TITLE: A TRANSMISSION COMPONENT

FIELD OF THE INVENTION

This invention relates to a transmission component, in particular a shift drum or shaft, for use in motorcycle and other vehicle transmissions as well as a transmission system employing the component.

BACKGROUND TO THE INVENTION

Transmission systems employing drum shift between gears are known in the automotive industry. For example, US Patent No. 4754662 assigned to Honda discloses a typical example of conventional transmissions for motorcycles comprising a drum shift, or drum shifter, having cam grooves formed in an outer peripheral surface thereof, and shift forks mounted on a shaft for sliding movement therealong and received at one ends respectively in the cam grooves.

The other ends of the shift forks are engaged with shifters or shift collars mounted on an input shaft and an output shaft, and input and output gears are mounted on the input shaft and the output shaft, respectively. In operation, a gear shift is manually operated to arguably move the shift drum, so that the shifters are moved by the co-acting shift forks to drivingly connect the input and output shafts together through the meshed input and output gears, thereby transmitting the rotation of a crankshaft of an engine of the motorcycle to the output shaft at a selected gear or transmission ratio, the output shaft having a drive sprocket around which a drive chain extends to drive a rear wheel of the motorcycle in a well known manner.

Such transmissions are preferred for use in motorcycles, gear shift being achieved by movement of a gear shift pedal which enables the gears to be selected only in a rising or descending sequence and contrast with transmissions typically used in automatic applications.

For example, WO 2007/082572 describes a modular system for producing at least one first and second transmission device, in co-operation with a differential, for

a motor vehicle. The two transmission devices have an input drive train and an output drive train which can be coupled in terms of drive. As a result of a shaft in the input drive train which is formed either for the first transmission device as a two part common shaft or for the second transmission device which are arranged co axially with respect to one another, these devices may be produced with a very high number of common parts. The dual transmission devices are notable and the construction is not consistent with ¹ a drum shaft transmission.

Similarly, US 4922599 provides a five speed automotive transmission using the vast majority of the parts of the four speed transmission but modifies the four speed transmissions by extending the main shaft and the pinion shaft by threading shaft extensions onto both of these shafts. This is a transmission applied to powerful automobile engines and is readily distinguishable from the transmissions used in motorcycles which are conventionally of the drum shift type.

In the manufacture of motorcycles, in particular, assembly lines must cater for different speed transmissions for different vehicles. That is, a choice may be made as to whether the motorcycle transmission will be manufactured as a A-, 5or 6 speed transmission. Accordingly, it is necessary to have on hand drum shifters specifically manufactured for each speed: A-, 5- and 6- speed transmission. This requires an inventory of shift drums be maintained at the assembly plant and this creates an expense that it would be desirable to avoid.

SUMMARY OF INVENTION

The invention therefore provides a shift drum for an automotive transmission comprising a first module configured to accommodate a pre-selected baseline number of transmission ratios wherein the first module is provided with connection means for connection with a second module configured to accommodate one or more transmission ratios above the baseline number of transmission ratios.

This modular construction may avoid the need to hold shift drums having five and six transmission ratios, or gears, these shift drums potentially being held in inventory for a long period of time. It will also be appreciated that shift drums having provision for five or six transmission ratios will be larger than those required for four

ransmission ratios and so the manufacturing cost will be higher. The modular construction allows shift drum first modules, having for example three or four transmission ratios excluding reverse and neutral, to be manufactured according to the assembly plant's schedule for total production of vehicles. Then, it is only necessary to manufacture second shift drum modules in such numbers as required to meet the production schedule for vehicles requiring 5- or 6- speed transmission. In this way, inventory and manufacturing costs may be reduced. Still further inventory reduction may be achieved by using common transmission housings or casings which will accommodate the desired range of transmission ratios, for example four to six transmission ratios.

The baseline number of transmission ratios provided for in the first shift drum modules may be selected as desired at the assembly plant. Three or four baseline transmission ratios, excluding reverse and neutral, may be provided for but this is flexible with the application. Similarly, the second modules may be provided with one, two, or three transmission ratios above the base line, to cater for 5- or 6- speed transmissions. Costs of manufacture are still controlled since shift drums need no longer be manufactured specifically for 5- and 6- speed transmissions, thus saving material and manufacturing costs.

The first and second modules are connected end to end, the length of the second module being desirably significantly shorter in length than the first module.

The second modules may form another aspect of the invention. In this aspect, there is provided a supplementary shift drum module configured to accommodate a p re-determined number of transmission ratios and comprising connection means for connecting it directly or indirectly, to a first shift drum module configured to accommodate a pre-selected baseline number of transmission ratios for a vehicle transmission. That is, the supplementary or second shift drum module may be connected directly to the first shift drum module or may be connected to a further supplementary shift drum module, that is, indirectly to the first shift drum module.

In a further aspect, the present invention provides a transmission system comprising:

a casing; an input shaft and an output shaft; and a shift drum for selecting a desired transmission ratio.

Wherein the shift drum comprises a first shift drum module configured to accommodate a pre-selected baseline number of transmission ratios connected to a second shift drum module configured to accommodate one or more transmission ratios above the baseline number of transmission ratios.

In a still further aspect, there is provided a method of assembling motor vehicles having a drum shift transmission comprising fitting the motor vehicle with a transmission including a shift drum comprising at least one module having a baseline number of transmission ratios, said module being connectable to a second module having a number of transmission ratios as required above the baseline number of transmission ratios.

Such an assembly method saves manufacturing costs for drum shifter transmissions as above described. It also provides for the possibility of retrofitting or "after market" use.

