assemblies is in a neutral condition, the one of the first and second gear selector assemblies is arranged to move the unloaded set of engagement members out of engagement with the currently engaged gear prior to the other of the first and second gear selector assemblies selecting the other of the first and second gear elements.

11. A transmission system according to any one of the preceding claims, wherein each of the selector assemblies includes an actuator system arranged to actuate axial and radial movement of the first and second sets of engagement members.

12. A transmission system according to claim 11, wherein the actuator system includes a first actuator for controlling axial actuation of the first set of engagement members and a second actuator for controlling axial actuation of the second set of engagement members.

13. A transmission system according to claim 11 or 12, wherein the actuator system includes a first control device for controlling radial actuation of the first set of engagement members and a second control device for controlling radial actuation of the second set of engagement members.

14. A transmission system according to claim 13, wherein the first set of engagement elements is mounted on a first support means and the actuator system includes a first biasing means for biasing the first set of engagement members in the direction of engagement with the adjacent gear element and wherein the first control device is arranged to control the radial position of the first set of engagement elements by acting against the first biasing means.

15. A transmission system according to claim 14, the first control device is arranged to control the radial positions of the first set of engagement members by adjusting its axial position relative to the first set of engagement elements.

16. A transmission system according to any one of claims 13 to 15, wherein the second set of engagement elements is mounted on second support means and the actuator system includes second biasing means for biasing the second set of engagement members in the direction of engagement with the adjacent gear element and wherein the second control device is arranged to control the radial position of the second set of engagement elements by acting against the second biasing means.

17. A transmission system according to claim 16, the second control device is arranged to control the radial positions of the second set of engagement members by adjusting its axial position relative to the second set of engagement elements.

18. A transmission system according to any one of claims 7 to 17, wherein one of the first and second gear selector assemblies is arranged to select only odd gears and the other of the first and second gear selector assemblies is arranged to select only even gears.

19. A transmission system according to any one of the preceding claims, including at least one subsequent gear rotatably mounted on the first shaft, wherein at least one of the first and second gear selector assemblies is arranged to lock the or each subsequent gear element for rotation with the first shaft from operational modes that include the following modes: lock the or each subsequent gear element for rotation with the first shaft in the forward and reverse torque directions; lock the or each subsequent gear

element for rotation with the first shaft in the forward torque direction and not lock in the reverse torque direction; lock the or each subsequent gear element for rotation with the first shaft in the reverse torque direction and not lock in the forward torque direction.

20. A transmission system according to any one of claims 5 to 19, wherein first and second sets of engagement members of the first gear selector assembly are arranged to move axially past the first and second sets of engagement members of the second gear selector assembly.

21. A transmission system according to any one of the preceding claims, wherein each gear element that is arranged to be selected by the first and/or second gear selector assembly includes a set of internal drive formations.

22. A transmission system according to any one of the preceding claims, including a control system for controlling operation of the first and second selector assemblies.

23. A transmission system according to any one of the preceding claims, wherein the first and second gear selector assemblies can select the following operational mode: not lock the gear element for rotation in the forward and reverse torque directions. This provides a neutral condition.

24. A transmission system including a first shaft, first, second and third gear elements rotatably mounted on the first shaft and at least a first gear selector assembly arranged to selectively lock each of the first, second and third gear elements for rotation with the first shaft from operational modes that include the following operational modes: lock the gear element for rotation with the first shaft in the forward and reverse torque directions; lock the gear element for rotation with the first shaft in the forward torque direction and not lock in the reverse torque direction; lock the gear element for rotation with the first shaft in the reverse torque direction and not lock in the forward torque direction.

2008/003914

29

25. A transmission system according to claim 24, including at least one additional gear element rotatably mounted on the first shaft, wherein the first gear selector assembly is arranged to selectively lock the or each additional gear element for rotation with the first shaft from operational modes that include the following operational modes: lock the or each additional gear element for rotation with the first shaft in the forward and reverse torque directions; lock the or each additional gear element for rotation with the first shaft in the forward torque direction and not lock in the reverse torque direction; lock the or each additional gear element for rotation with the first shaft in the reverse torque direction and not lock in the forward torque direction.

Description:

Transmission system

The present invention relates to transmission systems, in particular to dog-type and under- gear type transmission systems.

In conventional single clutch synchromesh and non-synchromesh transmission systems for vehicles it is necessary to disengage the transmission from the power source, such as an engine or motor, by operating the clutch before the current gear is deselected and the new gear is engaged. If the power is not disengaged when attempting to engage a new gear the synchromesh is unable to engage the new gear wheel or has to be forced into engagement with the risk of damaging the transmission and creating torque spikes in the transmission. This is because in most cases the speed of the engine is not matched to the speed of the new gear. For motor vehicles such as cars having conventional gearboxes and powered by an engine, the selection of a new gear ratio typically takes between 0.5 and 1 second to complete. So, for example, when a higher gear is selected the time delay allows the engine to reduce its speed [due to its own inertia] to more closely match the speed of the new gear before the clutch re-connects the engine and the transmission, thereby reducing the possibility of torque spikes occurring when the power is reapplied.

An instantaneous transmission system is arranged such that a new gear can be selected before the current gear is disengaged under power for at least some shift types. These transmission systems include at least one instantaneous gear selector mechanism, which typically has four modes of operation with respect to each of the rotatably mounted gear wheels associated with it:

Fully engaged in both torque directions (fully in gear);

Disengaged in both torque directions (neutral);

Engaged in the forward torque direction while disengaged in the reverse torque direction;

Disengaged in the forward torque direction while engaged in the reverse torque

direction.

It is the last two modes that enable a discrete ratio gearbox to have the ability to shift up or down ratios instantly under load without torque interruption. In some embodiments it is not necessary to have a neutral mode.

