WO2009047520A2 - Improvements in park brakes of vehicles - Google Patents

Abstract

A park brake mechanism of a vehicle has a bi-stable latch giving alternative stable conditions of engagement and disengagement in response to a repeated uni-directional input pulse. Various embodiments are disclosed.

Description

Improvements in Park Brakes of Vehicles

This invention relates to a park brake of a vehicle, and particularly to a positive latch park brake of the kind consisting of a pawl and toothed wheel.

Positive latch park brakes are intended to latch a shaft against rotation, and are typically provided in a final drive portion of a rotary vehicle transmission. In a common arrangement, a toothed wheel of a rotary transmission component is engageable on demand by a pawl pivoted on the transmission casing.

Mechanical actuation of such a pawl can be problematic if the park brake mechanism is located in an inaccessible region of the transmission, and accordingly a suitable compact engagement and release arrangement is required.

According to the invention there is provided a park brake mechanism comprising a toothed wheel, a pawl engageable on demand with said wheel to prevent rotation thereof, and an actuator for said pawl, wherein said actuator comprises a bi-stable latch having alternate stable conditions and adapted to change condition in response to a unidirectional input pulse.

Such an actuator is substantially simplified due to the unidirectional actuation, which means that the input pulse is insensitive to the state of engagement or disengagement of the park brake mechanism. Accordingly the usual reversing actuator is not required, which can reduce control requirements, and the design envelope which is required to physically contain the latching actuator and bi-stable latch.

A fluid actuator is advantageous for example a hydraulic cylinder or ram, because it can be disabled when not required, and thus saves energy. The bi-stable states require no external energy sources so long as a change of state is not required.

Preferably the bi-stable latch comprises input and output members having interengaging teeth, and a ground member to support the output member in either stable condition. The input member in the preferred embodiment is actuated to separate the output and ground members, and permit transition thereof between the bistable conditions. The input and output members are preferably relatively rotatable, and said relative rotation is preferably unidirectional. Such an actuator has alternate length conditions.

In the preferred embodiment the ground member is intermediate the input and output members.

Preferably the input, output and ground members have interengageable teeth whereby engagement of angled end faces thereof imparts a relative rotational force therebetween. The side faces of said teeth preferably comprise abutment faces to restrain relative rotation.

In the preferred embodiment, the pawl is resiliently urged to the disengaged condition and includes a ramp face opposite the pawl tooth for engagement by an operating cam. The cam is preferably a pivotable finger having a roller at the free end for engagement with said ramp face. A stop is provided in the preferred embodiment to limit pawl engaging movement of the finger, and a light spring provided to bias said finger away from the stop.

A relatively heavy spring is provided between the actuating mechanism and the cam finger in order to provide for relative movement of the movable parts of the actuating mechanism.

The actuating input member is in the preferred embodiment movable on an axis about which relatively rotatable members of the actuating mechanism are disposed. The input member may include a mechanical actuator, for example a cable, or a fluid actuator such as a fluid ram. In the preferred embodiment, the ram includes a hydraulic chamber having an inlet for fluid under pressure, and a restricted exhaust. Such an exhaust may permit a pressure pulse to exhaust as part of normal operation, or to ensure that hydraulic locking of the actuator is avoided. In an alternative embodiment, the bi-stable latch may comprise a reciprocal linear actuating element, a guide member movable by said element against a return bias, and a guide for the guide member, said guide defining two or more stable positions of the guide member in the movement direction of said linear actuating element, and the linear actuating element having opposite ramp faces to determine transverse movement of the guide member.

The actuating element preferably defines a contact edge for the guide element and comprising two peaks having ramp faces on either side thereof, the ramp faces being adapted to urge the guide element transversely.

The guide preferably comprises a continuous slot extending in the movement direction of the actuating element, and defining two or more return portions into which the guide element is in use urged by the return bias.

In an alternate embodiment, the bi-stable latch comprises a pivotable latch plate defining an unidirectional track and having alternate stable conditions corresponding to different angular positions of said latch plate. Said latch plate is in the preferred embodiment pivotable about an axis parallel to the axis of a pivotable pawl.

