BACKGROUND TO THE INVENTION AND PRIOR ART

The present invention relates to an operating cylinder for a gearbox according to the preamble of claim 1. The invention relates also to a gearbox, which is provided with such an operating cylinder, according to the preamble of claim 10. The invention also relates to a vehicle, which is provided with such a gearbox, according to the preamble of claim 11. In vehicles, particularly in heavy vehicles, e.g. trucks, the gearbox is provided with one or more operating cylinders which have the function of changing the gear ratio across the gearbox. The operating cylinder controls the engagement and disengagement of gearwheels in the gearbox so that appropriate gear ratios are achieved. The cylinder is preferably of pneumatic type and operated by compressed air, although other fluids might be used to operate it, e.g. hydraulic oil.

The gearbox in trucks often takes the form of an automatic gearbox comprising a large number of interacting components for effecting the automatic gearchanging. The operating cylinder therefore needs to be as small as possible so that there is room for all of the components which need to be accommodated in the gearbox. One or more electronic units of the vehicle receive information via sensors about its propulsion and deliver signals to, inter alia, valves which supply/remove compressed air or hydraulic oil to/from the operating cylinder, which thus controls the engagement and

disengagement of gearwheels in the gearbox so that appropriate gear ratios for the vehicle's specific operating state are effected. GB-A-759235 refers to an operating cylinder for a motor vehicle gearbox, which cylinder can be put into three different operative positions by means of a pressurised fluid, e.g. compressed air or hydraulic oil, which is supplied through apertures formed in a cylinder housing. A piston in the cylinder is journalled against the inside of the cylinder housing by annular shell parts which have recesses for seals. The seals thus in this version absorb substantial force. At the same time, the cylinder housing needs to be of considerable length to allow it to have a long enough bearing surface for the annular shell parts. US-A-4593606 refers to an operating cylinder for a gearbox for a vehicle. The cylinder is provided with an extra piston, making it possible for a main piston to be put into an intermediate position. The main piston is provided with a plain bearing running round its outside and abutting against the inside of the cylinder housing. To allow the main piston to be journalled against the inside of the cylinder housing, the cylinder housing needs to be of sufficient length.

SUMMARY OF THE INVENTION

Despite known solutions, there is a need to further develop an operating cylinder which is of smaller axial extent than existing operating cylinders, thereby reducing the space requirement for the cylinder in the gearbox so that there is room in the gearbox for all of the latter' s necessary components.

The object of the present invention is to propose an operating cylinder with small dimensions for a gearbox.

Another object of the invention is to propose a compact operating cylinder with low manufacturing cost for a gearbox. These objects are achieved with an operating cylinder for a gearbox of the kind mentioned in the introduction, which is characterised by the features indicated in claim 1. A further object of the invention is to propose a gearbox with an operating cylinder with small dimensions. This object is achieved with a gearbox which is provided with an operating cylinder of the kind mentioned in the introduction and is characterised by the features indicated in claim 10.

A further object of the invention is to propose a vehicle with a gearbox with an operating cylinder with small dimensions.

This object is achieved with a vehicle which is provided with a gearbox of the kind mentioned in the introduction and is characterised by the features indicated in claim 11. Such an operating cylinder is of smaller axial extent than existing operating cylinders, thereby reducing the space requirement for the operating cylinder in the gearbox so that there is room in the gearbox for all of the latter' s necessary components. Vehicle gearboxes usually have a plurality of gear steps, making it necessary to provide two or more operating cylinders in the gearbox. This further increases the need for the respective cylinders to be of small dimensions. The possibility of the cylinder being of smaller dimensions also reduces the material requirement, leading to reduced manufacturing costs for the cylinder. The gearbox may as a whole be compact in cases where the operating cylinder is of small dimensions, leading to reduced weight of the vehicle and hence also to less fuel consumption.

Further advantages of the invention are indicated by the detailed description set out below.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention is described below by way of example with reference to the attached drawings, in which Fig. 1 is a schematic side view of a vehicle with an operating cylinder and a gearbox according to the present invention,

Fig. 2 is a sectional view of the operating cylinder according to a first

embodiment of the present invention in a first extreme position,

Fig. 3 is a sectional view of the operating cylinder according to the first

embodiment of the present invention in an intermediate position,

Fig. 4 is a sectional view of the operating cylinder according to the first

embodiment of the present invention in a second extreme position, and

Fig. 5 is a sectional view of the operating cylinder according to a second

embodiment of the present invention in a first extreme position.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Fig. 1 is a schematic side view of a vehicle 1, e.g. a truck. The vehicle is provided with a combustion engine 2 connected to a gearbox 4. The gearbox is connected to the vehicle's tractive wheels 6 via a propeller shaft 8. The gearbox is provided with an operating cylinder 10 which has the function of moving one or more gearwheels 12 in the gearbox with the object of changing the gear ratio across the gearbox. As gearboxes for vehicles usually comprise a plurality of gear steps, two or more operating cylinders need to be provided in the gearbox. The cylinder 10 is preferably of pneumatic type and operated by compressed air, although other fluids might be used to operate it, e.g. hydraulic oil.