Examples of known instantaneous transmission systems are disclosed in WO2004/099654, WO2005/005868, WO2005/005869, WO2005/024261, WO2005/026570, WO2006/095140, WO2006/123128, WO2006/123166, WO2007/132209, WO2008/062192, and WO2008/096140. A limitation with known instantaneous transmission systems is that each gear selector device can only selectively engage two gear wheels. Typically each gear selector device is mounted between a pair of gear wheels and the maximum extent of movement for the selector device is limited by those gear wheels. When performing shifts requiring the operation of two selector assemblies, there is a possibility of the transmission system locking up under certain shift conditions, particularly if there is a torque reversal during a shift.

For powered grass cutting devices, such as sit on lawn mowers, a sequential type transmission system is often used to select gears. However because of the selector assembly used, there is considerable wear on the engagement elements which often leads to component failure, and ultimately failure of the transmission. Typically transmissions systems may only last for one year and then have to be replaced thus leading to high maintenance costs.

Another problem for powered grass cutting devices is that when on hills, the power interruption during a shift can cause the grass cutting device to roll backwards. Since these devices are essentially off road vehicles, it is not uncommon for them to be come stuck in gullies or ditches due to the uncontrolled movement. This is very frustrating for the operator and may require another vehicle to rescue it, which is time consuming and costly.

Accordingly the present invention seeks to provide a transmission system that mitigates at least one of the aforementioned problems, or at least provides an alternative arrangement to known systems.

According to one aspect of the present invention there is provided a transmission system including a first shaft, a first gear element rotatably mounted on the first shaft and a first gear selector assembly including first and second sets of engagement members that are arranged to selectively lock the first gear element for rotation with the first shaft from operational modes that include the following operational modes: lock the first gear element for rotation with the first shaft in the forward and reverse torque directions; lock the first gear element for rotation with the first shaft in the forward torque direction and not lock in the reverse torque direction; lock the first gear element for rotation with the first shaft in the reverse torque direction and not lock in the forward torque direction; and wherein the first gear selector assembly is arranged to adjust the radial positions of the first and second sets of engagement members relative to the first shaft to selectively engage the first gear element to select the operational modes.

The invention provides a new approach to selectively engaging gear elements in a manner that includes important operational modes that enable instantaneous shifts to be performed.

Advantageously the first shaft includes a hollow portion for housing the first and second sets of engagement members and a plurality of apertures, and wherein the first gear selector assembly is arranged to adjust the radial positions of the first and second sets of engagement members independently of each other, from a first operational condition wherein the first and/or second set of engagement members is substantially retracted into the hollow portion to a second operational condition wherein the first and/or second set of engagement members protrude through the apertures to engage the first gear wheel.

The hollow portion is located along the first shaft such that at least a part of it is substantially aligned with the first gear element. The hollow portion includes an axial cavity and the first and second sets of engagement members are arranged to move axially within the axial cavity. In preferred arrangements the first shaft is tubular and this enables the first gear selector assembly to move the first and second sets of engagement members into positions wherein they are axially aligned with the first gear element so that they can select the first gear element internally.

The transmission system can include a second gear element rotatably mounted on the first shaft and second gear selector assembly including first and second sets of engagement members that are arranged to selectively lock the second gear element for rotation with the first shaft from operational modes that include the following operational modes: lock the second gear element for rotation with the first shaft in the forward and reverse torque directions; lock the second gear element for rotation with the first shaft in the forward torque direction and not lock in the reverse torque direction; lock the second gear element for rotation with the first shaft in the reverse torque direction and not lock in the forward torque direction, and wherein the second gear selector assembly is arranged to adjust the radial positions of the first and second sets of engagement members relative to the first shaft to selectively engage the second gear element to select the operational modes.

Advantageously the first and second sets of engagement members of the second gear selector assembly are housed in the first shaft. Advantageously the first and second selector assemblies are arranged to adjust the axial positions of their first and second sets of engagement members within the shaft to enable them to align with the gear elements such that the first selector assembly can lock the second gear element for rotation with the first shaft using the same operational modes that it has for the first gear element, and the second gear selector assembly can lock the first gear element for rotation with the first shaft using the same operational modes that it has for the second gear element.

Advantageously the first and second sets of engagement members of each selector assembly are arranged to move along substantially radial paths when adjusting their radial positions.

Advantageously the first gear selector assembly can be arranged to selectively engage the second gear element and the second gear selector assembly can be arranged to selectively engage the first gear element.

When one of the first and second gear elements is locked for rotation with the first shaft in the forward and reverse torque directions by one of the first and second gear selector assemblies, the other of the first and second gear selector assemblies is arranged to select the other of the first and second gear elements before the one of the first and second gear selector assemblies

fully disengages the one of the first and second gear elements for at least some shift types. This enables instantaneous shifts to be performed for some shift types. That is, a new gear can be selected without drive being interrupted. For example, when the first gear element is locked for rotation with the first shaft in the forward and reverse torque directions by the first gear selector assembly the second gear selector assembly is arranged to select the second gear element before the first gear selector assembly fully disengages the first gear element. Thus the gear selector assembly is arranged to selectively lock the first and second gear elements for rotation with the first shaft simultaneously, at least momentarily. Typically, this is only happens for a very short period of time during the shift, since when the new gear has been selected the loaded element set becomes unloaded and the control system is arranged to disengage it from it gear element and move it into engagement with the new gear element. This is an instantaneous gearshift. Similarly when the second gear element is locked for rotation with the first shaft in the forward and reverse torque directions by the second gear selector assembly the first gear selector assembly is arranged to select the first gear element before the second gear selector assembly fully disengages the second gear element.

