BACKGROUND OF THE INVENTION AND PRIOR ART The invention relates to an actuator for a synchronizing unit in a gearbox according to the preamble of claim 1.

Conventional gearboxes comprise a main shaft, a counter shaft, a plurality of gearwheel pairs each comprising a primary gearwheel fixedly arranged on the countershaft and a secondary gearwheel rotatably arranged on the main shaft. The primary gearwheel and the secondary gearwheel of the gearwheel pairs are constantly in meshing engagement with each other. The main shaft may support synchronizing units arranged at the vicinity of the secondary gearwheels of the respective gearwheel pairs. Each synchronizing unit is used to selectively synchronize and lock one of the secondary gearwheels on the main shaft when a gear is to be engaged in the gearbox.

Each synchronizing unit comprises a displaceable coupling sleeve and a synchronizing ring which are rotationally fixedly arranged on the main shaft such they rotate at the same speed as the main shaft. The synchronizing ring is provided with a conically shaped friction surface adapted to come in contact with a correspondingly conically shaped friction surface of a coupling ring which is fixedly connected to a secondary gearwheel on the main shaft. When a gear is to be engaged in the gearbox, an actuator provides a movement, via a shift fork, which displaces the coupling sleeve in an axial direction from a neutral position to a synchronization position. During this movement the pre-synchronizing process takes place, where the coupling sleeve is temporarily halted by e.g. a spring system to allow for removal of the oil present between the conical friction surfaces, to allow indexing of the synchronizing ring which will prevent the coupling sleeve to pass the synchronizing position until a synchronous speed between the synchronizing ring and the coupling ring is achieved, as well as to allow mechanical contact between the conical friction surfaces resulting in a torque that seeks to synchronize the rotational speed of the synchronizing ring and the coupling ring. The synchronizer ring and the coupling ring relatively quickly obtain a synchronous speed. When the synchronous speed has been reached, the coupling sleeve is displaced further in the axial direction to a locking position in which it comes in engagement with a locking element of the coupling ring. In the locking position, the coupling sleeve provides a rotary locking of the coupling ring and the secondary gearwheel on the main shaft.

Usually a pneumatic actuator is used to displace the coupling sleeve from the neutral position to the synchronizing position and from the synchronizing position to the locking position. The last part of the movement towards the synchronizing position can be defined as a pre-synchronizing process. During the pre-synchronizing process, oil is to be evacuated from the friction surfaces of the synchronizing ring and the coupling ring before they come into contact with each other. In order to provide a favourable pre-synchronizing process in which the oil is evacuated in a robust manner, the e.g. spring system needs to be able to halt the coupling sleeve for a sufficiently long time, a process which is aided by a low force and a slow increase of the force generated from the actuator. The synchronizing process starts when the friction surfaces have come into contact with each other. In order to achieve a favorable synchronizing process, the actuator has to act with a large force on the synchronizing ring such that the friction surfaces are pressed together with a large force. Consequently, it is difficult to design an actuator having the property of providing a favorable pre-synchronizing process and a favorable synchronizing process. SUMMARY OF THE INVENTION

The object of the present invention is to achieve an actuator of a synchronizing unit in a gearbox having a simple design and a reliable function at the same time as it provides a favorable pre-synchronizing process and a favorable synchronizing process.

The above mentioned objects are achieved by the actuator defined in the characterized part of claim 1. Thus, the actuator comprises a valve mechanism configured to provide a restricted flow area for a compressed medium flow to the first chamber when the friction surface of the first component is moved towards the friction surface of the second component. The restricted compressed medium flow to the first chamber of the actuator results in movement of the piston with a relatively slow speed. Since the volume of the first chamber successively expands during the movement of the piston, the pressure acting on the first piston will be low. Consequently, the piston obtains a movement with a low force. The movement of the piston is transmitted to the first component of the synchronizing unit such that it is moved towards the second component of the synchronizing unit with a correspondingly low speed and force. The low speed and force of the first component allows a robust evacuation of oil located between the friction surfaces of the first component and the second component during a pre-synchronizing process just before the contact surfaces come into contact with each other.