The latch plate preferably includes a guide element grounded on said mechanism and having a protrusion engageable in a track of the latch plate to determine the angular position thereof. Said guide element is preferably resiliently biased against said latch plate, for example by mounting on a cantilever leaf spring, and is disengageable from said latch plate on demand. The latter arrangement allows free pivoting of the latch plate and is useful where an alternate pawl engagement/disengagement mechanism is desirable, for example in case of failure of a primary latch plate actuator having a unidirectional input pulse.

Other features of the invention will be apparent from the following description of a preferred embodiment illustrated by way of example only in the accompanying drawings, in which:- Fig. 1 is a schematic diagram of the principal system components of a first embodiment of the invention.

Figs. 2-10 show schematically, in developed plan, one complete engagement and disengagement cycle of a bi-stable latch of the invention.

Figs. 11 -20 show schematically a second embodiment of a bi-stable latch, and

Figs. 21-24 show schematically a variant of the latch of Figs. 11-20.

Figs. 25-27 show an embodiment of the invention in sectional, complete and exploded form.

Figs. 28-45 show an alternative embodiment of the invention in sectional, complete and exploded form, from several viewing angles.

With reference to Fig. 1 , a toothed wheel 11 is typically part of a rotary final drive component of a vehicle transmission. Pockets 12 are formed around the periphery (in this embodiment 12 pockets are illustrated), and define upstanding teeth 13 therebetween.

A pawl 14 is pivoted on axis 15 to a ground element, for example a transmission casing, and is urged to a disengaged condition by return spring 16. In Fig. 1, the pawl 14 is shown in the engaged condition, in which the wheel 11 is latched against rotation.

A cam finger 17 is pivoted on axis 18 to the ground element, e.g. the transmission casing, and includes at the free end a roller 19 bearing on the outer face of the pawl 14. It will be appreciated that by pivoting the cam finger 17 clockwise, the pawl is moved into engagement with the wheel 11. If the cam finger is pivoted anticlockwise, the return spring 16 urges the pawl 14 to the disengaged condition. A light spring 21 biases the pawl clockwise, as illustrated. An actuator 22 comprises an actuation member 23 and bi-stable latch assembly 24. The actuation member is shiftable on demand to the right, as illustrated, against a light bias spring 25. Two methods of shifting the actuation member 23 are illustrated, a pull cable 26 (e.g. a bowden cable) and a fluid pressure ram 27. The ram 27 comprises an inlet 28 from a pressurized fluid source, a piston 39, and a throttled outlet 40 to permit pressurized fluid in the ram to drain to exhaust.

The latch assembly 24 is bi-stable, and switches sequentially between stable conditions upon each successive actuation of the actuation member 23. The latch assembly bears on the cam finger 17 via a relatively strong compression spring 29, and the bi-stable conditions correspond to engaged and disengaged conditions of the pawl 14. A stop 30 limits clockwise movement of the cam finger 17.

The latch assembly comprises three relatively rotatable toothed components, which for the purposes of further explanation are illustrated in a developed form in Figs. 2- 10. Rightwards movement of Fig. 1 is represented by downward movement of Figs. 2-10.

An input member 31 is provided on the ram side, an output member 32 on the pawl side, and an intermediate member 33 connected to ground.

The output member 32 has a plurality of equispaced projections in which are provided pairs of teeth 34,35. The intermediate member 33 has single teeth 36 engageable with the teeth of the output member. The input member 31 has equispaced projections on which are provided single teeth 37, also engageable with the teeth of the output member.

Fig. 2 shows the latch assembly in a condition corresponding to Fig. 1, i.e. with pawl and wheel engaged, and the cam finger urged against the stop 30 via the spring 29. The output member is urged by reaction of spring 29 against the intermediate member 33 with the teeth 36 thereof engaged between the teeth 34,35 of the output member. The interengaged ramp faces of teeth 35,36, in conjunction with the return force of spring 29, bias the side faces of the teeth into engagement. Movement of the input member 31 in response to movement of the actuation member takes up axial clearance between teeth 37 and teeth 34, and then pushes output member away from intermediate member to the position illustrated in Fig. 3 in which spring 47 is further compressed.