Advantageously the first and second selector assemblies are arranged such that when a gear element is locked for rotation with the first shaft in the forward and reverse torque directions and a driving force is transmitted, one of the first and second sets of engagement members drivingly engages the engaged gear element, and the other set of engagement members is then in an unloaded condition. In preferred embodiments the first and second selector assemblies are arranged such that when a gear element is locked for rotation with the first shaft in the forward and reverse torque directions, and a braking force is transmitted, the first set of engagement members drivingly engages the engaged gear element and the second set of engagement members is in an unloaded condition, and when a driving force is transmitted, the second set of engagement members drivingly engages the engaged gear element and the first set of engagement members is then in an unloaded condition.

When one of the first and second gear elements is locked for rotation with the first shaft in the forward and reverse torque directions by one of the first and second selector assemblies, and the other of the first and second selector assemblies is in a neutral condition, the one of the first and second gear selector assemblies is arranged to move the unloaded set of engagement

members out of engagement with the currently engaged gear prior to the other of the first and second gear selector assemblies selecting the other of the first and second gear elements. For example, when changing gear from the first gear element to the second gear element and the first gear element is fully engaged by the first gear selector assembly, one of the first and second sets of engagement members is drivingly engaged with the first gear element and the other is in an unloaded condition. The first gear selector assembly is arranged to move the unloaded set out of engagement with the first gear element before the second gear selector assembly engages the second gear element. When changing gear from the second gear element to the first gear element and the second gear element is fully engaged by the second gear selector assembly, one of the first and second sets of engagement members is drivingly engaged with the first gear element and the other is in an unloaded condition. The second gear selector assembly is arranged to move the unloaded set out of engagement with the second gear element before the first gear selector assembly engages the first gear element.

Each of the selector assembly includes an actuator system arranged to actuate axial and radial movement of the first and second sets of engagement members. The actuator system can include a first actuator for controlling axial actuation of the first set of engagement members and a second actuator for controlling axial actuation of the second set of engagement members. The actuator system can include a first control device for controlling radial actuation of the first set of engagement members and a second control device for controlling radial actuation of the second set of engagement members.

In preferred embodiments the first set of engagement elements is mounted on a first support means and the actuator system includes a first biasing means for biasing the first set of engagement members in the direction of engagement with the adjacent gear element and wherein the first control device is arranged to control the radial position of the first set of engagement elements by acting against the first biasing means. The first control device is arranged to control the radial positions of the first set of engagement members by adjusting its axial position relative to the first set of engagement elements. Advantageously the support means can include a first set of resilient support arms each including an engagement element from the first set of enragement members mounted thereon and the control device is arranged to move from a first condition wherein the first set of engagement elements is substantially

retracted into the first shaft to a second condition wherein the first set of engagement elements engage the adjacent gear element. Advantageously the first control device includes a first set of roller elements that are arranged to act on the first support means.

The second set of engagement elements and the second control device can be arranged similar to the first set of engagement elements.

Advantageously the transmission system can include at least one subsequent gear rotatably mounted on the first shaft, wherein at least one of the first and second gear selector assemblies is arranged to lock the or each subsequent gear element for rotation with the first shaft from operational modes that include the following modes: lock the or each subsequent gear element for rotation with the first shaft in the forward and reverse torque directions; lock the or each subsequent gear element for rotation with the first shaft in the forward torque direction and not lock in the reverse torque direction; lock the or each subsequent gear element for rotation with the first shaft in the reverse torque direction and not lock in the forward torque direction. Typically, each rotatably mounted gear element is part of a gear train that transfers drive between the first shaft and a second shaft. Preferably transmission includes between three and twenty gear trains (cars tend to have four to eight gear trains, plus reverse and lorries around twelve to twenty gear trains plus reverse). For example, the first gear element can be part of a first gear train that includes a gear element fixed for rotation with the second shaft. The second gear element can be part of a second gear train that includes another gear element fixed for rotation with the second shaft.

Advantageously the first and/or second gear selector assemblies can be arranged to select the gear elements sequentially. This reduces the magnitude of the torque spikes produced when a new gear is selected since it minimises the relative rotational speed between the engagement elements and the new gear element. Advantageously the transmission system can be arranged such that one of the first and second gear selector assemblies is arranged to select only odd gears and the other of the first and second gear selector assemblies is arranged to select only even gears.

Advantageously the first and second sets of engagement members of the first gear selector assembly are arranged to interleave with the first and second sets of engagement members of the second gear selector assembly, thereby enabling the sets of engagement elements to move axially past each other. This enables the, for example, the first gear selector assembly to select a third gear element that is rotatably mounted on the first shaft while the second gear selector assembly engages the second gear element; and the second gear selector assembly to engage a the second gear element while the first gear selector assembly is engaged with the third gear element. When the first shaft is viewed from one end (or in cross-section) the sets of engagement members from the first gear selector assembly are arranged alternately with the sets of engagement members from the second gear selector assembly.

Advantageously each gear element that is arranged to be selected by the first and/or second gear selector assembly includes a set of internal drive formations. The set of drive formations is arranged such that they are engageable by the first and second sets of engagement elements from each selector assembly. Advantageously the set of drive formations comprises a set of internal dogs, and includes from one to six dogs, preferably two to four dogs, and more preferably still two dogs. Advantageously the first set of drive formations includes an equivalent number of drive formations as there are engagement elements in the first set of engagement members.

Advantageously the first and second gear selector assemblies can select the following operational mode: not lock the gear element for rotation in the forward and reverse torque directions. This provides a neutral condition.

Advantageously the first and second sets of engagement members are arranged to rotate, in use, with the first shaft. Preferably the first shaft is an input shaft and the second shaft is an output shaft and drive is transferred from the input shaft to the output shaft.

Advantageously the transmission system can include a control system for controlling operation of the first and second selector assemblies. Preferably the control system is an electronic transmission control unit that is programmed to control operation of the selector assemblies. This can prevent transmission lock up occurring by appropriate sequence control.

