WO1991011217A1

WO1991011217A1 - Safety device

Info

Publication number: WO1991011217A1
Application number: PCT/GB1991/000113
Authority: WO
Inventors: George Patrick Sharp
Original assignee: Denis Ferranti Meters Limited
Priority date: 1990-01-27
Filing date: 1991-01-25
Publication date: 1991-08-08
Also published as: AU7072691A; GB9101629D0; GB9001908D0; GB2240757A

Abstract

A safety device comprising a housing (15), a drum (2) rotatably mounted in the housing (15), a cable (1) wound on the drum, and a brake device which is arranged to brake the drum (2) if the rate of rotation of the drum (2) exceeds a predetermined threshold. A pawl (20) is mounted on the drum (2) and adapted to engage a tooth (13) defined by the brake device, engagement between the tooth (13) and pawl (20) causing the brake device to provide frictional resistance to rotation of the drum (2). The pawl (20) is engaged by a cam (11) and a spring (21) arranged such that during rotation of the drum (2) relative to the brake device the pawl (20) is moved cyclically between first and second position. The cam (11) moves the pawl (20) into the first position in which the pawl (20) can engage the tooth (13) and the spring (21) biases the pawl (20) towards the second position in which the pawl (20) cannot engage the tooth (13). The pawl (20) is mounted on the drum (2) such that rotation of the drum (2) results in centrifugal force biasing the pawl (20) towards the first position once the predetermined drum speed threshold is exceeded.

Description

SAFETY DEVICE

The present invention relates to a safety device.

Safety devices are known in which a drum is rotatably mounted in a housing and a cable is wound on the drum. The cable can be attached to a person to be protected against injury in the event of a fall, or to any other load the descent of which is to be controlled.

US Patent specification No. 4489919 describes one example of a safety device of the type to which the present invention relates. This US patent incorporates a winch with a manually actuable handle which can be used to raise or lower a load supported on a cable coiled around the winch drum. If a load is being lowered by the winch and control of the winch handle is lost, the load could fall rapidly. To prevent this happening, the winch incorporates a braking device which is engaged in the event of the drum rotating at a rate above a predetermined rate. Thus if for any reason a load falls rapidly its descent is braked and the load is brought to a halt in a controlled manner. Deceleration forces applied to the load are a function of the braking force. Thus the braking force can be selected for example to prevent serious injury to a worker protected against a fatal fall by connection to such a winch device.

The. braking mechanism described in US patent No. 4489919 includes an array of pawls which are arranged to engage in teeth defined by the braking mechanism when braking is required. The pawls are moved into the teeth engaging position by centrifugal force only when the drum is rotated rapidly. In normal operation of course the pawls never move into the brake engaging position and it is therefore possible over time for the braking mechanism to become inoperative as a result of the pawls becoming stuck in their normal inoperative position. For example, contamination or corrosion of the pivots supporting the pawls can render the braking device inoperative with potentially disastrous consequences. The risk of this occurring can be reduced by for example using sophisticated bearings to support the pawls but this obviously increases manufacturing costs without completely solving the problem.

Another problem associated with known devices of the type described in US patent 4489919 is that of providing a reliable mechanism for controlling the rate of descent of a load which is to be lowered. In the arrangement described in US 4489919, a load can only be lowered if a ratchet mechanism that normally prevents a load falling is deliberately disengaged. The load must then be supported directly by the winch handle. It is true that if control of the handle is lost the safety braking mechanism will bring the load to a halt but this is not satisfactory in many circumstances. For example if the load is an injured person the force which must be applied to the winch handle to control that person's descent is considerable and this can be a major problem if that force must be maintained for a prolonged period. Furthermore there are obvious problems in having a ratchet mechanism which must reliably prevent the load from falling in normal use but which must be disengageable to permit an intended lowering of a load.

Another problem associated with known safety devices of the type to which the present invention relates arises when such devices are used as fall arresters. The principle of known fall arresters is that a person to be protected against an accidental fall is connected to a cable wound on a drum of the arrester. In normal circumstances movement of the worker is not impeded as the cable is payed out by and reeled back in by a simple spring mechanism that applies a low tension to the cable. If a person supported by such a device falls his descent is brought to a halt in a controlled manner by a centrifugally operated braking device. It is known to provide a warning indicator in such devices which sounds a warning in the event of the tension in the cable exceeding a predetermined limit, for example as occurs when the cable is supporting a worker's body. This is important for example to indicate to other workers when one of their colleagues has been injured and is hanging unconscious from the cable. There is a problem with known fall arresters of this type however in that there is no means for indicating to a worker when he is operating close to the full extent of the cable. If for example he was to fall when all but the last few tens of centimetres of cable had been payed out he would fall freely for those few tens of centimetres and be brought to a sudden halt when the cable has been fully payed out rather than being brought gradually to a halt by the braking mechanism.

