GB2153461A - Disc brake adjustment - Google Patents

Abstract

The brake 2 for a railway vehicle has a yoke 26, first and second brake levers 16,18 respectively pivotable about respective first and second fulcra 22,24 mounted on the yoke 26. Each fulcrum is disposed intermediate the ends of the respective brake lever. First and second pads 8,10 are pivotally mounted respectively on ends of the first and second brake levers and a brake actuator 34 is located between the other ends of the brake levers to pivot them relatively about the fulcra to press the first and second pads against disc rings 4,6 respectively. The distance between the fulcra is variable by movement of at least one fulcrum e.g. 24 relatively to yoke 26 and to compensate for wear of the pads and/or the rings 4,6 the said distance is varied in response to detection of a movement of the pads between brake released and brake applied positions in excess of a predetermined amount. Actuator 34 is shown as a pneumatic motor with an excess movement detector comprising a movable arm 64 extending into actuator 34 with its outer end pivotably connected to cable 66, operable via a clutch arrangement 76 to move fulcrum 24 along slots 56A, 56B. A number of embodiments are disclosed e.g. in Figure 10 excess movement is detected at actuator 34 and air operates a piston and cylinder unit (138) mounted on clutch (276). <IMAGE>

Description

SPECIFICATION Disc brake This invention concerns disc brakes.

The disc brakes are ofthetype (hereinafter identi fied as the "type referred to") comprising a bridge piece, first and second brake levers respectively pivoting about respective first and second spaced fulcra mounted on the bridge piece, each fulcrum being disposed intermediate the ends of the respective brake lever so that a first portion of the lever extends from a first end ofthe leverto the fulcrum and a second portion ofthe lever extends from the fulcrum to the otherorsecond end ofthe lever, brake rotor means rotatable about an axis and having first and secondoppositelyfacing brakingsurfaces,firstand second friction pads pivotally mounted respectively on the first and second brake levers on the first portions thereof, and brake actuating means acting on the second portions of the brake levers to move them relatively to one another about the fulcra to press the first and second friction pads againstthefirst and second braking surfaces respectively.

In a known disc brake ofthe type referred to used on railway vehicles, the two fulcra are a fixed distance apart, the brake rotor means (which may be conveniently referred to as a disc) can be either a disc unit fastwith the axle of a railwayvehicle wheel or comprise two rings secured to either side of the web of the wheel, the friction surfaces on the disc face away from one another, and the disc is disposed between the brake pads which when braking is demanded are moved towards one another and pressed againstthe friction surfaces in response to operation ofthe brake actuating means acting to movethesecond ends of the brake levers further apart. The brake actuating means can be a pneumatic motor or actuator comprising a cylinder unit containing a diaphram or piston connected to a rod extending from the cylinder unit.

The rod is pivotally mounted on the second end of one brake lever and the cylinder unit is pivotally mounted on the second end ofthe other lever with a substantial portion ofthe cylinder unitjutting out beyond the outer side of that other lever. An application of compressed airto the actuator causes the rod and the cylinder unit to move relatively to each other to extend the length ofthe motorandthus cause brake application. When brake release is demanded pneumatic pressure in the actuator is relieved and spring means acts to move the second ends ofthe brake levers nearerto each other to move the brake padsfurther apart.

The actuator also comprises a slack adjuster. When the brake pads and friction surfaces becomeworn detector means observes when the second ends ofthe brake levers are moved during an application of the brake more than a predetermined distance from their brake released positions, and when this excess travel is observed the slack adjuster operates to extend the effective length of the rod. This causes the brake pads to be maintained only short distances from the friction surfaces, but as the brake becomes increasingly worn the positions occupied bythesecond ends of brake levers in the brake released state are progressively further apart. So the positions occupied by those second ends in the brake applied state also become further apart as brake wear increases.Therefore an increasing amount of working space is needed to accommodate the brake in its released state and to allow the second portion of the brake levers to move apart during brake application. Also, the brake is often mounted so it can twist and pivot relatively to the rail vehicle. This can allow some shifting of the brake bodily in one direction or other as pad wear is compensated for. That compenstion can also cause a distortion of the geometry ofthe brake such as skewing of the brake levers from their desired position relatively to the brake rotor, leading to a loss of braking efficiency.Because a disc brake on a railway vehicle is usually mounted in proximity to a rail wheel and underneaththevehicle,spaceforthebrakeis often restricted by the presence of other equipment including, for example a traction motor, necessaryto thevehicle.Thereforethe re-positioning ofthe brake levers in compensating for pad wear and/orthe bodily shifting ofthe brake and/or the skewing of the levers can result in the second portions of the brake levers and the jutting actuator cylinder becoming fouled by the other equipment with consequent loss of braking effect.