According to another aspect of the invention there is provided a transmission system including first and second rotatable shafts, said first shaft having an axial bore, means for transferring drive from one of the shafts to the other shaft including a first gear train having a first gear element rotatably mounted on the first shaft, a second gear train having a second gear element rotatably mounted on the first shaft, and a third gear train having a third gear element rotatably mounted on the first shaft, wherein each of the first, second and third gear elements includes internal drive formations, first and second gear selector assemblies for selectively transmitting torque between the first shaft and the first gear element, between the first shaft and the second gear element and between the first shaft and the third gear element, wherein the first and second selector assemblies each include first and second sets of engagement members and an actuator system for controlling the axial positions of the first and second sets of engagement members along the axial bore and for controlling the radial positions of the first and second sets of engagement members relative to the first shaft to move them into and out of engagement with the first, second and third gear elements via apertures formed in the first shaft, and wherein when one of the first, second and third gear elements is engaged by the first and second sets of engagement members of the first gear selector assembly, and a driving force is transmitted, one of the first and second sets of engagement members drivingly engages the drive formations on the engaged gear element, and the other set of engagement members is then in an unloaded condition and the actuator system is arranged to move the unloaded set of engagement members out of engagement with the engaged gear element prior to the second gear selector assembly selecting one of the unengaged gear elements.

With this arrangement, the transmission system can perform instantaneous gear shifts for multiple gear trains using only two gear selector assemblies for at least some shift types, without the possibility of lockup occurring if a torque reversal occurs during the shift. Multiple gears can be selected since the sets of engagement elements can be moved along the axial bore into alignment with each gear mounted on the first shaft. For example, a transmission system including five or more gear trains can be accommodated by use of just two gear selector assemblies.

Advantageously each selector assembly is arranged such that when a braking force is transmitted the first set of engagement members drivingly engages the engaged gear element,

and the second set of engagement members is in an unloaded condition, and when a driving force is transmitted the second set of engagement members drivingly engages the engaged gear element, and the first set of engagement members is then in an unloaded condition.

Preferably the selector assembly is arranged such that when the first and second sets of engagement members engage one of the first and second gear elements the backlash when moving between acceleration and deceleration is less than or equal to five degrees.

Preferably the actuator system for each gear selector assembly is arranged to bias the engaged set of engagement members out of engagement with the engaged gear element.

According to another aspect of the invention there is provided a transmission system including a first shaft, first, second and third gear elements rotatably mounted on the first shaft and at least a first gear selector assembly arranged to selectively lock each of the first, second and third gear elements for rotation with the first shaft from operational modes that include the following operational modes: lock the gear element for rotation with the first shaft in the forward and reverse torque directions; lock the gear element for rotation with the first shaft in the forward torque direction and not lock in the reverse torque direction; lock the gear element for rotation with the first shaft in the reverse torque direction and not lock in the forward torque direction.

The transmission system can include at least one additional gear element rotatably mounted on the first shaft, wherein the first gear selector assembly is arranged to selectively lock the or each additional gear element for rotation with the first shaft from operational modes that include the following operational modes: lock the or each additional gear element for rotation with the first shaft in the forward and reverse torque directions; lock the or each additional gear element for rotation with the first shaft in the forward torque direction and not lock in the reverse torque direction; lock the or each additional gear element for rotation with the first shaft in the reverse torque direction and not lock in the forward torque direction.

Advantageously the transmission system can be further arranged according to any configuration of the transmission system described herein.

According to another aspect of the invention there is provided a grass cutting device including a transmission system according to any configuration described.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, wherein:

Figure 1 is a sectional view of a general arrangement of a first transmission system in accordance with the present invention;

Figure 2 is a sectional view of an input shaft with a gear wheel rotatably mounted thereon and a selector mechanism for selectively locking the gear wheel for rotation with the shaft;

Figure 3 is an isometric view of the input shaft;

Figures 4 to 7 illustrate diagrammatically an accelerating upshift from first gear to second gear (parts of an actuator system are omitted for clarity);

Figure 8 is a sectional view of a general arrangement of a second transmission system in accordance with the present invention;

Figure 9 is a sectional view of an input shaft with a gear wheel rotatably mounted thereon and a selector mechanism for selectively locking the gear wheel for rotation with the shaft; and

Figure 10 is an isometric view of the input shaft.

Figure 1 shows a transmission including an output shaft 1, an input shaft 3 and first, second, third, fourth, fifth and sixth gear trains (or gear ratios) 5,7,9,11,12,14 (1 st , 2 nd , 3 rd , 4 th , 5 th and 6 th ) arranged to transmit drive between the input and output shafts 3,1. The first gear train 5 comprises a first gear wheel 13 rotatably mounted on the input shaft 3 via a bearing and a second gear wheel 15 fixed to the output shaft 1 in mesh with the first gear wheel 13. The second gear train 7 comprises a third gear wheel 17 rotatably mounted on the input shaft 3

and a fourth gear wheel 19 fixed to the output shaft 1 in mesh with the third gear wheel 17. The third gear train 9 comprises a fifth gear wheel 21 rotatably mounted on the input shaft 3 and a sixth gear wheel 23 fixed to the output shaft 1 in mesh with the fifth gear wheel 21. The fourth gear train 11 comprises a seventh gear wheel 25 rotatably mounted on the input shaft 3 and an eighth gear wheel 27 fixed to the output shaft 1 in mesh with the seventh gear wheel 25. The fifth gear train 12 comprises a ninth gear wheel 16 rotatably mounted on the input shaft 3 and a tenth gear wheel 18 fixed to the output shaft 1 in mesh with the ninth gear wheel 16. The sixth gear train 14 comprises an eleventh gear wheel 22 rotatably mounted on the input shaft 3 and a twelfth gear wheel 24 fixed to the output shaft 1 in mesh with the eleventh gear wheel 22.

