GB2082254A - Force actuator - Google Patents

Abstract

A force actuator, for example for a vehicle braking system comprises a primary member 5 movable for initiating force actuation and a secondary member 20 movable to apply a force. The members 5, 20 are coupled by displacement magnifying means 15-18 through a force- responsive link 19, 30, 31 to move the secondary member 20 in response to, and at a speed in excess of, movement of the primary member 5. The force- responsive link 19, 30, 31 is arranged to override the coupling when the applied force exceeds a predetermined value to effect direct connection through clutch faces 23, 24 of the secondary member 20 to the primary member 5. In application to a braking system the coupled movement of the secondary member can quickly take up slack in the brake linkage or rigging whereupon full braking force can be transmitted by the direct connection of the members. Preferred displacement magnifying means comprises first-order levers 15 and a preferred force responsive link includes a spring 31 which when unable to transmit the applied force yields so that primary member 5 moves into engagement with the member 20 of faces 23, 24. The primary member 5 may be moved by fluid pressure. Mechanical means may be included for retracting the members to release an applied force. <IMAGE>

Description

SPECIFICATION Force actuators This invention relates to force actuators. By a force actuator is meant a device for applying a force by means of a member movable in the direction in which the force is to be applied relative to a fixed datum. One example of a force actuator to which the invention is particularly applicable is a vehicle brake actuator, which commonly comprises a rod movable axially in a casing secured to a part of the vehicle. The rod is connected to or engages a braking element or a linkage or rigging connected to a braking element and when desired is moved axially in the casing, for example by fluid pressure, cam or spring means, to cause the braking element to engage a wheel of the vehicle or a brake part connected to a wheel.The braking element must engage the wheel or brake part with a certain force in order to effect braking and this force is transmitted by the moving rod of the brake actuator.

Force actuators are used in many circumstances in which it is desired to apply a force to an object and in many mechanisms where it is desired to apply a force to one part of the mechanism in relation to a fixed datum which may be the ground or another part of the mechanism.

In the use of force actuators it is often necessary for the movable member to move some initial distance from its original position in relation to the datum before the full available force can be applied. For example in a vehicle brake the braking element may have to be moved some distance from its rest position to engage the vehicle wheel or brake part, or slack in the brake linkage or rigging may have to be taken up by initial movement of the movable member before any force can be applied to the braking element against the wheel or brake part. Likewise in other circumstances some slack or clearance may have to be taken up by initial movement of the movable member before the desired force can be applied.

The initial movement of the movable member may be a substantial proportion of its available movement and may cause undesirable delay before the desired force is applied. This is of critical importance in vehicle braking systems, for it is desirable that the braking elements be forced against the wheels or brake parts with the minimum delay after initiating operation of the force actuator. Wear of the braking elements or of the linkage or rigging, or of parts of other mechanisms, can increase the initial movement required.

The invention provides a force actuator in which such critical movement can be effected quickly.

According to the invention a force actuator comprises a primary member movable for initiating force actuation and a secondary member movable to apply a force, displacementmagnifying means coupling the members through a force responsive link to move the secondary member in response to, and at a speed in excess of, movement of the primary member, the force responsive link being arranged to override the coupling when the applied force exceeds a predetermined value to effect direct connection of the secondary member to the primary member whereby force applied by the primary member is directly transmitted by the secondary member.

Conveniently, the force-responsive link is a spring transmitting and thus deformed by the applied force, complementary abutment means on the primary and secondary members being held apart by the spring when the applied force is below the predetermined value but being permitted to abut by deformation, the spring under greater applied force and thus to transmit movement from the primary member to the secondary member.

The invention includes a vehicle braking system including such a force actuator.

In such a vehicle braking system it will be understood that initial movement of the secondary member which may be necessary for the reasons described above can be effected by a smaller movement of the force-initiating primary member and thus with minimal delay. The ratio of the displacement-magnifying means should be such that only a small part of the available movement of the primary member is necessary to effect this initial movement. The force necessarily applied by the secondary member to enable this initial movement to be effected should be somewhat less than the predetermined force which will cause the force-responsive link to override the coupling. Then, when the braking element is in engagement with the vehicle wheel or brake part, the applied force can be increased to cause the force-responsive link to override the coupling.

