GB2314070A

GB2314070A - Clamping device

Info

Publication number: GB2314070A
Application number: GB9711942A
Authority: GB
Inventors: David Stewart Cameron
Original assignee: Individual
Current assignee: Individual
Priority date: 1996-06-10
Filing date: 1997-06-09
Publication date: 1997-12-17
Anticipated expiration: 2017-06-09
Also published as: GB2314070A8; GB9612136D0; GB9711942D0; GB2314070B

Abstract

A clamping device 10, intended for a lift, comprises a frame and at least two clamp members 20, 22 secured to the frame, each clamp member having a clamping surface 20a, 22a adapted to grip an element, e.g. a guide rail (24 see fig 5). At least one of the members is movable towards another member to grip the element between the surfaces, the movable members being pivotally mounted to the frame via parallelogram linkages 26, 28, 50, 52, so that the movable members can be swung into engagement with the element while keeping the surface at the same orientation throughout. Biassing means 40, such as disc springs, may be included on the frame to bias the movable members towards the element to be gripped, and these may have means (46, 48, 70, 72 see fig 4), to adjust the magnitude of the biassing force provided. These may adjust the angle of the clamping surfaces with respect to the element, or the clearance between the surfaces and the element. Stop means for limiting the pivotal movement of each movable clamp may also be provided. A method for using such a clamp for braking a lift is also disclosed.

Description

CLAMPING DEVICE This invention relates to a clamping device. More particularly, the invention relates to a clamping device that can be used for safety purposes in a lift.

In many countries there is a legal obligation to provide a safety device in lifts to brake downward movement of the lift carriage in the event of rapid downward acceleration. Many countries have proposed similar legislation to brake upward movement of the lift carriage in the event of rapid upward acceleration.

Safety devices for this purpose include a damping device fixedly secured to the lift carriage; the damping device is provided with a clamp for gripping a lift cable or a guide rail external to the carriage. A governor is also provided, which is responsive to the downward acceleration or speed of the lift carriage. If the acceleration or speed exceeds a predetermined value the governor actuates the damping device to cause the damp to grip the guide rail. This causes the lift carriage to decelerate, and come to a halt.

Known damping devices for lifts generally have either one movable clamp and one fixed damp, or two movable damps, with the guide rail disposed between the clamps. In the former arrangement, the governor moves the movable clamp towards the fixed damp to grip the rail between the clamps. In the latter arrangement, the governor causes both damps to move towards one another to grip the rail between the clamps. In both arrangements, it is highly desirable for the clamping device to be self-actuating once the governor has provided the initial impetus to bring the clamps in contact with the rail.

In order to be self-actuating, the gripping force must be provided by the frictional engagement of the clamps with the rail. Thus, the dynamic coefficient of friction between the clamp and the rail is of critical importance. The value of the dynamic coefficient of friction depends upon a number of factors including the contact pressure, the surface roughness, the presence of any lubrication and the material properties of the clamp and the rail. In order to be self-actuating, the frictional force generated between the damp and the rail must be greater than the force resisting engagement.

Fig. 9 is a schematic diagram showing the arrangement in a conventional fixed geometry tapered wedge device (which is known in the art). In this diagram a guide rail 200 is to be damped between damps 202. Each damp 202 can be pushed in the direction of the rail 200 by an arrangement comprising a low friction slide 204, a housing 206 for the slide and a spring 208. The springs 208 act to apply a spring force F to the damps 202 during activation of the device. Each clamp 202 is in the form of a wedge having a surface 202a at an angle + to a plane perpendicular to the direction of F. In order for the device to be self actuating the coefficient of friction must be greater than tan(.

Known self-actuating safety devices for lifts typically have spring loaded clamps that are brought into engagement with a guide rail by the tripping of an overspeed governor. The spring loading may be achieved, for example, by leaf springs, disc springs or compression springs. These springs must generate sufficient clamping force to decelerate the lift within certain retardation limits (these retardation limits are usually set by Statute in any given country, and may vary from country to country). It is common to provide an ancillary means to engage the clamps fully with the rail and induce the spring loading.