The gear wheels 13,17,21,25,16,22 are mounted on the input shaft 3 in close proximity to each other. In some applications, gear ratios of adjacent gear trains are similar such that the rotational speeds of adjacent gear wheels are similar. For example, in some applications the next highest (or lowest) gear element along the shaft may only differ by one or a few gear teeth when compared with the previous gear element. This helps to reduce the magnitude of torque spikes that occur when a new gear is selected.

The input shaft 3 is tubular and includes eight longitudinal slots 3a that extend along the shaft substantially parallel to each other (see Figure 4). The slots are evenly distributed around the circumference of the shaft such that in the cross-section of the input shaft 3, they are spaced by approximately 45 degrees from each other. The length of the slots 3 a is such that all of the gear wheels mounted on the input shaft are exposed to the slots 3 a.

First and second selector mechanisms 29,31 are mounted in the input shaft 3 on a support shaft 2. The first selector mechanism 29 enters the input shaft 3 from a first end and the second selector mechanism 31 enters the input shaft 3 from a second end. The first and second selector mechanisms 29,31 are arranged to selectively engage the gear wheels 13,17,21,25,16,22 that are mounted on the input shaft 3 to selectively lock each of them for rotation with the shaft from the following operational modes: fully engaged in clockwise and anti-clockwise directions (fully in gear); disengaged in both clockwise and anti-clockwise directions (neutral); engaged in the clockwise direction while disengaged in the anti-

clockwise direction; and engaged in the anti-clockwise direction while disengaged in the clockwise direction, as explained further below.

The first selector mechanism 29 and the first gear wheel 13 will now be described. The first and second selector mechanisms 29,31 are similar and therefore the following description of the first selector mechanism 29 is applicable to the second selector mechanism 31. Also, the first gear wheel 13 is similar to the third 17, fifth 21, seventh 25, ninth 16 and eleventh 22 gear wheels, except with regard to size and number of gear teeth, and therefore the description of the first gear element 13 is applicable to those gear wheels.

Figure 2 shows the first gear wheel 13 having two dogs 20 (drive formations). Each dog 20 is preferably formed integrally with the first gear wheel 13, but this is not essential. The first gear wheel 13 is annular and each dog 20 is defined by creating two recesses or under cut portions 201 in the curved inner face of the annular gear wheel 13, which increase in depth gradually as they approach the dog 20. Each recess 201 has a helical lead in. Each dog 20 has two drive faces 20a and extends through an angle of approximately 90°.

The gear selector mechanism 29 is arranged to selectively engage the dogs 20 located on each of the gear wheels 13,17,21,25,16,22. The selector mechanism 29 includes first and second sets of engagement elements 35,36 and an actuator assembly 38.

The first set of engagement elements 35 includes a ring 135 having first and second resilient arms 135al35b protruding therefrom. Each arm 135a,135b includes a root portion that is connected to the ring 135 and a remote end that can move relative to the ring 135. The arms 135a,135b are located on the ring 135 such that they are spaced by approximately 180 ° and are arranged such that when in an unloaded state the arms splay outwards such that they are inclined to the plane of the ring and are capable of being loaded against their resiliency such that they are forced inwards and are arranged substantially perpendicular to the plane of the ring. Each arm 135a,135b includes an engagement element 28 towards its remote end that is arranged to protrude substantially perpendicularly outwards from the arm 135a, 135b. The arms 135a,135b are arranged to fit into the input shaft 3 such that the engagement elements 28 can move along a first pair of substantially opposite facing slots 3a formed in the input shaft.

The resiliency of the first and second resilient arms enables the engagement elements 28 to adjust their radial positions when the arms move from a first operational position wherein the arms 135a, 135b are not compressed to a second operational position wherein the arms 135a, 135b are compressed. Movement from one of the first and second operational positions to the other of the first and second operational positions is controlled by the actuator assembly 38, which is described below.

The second set of engagement elements 36 includes a ring 136 having first and second resilient arms 136al36b protruding therefrom. The ring 36 and arms 136a, 136b are similarly arranged to the equivalent components in the first set of engagement elements 35. The arms 136a, 136b are arranged to fit into the input shaft 3 such that the engagement elements 30 can move along a second pair of substantially opposite facing slots 3a formed in the input shaft, wherein the second pair of slots is offset from the first pair of slots by approximately 90°.

Each engagement element 28,30 has a first (engagement) end 28a,30a that is arranged to engage the dogs 20 formed in the first gear wheel 13 and a second (root) end that is connected to a resilient arm. The engagement ends 28a,30a of the first and second sets of engagement members 35,36 typically have the same configuration but are opposite handed, for example the engagement ends 28a of the first set of engagement elements 35 are arranged to engage the dogs 20 when the power is off and the input shaft is allowed to decelerate (reverse torque direction) and the engagement ends 30a of the second set of engagement elements 36 are arranged to engage the dogs 20 when the power is on such that the input shaft 3 is positively driven (forward torque direction).

The engagement end 28a,30a, of each engagement element 28,30 includes an engagement face 43 for engaging the dog 20, a ramp 45, and an end face 42. The end faces 42 limit the extent of radial movement of the engagement elements 28,30 by abutting the inside of the gear wheel 13. The engagement faces 43 may be angled to complement the sides of the dogs 20a so that as the engagement elements 28,30 rotate into engagement, there is face-to-face contact to reduce wear. Each ramp 45 slopes away from the end face 42. The angle of inclination of the ramp 45 is such that the longitudinal distance between the edge of the ramp furthest from the end face 42 and the plane of the end face 42 is larger than the height of the dogs 20. This ensures that the transmission does not lock up when there is relative rotational

movement between the engagement elements 28,30 and the dog 20 that causes the ramp 45 to move towards engagement with the dog 20. The dogs 20 do not crash into the sides of the engagement elements 28,30 but rather engage the ramps 45. As further relative rotational movement between the dogs 20 and the engagement elements 28,30 occurs, the dogs 20 slide across the ramps 45 and the ramps cause the engagement elements 28,30 to move radially inwards into the input shaft 3 away from the dog 20 so that the transmission does not lock up.