Because of the ratio of the displacementmagnifying means the force applied by the secondary member during this initial movement and whilst the members are coupled through the force-responsive link is less than the initiating force applied to the displacement-magnifying means by the primary member. However, when the applied force is increased to cause the forceresponsive link to override the coupling the direct connection of the members enables the whole force applied by the primary member to be transmitted to the braking element or the brake linkage or rigging by the secondary member.

An embodiment of the invention is illustrated by way of example by the accompanying drawings in which: Fig. 1 is a sectional elevation of a springoperated force actuator for a railway vehicle braking system, showing the parts in the normal condition in which the vehicle brakes are fully released.

Fig. 2 is a similar view showing the parts in the condition when maximum movement of the forceapplying secondary member has been effected.

Fig. 3 is a similar view showing the operation of release means to retract the members for r.eleasing the brakes.

Figs. 4 and 5 illustrate cams included in the release means.

The force actuator consists of a cylindrical housing 1 having at one end a conical cover 2 and at the other end a spring thrust plate 3. Within the right-hand portion of the housing part 1, there is a cylinder 4 within which a primary member in the form of a piston 5 having a piston seal 6 and piston spacer 7 is sealingly slidable. Captive between the piston 5 and the base of a recess in the thrust plate 3 there is a heavy spring 8, shown in the fully compressed position as a result of fluid pressure in the pressure chamber 9 to the left of the piston urging the piston to the right.The piston 5 has an internal coaxial trunk 10 which extends from the piston to the left in the drawing and is sealingly slidable in a bore within an internal wall 12 of the housing 1 defining the pressure chamber 9 and, within the piston, extends into the region 13 within which the spring 8 is housed for form part of a release mechanism as will be described. The part of the trunk 10 of the piston 5 which extends through the wall 12 has secured to it a lever plate 14 carrying three equi-angularly spaced first order pivoted levers 1 5 (only one of which is shown, for simplicity) the outer relatively short limbs 1 6 of which bear against fixed abutments formed by thrust shoulders 1 7 within the cover 2.The relatively long limbs 18 of the levers 1 5 all bear against an annular thrust plate 1 9 which is slidable on a secondary member in the form of a rod 20 coaxial with the piston 5 and extending through the lever plate 14 into the region within the trunk 10 of the piston wherein it has a helical threaded portion 21. This helical threaded portion has a complementarily threaded clutch element 22 rotatably carried thereon. The clutch element 22 has an external clutch face 23 which is clutchable with an internal clutch face 24 of the trunk 10. In the position shown, the clutch faces 23 and 24 are just separated or unclutched by virtue of engagement of a thrust race 25 with the end surface 26 of a rotatable release member 27.In order to maintain the clutch element 22 as far to the right as possible, subject to engagement with the thrust race 25, there is provided a spring 44 between the lever plate 14 and a second thrust race 45 carried on the clutch element. The lefthand end of the secondary mover 20 has an output part 29 of square cross section which extends through a complementary hole in the cover 2 for connection to a brake linkage or rigging. Inwardly of the part 29, the rod 20 has a reduced portion carrying a collar 28 against which a thrust plate 30 is urged by a spring 31, the other end of which is seated against the aforementioned thrust plate 1 9 which is engaged by the levers 1 5.

A similar but longer spring 32 is provided between the thrust plate 19 and a recess 36 in the cover 2, serving as a return spring for retracting the rod 20 as will be described.

Rotation of the secondary member, rod 20 is prevented by the engagement of its squaresection output part 29 in the complementary hole in the cover 2. Rotation of the piston 5 within the cylinder is prevented by means of a locating pin 43 carried on the wall 1 2 and passing through the lever plate 14 fixed to the left-hand end of the piston trunk 10.

Referring now to the operation of the actuator, it is observed that the spring 31 must be of such strength as to be able to overcome without substantial deformation all resistance in the brake linkage or rigging in normal operation up to the point where the brake blocks make engagement with the wheel or wheels of the vehicle. This spring 31 forms a force-responsive link through which the primary and secondary members 5 and 20 are coupled by the displacement-magnifying levers 1 5. In order to apply the brakes, the normally supplied full reservoir pressure in the pressure chamber 9 is reduced to permit the force of the spring 8 to move the piston 5 leftwards to apply a resultant force to the lever plate 14.The short limbs 1 6 of the levers 1 5 are thus forced against the thrust shoulders 17 in the cover, causing anti-ciockwise rotation (as seen in Fig. 1) of the respective levers 1 5. The ends of the long limbs 1 8 of the levers 1 5, bearing against the thrust plate 19, move the plate 1 9 to the left. This movement of the thrust plate 1 9 approximates to the multiple of the distance moved by the primary member, piston 5, and the ratio between the lengths of the arms 18 and 16 of the displacement-magnifying levers 1 5. Because the spring 31 is able to overcome resistance in the brake rigging without substantial deformation, the movement of the thrust plate 1 9 is substantially entirely transmitted through the spring 31 to the thrust plate 30 and thus to the secondary member, rod 20.On the other hand, because of the ratio of the displacement magnifying levers, the force applied to the thrust plate 1 9 and thus through the spring 31 to the rod 20, is substantially less than the force applied to the fulcrums of the levers by the primary member, piston 5.