Some clamping devices rely on the governor to generate a minimum pull-through force to assist the engagement of the clamp with the rail. These devices have the disadvantage of complicating the governor design. Other clamping devices rely on the impact speed between the clamp and the rail to drive the clamp into full engagement. These devices often operate inconsistently. Other clamping devices use a hardened roller to dig into the rail and help drive the clamp into full engagement; however, these devices can damage the guide rail. It is also known to provide sharpened and hardened clamps to bite into the guide rail; these devices can also damage the guide rail.

It would be desirable to provide a clamping device that is completely self-actuating in that it does not require the governor to drive the clamp hard into the rail, and does not rely on the impact speed between the clamp and the rail. It would also be desirable to provide a clamping device that does not damage the rail when it engages the rail.

The present invention aims to solve the problems in the prior art by providing a self-actuating damping device, that causes little or no damage to an element to be damped. This is achieved by providing a damping device comprising at least two damps each having a clamping surface for gripping an element to be damped, wherein at least one of the damps can be swung into engagement with the element, along an arcuate path, without any change in the orientation of the clamping surface.

According to one aspect of the present invention, there is provided a damping device comprising a frame, and two damps secured to the frame, each clamp having a clamping surface adapted to grip an element, wherein at least one of the clamps is movable towards another of the damps to grip the element between the damping surfaces, and wherein the or each movable damp is pivotally mounted to the frame with two separate pivotable linkages, whereby the or each movable clamp can be swung into engagement with the element without any substantial change in the orientation of its clamping surface.

In the preferred construction, each pivotable linkage is pivotally mounted to the frame at a respective frame pivot point, and is pivotally mounted to a respective movable damp at a respective clamp pivot point. Each pivotable linkage has an axis extending between its respective frame pivot point and clamp pivot point.

As the or each movable clamp is swung into engagement with the element, each linkage axis pivots about its respective pivot point. At the point when the or each movable clamp first engages the element to be gripped each linkage axis will be at a specific angle to an axis normal to the surface of the element. This particular angle will hereinafter be referred to as the "initial engagement angle".

When the or each movable damp is brought into contact with the element to be gripped, the surface of the damp will be arranged at a specific angle to the surface of the element. This angle will hereinafter be called the "contact angle".

The contact angle can be zero, or it can be positive, or it can be negative. Whether a contact angle is positive or negative is purely a matter of convention.

Provided that the damping surfaces are at the correct orientation relative to the element to be gripped, the damps will be self actuating once they have been brought into engagement with the element. The correct orientation will depend on the dynamic coefficient of friction between the damping surfaces and the element, and also on the initial engagement angle. Specifically, the tangent of the initial engagement angle must be less than the coefficient of friction: for example, for a dynamic coefficient of friction of 0.1, the initial engagement angle would need to be less than 5.7 ; for a dynamic coefficient of friction of 0.2, the initial engagement would need to be less than 11.3 .

As discussed above, the value of the dynamic coefficient of friction can be affected by a number of factors, including any lubrication provided on the element.

Thus, for any given element the dynamic coefficient of friction may vary. In order to compensate for this, in accordance with a particularly advantageous feature of the invention, means is provided to adjust the initial engagement angle and/or the contact angle is adjustable. This can be achieved by making the position of the pivotable linkages of the or each movable clamp adjustable relative to the frame.

It is especially preferred that the damping device includes biasing means on the frame, said biasing means being adapted to bias the or each movable clamp in a direction towards the element to be gripped, as the or each movable clamp engages the element.

Preferably, each linkage is pivotally mounted to a shaft secured to the frame. It is desirable that each shaft is adjustably mounted to the frame, whereby the initial engagement angle and/or the contact angle can be adjusted. Preferably, each shaft extends through the frame and is provided with the biasing means on one side of the frame adapted to push the shaft in a direction towards the element to be gripped, and tensioning means on the other side of the frame for adjusting the preload applied by the biasing means. More specifically, the tensioning means comprises a nut, which is mounted to the shaft by a screw threaded connection, and is rotatable against the load applied by the biasing means, thereby adjusting the position of the shaft, the linkage to which it is connected and the clamp to which the linkage is connected.

In a preferred construction the contact angle and/or the engagement angle are adjustable independently of the spring force of the spring.