The arrangement of the gear selector mechanism 29 is such that it inherently prevents lockup of the transmission occurring when selecting a new gear.

When the engagement elements of the first and second sets 35,36 are interleaved, as in Figure 2, the engagement faces 43 of the engagement ends 28a of the first set of engagement elements 35 are adjacent the engagement faces 43 of the engagement end 30a of the second set of engagement elements 36, that is they face in opposite directions. When the first and second sets of engagement elements 35,36 are fully engaged with a gear, a dog 20 is located between one pair of adjacent engagement faces 43. The dimensions of the dog 20 and the ends of the elements are preferably such that there is little movement of each dog between the engagement face 43 of the acceleration element and the engagement face 43 of the deceleration element when the gear moves from acceleration to deceleration, or vice versa, to ensure that there is little or no backlash in the gear.

When both the first and second sets of engagement elements 35,36 engage a gear wheel drive is transmitted between the input and output shafts 3,1 whether torque is in the forward or reverse torque direction. During operation of the transmission system, one of the first and second sets of engagement members 35,36 drivingly engages the engaged gear wheel and the other set is not loaded. Which set depends on the direction of torque, i.e. whether it is in the forward or reverse torque directions.

The actuator assembly 38 controls the axial and radial movement of the first and second sets of engagement elements 35,36. The actuator assembly 38 is arranged such that the first and second sets of engagement elements 35,36 can move independently of each other. The actuator assembly 38 includes first and second axial actuators 140,142 (shown schematically

in Figure 1) and first and second axial actuator elements 144,146. The first and second axial actuators 140, 142 are force generator actuators and are preferably part of an electrical system for example, an electro-mechanical system or an electro-hydraulic system. The first and second axial actuator elements 144,146 are preferably in the form of independently controllable forks. Movement of the first set of engagement elements 35 is controlled by movement of the first actuator element 144, which is controlled by the first actuator 140. Movement of the second set of engagement elements 36 is controlled by movement of the second actuator element 146, which is controlled by the second actuator 142.

The actuator assembly 38 also includes a first control mechanism 200 for controlling the radial position of the first set of engagement elements 35. The first control mechanism includes a first radial actuator 202, a first radial actuator element 204, a support ring 206, first and second control arms 208,210 and first and second roller elements 212,214. The first control mechanism 200 is arranged such that the first and second roller elements 212,214 impinge on the resilient arms 135al35b respectively. The first radial actuator element 204, the support ring 206, the first and second control arms 208,210 and the first and second roller elements 212,214 are arranged to move as a unit when actuated by the first radial actuator 202 to adjust its axial position relative to the first set of engagement members 35. Thus the roller elements 212,214 can move along the resilient arms 135a,135b and the arrangement is such that the axial position of the roller elements 212,214 along the arms determines the radial positions of the engagement elements 28. When the first and second roller elements 212,214 are moved towards the engagement elements 28 the engagement elements are forced to move radially inwards and hence out of engagement from the adjacent gear wheel against the resiliency of the arms 135a,135b. When the first and second roller elements 212,214 are moved towards the root portion of the arms 135a,135b the engagement elements 28 move radially outwards, and if conditions allow, into engagement with the adjacent gear wheel under the resilient action of the arms 135a,135b. It will be appreciated by the skilled man that the ring 206 and the control arms 208,210 can be replaced by a tubular member, to provide a similar control function. In some embodiments, the tubular member can also replace the roller elements.

The actuator assembly 38 includes a second control mechanism 300 having a second radial actuator 302, a second radial actuator element 304, a support ring 306, first and second

control arms 308,310 and first and second roller elements 312,314. The second control mechanism 200 is arranged similarly to the first control mechanism 100, however it is arranged to control the radial positions of the second set of engagement elements 36.

Operation of the actuators 140,142,202,302 and hence the axial and radial movement of the first and second sets of engagement elements 35,36 is preferably controlled by a transmission control unit (not shown). The transmission control unit may include sensors (not shown) for determining the operational condition of selector mechanism 29 in the transmission. Typically these monitor the positions of the actuator members and hence the positions of the sets of engagement elements, for example whether they are engaged with a gear wheel or not. The sensors can be included in the actuators and may be, for example, Hall effect type sensors.

The transmission control unit is preferably in the form of an electronic logic control system driven by a processor, which runs software that is arranged to control operation of the first and second actuators and hence the first and second sets of engagement elements. The sequence programming is typically arranged to control movement of the gear selector mechanism 29 together with controlling the direction of torque in the transmission such that it prevents conflict shifts occurring. Being able to control the actuation of the first and second sets of engagement elements 35,36 totally independently by use of first and second actuators has the advantage that the magnitude and the timing of application of the biasing force applied by each actuator can be independently controlled. This means that even at low rotational gear speeds the engagement elements sets 35,36 do not accidentally disengage from the engaged gear wheel and thus no loss of drive is experienced.

Alternatively, the operation of the transmission can be controlled by cable.

The second selector assembly 31 is located in the input shaft 3 such that the first set of engagement elements 35 is located in a third pair of slots 3 a and the second set of engagement elements 36 are located in a fourth pair of slots 3a, the arrangement being such that the first and second sets of engagement elements from the first selector assembly 29 are arranged alternately with the first and second sets of engagement elements 35,36 of the second selector assembly (see Figure 2 — sets of engagement elements from the first selector mechanism are shown in cross-hatching). The combined operation of the first and second selector

mechanisms 29,31 enables a new gear to be selected while the current gear is still engaged for at least accelerating upshifts and decelerating downshifts without loss of drive and thus instantaneous shifts under can be performed. Operation of the first and second selector mechanisms 29,31 will now be described for a gear change between the first and second gears. However the steps are applicable to gear changes between any two gears.