Whilst the invention is applicable to a variety of disc brakes ofthe type referred to, one object is to provide a dissbrakesuitablefor use on a railway vehicle and capable of being constructed so the aforesaid drawbacks are at least mitigated.

According to the invention there is provided a disc brake ofthe type referred to wherein the distance between the fulcra is variable by movement of at least one fulcrum relativelyto the bridge piece, and to compensate forwear ofthe brake pads and/or the friction surfaces said distance is varied in response to detection of a movement ofthe brake pads between brake released and brake applied positions in excess of a predetermined amount.

The detection of the excess movement ofthe pads may be performed by detection of a movement ofthe brake levers between brake released and brake applied positions in excess of a predetermined amount.

When the fulcrum is moved the brake leverthereon and particularly the first portion ofthe lever carrying the brake pad is moved towards the corresponding friction surface to bring the pad closerthereto in the brake released state to compensate for wear. As the disposition of the actuating means on the brake levers and the latter own the bridge piece can allow the brake pads a self-centering tendency with respect to the The drawing(s) originally filed (were) informal and the print here reproduced is taken from a later filed formal copy.

The claims were filed later than the filing date within the period prescribed by Rule 25(1) of the Patents Rules 1982.

brake rotor means during brake application, the moving of one pad towards its corresponding friction surface on the rotor means to compensate for wear causes both pads to be closet to their respective friction surfaces, afterthe next release ofthe brake, than beforethefulcrum was moved.

In the case where the friction surfaces face away from one another on a brake disc between the pads, the distance betweenthefulcra is reduced upon detection of said excess movement.

By compensating for pad and friction surface wear by moving one or both fulcra relativelyto the bridge piece, then throughoutthe life ofthe brake pads and friction surfaces the second ends ofthe brake levers can remain in approximately the same positions, in the brake released state and each moves to approx- imatelythe same position when the brake is applied.

This means the brake requires substantially the same space to work in whetherthe pads and friction surfaces are new orworn,the brake does not tend progressively to shift considerably bodily in one direction or another as compensation forweartakes place, nor do the levers tend to wildly skew so the desired geometry of the brake is maintained to a large extent. Also the actuating means does not need to include a conventional slack adjuster. The actuating means can be disposed between the brake levers in the sense that it need notjut beyond the outerside of either.

The bridge piece may be a U-shaped yoke in which each fulcrum is disposed at or adjacent an end of each arm ofthe U.

The movement ofthe or each movablefulcrum may be along slot means with which the bridge piece is provided and in which slot means the movable fulcrum is disposed. The slot means extends trans verselyto the axis ofthe movable fulcrum therein, and the direction of said slot means may be towards the otherfulcrum.

The arrangement is such, that on the or a said movable fulcrum being moved along the slot means to any given position, it is retained at that position until it is again moved in responseto detection of said excess movement ofthe brake pads.

Means detecting the excess movementofthe brake pads or levers can be arranged to provide a signal or response to cause fulcrum moving means to move a said movablefulcrum along its slot towards the other fulcrum. Each movement of said movable fulcrum may be through a predetermined increment of length.

The fulcrum moving means may comprise a rotatable device caused to rotate-in a given direction of occurance of said signal or response to move the fulcrum along its slot means. Said device may be a cam arranged to move the fulcrum, for example by pushing it as the cam rotates in the given direction, or thefulcrum moving means may comprise a screwjack comprising the device such thatthe length of the jack is varied to move the fulcrum as the device is rotated in said given direction.

Rotation of the device in said given direction may be by a driving mdans operable in consequencetothe occurance of said signal or response. For example the driving means may comprise a mechanical connection between the brake actuating means and the device, so the connection can transmit motion from the actuating means to said device. Said mechanical connection may comprise a flexible drive or Bowden cable arrangement. Or, for example, the driving means may comprise a pneumatic or fluid pressure system operational in response to detection of said excess movement. Or, for example, the driving means may comprise a mechanical connection, for example comprising interconnected pivotable levers, between a brake lever and the device, and, for example, said driving means may be adapted to detect said excess movement.One way drive or clutch means may be provided to ensure only rotary drive in said given direction is transmitted to the rotatable device (ofthe fulcrum moving means) by the driving means when said excess movement has been detected.