The gearbox 4 in a truck often take the form of an automatic gearbox 4 comprising a large number of interacting components for effecting the automatic gearchanging. The operating cylinder 10 needs therefore to be as small as possible so that there is room for all of the components which need to be accommodated in the gearbox 4.

Fig. 2 is a sectional view of the operating cylinder 10 according to a first embodiment of the present invention in a first extreme position. It has a cylinder housing 14 provided with respective first and second endwalls 16 and 18 to form a space 20 in which a piston 22 is arranged to be able to move axially to and fro. A piston rod 24 is connected to the piston via a fastening element 26. The piston rod is arranged to extend through an aperture 28 in the first endwall 16. At its end which points away from the piston, the piston rod is provided with a fastening device 30 for connecting an arm which moves the gearwheels 12 in the gearbox for gear changes.

The piston 22 divides the space 20 formed by the cylinder housing 14, the first endwall 16 and the second endwall 18 into respective first and second spaces 34 and 36. A first port 38 is in communication with the first space 34, and a second port 40 is in communication with the second space 36. A fluid for moving the piston relative to the cylinder housing 14 is supplied and removed through the first and second ports 38, 40. These ports are connected to a pressure source 42 for fluid via first and second valves 44, 46 which are preferably controlled by signals from a control unit 48. Also connected to the control unit 48 is a sensor 50 which is fitted in the second endwall 18 and detects the position of the piston in the cylinder housing. This is achieved by means of a pin 52 which is connected to the piston and extends into an elongate cavity 54 in the sensor 50. The sensor 50 is preferably provided with an undepicted coil which registers by induction the position of the outer end of the pin relative to the coil. Using the sensor 50 to detect the pin's position in the elongate cavity makes it possible to determine the position of the piston 22 in the cylinder housing 14.

A third port 56 provided in the cylinder housing 14 is in communication with surrounding atmospheric pressure. This third port is in contact with a third space 58 formed between the piston 22, the cylinder housing 14 and respective first and second radial seals 60, 62 situated between them. These radial seals are designed to slide on the piston and the cylinder housing during axial movements of the piston relative to the cylinder housing. They prevent pressurised fluid from flowing between the first, second and third spaces 34, 36, 58. The third port 56 has the function of equalising with atmospheric pressure any negative or positive pressures which occur in the third space 58.

The first and second radial seals 60, 62 are provided with stiffening elements 64 intended to cooperate with a flange 68 which runs round the piston. These radial seals can therefore also serve as secondary pistons and transmit the force from the pressurised fluid to the piston's flange 68 in order thereby to exert axial force upon the piston.

The piston 22 is provided with an axial cavity 70 arranged to abut by means of a plain bearing 72 against a protrusion 74 situated on the second endwall 18. The cavity 70 is preferably of substantially circular shape with a centreline 76 which substantially coincides with the piston centreline 78. The protrusion 74 is preferably of

substantially circular shape with a smaller diameter than the cavity 70. In cases where both the cavity 70 and the protrusion 74 are of circular shape, the plain bearing 72 will also be substantially circular to provide the piston 22 with stable journalling. To provide the piston 22 with stable journalling along the whole of its travel, the axial extent of the cavity 70 and the protrusion 74 will be equal to or greater than the piston's 22 travel. The first endwall 16 is provided at its aperture with a piston rod bearing 80 which in conjunction with a piston rod seal 79 journals and absorbs radial forces between the piston rod 24 and the first endwall 16. The combined effect of the plain bearing 72 between the piston 22 and the protrusion 74 on the second endwall 18 in conjunction with the piston rod bearing 80 is that the piston and the piston rod can with great stability move to and fro in the cylinder housing 14. The plain bearing 72 between the piston 22 and the protrusion 74 on the second endwall also make it possible to reduce the axial extent of the operating cylinder 10, thereby reducing the space requirement for the cylinder 10 in the gearbox 4 so that there is room in the gearbox 4 for all of the latter' s necessary components. The fact that the cylinder 10 can be of smaller dimensions also reduces the material requirement, leading to lower manufacturing costs for the cylinder 10. The gearbox 4 as a whole may be of compact construction in cases where the cylinder 10 is of small dimensions, leading to reduced weight of the vehicle 1 and hence also to less fuel consumption.