The first and second engagement element sets 35,36 start in a neutral position (see Figure 1), that is, the first and second control mechanisms 200,300 for the first and second selector mechanisms 29,31 are positioned such that the first and second sets of engagement members 35,36 are withdrawn into the input shaft 3 and are not engaged with any of the gear wheels 13,17,21,25,16,22.

To select first gear 5 with the first selector mechanism 29, the second axial actuator 142 is actuated to slide the second set of engagement members along the second pair of slots 3 a in the input shaft until it aligns with the first gear wheel 13. The second control mechanism 300 is actuated to move the second set of engagement members 36 into engagement with the first gear wheel 13 (see Figure 4). This involves moving the roller elements 314,312 towards the root portion of the arms 136b thereby enabling the resiliency of the arms 136b to adjust the radial positions of the engagement elements 30 outwards from the input shaft 3 via the second pair of slots 3a, such that relative rotational movement between the input shaft 3 and the first gear wheel 13 enables the second set of engagement elements 36 to engage the dogs 20. The inner curved surface of the first gear wheel causes the engagement elements to be compressed into input shaft 3 slightly. However relative rotation between the first gear wheel 13 and the input shaft 3 causes the recesses 201 to align with the second engagement elements 36 and the resiliency of the arms 136b pushes the elements 30 radially outwards into the recesses 201 to engage the dogs 20. The first control mechanism 200 is similarly actuated to enable the first set of engagement elements 35 to move into engagement with the first gear element 13.

When the first gear wheel 13 is fully engaged, that is, the dogs 20 are located between one of the first and second engagement elements 28,30, power is transferred from the input shaft 3, to the first gear wheel 13 by the first engagement element set 35 when decelerating (reverse torque direction) and the second engagement element set 36 when accelerating (forward

torque direction) via the dogs 20. Power is transmitted to the output shaft 1 put via the gear wheel 15.

Whilst accelerating using the first gear wheel pair 5, the engagement faces 43 of the first engagement element set 35 are not loaded, whilst the engagement faces 43 of the second engagement element set 36 are loaded. When a user, or an engine management system (not shown) wishes to engage the second gear wheel pair 7 signals are sent to the transmission control unit to actuate the first selector assembly 29 to move the unloaded set of engagement elements (the first set 35 in this case) out of engagement with the first gear wheel 13. This is achieved by actuating the first control mechanism 200 to move the first set of engagement elements into the input shaft 3 (see Figure 5).

The transmission control unit also sends signals to the second control mechanism 300 to bias the second set of engagement elements 36 out of engagement with the first gear element 13. However, because the second set of engagement elements 36 is loaded, i.e. is driving the first gear wheel 13, it cannot be disengaged from the first gear wheel 13, and therefore the engagement elements of the second engagement element set 36 remain stationary.

The transmission control unit instructs the second selector mechanism 31 to engage the third gear wheel 17 after the unloaded set of engagement elements from the first selector mechanism 29 has been removed from the first gear wheel 13. This involves actuating the second axial actuator 142 to slide the second set of engagement members along the fourth pair of slots 3 a in the input shaft until it aligns with the third gear wheel 17 (if it is not already in position). The second control mechanism 300 is actuated to enable the second set of engagement members 36 to move radially outwards into engagement with the third gear wheel 17. This involves moving the roller elements 314,312 towards the root portion of the arms 136b thereby enabling the resiliency of the arms 136b to adjust the radial positions of the engagement elements 30 outwards from the input shaft 3 via the fourth pair of slots 3a, such that relative rotational movement between the input shaft 3 and the third gear wheel 17 enables the second set of engagement elements 36 to engage the dogs 20 (see Figure 6). The second set of engagement elements 36 then begins to drive the third gear wheel 17. Energy is transmitted from the input shaft 3 to the output shaft 1 by way of the second gear wheel pair 7. As this occurs, the second set of engagements 36 in the first selector mechanism 29 cease

to be loaded, and are free to disengage the dogs 20 in the first gear element 13. The second set of engagement elements 36 are caused to withdraw into the input shaft 3 under the loading of the second control mechanism 300 thereby completing the disengagement of the first gear wheel 13 from the input shaft 3 (see Figure 6). The first set of engagement elements 35 of the second selector mechanism 31 are moved into engagement with the third gear element 17 by operation of the first control mechanism 200 thereby completing engagement of the third gear wheel 17 (see Figure 7). This method for selecting gear wheel pairs substantially eliminates torque interruption since the second gear wheel pair 7 is engaged before the first wheel pair 5 is disengaged, thus momentarily, the first and second gear wheel pairs 5,7 are simultaneously engaged.

When a gear wheel is engaged by both the first and second engagement element sets 35,36 it is possible to accelerate or decelerate using a gear wheel pair with very little backlash occurring when switching between the two conditions. Backlash is the lost motion experienced when the dog moves from the engagement face 43 of the acceleration engagement element to the engagement face 43 of the deceleration engagement element when moving from acceleration to deceleration, or vice versa. A conventional dog-type transmission system has approximately 30 degrees of backlash. A typical transmission system for a car in accordance with the current invention has backlash of less than five degrees.

Backlash is reduced by minimising the clearance required between an engagement member and a dog during a gear shift: that is, the clearance between the dog and the following engagement member. The clearance between the dog and the following engagement member is in the range 0.5mm - 0.03mm and is typically less than 0.2mm. Backlash is also a function of the retention angle, that is, the angle of the engagement face 43, which is the same as the angle of the undercut on the engagement face of the dog. The retention angle influences whether there is relative movement between the dog and the engagement face 43. The smaller the retention angle, the less backlash that is experienced. The retention angle is typically between 2.5 and 15 degrees, and preferably is around 5 degrees for lawn mower applications.