As envisaged above both fulcra may be moved relative to the bridge piece. Both fulcra may be moved simu Itaneously in respective said slot means and each fulcrum may be moved by operation of respective driving means and respective fulcrum moving means.

The invention will now be further described, byway of example, with reference to the accompanying diagrammatic drawings in which Fig. lisa plan view of a first embodiment of brake formed according to the invention in which the brake pads are unworn; Fig. 2 is a side view of the brake in Fig. 1; Fig. 3 is a section on an enlarged scale on line Ill-Ill in Fig.1; Fig. 4 is a section on line IV-IV in Fig. 3; Fig. 5 is a section on line V-V in Fig. 3; Fig.6 is a plan view ofthe brake in Fig. 1 when the brake pads are considerably worn; Fig. 7 shows a modification to the brake in Fig. 1; Fig. 8 is a plan view of another embodiment of disc brakeformed accordingto the invention;; Fig. 9 shows on an enlarged scale a fragment partly in section ofthe brake in Fig. 8; Fig. 10 is a plan view of a modification to the brake in Fig. 8; Fig. 11 is a side view of another modification ofthe brake in Fig. 8 and Fig. 12 is a plan view ofthe brake in Fig. 11.

In the drawings like reference numerals refer to like or comparable parts.

Referring to Figs. 1 to 6, the disc brake 2 is shown mounted (by means to be described) in association with a wheel W of a railway vehicle in which the brake rotor or disc comprises two friction rings 4 and 6 affixedly mounted on either side of a web of the wheel and concentric with the wheel axis. Associated with each disc ring is a respective brake pad 8 or 10 of friction material each on a shoe 12 or 14 supported by ends of respective brake levers 16,18 on which each is pivotally mounted in known manner by means of a pivot pin 20 extending transverse to the direction of the brake lever. In this example, the brake levers are substantially horizontal in a plane which is substantially radial of the wheel, and the pins 20 are vertical.

The brake lever 16 comprises a pair of superposed lever arms 1 6A, 1 6B which may be fast one with another and pivot simultaneously about a fulcrum 22.

The brake lever 18 can be similarly formed with lever arms 18A, 18B pivoting aboutafulcrum 24.Thefulra or pivot pins 22, 24are parallel with the pins 20 and are mounted on the ends of arms of a U-shaped bridging piece or yoke 26 comprising superposed plates 26A, 26B which can be rigidly affixed to one another, the brake levers being disposed between the yoke plates.

The fulcrum 24 (see Fig. 3) is constituted by a pin formed by upper and lowertrunnions 28 and 30 which are engaged in the upper and lower yoke plates 26A and 26B and are integral and co-axial with an intermediate cylindrical enlargement 32 between the two lever arms of the brake lever pivoting on that fulcrum.

A brake actuator34formed by a pneumatic motor is wholly located between the ends ofthe brake levers remote from the pads 8, 10. The actuator 34 has a cylinder36 containing a piston or diaphram connected with a rod 38. Cylinder 36 and rod 38 are respectively pivoted by pins 40to the brake levers 16 and 18, the axes of those pins being parallel to the fulcra 22,24 and pins 20. On a demand four braking, supply of compressed airtothe actuator causes a relative movement apart ofthe cylinder 36 and rod 38 thus pivoting the brake levers aboutthe fulcra 22,24 and pressing the brake pads 8, 10 against the disc4,6.

When brake release is demanded air pressure in the cylinder is relieved, allowing spring means to urge the pins 40 closertogetherthus pivoting the brake levers to relieve the pressure of the brake pads on the disc 4, 6. The actuatorcan comprisethespring means acting to telescope the cylinder and rod together for brake release.

The brake may be mounted on a rail vehicle or bogie frame by any desired method. In the example each brake shoe is pivotally connected at 42 to a drag link 44 pivotally connected at 46 to the vehicle or bogieframe.

The drag links may be arranged to maintain the brake pads parallel with the brake disc. Also in the example the yoke is mounted on the vehicle or bogieframe by a pair of brackets 48 on the yoke. If desired the yoke may be mounted to pivot relatively to the vehicle or bogie frame.