As the rod 20 is being moved to the left as described, the clutch element 22, being held by the release member 27 with its clutch face 23 just clear of the clutch face 24 on the trunk 1 0, is free to rotate on threaded part 21 of the rod 20. The first movement of the piston 5 and rod 20 moves the thrust race 25 away from the end 26 of the release member 27. The clutch element can then rotate until its clutch face 23 engages the clutch face 24. Thereafter it tends to be moved to the left with the rod 20 but is urged towards the clutch face 24 by the spring 44. It therefore rotates with the clutch faces in rubbing contact as the rod moves through it to the left.

When the initial movement of the rod 20, necessary to take up any slack in the brake rigging and move the braking elements or blocks into engagement with the vehicle wheels, has been completed, further movement of the rod 20 is resisted, but the force applied to the blocks by the rod 20 must be increased to effect braking.

Further movement of the piston 5 causes the thrust plate 1 9 to be moved to the left relative to the rod 20, compressing the spring 31 and thus increasing the force applied by the rod 20. After a short movement of the piston 5 the clutch faces 23 and 24 are urged together, since there is no longer any tendency for the clutch element 22 to be moved to the left. The clutch element 22 is thereby prevented from further rotation and further movement of the piston 5 to the left moves the rod 20 at the same speed. Thus the coupling of the members 5 and 20 through the spring 31 is overridden and the members are directly connected through the clutch faces 23 and 24.

The majority of the available force from the piston 5 (being the difference between the force applied by the spring 8 and the force applied by fluid pressure in the chamber 9) is directly transmitted to the rod 20 and thus to the brake blocks. Further reduction of the fluid pressure increases this available force until with zero pressure in the chamber 9 the maximum braking force is applied.

The force at which the clutch faces 23 and 24 are brought together is predetermined by their initial spacing and the geometry of the mechanism.

Fig. 2 shows the effective full stroke condition of the actuator, such as might occur if the actuator were used with a brake system in which substantial initial movement was required to bring the brake blocks into engagement with the vehicle wheels. It will be observed that although the piston 5 has moved through the majority of its available movement as indicated by the clearance between the end of its skirt and the spring thrust plate 3, this movement is under one fifth of the distance moved by the rod 20 due to the full anticlockwise rotation of the levers 1 5. The clutch element 22 has passed from end to end of the screwed part 21 of the rod 20 during the substantial initial movement.Engagement of the brake blocks with the wheels has produced a reaction to resist further movement of the rod 20 and further movement of the piston 5 has moved the thrust plate 1 9 off its seating on the rod 20 to compress the spring 31 and cause the clutch faces 23 and 24 to engage. The piston 5 is thus applying braking force directly to the rod 20 for transmission to the brake blocks.

To release the brake blocks from the wheels, fluid pressure is increased in the pressure chamber 9. The piston 5 is moved to the right in the drawing as the pressure overcomes the force of the spring 8. Initially, the clutch faces 23 and 24 remain in engagement and the rod 20 moves with the piston 5 under the influence of the return spring 32. When the thrust race 25 of the clutch element 22 again engages with end 26 of the release member 27, further movement of the piston 5 separates the clutch faces. The clutch element is then free to rotate on the rod, permitting the rod to be retracted fully by the return spring 32 until the condition of the actuator shown in Fig. 1 is restored.

The actuator includes alternative manuallyoperated release means comprising a cam 33, the form of which is shown in Fig. 4, secured by a circlip within the end of the trunk 10 around the release member 27. The cam 33 engages a complementary cam 34, the form of which is shown in Fig. 5, on the release member 27. The release member extends through a bore 38 in the thrust plate 3 and has secured to it a handle 39 seated against a thrust race 40 received in the outer wall of the thrust plate. The handle is urged into the position shown in Figs. 1 and 2 by a captive coil torque spring 41.