To select the third (or subsequent) gear train 9, the first selector mechanism 29 is moved axially along the input shaft 3 until the sets of engagement members are axially aligned with the third gear train 9. The first selector mechanism 29 is able to reach past the second selector

mechanism 31 since the respective sets of engagement members 35,36 are angularly offset from each other. The arms 135a,135b,208,210,308,310 for the first selector mechanism 29 are shown with break lines in Figures 1, and 4 to 7 and therefore the full length of the arms is not shown. The arms 135a,135b,208,210,308,310 are sufficiently long to enable all of the gears to be selected.

Transition from the second gear wheel pair 7 to the first gear wheel pair 5 whilst decelerating is achieved by a similar process. Initially the second gear wheel pair 7 is fully engaged by the second selector mechanism 31.

Whilst decelerating in the second gear wheel pair 7 the engagement surfaces 43 of the second set of engagement elements 36 are not loaded, whilst the engagement surfaces 43 of the first set of engagement elements 35 are loaded. When a user, or an engine management system (not shown) wishes to engage the first gear wheel pair 5, the transmission control unit actuates the second control mechanism 300 to move the unloaded second set of engagement elements 36 out of engagement with the third gear wheel 17 and into the input shaft 3.

The transmission control unit also instructs the first control mechanism 200 to act on the first set of engagement elements 35, however since they are drivingly engaged with the dogs 20 on the third gear wheel 17, the first set of engagement elements 35 remains engaged.

The transmission control unit instructs the first selector mechanism 29 to engage the first gear wheel 13 after the unloaded set of engagement elements from the second selector mechanism 31 has been removed from the third gear wheel 17. The first selector mechanism 29 moves the sets of engagement elements 35,36 slide axially until they align with the first gear wheel 13, if they are not already aligned. The first control mechanism 200 is actuated to enable the first set of engagement members 35 to engage the dogs 20 on the first gear wheel 13. Drive is transmitted between the input shaft 3 and the first gear wheel 13 via the first set of engagement elements 35 and drive is transmitted from the input shaft 3 to the output shaft 1 by way of the first gear wheel pair 5. As this occurs, the first set of engagement elements 35 of the second selector mechanism 31 cease to be loaded by the third gear wheel 7 and the second control mechanism 200 withdraws the first set of engagement elements 35 into the input shaft 3 thereby completing disengagement of the third gear wheel 17. The first selector

mechanism 29 operates its second control mechanism 200 to move the second set of engagement elements 36 into engagement with the first gear wheel 13, thereby completing engagement of the first gear wheel 13 with the input shaft 1.

Kick-down shifts, that is a gear shift from a higher gear ratio to a larger gear ratio but where acceleration takes place, for example when a vehicle is travelling up a hill and the driver selects a lower gear to accelerate up the hill, have a brief torque interruption to allow disengagement of the engaged gear elements prior to the shift.

A second embodiment of the invention is shown in Figures 8 to 10. This is a more simplified version of the invention which includes only one (a first) selector mechanisms 529 mounted in the input shaft 503. The selector mechanism 529 enters the input shaft 503 from a first end. The selector mechanism 529 is arranged to selectively engage the gear elements 513,517,521,525,516,522 that are mounted on the input shaft 503 to selectively lock each of them for rotation with the shaft from the following operational modes: fully engaged in both torque directions (fully in gear); disengaged in both torque directions (neutral); engaged in the forward torque direction while disengaged in the reverse torque direction; and disengaged in the forward torque direction while engaged in the reverse torque direction.

In this embodiment, to shift between gears, it is necessary to fully disengage the currently engaged gear wheel before the new gear wheel is engaged. The gear selector device 529 includes an actuator mechanism 538 to control operation of first and second sets of engagement elements 535,536 in order to select between the operational modes for each gear wheel 513,517,521,525,516,522. The actuator mechanism 538 includes a cradle 540 having spring elements 542 located therein and a shift rod 544 and is arranged to control the axial positions and the biasing loads applied to the first and second sets of engagement elements 535,536. However, instead of using a control mechanism to control the radial positions of the first and second sets of engagement elements 535,536, radial positions are automatically adjusted by axial movement of the sets of engagement elements 535,536 since the engagement elements have a rounded or sloped leading face, such that when it comes into contact with the body of an adjacent gear element, it is forced into the input shaft 503 until the axial position is such that the resiliency of the arms enables the sets of engagement elements 535,536 to move into engagement with the new gear wheel.

Figures 9 and 10 show the input shaft 503 having slots 503a formed therein, the first gear wheel 513 having a dog 520 formed by recesses 501 and the arrangement of the engagement elements 528,530, in particular the engagement ends 528a,530a, which include engagement faces 543, ends 542 and ramps 545.

This type of transmission is particularly suited to low torque and / or low speed applications, for example in lawn mowers. It is particularly useful to provide powered gear shifts, for example on inclines so that the lawn mower does not roll backwards down the hill. It can also be used in any other application where the gear ratios are close, preferably where the number of gear teeth on adjacent rotatably mounted gear wheels differ by a low number, for example one to three.

It will be apparent to the skilled person that modifications can be made to the above embodiments that fall within the scope of the current invention. For example, for the second embodiment, if an actuator assembly is used that enables total independent movement of the first and second sets of engagement elements 35,36, the transmission can be arranged with a first being actuated from one end of the input shaft and the other set from the other end of the input shaft, and the sets can extend into the input shaft from different ends.

For both embodiments, as an alternative to using resilient arms, the arms can be pivotally attached to the support rings and a separate resilient member, or some other means, can be used to bias the engagement elements into engagement with the gear elements.

Use of instantaneous type gear selector mechanism leads to improved performance, lower fuel consumption and lower emissions since drive interruption during gear changes is substantially eliminated. Also the system is a more compact design than conventional gearboxes leading to a reduction in gearbox weight.




 
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