At end 50 of the yoke arm mounting the fulcrum 24, both the plates 26A and 26B are provided with planar transverse parts 52,54 respectively, projecting to either side of the yoke arm and each formed with a respective slot 56A, 56B respectively receiving the trunnions 28,30. The slots 56A, 56B are directly one above the other, parallel and directedtowardsthe other fulcrum 22.Journal led in parts 52 and 54 is a shaft 58fast with an eccentricorsnail-cam 60 which when progressively rotated anti-clockwise (see Fig. 5) acts to push the cylindrical enlargement 32 progres lively to the left in Fig. 5, thus the cam can move the fulcrum 24 along the slots 56A, 56B towards the fulcrum 22 to reduce the distance between the fulcra.

Means (not shown), for example resilient biassing, can be provided to inhibitthefulcrum 24 moving towards the fulcrum 22 except by urging ofthe cam. In the example, cam 60 acts on a plate 62 affixed to the enlarged part 32. However, the cam may directly contactthe part 32, or a snail-cam formation may interact in some other way with thefulcrum 24 to move it.

An excess movement detector to observe when (dueto wear of the pads 8,10 and disc rings 4,6) the pins 40 have to be movedfurtherthan a predetermined distance apart to apply the brake, can comprise a movable arm 64which extends into the actuator 34 and has its outer end pivotally connected to a wire or cable 66 of a flexible drive or Bowden cable 68 with a casing tube 70 fixed at its ends to brackets 72,74 on the cylinder and yoke respectively.The arm 64 is arranged to co-actwith the actuator so that (i) onlywhen brake application requires the pins 40 to be moved apart in excess of a predeternined amount does relative movement between the acutator cylinder and the actuator piston, diaphragm or rod 38 cause the arm 64 to move relative to the actuator to push the wire 68 through the casing 70, and (ii) only on the next release of the brake does relative movement between the actuator cylinder and the actuator piston, diaphragm or rod 38 cause the arm 64to move back relative to the actuatorto pull the wire 68 backthrough casing 70.

The other end of the wire 68 is pivotably connected by a shackle 72to a pin 74 off-setfrom the axis ofthe cam shaft 58 to drive a grip-rollerfree wheeling clutch arrangement76 connected to the cam shaft. When the wire 68 is pushed through the casing 70, the clutch arrangement is rotated through an arc in the clockwise direction of arrow in Fig. 1 about the axis ofthe shaft 58, but the clutch arrangementfreewheels and the shaft remains stationary. On the wire being pulled backthrough casing 70, the pin 74 and clutch arrangement are rotated by the pull backthrough su bstantially the same arc in the anti-clockwise direction of arrow Y.But this time the clutch locks and rotates the shaft 58 causing the cam 60 to push the fulcrum 24 an incrementalongtheslots56A, 56B towards the fulcrum 22 so the worn brake pads move closerto the disc to compensate for wear. Fig. 6 shows a situation where progressive brake wear has caused successive compensations resulting in fulcrum 24 coming to a position atthe inner ends ofthe slots 56A, 56B,so new brake pads and possibly disc rings are required.

Clutch arrangement 76 has a pair of clutches 76A and 76B each based on the known grip-roller free wheeling clutch principle. Upper clutch 76Acomprises an outer race 78 with an integral upper cover 80 freely surrounding the cam shaft 58, and an inner race or spider82 keyed at 84to the cam shaft and forming with the outer race wedge-shaped spaces containing rollers 86 and their biasing springs 88. The lower clutch 76B comprises a similarly disposed inner race 90 and rollers 92 with biasing springs. The outer race 94 sits loose on a base plate 96 and has a cylindrical wall 98 freely surrounding the outer race 78.100 and 102 are retaining clips and 104 a shim. The base 96 locked to part 52 covers the slot 56A and carries a pivoting latch 106 biased into its locking position buy a leaf spring 108 to engage in a notch between adjacent ones of a ring of castellations 110 on the wall 98 to lock the outer race 94 against rotation.

When the pin 74 is pushed in direction X in Fig. 1 and likewise rotates the outer race 78, the inner races 82, 90 and cam shaft 58 remain stationary. When the pin 74 is pulled in direction Yin Fig. 1 and likewise rotates the outer race 78, the rollers 86 wedge with the inner race 82 and rotate it in direction Yto rotate the cam shaft and cam 60 which moves fulcrum 32, but outer race 94 remains locked by the latch 106. In these circumstances cam 60 can only rotate in anti-clockwise direction Y. If it attempts to rotate clockwise, the rollers 92 wedge againstthe stationary outer race 94 so clutch 76B acts as a holding clutch forthe cam.

