GB2229420A

GB2229420A - Power operated device

Info

Publication number: GB2229420A
Application number: GB9005004A
Authority: GB
Inventors: Alan Archer
Original assignee: Qualter Hall and Co Ltd
Current assignee: Qualter Hall and Co Ltd
Priority date: 1989-03-08
Filing date: 1990-03-06
Publication date: 1990-09-26
Also published as: GB9005004D0; GB8905202D0

Abstract

A power operated device for moving a rail mounted vehicle, and which is capable of being mounted on one of the rails and of acting against the vehicle, includes a hydraulic piston and cylinder assembly (10) and clamping means (16 and 18) for clamping the cylinder and piston rod with respect to the rail, alternately, and enabling the device to inch itself along the rail to act against one of the vehicle wheels. The device may be used for pushing a mine car out of a mine-shaft cage. Wedges may be used for the clamping means, or spring-loaded sprags, pivotting about vertical axes, and the front end of the device may carry a freely rotatable pusher wheel (26). <IMAGE>

Description

Power operated device.

The invention relates to a power operated device by means of which a mine car can be very easily unloaded from within a mine-shaft cage.

It is a particular problem that mine cars cannot always easily be unloaded from within a mine-shaft cage. The problem occurs at the end of a working shift, for example, when men instead of mine cars are to be carried in the cage and a mine car within the cage cannot therefore be pushed out of the cage by a mine car entering the cage. The mine cars are often too heavy to be safely pushed by man power and there is a tendency towards the use of bigger and therefore much heavier mine cars so that the problem is growing more acute. Heavily loaded mine cars are sometimes unloaded with the aid of a winch, but this is still a dangerous and arduous task.

The invention has for its object to alleviate the problem referred to.

According to one aspect of the invention, there is provided a power operated device for moving a rail mounted vehicle, the device comprising a fluid pressure piston and cylinder assembly for mounting on one of the rails and extending along it, valve means for causing said piston and cylinder assembly to be alternately extended and retracted, and means for clamping the cylinder and a piston rod of the assembly alternatively with respect to the rail in dependence upon said extension and contraction, so that, in operation, the device is caused to inch itself along said one of the rails and to act against a part of the vehicle.

According to another aspect of the invention, there is provided a power operated device for moving a rail mounted vehicle, the device comprising a fluid pressure piston and cylinder assembly for mounting on one of the rails and extending along it; valve means for causing said piston and cylinder device to be alternately extended and retracted; and means for clamping the cylinder and a piston rod of the assembly alternatively with respect to the rail, said means including respective wedge means each capable of operating in one direction only whereby, as the piston and cylinder assembly is alternately extended and retracted first one of said wedge means and then the other is operative, alternately, to cause the device to inch itself along said one of the rails and to act against a part of the vehicle.

The means provided for clamping the cylinder and the piston rod end of the assembly with respect to the rail, alternately, may be constituted by respective wedge members located in respective tapering cavities defined by depending side walls of the device and a surface of a rail on which the device is located, the clamping means constituted by the respective wedge members being capable of operating in one direction only, that is to say in each case being self-tightening when a load is imposed on it tending to force it away from the vehicle, the wedge member concerned being automatically slackened when the one clamping means are advanced during the extension of the piston and cylinder assembly and when the other clamping means are advanced during the subsequent retraction of the assembly.The wedge means may each have a facing of friction material for contacting the surface of the rail, and may have respective release levers which extend through slots in the side walls of the respective cavities in which the wedge members are located. Respective springs may be provided to urge the wedge members into engagement with the surface of the rail with which they co-operate.

Alternatively, the means provided for clamping the cylinder and the piston rod end of the assembly with respect to the rail, alternately, may be constituted by means for engaging side surfaces of an upstanding flange extending in parallel relation to said rail. In this case, the device may be provided with respective housing assemblies at the front and rear ends of the piston and cylinder assembly, one being carried by the cylinder and the other by the piston rod, each housing assembly accommodating a clamping means constituted by a sliding sprag element capable of pivoting about a vertical axis and acted upon by spring means urging a free end thereof into engagement with a side surface of the upstanding flange extending in parallel relation to the rail, respective manually operable levers being provided to pivot the sliding sprag elements away from said upstanding flange.A bridge piece of each housing assembly may extend over the upstanding flange to provide a pressure plate at the side of the upstanding flange remote from the associated sliding sprag element. Each sliding sprag element may be provided with a stepped abutment portion adapted to engage an appropriate one of a series of equally spaced apertures or depressions in the side surface of the upstanding flange extending in parallel relation to the rail. Resilient means may be provided for urging each sliding sprag element into engagement with the side surface of the upstanding flange. Each sliding sprag element may be reversible or provided with a pair of stepped abutment portions capable of being brought into operation selectively, whereby the device can be caused to advance forwardly or in reverse.