It is an object of the present invention to obviate or mitigate one or more of the problems outlined above.

According to the present invention there is provided a safety device comprising a housing, a drum rotatably mounted in the housing, a cable wound on the drum, a brake device, and at least one pawl mounted on the drum and adapted to engage at least one tooth defined by the brake device, engagement between said at least one tooth and pawl causing the brake device to provide frictional resistance to rotation of the drum, wherein said at least one pawl is engaged by a cam and biasing means arranged such that during rotation of the drum relative to the brake device the pawl is moved -cyclically between first and second position, the cam moving the pawl into the first position in which the pawl can engage the said at least one tooth, and the biasing means biasing the pawl towards the second position in which the pawl cannot engage the said at least one tooth, said at least one pawl being mounted on the drum such that rotation of the drum results in centrifugal force biasing the pawl towards the first position.

The brake device may comprise a disc of friction material sandwiched between a surface supported by the casing and a brake disc, the brake disc supporting the said at least one tooth which is engageable by the said at least one pawl.

The arrangement is such that rotation of the drum in the normal manner, which does not engage the brake device, results in the or each pawl being "exercised", that is moved to the position which it should assume if the drum is rotated at a speed indicating a braking requirement. If for any reason the pawl should become stuck in one position, the cam ensures that it can only become stuck in a position in which it will engage the brake device. Thus failure of the pawl mechanism is immediately indicated as the cable cannot be pulled out from the safety device. The device is therefore inherently fail-safe.

Two pawls may be provided arranged on opposite sides of a central cam defined by the brake device. Thus as the pawls rotate with the drum they are acted upon by the cam. The cam pushes the pawls outwards against a return force provided by coiled springs.

According to a second aspect of the present invention, there is provided a safety device comprising a housing, a drum rotatably mounted in the housing, a cable wound on the drum, and a winch device for controlling rotation of the drum to wind cable onto or off the drum, wherein the winch device comprises a rotatable shaft, a gearwheel freely rotatable on the shaft and arranged to engage the drum such that rotation of the gearwheel causes rotation of the drum, an actuator mounted on the shaft such that the gearwheel is positioned between the actuator and a flange on the shaft, and a brake device mounted on the shaft between the actuator and the flange to engage the gearwheel, the actuator being movable relative to the shaft in one direction to increase the pressure on the brake device, and movable in the other direction to reduce the pressure on the brake device, whereby slippage of the gearwheel relative to the shaft can be controlled by movement of the actuator.

The shaft may support a toothed wheel engageable by a pawl such that the shaft can only be rotated in one direction. That one direction corresponds to the direction of rotation of the drum which causes the cable to be wound onto the drum. If a cable is to be wound off the drum, the actuator is moved such that the pressure on the brake device is reduced, thereby enabling the gearwheel to rotate on the shaft. The user can simply control the braking force, and thus the speed of descent of a load attached to the cable, by manipulation of the actuator. The load is supported by the brake device rather than by force applied directly to the actuator.

The actuator may comprise a member which is screwed onto a threaded portion of the winch shaft. The thread sense is such that rotation of the actuator in a direction permitted by the ratchet mechanism screws the actuator onto the shaft, thereby increasing the pressure on the brake device and increasing the torque which can be applied to the drum by the gearwheel. If however it is desired to allow a load to descend, the actuator is rotated in the opposite direction and the user can then closely control the rate of descent by small adjustments to the angular position of the actuator.

According to a third aspect of the present invention, there is provided a safety device comprising a housing, a drum rotatably mounted in the housing, a cable attached at one end to the drum, and a brake device for braking rotation of the drum if the rate of rotation of the drum exceeds a predetermined rate, wherein a member is secured to the cable a predetermined distance along the cable from its point of attachment to the drum, and means are provided to signal the passage of the member out of the housing.

The signalling means may comprise a switch positioned so as to be sensitive to variations in the diameter of the cable and the member may be a simple body secured to the cable or a sheath extending from the end of the cable secured to the drum. The switch may be actuated by a roller across which the cable runs and may serve the dual purpose of indicating on the one hand when nearly all of the cable has been wound off the drum and on the other hand when the tension in the cable has exceeded a predetermined limit.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a sectional view through the embodiment of the present invention;

Fig. 2 is a view on the lines II-II of Fig. 1, the section line of Fig. 1 being indicated by lines I-I; and

Fig. 3 illustrates the relative positions of various components shown in Fig. 1.