When new brake pads and/or disc rings are fitted there is a need to restore the cam 60 to the position shown in Figs. 3 and 5, by rotating it clockwise in direction X. This is done by releasing latch 106 and rotating the whole of clutch 76B (and with it the inner race of clutch 76A) clockwise and then re-locking the outer race 94 with the latch.

In Fig. 7, fulcrum 24' replaces fulcrum 22 of Figs. 1 to 6. Fulcrum 24' is similarto the fulcrum 24 and is shifted towardsthatfulcrum during compensation for brake wear in like manner to the technique used for shifting fulcrum 24 involving the use of a second Bowden cable 68' and clutch arrangement 76'.

In Figs. 8 and 9, the Bowden cable 68 acts on a clutch arrangement 276 secured to and spanning the two yoke plates 26A, 26B (only one shown) between which is a clevis 112 of which the bifu rcationsrespectively engage the trunnions 28,30 ofthe fulcrum 24. The clevis 112 is mounted on athreaded bolt 114engaging a threaded nut 116. Clutch arrangement 276 based on the known grip-rollerfreewheeling clutch principle comprises an inner race 118 keyed to the nut 116 forming wedge-shaped spaceswith an outer race 120 intheform of a capforthe nut 116 and rotatablewithin a casing 122.The wedge-shaped spaces contain rollers 124 and their biasing springs, the inner race 118 and the rollers being between shim rings 126 and 128 pressed by Belville washer stack 130 and a threaded locking ring 132. Wire 66 is attached to the outer race 120through a shackle 272 pivoted to the race at a point off-setfrom the axis ofthe bolt 114. When due to brake wearthe movement apart ofthe pins 40 during brake application exceeds the predetermined amountthe wire 66 pushes the outer race 120 which rotates through an arc about the axis of bolt 114.During the next brake release the wire pulls the outer race through that arc in reverse direction causing the rollers 1 24to engage and rotate the inner race causing the nut 1 16to rotate and pay out some of bolt 114 therethrough. So the clevis 112 moves the fulcrum 24 along the slots 56A, 56B towards the fulcrum 22 to compensate forthe brake wear. When new brake pads and/or disc rings are fitted the bolt 114 can be retracted into the nut 116 by rotating the latter using an applied tool afterthe outer race 120 is removed from the casing 122 after releasing the retaining washer 134 and ring clip 136.

In Fig. 10 the outer race of the clutch arrangement 276 is connected to a piston of a piston and cylinder unit 138 mounted on the clutch 276. When the piston is driven in one direction by compressed airthe outer race is rotated by the piston but does not rotate the inner race. On the piston being driven in the opposite direction by return or spring means it rotates the outer race in reverse direction to rotate the inner race thus paying out moie of the bolt 1 14to move the fulcrum 24 towardsthefulcrum 22. The unit 138 is connected by pipe 140 to a port in the cylinder of actuator 34.

Normally that port is open to atmosphere, but is communicated with the pressurised air in the actuator cylinder by movement of the actuator piston or diaphragm past the port only when the pins 40 have to be moved more than a predetermined distance apart to apply the brake due to wear, then air under pressure operates the unit 138. On the next release of the brake, the return movement of the actuator piston or diaphragm uncovers the port to atmosphere allowing the force ofthe return means to move the fulcrum 24.

In Figs. 11 and 12 a mechanical linkage 150 is used to detect excess movement of the brake pads and cause the clutch 276 to operate so as to move the fulcrum 24.