A directional control valve may be mounted on the cylinder of the piston and cylinder assembly, the valve having control spindles projecting from its opposite ends, hydraulic hoses being connected to the control valve for the supply and exhaust of fluid under pressure to and from the piston and cylinder assembly, and respective means being provided for applying end pressure to the two control spindles of the directional control valve, alternately, on the completion of extension and retraction movements of the piston and cylinder assembly, during an automatic mode of operation of the device, whereby the assembly is automatically extended and retracted.The respective means for applying end pressure to the two control spindles of the directional control valve may be constituted by an abutment member carried by a slide member which is slidable on the exterior surface of the cylinder, said slide member being connected to the piston rod end for movement with said piston rod, and by an abutment arrangement connected to a carrier element located on the piston'rod end, the abutment arrangements preferably being capable of being displaced to an inoperative position so that the device is brought to a halt when the piston and cylinder assembly reaches its fully retracted condition.

The power operated device described above may be provided in combination with a control valve hydraulically connected to the device by way of flexible hoses, said control valve being capable of being hand held so that the device can be operated in a non-automatic mode.

A leading end of the device may be provided with a freely rotatable pusher wheel for acting against a wheel of a vehicle, and at least the leading end of the device will preferably be provided with a roller for acting downwardly on the rail.

In order that the invention may be fully understood and readily carried into effect, the same will now be described, by way of example only, with reference to the accompanying drawings, of which: - Figure 1 is a side view of a device embodying the invention and being used for moving a mine car mounted on rails, Figure 2 is a plan view, Figure 3 is a sectional view on the line 3-3 in Figure 1, Figure 4 is a sectional view on the line 4-4 in Figure 1, and Figure 5 is an end view looking in the direction of arrow 5 in Figure 1, Figure 6 is a view similar to Figure 1 and illustrating a somewhat modified device, Figure 7 is a plan view thereof, Figure 8 is a sectional view on the line 8-8 in Figure 6, Figure 9 is a sectional view on the line 9-9 in Figure 6, Figure 10 is a sectional view on the line 10-10 in Figure 6, Figure 11 is an end view, looking in the direction of arrow 11 in Figure 6, and Figures 12 and 13 are views which will be referred to presently when describing further possible modifications.

Referring now to Figures 1 to 5 of the drawings, the power operated device there illustrated is constituted by a double acting hydraulic piston and cylinder assembly, generally indicated 10, for mounting above one of a pair of rails on which a mine car is mounted.

At the leading end of the device, that is to say the end of the cylinder remote from the piston rod end, there is a housing assembly, generally indicated 20, between upstanding side walls 22 and 24 of which a freely rotatable pusher wheel 26 is located by a spindle 28. Located in contact with the pusher wheel is a support roller 30, said support roller being freely rotatable on a further spindle 32. The support roller 30 is adapted to roll on the top surface of the rail above which the piston and cylinder assembly is mounted, and is located in such a position in relation to the pusher wheel 26 that the latter is spaced from contact with the rail, as shown. At the other end of the device, that is to say on the piston rod end, there is a further housing assembly, generally indicated 34, within which is located a further support roller 36 freely rotatable on a spindle 38.

The arrangement is such that when it is desired to displace a mine car along the rails on which it is mounted, the device can be mounted, as shown, above one of the rails with the pusher wheel 26 in contact with one of the wheels of the mine car. As the leading end of the device is caused to advance, as will presently be described, the pusher wheel acts against the mine car wheel and can rotate freely as the mine car wheel rotates.

Simultaneously, the pusher wheel reacts against the support roller 30 as the latter rolls along the top surface of the rail.

Means are provided for clamping the cylinder 12 and the remote end of the piston rod 14 of the assembly 10 to the rail, alternately, so that as the assembly is alternately extended and retracted the device will be caused to progress along the rail, the mine car being pushed ahead of it as the piston and cylinder assembly is extended. The clamping means are indicated 16 and 18, respectively.