In a preferred embodiment, there are two clamps and both damps are movable and are pivotally connected to the frame with two separate pivotable linkages. In this preferred embodiment, there would be two damps, four pivotable linkages and four adjustable shafts.

It is desirable in some circumstances to provide two or more pairs of two damps, each pair of damps being arranged to grip a part of the element axially spaced from the part gripped by the other clamps.

The or each movable damp is movable between a first position, in which it cannot engage the element to be gripped, and a second position, in which it can engage the element to be gripped. It is desirable that when the or each movable clamp is biased towards the first position, when not in engagement with the element to be gripped. In the preferred embodiment, this biasing force is provided by gravity.

It is preferred that the frame comprises two opposite side walls (through which the shafts can be positioned) and a top and bottom wall. It is also preferred that the frame is open at the front and back, in order to facilitate maintenance. It is preferred to provide stop means to limit the movement of the or each movable clamp.

The stop means may comprise an upper surface of the bottom wall and a lower surface of the top wall of the frame. Preferably, the stop means further comprises a stop member at the upper end of the or each movable damp, each stop member being adapted to abut the lower surface of the top wall of the frame; this provides a limit on the second position of the or each movable clamp. The position of the stop members is preferably adjustable in order to enable the limit on the second position of the or each movable clamp to be adjusted.

An actuating element is normally provided in order to initiate movement of the or each movable damp from the first position to the second position. However, it is a feature of the present invention that the clamping device is self-actuating, so once the clamps come into contact with the element, further operation by the actuating element is not necessary.

The actuating element is typically connected to some kind of control system, the purpose of which is to control actuation of the actuating element. For example, when the damping device is to be used in a lift, a governor can be provided to actuate the element in an emergency in order to stop the lift.

In a preferred embodiment the or each movable damp comprises a body, to which the pivotable linkages are secured, and an engagement member for engaging and gripping the element to be gripped. Advantageously, the engagement member is movably mounted to the body of the damping device, whereby the angle between the engagement member and the element to be gripped, relative to the angle between the body and said element, can be adjusted as the engagement member moves into engagement with said element. The engagement member may be provided with one or more friction pads thereon, which are adapted to engage the element to be gripped.

According to another aspect of the present invention, there is provided a clamping device comprising a frame, and two clamps secured to the frame, each clamp having a clamping surface, wherein at least one of the clamps is movable towards another of the damps to grip the element between the damping surfaces, and wherein the or each movable clamp is mounted to the frame with two separate linkages, wherein the position of each linkage relative to the frame is adjustable in order to change the initial engagement angle and/or the contact angle.

According to another aspect of the invention there is provided a method of braking a lift by applying a braking force to an elongate element associated with said lift, the arrangement being such that the application of the braking force to said element causes the lift to reduce its speed or stop, wherein said braking force is applied by bringing a damping surface of the damp into frictional engagement with the elongate element by moving the clamping surface towards the elongate element along an arcuate path such that the orientation of the damping surface, relative to the elongate element, is substantially constant.

Preferably the method involves at least two of said damps.

The damping device according to the invention is self-actuating, and enables an element to be reliably gripped without damaging it. In addition, the orientation of the damps can be adjusted to suit prevailing conditions. The ability to swing the damps into and out of engagement with the element to be gripped allows relatively large clearances to be maintained between the damps and the element during normal operation; this reduces the risk that the damping device will selfactuate inadvertently. The clamping device according to the invention can easily be modified to brake downward or upward movement of a lift; the device simply needs to be adjusted to make sure that the clamps hang in the first position under the influence of gravity.

Reference is now made to the accompanying drawings, in which: Fig. 1 is a top plan view of an embodiment of a clamping device according to the invention; Fig. 2 is a front view of the damping device shown in Fig. 1, with two clamps in a first position; Fig. 3 is a side view of the clamping device shown in Fig. 2; Fig. 4 is a view similar to Fig. 2, with the clamps in a second position; Fig. 5 is a view along lines 5-5 of Fig. 4; Fig. 6 is a schematic view of an embodiment of a clamping device according to the invention; Fig. 7 is a front view of another embodiment of clamping device according to the invention; Fig. 8 is a front view of a further embodiment of a clamping device according to the invention; and Fig. 9 is a schematic view of a damping device according to the prior art.