By operation of the manually-operable release means the rod 20 may be retracted from a position in which force is applied to the brake rigging without the application of fluid pressure to the pressure chamber 9. The actuator may be considered in the condition shown in Fig. 2.

Rotation of the handle 39 through approximately 1 800 to the position shown in Fig. 3 causes interaction between the cams 33 and 34 to retract the piston 5 against the spring 8 to the position shown in Fig. 3. The rod 20 is initially retracted with the piston by the return spring 32 until the clutch faces 23 and 24 are separated as previously described. The clutch element 22 is then free to rotate and the rod 20 is fully retracted as before. The actuator is then in the condition shown in Fig. 3 and it will be noted that the retraction of the piston 5 by the cams 33 and 34 is just less than its full retraction, a clearance being left between the skirt of the piston and the thrust plate 3.

Retraction of the rod 20 by the manuallyoperable release means may be utilised to enable the brakes to be released on a parked vehicle without fluid pressure available or to enable one brake to be released for maintenance in a vehicle in which the other brakes are applied.

After such retraction, if the handle 39 is returned to its position shown in Figs. 1 and 2 the piston 5 moved to the left by the spring 8 and the actuator proceed through the sequence of events previously described for a normal brake application. Alternatively, if the handle is not restored to its original position manually, subsequent application of fluid pressure to the pressure chamber 9 moves the piston slightly to the right in the drawings, taking up the small clearance previously mentioned and separating the cams 33 and 34. The handle is thereupon returned to its original position by the spring 41.

From the above it will be understood that the invention provides not only a new kind of force actuator on which a substantial stroke is effected by a small movement of a primary initiating member, enabling full available force to be applied in minimal time, but also avoids the need for slack adjustment in brake systems or other mechanisms where slack has normally to be kept to a minimum to avoid excessive stroke of the actuating means.

Claims (11)

1. A force actuator comprising a primary member movable for initiating force actuation and a secondary member movable to apply a force, displacement-magnifying means coupling the members through a force-responsive link to move the secondary member in response to, and at a speed in excess of, movement of the primary member, the force-responsive link being arranged to override the coupling when the applied force exceeds a predetermined value to effect direct connection of the secondary member to the primary member whereby force applied by the primary member is directly transmitted by the secondary member.

2. A force actuator as claimed in Claim 1 wherein the force-responsive link is a spring transmitting and thus deformed by the applied force, complementary abutment means on the primary and secondary members being held apart by the spring when the applied force is below the predetermined value but being permitted to abut by deformation of the spring under greater applied force and thus to transmit movement from the primary member to a secondary member.

3. A force actuator as claimed in Claim 1 or Claim 2 wherein the primary member coaxially surrounds the secondary member and both are movable axially, the displacement-magnifying means comprising a first-order lever having a fulcrum carried by the primary member, a short limb arranged to engage a fixed abutment and a long limb connected to the force-responsive link.

4. A force actuator as claimed in Claim 3 wherein the complementary abutment means comprise a clutch element screwed on the secondary member and a complementary clutch face on the primary member, spring means normally locating the clutch element close to the clutch face and permitting rotation of the clutch element to accommodate axial movement of the secondary member relative to the primary member, overriding of the coupling for the forceresponsive link causing the clutch face to engage the clutch element to resist further rotation thereof and thus enable direct axial movement of the secondary member by the primary member.

5. A force actuator as claimed in Claim 4 wherein the movement of the secondary member by or in response to movement of the primary member is uni-directional, including a return spring to move the secondary member in the opposite direction.

6. A force actuator as claimed in Claims 4 and 5 including release means operative to retract the primary member when it has been moved to initiate application of a force greater than the predetermined value and thereby separate the clutch face from the clutch element, permitting the clutch element to rotate to allow the secondary member to move in the said opposite direction under the influence of the return spring.

7. A force actuator as claimed in Claim 6 including a spring to disengage the release means from the primary member when the latter is further retracted after operation of the release means.

8. A force actuator as claimed in any preceding claim wherein movement of the primary member can be effected by fluid pressure.

9. A force actuator as claimed in Claim 8 wherein the primary member is urged in the direction of movement to initiate force actuation by a spring and may be retracted by fluid pressure.

10. A force actuator substantially as hereinbefore described with reference to an as illustrated by the accompanying drawings.

11. A vehicle braking system including a force actuator as claimed in any preceding claim.