The movement of the piston is stopped at least temporarily when the contact surface of the first component has come into contact with the contact surface of the second component of the synchronizing unit. The expansion of the first chamber ceases when the movement of the piston is stopped. However, the restricted compressed medium flow to the first chamber continuous resulting in a pressure raise in the first chamber. In order to provide a faster pressure raise in the first chamber, the valve mechanism is configured to provide a larger flow area and a larger compressed medium flow to the first chamber when the first component has reached the synchronizing position. The larger medium flow to the first chamber results in a fast pressure raise in the first chamber. As a consequence, the piston provides a large force pressing together the friction surfaces of said components substantially immediately after the first component has reached the synchronizing position. The larger supply of the compressed medium flow to the first chamber results in a fast synchronizing process. According to an embodiment of the invention, the valve mechanism comprises a first inlet passage, a second inlet passage and a valve member configured to provide the restricted compressed medium flow to the first chamber via the first inlet passage and to provide the larger compressed medium flow to the first chamber via the first inlet passage and the second inlet passage. The first inlet passage and the second inlet passage may receive a compressed medium flow from a common line. The flow areas of the first passage and the second passage are dimensioned such that the first chamber receives a suitable restricted compressed medium flow during the pre-synchronizing process and a suitable larger compressed medium flow during the synchronizing process.

According to an embodiment of the invention, the first inlet passage is connected to a compressed medium source in the form of a compressed air source comprising a compressed air line provided with a valve configured to connect the first inlet passage with the compressed air source or air with ambient pressure. When the valve connects the first chamber with air of ambient pressure, the actuator is in an inactivated state and the synchronizing unit in a neutral position. When the valve connects the first chamber with the compressed air source, compressed air is supplied to the first chamber, via the first inlet passage, which moves the first component of the synchronizing unit from the neutral position to the synchronizing position. When the first component has reached the synchronizing position the valve mechanism supply compressed air to the first chamber via the first inlet passage and the second inlet passage which provides a fast synchronizing process. However, it is possible to use another medium man compressed air such as, for example, a hydraulic oil.

According to an embodiment of the invention, the second inlet passage is connected to the compressed medium source via an opening, wherein the valve member is configured to close the opening when the restricted compressed medium is to be supplied to the first chamber and to expose the opening when the larger compressed medium flow is to be supplied to the first chamber. The valve member may be configured to be moved from the closed position to the open position when the pressure in the first chamber exceeds a predetermined pressure. When the contact surfaces of the components come into contact with each other, the piston is stopped and the restricted compressed medium flow to the first chamber results in a pressure raise in the first chamber. The pressure raise in the first chamber indicates that the contact surfaces have come into contact with each other. Preferably, said increased pressure in the first chamber is used to initiate adjustment of the valve member from the closed position to the open position.

According to an embodiment of the invention, the valve member comprises a movably arranged valve body which in the closed position closes the opening and in the open position exposes said opening to the second inlet passage. Such a valve body may be given a very simple design. The valve body may comprise a surface to be in contact with the pressure in the first chamber and an opposite surface in contact with a valve spring. In this case, the valve spring moves the valve body to the closed position when the pressure in the first chamber is lower than the predetermined pressure value. When the pressure in the first chamber exceeds the predetermined pressure value, the valve body is moved by the pressure in the first chamber from the closed position to the open position against the action of the valve spring. In this case, the valve mechanism is automatically set in the open position when the contact surfaces of the components come into contact with each other and the pressure in the first chamber starts to rise. No control units needs to be used controlling the valve member. According to an embodiment of the invention, said opening to the second inlet passage is formed between a wall element dividing the first inlet passage from the second inlet passage and a wall of a valve housing enclosing the valve spring and at least a part of the valve body. In this case, the first inlet passage and the second passage may be arranged in parallel divided by the wall element. To define the opening by means of an already existing wall of the valve housing allows a simple design of the valve mechanism.

According to an embodiment of the invention, the flow area of the first inlet passage is smaller than the flow area of a line conducting the compressed medium from the compressed medium source to the actuator. This is a prerequisite for supplying a restricted compressed medium flow to the first chamber, via a compressed media line, from a compressed medium source containing the compressed medium with a substantially constant pressure. The flow areas of the first inlet passage and the second inlet passage are dimensioned in a suitable manner. The total flow area of the first inlet passage and the second inlet passage may correspond to the flow area in the compressed medium line.

According to an embodiment of the invention, the shift mechanism is a shift fork and a coupling sleeve transmitting a movement from the actuator to a first component in the form of a synchronizing ring and a second component in the form of a coupling ring. The actuator displaces the coupling sleeve in an axial direction from a neutral position towards a synchronizing position via the shift fork. The coupling sleeve brings the synchronizing ring to the synchronizing position in which the friction surface of the synchronizing ring comes in contact with the friction surface of the coupling ring.

When the synchronizing ring and the coupling ring have reached a synchronous speed, the coupling sleeve is displaced further in the axial direction to a locking position with the coupling ring in which the coupling ring and the secondary gearwheel are rotary locked on the main shaft.