Referring first to Fig. 6 a damping device generally designated 10 comprises two movable damps 20 and 22 which are pivotally secured to a frame (not shown in Fig. 6) by pivotable linkages 26, 28, 50 and 52. The damps 20 and 22 are adapted to damp an element to be damped, such as a guide rail 24 of a lift (for the purposes of darity, the guide rail 24 is not shown in all the drawings). The damp 20 has a damping surface 20a, while the clamp 22 has a clamping surface 22a; the rail 24 has a surface 24a.

The linkage 26 is pivotally secured to the damp 20 at a clamp pivot point 26a, and is pivotally secured to the frame at a frame pivot point 26b. The linkage 28 is pivotally secured to the damp 20 at a clamp pivot point 28a, and is pivotally secured to the frame at a frame pivot point 28b. The linkage 50 is pivotally secured to the damp 22 at a clamp pivot point 50a, and is pivotally secured to the frame at a frame pivot point 50b. The linkage 52 is pivotally secured to the clamp 22 at a clamp pivot point 52a, and is pivotally secured to the frame at a frame pivot point 52b.

The linkage 26 is operatively connected to a spring 40, the linkage 28 is operatively connected to a spring 44, the linkage 50 is operatively connected to a spring 64, and the linkage 52 is operatively connected to a spring 68. Each of the springs 40, 44, 64 and 68 applies a force F to the damps 20 and 22 as they come into contact with the rail 24.

The angle between the surface 20a and the surface 24a (and also the angle between the surface 22a and the surface 24a) is designated o in Fig. 6; this angle is the contact angle. In Fig. 6 the clamps 20 and 22 are shown after they have just come into contact with the rail 24. Each linkage 26, 28, 50 and 52 has a linkage axis that extends between its respective frame pivot point and clamp pivot point; in Fig. 6 these axes coincide with the lines representing the linkages 26, 28, 50 and 52.

When the damps 20 and 22 first come into contact with the rail 24, the linkage axes will be at a particular angle to an axis normal to the surface 24a; this angle is designated + in Fig. 6 and it is called the initial engagement angle.

The contact angle e can be either positive or negative. It will be assumed in the present application that the contact angle e is positive when the clamps 20 and 22 converge in the direction of relative movement of the rail 24 (a positive contact angle 8 its shown in Fig. 6, where the rail 24 moves upwardly relative to the damps 20 and 22). If the contact angle o is positive, as shown in Fig. 6, the initial contact between the rail 24 and the damps 20 and 22 will produce a relatively low coefficient of friction. This will require a correspondingly low initial engagement angle + for the device 10 to self actuate. On the other hand, if the contact angle e is negative, the leading edge of the damps 20 and 22 will produce a higher coefficient of friction (it will tend to dig in), and a higher initial engagement angle + will be possible. By adjusting the initial engagement angle + and the contact angle o (which can be adjusted independently of one another), the device 10 can be made self actuating in a wide variety of different applications.

Referring now to Figs 1 to 5, the clamping device 10 is shown in more detail. The parts shown schematically in Fig. 6 are similar to several of the parts shown in Figs. 1 to 5, and like parts have been designated with like reference numerals. In Figs. 1 to 5, the clamping device 10 comprises frame having a top wall 12, a bottom wall 14, and two opposing side walls 16 and 18. The top wall 14 is provided with a slot 1 4b through which the guide rail 24 extends. A similar slot (not shown) is provided in the bottom wall 12.

As explained with reference to Fig. 6, an upper end of the clamp 20 is pivotally secured to one end of the linkage 26, while a lower end of the clamp 20 is pivotally secured to one end of the linkage 28. The other end of the linkage 26 is pivotally secured to a shaft 30, while the other end of the linkage 28 is pivotally secured to a shaft 32. The shafts 30 and 32 extend through the side wall 16. The shafts 30 and 32 are provided with stop surfaces 34 and 36 respectively. A spacer 38 and biasing means in the form of a plurality of disc springs 40 are provided on the shaft 30, between the stop surface 34 and the side wall 16; and a spacer 42 and biasing means in the form of a plurality of disc springs 44 are provided on the shaft 32, between the stop surface 36 and the side wall 16. Tensioning means is provided on the shaft 30 in the form a nut 46, and tensioning means is provided on the shaft 32 in the form of a nut 48. The nut 48 is provided with an inner screw threaded portion 48a, which is threaded over an outer screw threaded portion 32a on the shaft 32; the nut 46 is provided with a similar inner screw threaded portion (not shown), which is threaded over a similar outer screw threaded portion (not shown) on the shaft 30.