The shift drum, drum shifter or drum selector - all terms which may be used interchangeably - may be provided in the form of a hollow cylinder. An outer peripheral surface of the shift drum is cut with cam grooves, positions and sections of which correspond to the required number of transmission ratios. In the modular construction, both modules are also of cylindrical shape, the first module being cut with cam grooves provided with positions and sections correspondent with the baseline number of transmission ratios. The second shift drum module is cut with cam groove(s) correspondent with the required above baseline number of transmission ratios or gears.

The shift drum of the present invention may be applied in any type of vehicle but, given the conventional use of such drum shifters in motorcycles, application to motorcycles is highly advantageous.

The modular shift drum may be supported by a modular construction for input and/or output shafts as well, these shafts being provided with extensions according to requirements for transmission ratios above a baseline number. Production and cost advantages are the same as for shift drums. The provision for extensions for input and output shafts forms another aspect of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The modular shift drum of the present invention may be more fully understood from the following description of a preferred embodiment thereof made with reference to the accompanying drawings in which:

Figure 1 is a schematic sectional view of a 4- speed transmission incorporating a drum shifter.

Figure 2 is a sectional view of the first module of the shift drum incorporated in the 4- speed transmission of Figure 1.

Figure 3 is a schematic sectional view of a 5- speed transmission incorporating a modular drum shifter in accordance with one embodiment of the invention.

Figure 4 is a sectional view of the second module of the shift drum incorporated in the 5 - speed transmission of Figure 3.

Figure 5 is a sectional view of the gear box assembly of the first and second modules as shown in Figures 3 and 4.

DESCRIPTION

Referring now to Figure 1 , there is shown a 4- speed transmission, for a vehicle such as a motorcycle, having a casing 30 which houses an input shaft 12 and an output shaft 13. The input shaft 12 and output shaft 13 are disposed in parallel relation and have four forward input gears and four forward output gears respectively mounted on them. The forward input and output gears are in constant mesh.

A shift drum 19, of hollow cylindrical form, is mounted within casing 30 and rotates or moves in indexing movements in response to operation of a gear shift lever (not shown), such as a foot pedal, operated by a driver of a vehicle

incorporating the transmission. Linkages between the foot pedal and the shift drum 19 - which need not be described here at any length - transmit force acting on the foot pedal to rotate the shift drum 19 to select a required gear or transmission ratio.

Referring to Figure 2, the outer peripheral surface of the shift drum 19 is formed or cut with three longitudinally spaced cam grooves 30, 31 and 32. Three corresponding shift forks 40, 41 and 42 are provided, each of the shift forks 40, 41 and 42 being provided, at one end, with a pin 40a, 41 a, 42a slidably received in a respective one of the cam grooves 30, 31 and 32 in the shift drum 19. Shift forks 40-42 move along a guide shaft 50 in response to angular movement of the shift drum 19 as a gear or transmission ratio is selected. The opposite ends of shift forks 40, 41 and 42 are fitted to shift collars 60a, and 60b. As shift drum 19 rotates, shift collars 60a and 60b move - in response to movement of the shift forks - to engage forward input and output gears, correspondent with the selected gear or transmission ratio, thereby transmitting the rotation of the crankshaft to the output shaft 13 (through input shaft) with the required transmission ratio.

The shift drum 19 is of integral construction, being manufactured in one piece having cam grooves 30, 31 and 32 formed therein. The shift drum 19 is manufactured for an expected production run of 4- speed (four forward and one reverse) transmissions. However, production of 5- and 6- speed transmissions could be required at the assembly plant. Costs of manufacture and holding the drums in inventory correspondingly accrue.

Figures 3 and 4 show a different modular construction for a shift drum 119. In this case, a generally cylindrical first module or shift drum portion 119a is manufactured or cut with cam grooves 140, 141 and 142 suitable for a 4- speed drum shift transmission. That is, shift drum module 1 19a is configured with a baseline of four transmission ratios. Shift drum portion 119a is also provided with connection means 120 to enable end to end connection to the second module or shift drum extension 119b which is also of generally cylindrical form though of significantly shorter length than first module 119a. Further detail of the first shift drum portion 119a is apparent from Figure 3. The connection means may, for example, be a Woodruff key. Shift drum extension 119b is manufactured with a further cam

groove 143 enabling a fifth speed, gear or additional transmission ratio, above the baseline number, to be incorporated within the transmission. Further detail of the shift drum extension 119b is apparent from Figure 4. It will be appreciated that shift drum extensions 119b could be manufactured for use in 6- speed transmissions.

Shift drums 119 of modular construction aid the management of inventory and manufacturing costs for motorcycle assembly or component plant. First drum portions 119a may be used in any transmission having 4 speeds or higher. Thus, first drum portions 119a need only be manufactured on a "just in time" basis in accordance with the production schedule for 4- speed or higher transmissions. Shift drum extensions 119b are then manufactured in accordance with the need for 5- and/or 6- speed transmissions, again on a "just in time" basis. There is no requirement to have separate manufacturing lines for 4- , 5- and 6- speed transmissions. Only manufacturing lines for first and second modules are needed, thus saving manufacturing cost. Also, inventory costs fall as it should not be necessary to have holdings of first and second drum shift modules beyond the planned "just in time" schedule.

Again referring to Figure 4, input and output shafts 112 and 113 are likewise formed in a modular manner, with shaft extensions 112b and 113b, connected to first shaft portions 112a and 113a, being provided for the 4-speed transmission. It may be noted here that modular manufacture and assembly of transmissions in accordance with the various aspects of the invention does not cause any change to transmission casings or housings 30. These remain the same for 4 to 6 speed transmissions, though casings are often formed into upper and lower portions 30a and 30b, these portions being the same for both 5- and 6- speed transmissions and enabling the possibility of retrofitting higher transmission ratios if required by the customer.

Modifications and variations to the transmission component and system of the present invention may be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the current invention.