In the first embodiment of the invention depicted in Fig. 2, the plain bearing 72 is situated in a recess 82 which runs round the inside of the cavity 70 in the piston 22. This means that the plain bearing 72 will accompany the piston's 22 reciprocating movements. The recess 82 is preferably situated in the vicinity of an end surface 84 of the piston 22 which faces away from the piston rod 24, resulting in greater stability along the whole travel of the piston 22.

As mentioned above, the operating cylinder 10 moves the gearwheels 12 situated in the gearbox 4 during gear changes. As gear changes take place from one gear step to another, with an intermediate neutral position, the piston 22 has to be directable to three positions in the cylinder housing 14, comprising respective first and second extreme positions and an intermediate position.

To move the piston 22 from the first extreme position depicted in Fig. 2 to an intermediate position depicted in Fig. 3, pressurised fluid is supplied to both the first and second spaces 34, 36 in the cylinder housing 14 via both of the first and second ports 38, 40. The valves 44, 46 therefore open and pressurised fluid from the pressure source is supplied to the first and second spaces. The piston will thus be caused to move to the intermediate position depicted in Fig. 3. The radial seals 60, 62 abutting against the flange 68 which runs round the piston not only exert forces in opposite directions upon the piston but also act sealingly relative to the third space 58 formed between these two seals, so that the pressurised fluid is kept in the first and second spaces 34, 36 and cannot reach the third space 58 and therefore cannot leave the cylinder housing 14 through the third port 56. The inside of the cylinder housing is preferably provided with a notch 86 which runs round it and provides abutment surfaces 88 for the two radial seals 60, 62. The axial extent of this circular notch 86 preferably corresponds to that of the circular flange 68. The substantially central location of the circular notch 86 in the cylinder housing 14 rmeans that the

intermediate position depicted in Fig. 3 will be achieved by the fact that both the circular notch 86 on the cylinder housing and the circular flange 68 on the piston 22 are fixed between the respective radial seals 60, 62. In the intermediate position depicted in Fig. 3, the operating cylinder 10 has directed the gearwheels 12 situated in the gearbox 4 to a neutral position. The piston's 22 movement from the first position to the intermediate position has moved the pin 52 further into the cavity 54 in the sensor 50. The sensor 50 therefore sends signals to the control unit 48 about the position of the piston 22 in the cylinder housing 14, enabling the control unit to decide which gear ratio is set in the gearbox. For the sake of simplification, the control unit 48, the valves 44, 46 and the pressure source 42 with associated connections and lines have been omitted in Fig. 3.

To move the piston 22 from the intermediate position depicted in Fig. 3 to a second extreme position depicted in Fig. 4, pressurised fluid is supplied to the first space 34 through the first port 38, while at the same time the second space 36 is evacuated of fluid through the second port 40. To prevent negative pressure in the third space 58, fluid will flow in through the third port 56. The first valve 44 therefore opens and pressurised fluid from the pressure source 42 is supplied to the first space 34. The second valve 46 prevents fluid from being evacuated through the second port 40. The piston 22 will then move to the second extreme position depicted in Fig. 4. The first radial seal 60 will abut sealingly against the circular notch 86 in the cylinder housing 14 and prevent pressurised fluid from reaching the third space 58 formed between the radial seals 60, 62, with the result that the fluid is kept in the first space 34 and cannot reach the third space 58 and therefore cannot leave the cylinder housing through the third port 56. In the second extreme position depicted in Fig. 4 the operating cylinder 10 has moved the gearwheels 12 in the gearbox 4 to a second gear position. The piston's 22 movement from the intermediate position to the second extreme position has moved the pin 52 further into the cavity 54 in the sensor 50. The sensor therefore sends signals to the control unit 48 about the piston's position in the cylinder housing 14. For the sake of simplification, the control unit 48, the valves 44, 46 and the pressure source 42 with associated connections and lines have been omitted in Fig. 4. To return the piston 22 to the first extreme position, fluid is supplied to the second space 36, whereupon the first and third spaces 34, 58 are evacuated of fluid. There is preferably a passage 90 past the plain bearing 72 so that fluid can flow to and from the piston's axial cavity 70 via a transverse duct 91 in the second endwall 18, which duct is in communication with the second space 36. The second space 36 is thus in communication with the piston's axial cavity via the passage 90. The transverse duct 91 extends substantially radially, while the passage 90 extends substantially axially.

Fig. 5 is a sectional view of the operating cylinder 10 in a second embodiment of the present invention in a first extreme position. The first embodiment differs from the second in that the plain bearing 72 is situated on the protrusion 74, which makes it easy for the bearing to be removed and replaced by the second endwall 18 being separated from the cylinder housing 14 and being refitted after a new plain bearing 72 has been fitted on the protrusion 74.

Cited components and features indicated above may within the scope of the invention be combined with different cited embodiments.