An upper end of the damp 22 is pivotally secured to one end of the linkage 50, while a lower end of the clamp 22 is pivotally secured to one end of the linkage 52. The other end of the linkage 50 is pivotally secured to a shaft 54, while the other end of the linkage 52 is pivotally secured to a shaft 56. The shafts 54 and 56 extend through the side wall 18. The shafts 54 and 56 are provided with stop surfaces 58 and 60 respectively. A spacer 62 and biasing means in the form of a plurality of disc springs 64 are provided on the shaft 54, between the stop surface 58 and the side wall 18; and a spacer 66 and biasing means in the form of a plurality of disc springs 68 are provided on the shaft 56, between the stop surface 60 and the side wall 18. Tensioning means is provided on the shaft 54 in the form a nut 70, and tensioning means is provided on the shaft 56 in the form of a nut 72. The nut 72 is provided with an inner screw threaded portion 72a, which is threaded over an outer screw threaded portion 56a on the shaft 56; the nut 70 is provided with a similar inner screw threaded portion (not shown), which is threaded over a similar outer screw threaded portion (not shown) on the shaft 54. The disc springs 40, 44, 64 and 68 can be different in order to vary the operating characteristics.

The size and/or number of the spacers 38,42, 62 and 66 can be changed in order to allow the device 10 to be used with different thicknesses of guide rail 24.

The adjustment of the size and/or number of the spacers 38, 42, 62 and 66 performs the secondary function, in conjunction with the nuts 46, 48, 70 and 72, of adjusting the spring force of the springs 40, 44, 64 and 68.

The clamps 22 and 24 are provided with pins 74 and 76 respectively, which extend through a horizontal slot (not shown) in a U-shaped member 78. The U-shaped member 78 forms part of damp actuating mechanism, which also includes an actuating rod 80 and an actuating lever 82. For clarity the actuating mechanism is not shown in all the drawings. The lever 82 is fixed to the rod 80, and the rod 80 is fixed to the member 78, so that tuming the lever 82 causes the rod 80 to rotate, which causes the member 78 to pivot about the rod 80. The lever 82 is removably secured to the rod 80, so that it can be disconnected therefrom, and, if desired, resecured to an opposite end of the rod 80.

An upper surface 1 4a of the bottom wall 14 serves as a lower stop surface for the damps 20 and 22 - this is illustrated in Fig. 2. A stop member 84 provides a stop surface for the damp 20, and a stop member 86 provides a stop surface for the damp 22. The stop member 84 comprises a bolt screwed into a plate 88 that is fixed to a lower surface 1 2a of the top wall 12, and the stop member 86 comprises a bolt screwed into a plate 90 that is fixed to the lower surface 12a. The stop members 84 and 86 provide a limit on the upward movement of the clamps 20 and 22 - this is shown in Fig. 4.

The actuating lever 82 will be connected to a control system (not shown) for controlling actuation of the actuating mechanism.

The operation of the clamping device 10 will now be described. It will be assumed that the clamping device is being used as a safety device for a lift, and is secured to part of the lift carriage (not shown), so that the guide rail 24 can run between the clamps 20 and 22 as the lift moves up and down.

In normal operation the clamps 20 and 22 are in the first position, as shown in Fig. 2; in this position the guide rail 24 can run freely between the clamps 20 and 22.

In an emergency the control system operates to activate the actuating mechanism. This causes the actuating lever 82 to pivot about the axis of the actuating rod 80; the actuating rod 80 rotates with the lever 82, thereby pivoting the U-shaped member 78 about the axis of the rod 80. As the member 78 pivots, the pins 76 and 76 are lifted by the member 78, and slide in the slots provided in the member 78.