Referring to the drawings, the safety device comprises a safety cable 1 wrapped around the circumference of a drum 2. The drum 2 is rotatably supported on a fixed main shaft 3. One end of the main shaft defines an axial slot 4 which accepts one end of a clock type wound spring 5. The spring 5 engages with a collar 6 which is supported on the main shaft 3 such that the spring is coaxial with the drum 2. The spring 5 fits inside a spring housing 7 to which it is secured.

The spring housing 7 is fixed to one end of the drum 2. The other end of the drum 2 defines a radial flange 8 extending to substantially the same diameter as the spring housing 7. Thus the safety cable 1 is held in position between the spring housing 7 and the radial flange 8. The drum 2 is free to rotate only against the returning force of the wound spring 5 and a substantially constant take up pressure is thereby maintained on the safety cable 1.

A brake plate 9 is supported on the main shaft 3 adjacent to and coaxial with the drum flange 8. The brake plate 9 is substantially disc shaped and is rotatable on the main shaft 3. On its face adjacent to the drum 2 the brake plate 9 supports two annular protruding walls; one 10 close to the outer edge of the brake plate 9, the other 11 around the main shaft 3. A recess 12 is thus defined in the brake plate 9 between the two walls. The inner wall 11 protrudes along the main shaft 3; its outer surface defining a hexagon which as described below functions as a cam. The outer wall 10 has a smooth circular outer edge and an inner edge which defines a sawtooth of six teeth 13 in a well known ratchet design.

The main shaft 3 is supported between two parallel spaced walls 14 of a main housing 15, the said walls being perpendicular to the main shaft 3. The main shaft 3 extends through holes 16 in the housing walls 14 and is fixed by a pin at one end to said wall 14.

Adjacent to the brake plate 9 on the opposite face to the drum 2 a brake pad 16 is supported on the main shaft 3 coaxial with the brake plate 9. The brake pad 16 is of the form of a thin disc manufactured from friction material as commonly used in automobile braking systems. The thickness of the brake pad 16 is. reduced so that only approximately the radially outer half of the brake plate 9 is contacted by the pad 16. Between the brake pad 16 and the housing wall 14 a thin metal disc 17 of stainless steel or similar material is positioned, supported on the main shaft 3 coaxial with the brake pad 16. This is of substantially the same diameter as the brake plate 9. The brake plate 9 is held in close contact with the brake pad 16 by a pressure ring collar 18 which fits over the periphery of the brake plate 9 which extends beyond the outer wall 10 and is fixed to the housing wall 14 by nuts and bolts equi-spaced around the periphery of the collar 18. Belville-type washers mounted on the bolts between the nuts and the collar 18 are used to maintain a predetermined pressure between the brake pad 16 and the brake plate 9.

On the drum flange 8 adjacent to the brake plate 9 two fixed circular pins 19 are supported diametrically opposite each other at approximately one third of the radius of the drum flange 8. Each pin 19 supports a pawl 20 which is free to pivot on its respective pin 19. The pawls 20 reside in the recess 12 of the brake plate 9. The thickness of the pawls 20 is substantially the same as the depth of the recess 12. One end of each pawl 20 is defined to engage securely with the teeth 13 of the ratchet defined in the brake plate 9. The pawls 20 are positioned opposite each other on either side of the inner wall 11. The pawls 20 are fixed to each other at either end by means of two springs 21. Thus as the drum 2 rotates the pawls 20 rotate as a pair around the main shaft 3 whilst pivoting on their respective pins 19 in a manner defined by their contact with the hexagonal edge of the inner wall 11 which acts as a cam.

A winch shaft 22 runs parallel to the main shaft 3 between the housing walls 14 and is supported at either end by bearing members 23. The shaft 22 is positioned clear of the periphery of the above-described main shaft 3 assembly such that a toothed wheel 24 supported on the winch shaft 22 can engage with teeth defined on the outer rim of the drum flange 8. The winch shaft comprises a splined end 25, a middle portion 26 of approximately twice the diameter of the splined end 25 and a screw threaded end 27. The toothed wheel 2 is supported on the screw threaded end between two friction discs 28 made from the same material as the brake pad 16. The friction discs 28 are of substantially the same diameter as the middle portion 26 the end of which defines a flange against which one of the discs 28 bears. A splined member 29 is screw mounted along the remaining length of the screw threaded end 27 of the winch shaft 22. The splined member 29 defines a flange at one end which bears against one of the friction discs 28. The splined member 29 can be screw tightened against the friction disc 28 such that the toothed wheel 24 is unable to rotate between the two friction discs 28. The splined end 25 of the winch shaft 22 supports a ratchet wheel 31. The ratchet wheel 31 is engaged by a pawl (not shown) which prevents the shaft 22 from rotating in one direction.