Once clear of intermediate member 33, the angled faces of teeth 37,34 and the return force of spring 29 cause relative rotation of the output member (to the right as illustrated).

Release of the actuation force on the input member 31 causes it to return under the action of spring 25, leaving the output member 33 supported on the teeth 36 of the intermediate member (Fig. 4). The return force of spring 29 causes the output member 32 to rotate further (Fig. 5), by virtue of the angled flank faces of the abutting teeth 34,36, until the sides of teeth 36 about the sides of teeth 34. In this condition (Fig. 6), the latch assembly is in a stable configuration, with the output member 32 axially further away from the cam finger 17, so that the pawl is free to move to the disengaged condition by virtue of spring 16.

Momentary movement of the actuation member 23 may be by pull on the cable 26, or by pressure pulse via the inlet 28.

A second momentary movement of actuation member 23 results in the sequence illustrated in Figs. 7-10. Once again, the input member 31 advances to push the output member 32 away from the intermediate member (Fig. 7), until the teeth 34,35 clear the teeth 36 (Fig. 8). In this condition, the teeth 37 slip over teeth 35 (as the output member rotates) to engage against the side of teeth 34 (Fig. 9). Retreat of the actuation member results in the output member being supported on the intermediate member (Fig. 10), and in the engaged condition illustrated in Fig. 2.

The teeth of Fig. 10 are displaced one unit in relative rotation, and will continue to do so on each engagement/disengagement cycle by virtue of the circular array provided. The input to the actuation member is identical for both engaging and disengaging movement, thus simplifying the control mechanism for the park brake.

Note that the fluid feed to the ram 27 may exhaust via the inlet 28 by connecting the inlet to drain. A fluid restrictor 40 may remain desirable in order to permit fluid pressure in the ram to drain in case of failure of a control mechanism for applying fluid pressure to the inlet 28.

It will be appreciated that the pre-load and rate of the springs is selected to avoid conflict whilst allowing correct operation of the components at a low operational force. Any type of spring may be considered, e.g. coil, torsion, diaphragm, elastomer etc. The shape of the pockets 12 and the tooth of the pawl 14 is selected by conventional means to provide smooth engagement and disengagement of the pawl, on demand. As with most such park brake mechanisms, the pawl is arranged to rattle over the pockets above a predetermined rotational speed, so as to ensure that the pawl cannot engaged unless the shaft is stationery, or almost so.

Typically gross angular movement of the pawl is small, and usually less than 5°. Positional movement of the cam finger 17, or for example a microswitch at stop 30, provides an indication of engagement for a control system of a vehicle.

In the drawings, SWl indicates an engaged state of the pawl 14. SW2 indicates energization of the actuation member 23, with output member 32 advanced.

A second embodiment of a bi-stable latch is illustrated in Figs.11 -20.

With reference to Fig. 11, a reciprocal sleeve 41 is movable axially from position 'OFF' to position ON' against the effect of a 'load' provided by return spring 45. A somewhat serpentine but continuous cam track 42 is formed in a guide plate 43 which is attached to a ground element, typically the casing of a transmission. The sleeve 41 carries a pin 44 located in the track 42, and it will be understood that as the pin 44 travels around the track 42, the sleeve 41 will reciprocate both axially and in rotation. The track 42 defines two stable positions for the pin 44 with respect to the force of the return spring, these being at the peak 46 (as illustrated) of the track, and by virtue of a notch 47 in the base leg of the track. These stable positions correspond to 'Position OFF' and 'Position ON'.

A linear actuator for the pin 44 comprises a plunger 48 having a slot 49 adapted to receive a fixed guide 50.

The actuating end of the plunger comprises a plurality of ramp faces adapted to place a desirable side load on the pin 44 so as to cause it to move unidirectionally around the track 42 (anticlockwise as viewed).

In use, Fig. 11 illustrates the stable condition corresponding to 'OFF', with the pin 44 at the peak 46.

Fig. 12 illustrates initial movement of the plunger by which the ramp face 51 urges the pin 44 leftwards, as viewed. Further plunger movement forces the pin 44 downwards (Fig. 13) until it crosses a peak 52 of the plunger to an opposite ramp face 53, which urges the pin rightwards as viewed.