The upward movement of the pins 74 and 76 causes a corresponding upward movement of the damps 20 and 22. The damps 20 pivots relative to the linkages 26 and 28, while the damp 22 pivots relative to the linkages 50 and 52. This causes the damps 20 and 22 to move upwardly along a curved pathway - ie, to swing upwardly towards the guide rail 24. Importantly, the orientation of the damping surfaces 20a and 22a does not change during the upward movement, so that the surfaces 20a and 22a maintain a constant angle with respect to the guide rail 24 at all times.

After a certain amount of upward movement, the damps 20 and 22 contact the guide rail 24. At this point the damping device 10 becomes self actuating: upward movement of the guide rail 24 relative to the clamps 20 and 22 causes the clamps 20 and 22 to grip the rail more tightly so that the any relative movement between the guide rail 24 and the clamping device 10 is quickly prevented. At this point the lift carriage will have been brought to a complete stop by the clamping device 10. During full engagement of the damps 20 and 22 with the rail 24, the four independent spring links ensures that the correct pressure is maintained on the rail by the clamps.

The nuts 46, 48, 70 and 72, together with the disc springs 40, 44, 64 and 68 provide a means for adjusting the position of the damps 20 and 22 within the frame. The nuts 46 and 48 can be tightened to bring the clamp 20 closer to the side wall 16, and they can be loosened to move the clamp 20 further away from side wall 16. The nuts 70 and 72 can be tightened to bring the damp 22 closer to the side wall 18, and they can be loosened to move the clamp 22 further away from the side wall 18. Thus, by tightening or loosening the nuts 46, 48, 70 and 72 it is possible to adjust the clearance between the clamps 20/22 and the guide rails 24. This is important because it enables the initial engagement angle (p to be adjusted; this enables the device 10 to be adjusted to ensure that it is self actuating.

Furthermore, the nut 46 can be tightened more or less than the nut 48, and the nut 70 can be tightened more or less than the nut 72. This enables the contact angle o to be adjusted.

In general, the initial engagement angle + can be adjusted by tightening or loosening the nuts 46 and 48 (or the nuts 70 and 72) by the same amount. The contact angle o can be adjusted by tightening or loosening the nuts by different amounts. In practice, it is usual to set the contact angle 8 first, then the initial engagement angle +.

The damping device 10 is used for braking downward acceleration of a lift carriage. However, the device 10 can easily be adapted to brake upward acceleration of a lift carriage. It is necessary only to tum the device 10 upside down, and remove the stop members 84 and 86 from the top wall 14 (which is now the bottom wall). Normally the stop members 84 and 86 would be secured to the bottom wall 12 (which is now the top wall). In order to ensure that the device 10 can be used either way up, it is important to ensure that the shafts 30, 32, 54 and 56 are located sufficiently far from the top and bottom walls 12 and 14 that the gap between the damps 20 and 22 decreases upon movement from the first (ie unclamped) position to the second (ie clamped) position.

Fig. 7 shows another embodiment of a clamping device, which is generally designated 100. Most of the parts of the clamping device 100 are identical to the clamping device 10, and like parts have been designated with like reference numerals.

In the embodiment of Fig. 7 the damps 20 and 22 have been modified.

The damp 20 comprises a body 102 and an engagement member in the form of a gib 104, which is pivotally mounted to the body 102 such that the gib 104 can pivot about an axis parallel to the longitudinal axis of the gib 104. The gib 104 is provided with bronze pads 106, which have surfaces 1 06a adapted to engage the rail 24.

Similarly, the clamp 22 comprises a body 108 and an engagement member in the form of a gib 110, which is pivotally mounted to the body 108 such that the gib 110 can pivot about an axis parallel to the longitudinal axis of the gib 110. The gib 110 is provided with bronze pads 112, which have surfaces 11 2a adapted to engage the rail 24. The bronze pads 106 and 112 could be replaced with another friction material.