According to an embodiment of the invention, the actuator comprises a spring member configured to brake the movement of the piston in a pre-synchronizing position. In this case, the speed of the first component will be reduced in a pre-synchronizing position just before the contact surfaces come into contact with each other. Such a spring member ensures a low speed of the first component during the pre-synchronizing process allowing a robust evacuation of oil located between the friction surfaces of the first component and the second component during the pre-synchronizing process. The piston rod may comprise a protruding portion configured to come in contact with the spring member in the pre-synchronizing position. In this case, the spring member brakes the movement of the first component when it moves from the pre-synchronizing position to the synchronizing position. Thus, the first component is able to move from the neutral position to the pre-synchronizing position without being braked by the spring member and from a synchronizing position to a locking position without being braked by the spring member. The spring member may be designed to be compressed in a radially outward direction by the protruding portion of the piston rod.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, a preferred embodiment of the invention is described as an example with reference to the attached drawings, on which:

Fig. 1 shows a gearbox comprising an actuator according to the invention,

Fig. 2 shows the actuator in Fig. 1 in a first position

Fig. 3 shows the actuator in a second position and

Fig. 4 shows the actuator in a third position.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Fig. 1 shows a part of a gearbox 2 in a vehicle 1. The vehicle 1 can be a heavy vehicle. The gearbox 2 comprises a housing and an input shaft 3 driven by a not shown engine. The gearbox 2 comprises further a counter shaft 4 provided with a plurality of gearwheels 5-7 of different sizes. The gearwheels 5-7 are fixedly mounted on the counter shaft 4. The gearbox 2 comprises a main shaft 8 provided with a plurality of gearwheels 10- 11 of different sizes. The gearwheels 6-7 on the counter shaft 4 are in constant meshing engagement with a gearwheel 10-11 on the main shaft 8 so mat they form a number of gearwheel pairs in the gearbox 2. Each gearwheel pair includes a primary gearwheel 6-7 fixedly attached on the counter shaft 4 and secondary gear wheel 10-11 rotatably arranged on the main shaft 8 or the input shaft 3.

The gearbox 2 is equipped with a split gear which in a first split position connects the input shaft 3 with the counter shaft 4 via a first gearwheel pair 5, 9 and which in a second split position connects the input shaft 3 with the counter shaft 4 via a second gearwheel pair 6, 10. The gearwheel pair 6, 10 provides a ratio that defines a gear in the gearbox 2 and the gearwheel pair 7, 11 defines another gear in the gearbox 2.

The secondary gearwheels 9-11 are rotatably arranged on the main shaft 8 or the input shaft 3 by means of bearings 12 that can be needle bearings. A first synchronizing unit

13 and a second synchronizing unit 14 are disposed adjacent to the secondary gear wheels 9-11 of the main shaft 8 and on the input shaft 3. Each synchronizing unit 13,

14 is configured to synchronize and lock at least one of the secondary gear wheels 9-11 on the main shaft 8 or on the input shaft 3. The first synchronizing unit 13 has the task to establish the different split positions. The first synchronizing unit 13 is able to connect the input shaft 3 with the countershaft 4 in the gearbox, via the first gearwheels pair 5, 9 in the first split position and, via the second gearwheels pair 6 10, in the second split position. The second synchronization device 14 is configured to synchronize and lock the secondary gearwheels 10, 11 on the main shaft 8.

Each synchronizing unit 13, 14 comprises one displaceable coupling sleeves 15 and two synchronizing rings 16. The coupling sleeves 15 and the synchronizing rings 16 are rotationally fixedly connected to each other. The coupling sleeves 15 of the first synchronizing unit 13 is connected to the input shaft 3 by a driver 17. Thus, the coupling sleeve 15 and the synchronizing rings 16 of the first synchronizing unit 13 rotate with the same speed as the input shaft 3. The coupling sleeves 15 of the second synchronizing unit 14 are connected to the main shaft 8 by two drivers 17, 18. Thus, the coupling sleeve 15 and the synchronizing rings 16 of the second synchronizing unit 14 rotate with the same speed as the main shaft 8. In this case, the synchronizing units 13, 14 are double acting. Thus, each synchronizing unit 13, 14 comprises two synchronizing rings 16 and two coupling rings 19. Each synchronizing ring 16 is provided with a conically shaped friction surface 16a adapted to come in engagement with a correspondingly conically shaped friction surface 19a of a coupling ring 19. The coupling rings 19 are fixedly connected to a secondary gearwheel 9-11 on the main shaft 8.