At Fig. 14 the sleeve 41, which is moved by the pin 44, travels beyond 'ON', and the return spring 45 is fully compressed. Rightwards movement of the pin is arrested by a protrusion 54 of ramp face 53. The plunger 48 is at full axial travel.

At Fig. 15, the plunger 48 reverses, and the pin 44 is urged by return spring 45 into the notch 47, allowing the sleeve to retreat to 'ON' corresponding to the alternative stable condition. The plunger returns to the rest position (Fig. 16).

Figs. 17-20 illustrate a further reciprocation of the plunger 48 whereby the pin 44 is disengaged from the notch by ramp face 55 (Fig. 17), urged rightwards (Fig. 18), returns round the track by virtue of return spring 45 (Fig. 19), and adopts the first stable position again (Fig. 20) at which the plunger returns to the rest position. It will be understood that, as with the first embodiment a repeated unidirectional input movement, via the plunger 48, causes the sleeve to adopt one of two alternative positions in sequence. Such a mechanism can be used to actuate a park brake, for example via the cam finger arrangement described in Fig. 1. Suitable additional guide and support elements are not illustrated, but are within the skill of the skilled person, and dependant on the structure surrounding the bi-stable actuation device.

In a preferred embodiment, one or more duplicate bi-stable arrangements may be provided to eliminate unwanted transverse loads, and to reduce stress.

Figs. 21-24 illustrate a variant of the embodiment of Figs. 11-20, in which an additional stable condition 61 is provided in the return side of the track 42 to define an intermediate position for the sleeve 41.

In this arrangement the 'OFF' condition B corresponds to the intermediate position of the sleeve 41, and one further actuation movement of the plunger 48 is required to move the point/sleeve to a 'Ready to engage' condition C represented by peak 46. Such an arrangement ensures that a change of state of the sleeve could not occur in certain kinds of error conditions resulting in a single cycle of the plunger 48. Further intermediate conditions are of course possible, and within the competence of a suitably skilled person. Note that in Figs. 21-24, the shape of the contact portion of the plunger is unchanged.

The embodiments of Figs. 11-24 are illustrated schematically, and it will be understood that the plunger and sleeve elements, and the cam and track elements may take other physical forms which provide an unidirectional escapement.

Figs. 25-27 illustrate an embodiment of a bi-stable latch and park brake.

A toothed wheel 70 has a pawl 71 pivotable about shaft 72. The shaft 72 is supported in a mounting bracket 73, and the pawl 71 acted upon via a cam lever 74 having a position indicator 75 and roller 76. A hairpin return spring 77 is provided for the cam lever 74, which is mounted on shaft 78. A bi-stable latch assembly comprises a latch shroud 81 having a return spring 82 and a linear pulse generator comprising a hydraulic ram 83 having a piston 84 and return spring 85. A cam track 87 is provided around the latch shroud 81. A ram closure 86 has suitable connections to a source of pressure pulse.

Also provided is a manual override shaft 91 operable on the bi-stable latch shroud via a cam lever 92. A hairpin return spring 93 biases the override shaft 91 to the inactive condition. The override shaft has an input lever 94, and a backstop 95 on which is provided an operating pin 96 for the cam track of the latch shroud 81.

A proximity sensor 97 provides an indication of the operational state of the bi-stable latch, and thus an indication of whether the parking brake is engaged.

Operation is as previously described with repeated unidirectional pulsing of the ram 83 (hydraulic actuator) causing axial and rotary reciprocation of the shroud 81, and successive engagement and disengagement of the pawl.

An alternative embodiment is illustrated in Figs. 28-45.

A toothed wheel 170 has a pawl 171 pivotable about a shaft axis 172a. The corresponding shaft 172 is supported in a mounting bracket 173, and the pawl 171 is acted upon via a freely pivoting cam lever 174 with position indicator lever 175. A hairpin return spring 177 is provided for the cam lever 174, which is mounted on a shaft 178. A hairpin return spring 169 is provided for the pawl 171.