A gib stabiliser 114 is provided to support the bodies 102, 108 and/or the gibs 104, 110 at higher speeds. The gib stabiliser 114 comprises a plate 116 having an elongate slot 118 provided therein. The width of the slot 118 is slightly wider than the width of the bodies 102, 108 and/or the gibs 104, 110. Typically the slot will be 1 - 2 mm, or less, wider than the width of the bodies 102, 108 or of the gibs 104, 110. In Fig. 7 the width dimension extends in a direction out of the plane of the paper. The plate 116 of the gib stabiliser 114 is bolted to the bottom plate 14 by a bolts 120, which are screwed into threaded formations 122 provided on the bottom plate 14.

The gibs 104 and 110 are self-aligning as they come into contact with the rail 24. This ensures that when the pads 112 come into contact with the rail 24, the surfaces 11 2a lie flat against the rail 24.

Fig. 8 shows a further embodiment of a clamping device, which is generally designated 200. Most of the parts of the clamping device 200 are identical to the clamping device 10, and like parts have been designated with like reference numerals.

In the embodiment of Fig. 8 there are two sets of the clamps 20 and 22 arranged one set above the other. Each set of clamps is identical. The clamps 20 and 22 are different to the clamps in the device 10. The clamp 20 comprises a body 202 and an engagement member in the form of a gib 204, which is pivotally mounted to the body 202 such that the gib 204 can pivot about the longitudinal axis of the gib 204. The gib 204 is provided with a disk brake friction pad 206, which has a surface 206a adapted to engage the rail 24. Similarly, the damp 22 comprises a body 208 and an engagement member in the form of a gib 210, which is pivotally mounted to the body 208 such that the gib 210 can pivot about the longitudinal axis of the gib 210. The gib 210 is provided with a disk brake friction pad 212, which has a surface 212a adapted to engage the rail 24.

A gib stabiliser 214 is provided to support the bodies 202, 208 and/or the gibs 204, 210 at higher speeds. The gib stabiliser 214 comprises a plate 216 having an elongate slot 218 provided therein. The width of the slot 218 is slightly wider than the width of the bodies 202, 208 and/or the gibs 204, 210. Typically the slot will be 1 - 2 mm, or less, wider than the width of the bodies 202, 208 or of the gibs 204,210. In Fig. 7 the width dimension extends in a direction out of the plane of the paper. The plate 216 of the gib stabiliser 214 is bolted to the bottom plate 14 by a bolts 220, which are screwed into threaded formations 222 provided on the bottom plate 14.

The gibs 204 and 210 are self-aligning as they come into contact with the rail 24. This ensures that when the pads 212 come into contact with the rail 24, the surfaces 21 2a lie flat against the rail 24.

In the embodiment of Fig. 8, the nuts 46, 48, 70 and 72 serve the purpose only of adjusting the angles + and 0. Nuts 224, 226, 228 and 230 are provided in order to achieve tensioning of the springs 40, 44, 64 and 68 respectively.

This arrangement enables the spring compression (and hence the clamping force) to be adjusted independently of the angles 8 and +.

Whilst certain embodiments of the invention have been described above, it will be appreciated that the invention can be modified. For example, the stop members 84 and 86 can additionally, or instead, be provided on the bottom wall 14.

Furthermore, the stop members 84 ad 86 can instead be provided on the clamps 20 and 22, at the top and/or bottom thereof. Moreover, the disc springs 40, 44, 64 and 68 can be replaced with any other suitable springs, including leaf springs and compression springs.

The clamping device 10 has been described with reference to use in gripping a guide rail of a lift. The device 10 could be used to grip other elements in a lift; and it need not be used only in lifts - it can be used in any application requiring self-actuated clamping.

Claims

1. A damping device, comprising a frame, and two damps secured to the frame, each clamp having a clamping surface adapted to grip an element, wherein at least one of the damps is movable towards another of the damps to grip the element between the damping surfaces, and wherein the or each movable damp is pivotally mounted to the frame with two separate pivotable linkages, whereby the or each movable damp can be swung into engagement with the element without any substantial change in the orientation of its damping surface.

2. A damping device according to daim 1, wherein each pivotable linkage is pivotally mounted to the frame at a respective frame pivot point, and is pivotally mounted to a respective movable clamp at a respective damp pivot point, whereby each pivotable linkage has an axis extending between its respective frame pivot point and damp pivot point, and as the or each movable damp is swung into engagement with the element, each linkage axis pivots about its respective pivot point.