The screw threaded splined member 29 extends beyond the housing wall 14 and supports a handle 30 defining a circular hub 32 at the end of the winch shaft 22. The hub 32 makes contact with an external raised surface of the housing wall 14. An arm extends radially from the hub 32, parallel to but clear of the housing wall 14. Adjacent to the other face of the hub 32 is a knob 33 with a dome shaped head fitted to the winch shaft 22. Fitted over the knob 33 and extending to the hub 32 is a gaiter 34 made from flexible but durable material such as PVC, designed such that it can be compressed in a "bellows" manner in the axial direction.

The safety cable 1 extends from the drum, passes over a roller 35 and out through an exit hole 36 in the main housing 15 directly below the drum 2. The roller 35 is supported on a shaft 37 parallel to the main shaft 3 and fixed to the housing wall 14 at either end. In normal operation the roller 35 assumes the position shown in Fig. 2. The roller 35 and its shaft 37 can be displaced away from the position shown however, in a direction perpendicular to the shaft 37, if the cable tension is increased. Return springs (not shown) bias the roller 35 to the position shown. If the roller 35 is displaced a microswitch (not shown) is triggered which completes an electrical circuit causing an alarm (not shown) to sound. The alarm fits into a recess 38 in the main housing 15. The roller 35 is displaced in two circumstances. Firstly when a predetermined tension is exceeded on the safety cable 1, i.e. when a sharp fall occurs the safety cable 1 is jerked and the roller 35 is displaced. Secondly when a predetermined amount of safety cable 1 remains in the housing 14, e.g. one metre, an attachment (not shown) on the safety cable 1 passes over the roller 35 and displaces it. It is sufficient for the attachment to be long enough to increase the effective diameter of the cable enough for the roller 35 to be displaced, providing the attachment will not prevent the cable being payed out from the casing in the event of a fall.

The main housing 15 is in two similar halves. Each half covers half the length of the main shaft 3. Each half of the housing 14 describes an external flange 39 at the contact edges adapted for the purpose of fixing. An extended flange piece 40 opposite the exit point 36 of the safety cable 1 defines a hole 41 through both halves of the housing 15. This hole 41 is used to mount or hang the device. The end of the safety cable 1 external to the housing 14 may support for example an eye-loop from which a load can be hung.

By pressing the covered knob 33 the winch shaft 22 can be moved axially into the housing 15 so that the toothed wheel 24 engages with the teeth defined in the drum flange 8. The handle 30 can now be turned to activate the winding operation. To raise the safety cable 1 the handle 30 is turned clockwise forcing the drum 2 to turn anticlockwise. The pawls 20 describe a circle anticlockwise around the main shaft 3, rotating in the direction such that mating with the sawtooth ratchet is not possible. The drum 2 is free to rotate and the safety cable is wrapped around the drum. Thus once the handle 30 is rotated in a clockwise direction it is held in position by the ratchet wheel 31 engaging with the pawl (not shown) fixed in the housing 15.

To lower the safety line 1 the handle 30 is turned clockwise to raise the cable slightly. This disengages the pawls 20 if they are mated with the ratchet sawtooth edge. The handle

30 is then turned anticlockwise slightly and the ratchet wheel

31 bears against the pawl (not shown) preventing - the winch shaft 22 from rotating. This has the effect of unscrewing the splined member 29 slightly and the contact between the friction discs 28 and the toothed wheel 24 is loosened and the toothed wheel 24 is free to rotate about the winch shaft 22. The drum 2 is free to rotate under the weight of the object attached to the end of the safety line 1 against the returning force of the wound spring 5. If the lowering occurs at slow rate the pawls 20 do not engage with the teeth 13 of the sawtooth ratchet on the brake plate 9. As the drum 2 rotates the pawls 20 are biased towards the teeth 13 by the hexagonal edged cam but are then pulled away from the teeth 13 under the action of the returning springs 21. Normally the cable is payed out slowly, the rate being controllable by adjusting the braking force provided by the brake discs 28. This is simply achieved by small rotations of the handle 30. However, if the lowering should occur too quickly, the springs 21 will not have time to act; they are overcome by the inertia of the pawls which carries them into engagement with the teeth 12 of the brake plate 9. The brake plate 9 rotates against the brake pad 16 and is brought to rest by the frictional resistance. The drum 2 is thus brought to a "cushioned" arrest. The braking force applied to the drum 2 is determined by the pressure acting on the brake plate 9 by means of the pressure ring collar 18 and the Belville washer fixings.