A bi-stable latch mechanism comprises a latch plate 201, described in greater detail below, and mounted for pivoting on the shaft 178. A hairpin spring 202 urges the latch plate upwardly to the rest position, and a release lever 203 is fixed on the end of the shaft 178. Also fixed to shaft 178 is a manual override lever 210 which is illustrated to the left of the cam lever 174 but in use sits between two legs of the cam lever so as to bear on the inner face thereof.

A leaf spring 204 is upstanding from the shaft 172 and imparts a biasing force on the latch plate 201 so as to urge it towards the toothed wheel 170. The free end of spring 204 carries a guide pin 205 which bears directly on the outer face of the latch plate. The spring 204 is secured on the shaft 172 by boss 208 and roll pin 209.

A proximity sensor 197 provides an indication of the operational state of the bi-stable latch, and thus an indication of whether the parking brake is engaged.

A hydraulic actuator (linear pulse generator) comprises a hydraulic ram 183 having a piston 184 and return spring 185. A ram closure 186 has suitable connections to a source of pressure pulse. The piston 184 acts on an arm 207 of the latch plate 201.

Operation of the park brake is as follows, and with additional reference to Figs. 30-39.

BRAKE ON (Fig. 30 & Fig. 31) The pawl 171 is engaged with a tooth of the wheel 170 to prevent angular movement thereof. The piston 184 is fully retracted (i.e. ram unpressurized), and the latch plate urged upwardly by hairpin spring 202. The cam lever 174 is in the uppermost condition, where it tends to remain due to the geometry of the rear face of the pawl. The manual release lever 203 is in a non-operational position so that the manual override lever 210 does not bear on the cam lever 174.

The position of the guide pin 205 is shown at A in Fig. 29.

RESETTING TO OFF (Fig. 32 & Fig. 33)

To free the park brake, the hydraulic ram 183 is actuated, forcing the latch plate 201 down (pivoting around shaft 178) by virtue of the piston 184 bearing on the arm 207. The underside of arm 207 bears on the cam lever 174, also forcing it downwards (as viewed). The piston stroke is limited by contact between the cam lever 174 and the mounting bracket 173.

During this movement, the guide pin 205 tracks along the latch plate in the direction of arrow 211 (Fig. 29) against an upstanding cam form 213. The guide pin is prevented from travelling along the right hand track (Fig. 29) by a small step 212, of for example 2.5 mm. The guide pin reaches position B (Fig. 29) where it abuts an end stop 214 upstanding from the latch plate 201.

The pawl 171 pivots out of engagement with the toothed wheel 170, by virtue of the spring 169.

BRAKE OFF (Fig. 34 & Fig. 35)

Pressure is released from the piston 184 allowing it to return under the action of spring 185. The latch plate also rises under the action by spring 202. The guide pin tracks along the upper face of the cam form 213 to reach an abutment 215 constituted by a concave form (position C). In this condition the leaf spring 204 holds the latch plate 201.

The cam lever 174 follows the latch plate, by virtue of spring 177 to rest underneath, and in abutment. The pawl 171 has a profiled rear face to ensure no pivoting thereof during movement of the cam lever 174 during RESETTING to OFF motion.

RESETTING TO ON (Fig. 36 & Fig. 37) The hydraulic ram 183 is re-pressurized, and the piston 184 pushes arm 207 down, and hence cam lever 174. Piston stroke is again limited by contact of the cam lever with the mounting bracket 173.

During this motion the guide pin 205 moves out of the abutment 215 and against a right side wall 216 where it adopts position D.

BRAKE ON (Figs. 38 & 39) The hydraulic ram is re-released. The latch plate 201 rises with the piston 184, and the guide pin 205 tracks down the side wall 216 to return to position A. The pin rides up a ramp face 217 before dropping over step 212.

Upward movement of the cam lever 201 is stopped by a small peak at the back of the pawl 171. The operation cycle is complete.

Figs. 40-45 illustrate operation of a manual override mechanism, which is provided in case of hydraulic failure, or as an alternative operating mechanism.

MANUAL RELEASE DISENGAGED (Fig. 40 & Fig. 41)

In this condition the lever 203 is up, as viewed. In use it may be coupled to a bowden cable or the like.