3. A clamping device according to daim 2, wherein the arrangement is such that, at the point when the or each movable clamp first engages the element to be gripped, each pivotable linkage axis is at an angle + to an axis normal to the surface of the element, and tan(4)) is less than the coefficient of friction between the or each movable clamp and the element.

4. A damping device according to daim 3, further comprising means to adjust the angle +.

5. A damping device according to any daim 3 or 4, wherein the pivotable linkages are adjustably mounted to the frame, so that the position of each pivotable linkage relative to the frame can be adjusted independently of the position of the other pivotable linkages relative to the frame, whereby the angle l) can be adjusted.

6. A damping device according to any preceding daim, further comprising biasing means on the frame, said biasing means being adapted to bias the or each movable damp in a direction towards the element to be gripped, as the or each movable clamp engages the element.

7. A clamping device according to any preceding claim, wherein the or each pivotable linkage is pivotally mounted to a respective shaft, the or each shaft being secured to the frame.

8. A damping device according to daim 7, wherein the or each shaft is adjustably mounted to the frame, whereby the angle + can be adjusted.

9. A clamping device according to daim 7 or 8, when dependent upon claim 6, wherein the or each shaft extends through the frame and is provided with the biasing means on one side of the frame, said biasing means being adapted to push the or each shaft in a direction towards the element to be gripped.

10. A clamping device according to daim 9, further comprising means to adjust the magnitude of the biasing force provided by the biasing means.

11. A damping device according to daim 10, wherein the magnitude of the biasing force is adjustable independently of the angle 4).

12. A clamping device according to claim 10 or 11, wherein the means to adjust the magnitude of the biasing force comprises tensioning means provided on the or each shaft.

13. A clamping device according to claim 12, wherein the tensioning means comprises a nut, which is mounted to the or each shaft by a screw threaded connection, and is rotatable against the load applied by the biasing means, thereby adjusting the position of the shaft, the linkage to which it is connected and the clamp to which the linkage is connected.

14. A clamping device according to any preceding daim, comprising two of said damps and two pairs of said pivotable linkages, each damp being pivotally connected to the frame with a respective pair of said pivotable linkages.

15. A damping device according to any one of claims 1 to 11, comprising a first, second, third and fourth damps, and four pairs of said pivotable linkages, each clamp being pivotally connected to the frame with a respective pair of said pivotable linkages, wherein the first and second clamps are arranged to move together in order to grip a first portion of the element therebetween, and the third and fourth clamps are arranged to move together to grip a second portion of the element therebetween, the second portion of the element being axially spaced from the first portion.

16. A damping device according to any preceding daim, further comprising stop means for limiting the pivotal movement of the or each movable clamp.

17. A clamping device according to any preceding claim, wherein the stop means includes an upper and lower wall of the frame.

18. A clamping device according to daim 16 or 17, wherein the stop means is adjustable, whereby the limits of the pivotal movement of the or each clamp can be adjusted.

19. A clamping device according to any preceding claim, further comprising an actuating element for initiating movement of the or each movable clamp towards the element to be gripped.

20. A damping device according to claim 19, further comprising a control means for controlling actuation of the actuating element.

21. A clamping device according to any preceding daim, wherein the or each movable damp comprises a body, to which the pivotable linkages are secured, and an engagement member for engaging and gripping the element to be gripped.

22. A damping device according to claim 21, wherein the engagement member is resiliently mounted to the body of the damping device, whereby the angle between the engagement member and the element to be gripped, relative to the angle between the body and said element, can be adjusted as the engagement member moves into engagement with said element.

23. A lift braking device comprising a clamping device according to any preceding claim.

24. A method of braking a lift by applying a braking force to an elongate element associated with said lift, the arrangement being such that the application of the braking force to said element causes the lift to reduce its speed or stop, wherein said braking force is applied by bringing a clamping surface of the clamp into frictional engagement with the elongate element by moving the clamping surface towards the elongate element along an arcuate path such that the orientation of the clamping surface, relative to the elongate element, is substantially constant.

25. A clamping device substantially as herein described with reference to and as shown in Figs 1 to 8 of the drawings.