A control unit 20 controls the activation of the second synchronizing unit 14 during a gear exchange processes in the gearbox 2. The control unit 20 controls the activation of the synchronizing unit 14 by means of a schematically shown pneumatic actuator 21. The actuator 21 is connected to a compressed air source 22 via a compressed air line 23. The compressed air source 22 contains compressed air with a substantially constant pressure. The compressed air line 23 comprises a three way valve 24. The control unit 20 is able to adjust the three way 24 valve to a first position in which the actuator 21 is connected to ambient air of atmospheric pressure and to a second position in which the actuator 21 is connected to the compressed air source 22. The actuator 21 is connected, via a schematically shown shift fork 25, to the coupling sleeve 15 of the second synchronizing unit 14. The actuator 21 is, via the shift fork 25, able to displace the coupling sleeve 15 of the second synchronizing unit 14 in an axial direction in relation to the main shaft 8. In this case, the actuator 21is single-acting and it has the task to synchronize and lock the secondary gearwheel 10 on the main shaft 8.

Figs. 2 and 3 show the actuator 21 more in detail. The actuator 21 comprises a cylinder 30 provided with an inner space. A piston 31 is movably arranged in the inner space of the cylinder 30. The piston 31 divides the inner space of the cylinder 30 into a first chamber 32 and a second chamber 33. The second chamber 33 comprises a spring member 34 compressive in a radial direction. The spring member 34 is provided with a center opening. The piston 31 is fixedly connected to a piston rod 35 which transmit motions from the piston 31, via the shift fork 25, to the coupling sleeve 15 of the synchronizing unit 14. The piston rod 35 extends through the center opening of the spring member 34. The piston rod 35 comprises a protruding portion 35a having a larger diameter than the center opening of the axial spring member 34. The compressed air line 23 comprises a first inlet passage 36 and a second inlet passage 37 connected the first chamber 32 of the cylinder 30. A wall element 38 is arranged between the first inlet passage 36 and a second inlet passage 37. The compressed air line 23 is connected to the second inlet passage 37 via an opening 39. A valve member 40-42 controls the compressed air flow through the opening 39 to the second inlet passage 37 of the inlet passage. The valve member 40-42 comprises a valve housing 40 enclosing a valve spring 41 and a valve body 42. The valve housing 40 and the wall element 38 define the opening 39 to the second inlet passage 37. The cylinder 30 comprises at least one opening 43 through which the second chamber 33 is in constant contact with ambient air.

Fig. 2 shows the piston 31 in a first end position in the cylinder 30. The three-way valve 24 has connected the actuator 21 to ambient air of atmospheric pressure. A stop element 44 defines the first end position of the piston 31which is located at a distance from en end wall of the cylinder 30. The second inlet passage 37 has an opening located at a distance from the opening of the first inlet passage 36 in the first chamber 32. When atmospheric pressure prevails in the first chamber 32, the valve spring 41 is able to move the valve body 42 to a closed position in which it covers the opening 39 to the second inlet passage 37 of the inlet passage. When the secondary gearwheel 10 is to be engaged in the gearbox 2, the control unit

20 moves the three-way 24 valve from the first position to the second position such that compressed air is conducted from the compressed air source 22 to the first chamber 32 of the actuator 21 via the compressed air line 23. Since the valve body 42 covers the opening 39 to the second inlet passage 37, the compressed air flow enters the first chamber via the first inlet passage 36. Due to the fact that the first inlet passage 36 has a restricted flow area in relation to the compressed air line 23, a restricted compressed air flow is supplied to the first chamber 32. The supplied compressed air raises the pressure in the first chamber 32 to a level such that it provides a movement of the piston 31 from the first end position. The movement of the piston 31 is transmitted, via the piston rod 35 and the shift fork 25, to a corresponding movement of the coupling sleeve 15 of the synchronizing unit 14. The actuator 21 moves the coupling sleeve 15 and the synchronizing ring 16 from a neutral position.

The protruding portion 35a of the piston rod 35 comes in contact with the axial spring member 34 in a pre-synchronizing position which is shown in Fig. 3. The protruding portion 35a of the piston rod 35 comprises an inclined contact surface to come in contact with an inclined contact surface of the spring member 34. In the pre- synchronizing position, the protruding portion 35a presses the wall defining the center opening of the spring member 34 in a direction radially outwardly. The restricted compressed air in the first chamber 32 acts with a relatively low force on the piston 31. As a consequence the speed of the piston 31 will slowed down to a low speed when the protruding portion 35a of the piston rod 35 comes in contact with the spring member 34.The friction surface 16a obtains a corresponding movement with a low speed and a low force towards the friction surface 19a of the coupling ring 19. Such a movement results in an efficient evacuation of oil from the space between the friction surfaces 16a, 19a.

When the friction surfaces 16a, 19a have come in contact with each other, the actuator

21 has moved the coupling sleeve 15 to the synchronizing position. The contact between the friction surfaces 16a, 19a stops the movement of the coupling sleeve 15 and the piston 31. Since the movement of the piston 31 has stopped, the following