MANUAL BRAKE ON (Fig. 42 & Fig. 43)

In this condition the lever 203 is pulled down, and by virtue of being fast with shaft 178, cause pivoting of override lever 210, which is also fast with shaft 178. Lever 210 acts directly to urge cam lever 174 down, and hence to urge pawl 171 into engagement with wheel 170. A stud on the inside of lever 203 (not shown) acts against the right side face of leaf spring 205, as illustrated in Fig. 43 to disengage guide pin 205 from the latch plate 201. For this purpose the free end of spring 205 is canted slightly to the left, as also illustrated in Fig. 43. Thus the cam lever 174 is free to engage the pawl 171 with the wheel 170.

MANUAL BRAKE OFF (Fig. 44 & Fig. 45)

Further downward movement of lever 203 causes the override lever 210 to pivot the cam lever away from the pawl 171, which consequently allows the pawl 171 to disengage from the wheel 170. The leaf spring is urged further to the left as illustrated in Fig. 45, and may act as a return bias on the lever 203. Reverse movement of the lever 203 firstly engages and then disengages the pawl.

Although the embodiment of Figs. 28-45 is described in an orientation with the hydraulic ram upright, it will be appreciated that gravity forms no part of the operation thereof, and it could be mounted in any orientation. The toothed wheel 170 is conventionally fast with a transmission output shaft so that engagement of the pawl locks the output shaft against annular movement.

Modifications and improvements of the invention are possible within the scope of the invention as described herein, as will be apparent to a suitably skilled person.

Claims

Claims

1. A park brake mechanism of a vehicle and comprising a toothed wheel, a pawl engageable on demand with said wheel to prevent rotation thereof, and an actuator for said pawl, wherein said actuator comprises a bi-stable latch having alternate stable conditions and adapted to change condition in response to a unidirectional input pulse.

2. A mechanism according to claim 1 wherein said bi-stable latch comprises input and output members having interengaging teeth, and a ground member to support the output member in either stable condition.

3. A mechanism according to claim 2 wherein the input member is operable by said actuator to separate the output and ground members, and permit transition thereof between the bi-stable conditions.

4. A mechanism according to claim 2 or claim 3 wherein said input and output members are relatively rotatable.

5. A mechanism according to claim 4 wherein relative rotation of said input and output members is unidirectional.

6. A mechanism according to any of claims 2-5 wherein said ground member is intermediate the input and output members.

7. A mechanism according to claim 2 wherein said pawl is resiliency urged to a disengaged condition and includes a ramp face opposite a pawl tooth for engagement by an operating cam comprising a pivotable finger having a roller at the free end for engagement with said ramp face.

8. A mechanism according to claim 7 and further including a stop to limit pawl engaging movement of the finger, a light spring to bias said finger away from the stop, and a relatively heavy spring between the actuator and the cam finger to provide for relative movement thereof.

9. A mechanism according to claim 1 wherein said bi-stable latch comprises a reciprocal linear actuating element, a guide member movable by said actuating element against a return bias, and a guide for the guide member, said guide defining two or more stable positions of the guide member in the movement direction of said linear actuating element, and the linear actuating element having opposite ramp faces to determine transverse movement of the guide member.

10. A mechanism according to claim 9 wherein said actuating element defines a contact edge for the guide element, said edge comprising two peaks having ramp faces on either side thereof, the ramp faces being adapted to urge the guide element transversely.

11. A mechanism according to claim 9 or claim 10 wherein said guide comprises a continuous slot extending in the movement direction of the actuating element, and defining two or more return portions into which the guide element is in use urged by said return bias.

12. A mechanism according to claim 1 wherein said bi-stable latch comprises a pivotable latch plate defining an unidirectional track and having alternate stable conditions corresponding to different angular positions of said latch plate.

13. A mechanism according to claim 12 wherein said latch plate is pivotable about an axis parallel to the axis of a pivotable pawl.

14. A mechanism according to claim 12 or claim 13 wherein said latch plate includes a guide element grounded on said mechanism and having a protrusion engageable in a track of the latch plate to determine the angular position thereof.

15. A mechanism according to claim 14 wherein said guide element is resiliency biased against said latch plate and is disengageable from said latch plate on demand.

16. A mechanism according to claim 15 wherein said guide element is mounted on a cantilever leaf spring.