The linkage 150 comprises a lever arm 152fastwith the outer race 122 of the clutch 276. At its free end the arm 152 has a slot 154 in which is disposed a finger 156 formed at one arm of a bell-crank lever 158 which can swing about a pivot 160 mounted on the yoke 26 and parallel to fulcra 22,24. The longer arm 162 ofthe bell-crank lever extends along the brake lever 18 and is connected through a pivot 164 (parallel to pivot 160) to a bracket 166 fast with the lever arm 18A. When actuator 34 moves the pins 40 apartto applythe brake the finger 156 swings anticlockwise (Fig. 12) about the pivot 160.As long as the pads 8,10 do not move more than a predetermined amountforthe brake to be applied, the combination of the finger 156 and slot 154 act as a lost motion device in which the aforesaid anti-clockwise swing is insufficient for the finger to reach end 1 54A ofthe slot and so the lever 152 remains stationary during both brake application and brake release. But on the pads having to move morethan the predetermined amount at brake application, thefinger 156 engages the slot end 1 54A and pivots the lever through an arc as indicated by arrowX (Fig. 11) thus rotating the outer race 122.At brake release, the finger swings back about pivot 160to push the opposite end 154B ofthe slot causing the lever 152 and outer race 122 to rotate through an arc in the direction of arrow Y (Fig. 11). This rotation ofthe race 122 causes (as in the embodiment in Figs. 8 and 9) the fulcrum 24to move towardsthefulcrum 22. The progressive movement of the fulcrum 24 along the slot 56 as pad wear increases has the effect of pivoting the bell-crank lever 158 progressively clockwise in Fig. 12 about pivot 160 thus pushing finger 156 further into slot 154 and so the finger is disposed nearerto slot end 154 B when the brakes are in the released attitude. This would tend to progressively the delay the occurence of compensation for pad wear, but in a brake formed to practical dimensions intending to provide practical nominal clearances between the brake pads and rotors when the brake is in released attitude, any variation in compensation which occurs asthe pads become progressively worn is only slight.

It will be appreciated in Figs. 8to 10 or 11 and 12 that both fulcra can be similarly shifted, and thatthe clutch arrangements 76 and 76' in Figs. 1 to 7 can each be operated by a piston and cylinder unit 138, or by a mechanical linkage acted on by a brake lever.

Claims (11)

1. A disc brake comprising a bridge piece, first and second brake levers respectively pivoting about re spectivefirstand second spaced fulcra mounted on the bridge piece, each fulcrum being disposed intermediatethe ends of the respective brake lever so that a first portion ofthe lever extends from a first end ofthe lever to the fulcrum and a second portion of the lever extends from the fulcrum to the other or second end ofthe lever, brake rotor means rotatable about an axis and having first and second oppositely facing braking surfaces, first and second friction pads pivotally mounted respectively on the first and second brake levers on the first portions thereof, and brake actuating means acting on the second portions of the brake levers to move them relatively to one another about the fulcra to press the first and second friction padsagainstthefirstandsecond braking surfaces respectively, wherein the distance between the fulcra is variable by movement of at least one fulcrum relativelytothe bridge piece, and to compensatefor wear ofthe brake pads and/orthefriction surfaces said distance is varied in response to detection of a movement ofthe brake pads between brake released and brake applied positions in excess of a predetermined amount.

2. A disc brake according to claim 1, wherein excess movement of the pads is detected by means for detecting a movement of the brake levers between brake released and brake applied positions in excess of a predetermined amount.

3. A disc brake according to claim 1 or 2, wherein the actuating means is disposed between the brake levers.

4. A disc brake according to any preceding claim, wherein the bridge piece is a U-shaped yoke in which each fulcrum is disposed at or adjacent an end of each arm ofthe U.

5. A disc brake according to any preceding claim, wherein the or each movable fulcrum is movable along slot means with which the bridge piece is provided and in which slotmeansthe movable fulcrum is disposed, the slot means extending trans verselyto the axis ofthe movable fulcrum therein.

6. A disc brake according to claim 5, comprising means for detecting the excess movement of the brake pads or levers which is arranged to provide a signal or responseto causefulorum moving meansto move a said movable fulcrum along its slot towards the otherfulcrum.

7. A disc brake according to claim 6, wherein each movement of said movable fulcrum is through a predetermined increment of length.

8. A disc brake according to claim 7, wherein the fulcrum moving means comprises a rotatable device caused to rotate in a given direction by occurance of said signal or response ot move the fulcrum along its slot means.

9. A disc brake according to claim 8, wherein the rotatable device is a cam arranged to move the fulcrum.

10. A disc brake according to claim 8, wherein the rotatable device comprises a screw jack, the length of the jack being varied to move the fulcrum as the device is rotated.

11. A discbrake according to claim 8.or 10, comprising one way drive or clutch means arranged to ensure only rotary drive in said given direction is transmitted to the rotatable device when said excess movement has been detected.