Claims

CLAIMS:

1. A safety device comprising a housing, a drum rotatably mounted in the housing, a cable wound on the drum, a brake device, and at least one pawl mounted on the drum and adapted to engage at least one tooth defined by the brake device, engagement between said at least one tooth and pawl causing the brake device to provide frictional resistance to rotation of the drum, wherein said at least one pawl is engaged by a cam and biasing means arranged such that during rotation of the drum relative to the brake device the pawl is moved cyclically between first and second positions, the cam moving the pawl into the first position in which the pawl can engage the said at least one tooth, and the biasing means biasing the pawl towards the second position in which the pawl cannot engage the said at least one tooth, said at least one pawl being mounted on the drum such that rotation of the drum results in centrifugal force biasing the pawl towards the first position.

2. A safety device according to claim 1, wherein the brake device comprises a disc of friction material sandwiched between a surface supported by the casing and a brake disc, the brake disc supporting the said at least one tooth which is engageable by the said at least one pawl.

3. A safety device according to claim 1 or 2, comprising two pawls arranged on opposite sides of a central cam defined by the brake device, the pawls being arranged to rotate with the drum and to be acted upon by the cam so as to push the pawls outwards against a return force provided by coiled springs.

4. A safety device according to any preceding claim, comprising a winch device for controlling rotation of the drum to wind cable onto or off the drum, wherein the winch device comprises a rotatable shaft, a gearwheel freely rotatable on the shaft and arranged to engage the drum such that rotation of the gearwheel causes rotation of the drum, an actuator mounted on the shaft such that the gearwheel is positioned between the actuator and a flange on the shaft, and a further brake device mounted on the shaft between the actuator and the flange to engage the gearwheel, the actuator being movable relative to the shaft in one direction to increase the pressure on the further brake device, and movable in the other direction to reduce the pressure on the further brake device, whereby slippage of the gearwheel relative to the shaft can be controlled by movement of the actuator.

5. A safety device according to claim 4, wherein the shaft supports a toothed wheel engageable by a pawl such that the shaft can only be rotated in one direction.

6. A safety device according to claim 5, wherein the actuator comprises a member which is screwed onto a threaded portion of the winch shaft, the thread sense being such that rotation of the actuator in a direction permitted by the toothed wheel and pawl screws the actuator onto the shaft, thereby increasing the pressure on the further brake device and increasing the torque which can be applied to the drum by the gearwheel.

7. A safety device according to any preceding claim, wherein a member is secured to the cable a predetermined distance along the cable from its point of attachment to the drum, and means are provided to signal the passage of the member out of the housing.

8. A safety circuit according to claim 7, wherein the signalling means comprise a switch positioned so as to be sensitive to variations in the diameter of the cable and the member is a body secured to the cable or a sheath extending from the end of the cable secured to the drum.

9. A safety device comprising a housing, a drum rotatably mounted in the housing, a cable wound on the drum, and a winch device for controlling rotation of the drum to wind cable onto or off the drum, wherein the winch device comprises a rotatable shaft, a gearwheel freely rotatable on the shaft and arranged to engage the drum such that rotation of the gearwheel causes rotation of the drum, an actuator mounted on the shaft such that the gearwheel is positioned between the actuator and a flange on the shaft, and a brake device mounted on the shaft between the actuator and the flange to engage the gearwheel, the actuator being movable relative to the shaft in one -, direction to increase the pressure on the brake device, and movable in the other direction to reduce the pressure on the f brake device, whereby slippage of the gearwheel relative to the shaft can be controlled by movement of the actuator.

10. A safety device comprising a housing, a drum rotatably mounted in the housing, a cable attached at one end to the drum, and a brake device for braking rotation of the drum if the rate of rotation of the drum exceeds a predetermined rate, wherein a member is secured to the cable a predetermined distance along the cable from its point of attachment to the drum, and means are provided to signal the passage of the member out of the housing.