Referring in particular to Figures 1 and 3, the clamping means 16 are shown to be located beneath the housing assembly 20 and are constituted by a wedge member 40. The wedge member is located in a tapering cavity defined by a depending side wall 42 and a side surface of the rail on which the device is mounted, the wedge member being provided with a facing of friction material for contacting the side surface of the rail. A release lever 44 which extends laterally from the wedge member extends through a slot 46 in the side wall 42 so that the wedge member can be moved manually out of engagement with the side wall of the rail when required.In a length of metal tube 48 welded to the depending side wall 42 there is housed a compression spring 50 which acts against the release lever 44 to urge the wedge member into engagement with the side wall of the rail, the length of tube also being slotted as shown to allow the manual release of the wedge member when required. In Figures 1 and 5 it can be seen that the clamping means -1.8 are located beneath the housing assembly 34 secured to the piston rod end; these are of similar construction to the clamping means 16 and the same reference numerals denote like parts. In each case it will be seen that on the side of the rail opposite the wedge member the side wall 22 of the housing is extended downwardly to abut the side surface of the rail with a force equal and opposite to the clamping force imposed by the wedge member.It will be understood that the clamping means are capable of operating in one direction only, that is to say in each case the wedge member is self-tightening when a force is imposed on the clamping means of which it forms a part in a direction tending to force the clamping means away from the mine car wheel. The wedge member concerned is automatically slackened when the clamping means 16 are advanced during the extension of the piston and cylinder assembly and when the clamping means 18 are advanced during the subsequent retraction of the assembly.

A cam operated directional control valve, generally indicated 52, is mounted as shown on the cylinder of the piston and cylinder assembly and has control spindles 54 and 56 projecting from its opposite ends. As shown in Figure 2, hydraulic hoses 51 and 53 are connected to the control valve for the supply and exhaust of oil under pressure to and from the piston and cylinder assembly. The hydraulic connections are such that end pressure on the control spindle 56 causes the assembly to contract and end pressure on the control spindle 54 causes the assembly to be extended. Means for applying end pressure to the two control spindles, alternately, during an automatic mode of operation of the device are constituted by an upstanding abutment member 58 carried by a slide member 60 and by an abutment arrangement 62 which extends outwards as shown from a carrier element 64.The carrier element is provided with a handle 66 the purpose of which will be described presently.

The slide member 60 which carries the upstanding abutment member 58 is slidable on the cylindrical exterior surface of the cylinder of the assembly 10 and, as shown in Figure 4, is formed by upper and lower semi-cylindrical parts with flanges which are bolted together. The upper one of the semicylindrical parts is provided with an upstanding apertured part 68 which carries the upstanding abutment member 58. The part 68 is apertured so that it can straddle a conduit 70 extending from the directional control valve to the remote end of the cylinder. A pair of rods 72,72 which are welded at their one ends to the upper one of the semi-cylindrical parts of the slide member extend the full length of the assembly and are connected at their other ends to a collar 74 carried at the extreme end of the piston rod. During the operation of the device the pair of rods alternately push and pull the slide member along the cylinder between the positions shown in full and chain-dotted lines in Figure 1; the upstanding abutment member 58 thus reverses the control valve each time the piston and cylinder assembly reaches its maximum extension.

In the fully automatic mode of operation, during which the device operates continuously, the abutment arrangement 62 which extends outwards from the carrier element 64, reverses the control valve as the piston and cylinder assembly nears its fully retracted condition. However, if the handle 66 is moved rearwardly as shown in chain-dotted lines in Figure 1, the abutment arrangement 62 is displaced so that it cannot contact the control spindle 56 and the device is thereby brought to a halt when the piston and cylinder assembly reaches its fully retracted condition. The device can be hydraulically connected, by way of flexible hoses, to a manually operable and hand held control valve (not shown) so that the device can be operated in a non-automatic mode when required.

Thus there is provided a device by means of which a mine car can be moved safely and with a minimum of manual effort. When the device has been mounted on the rail in close proximity to the mine car, this having entailed the movement of the wedge members of the clamping means by manual operation so that the wedge members have been able to be brought into position alongside the rail, the subsequent operation of the device is either fully automatic or in response to the users operation of the manually operable control valve as previously described.

However, various modifications may be made. For example, in Figures 6 to 11 there is illustrated a device basically similar to that described above except that the clamping means 16 and 18 have been differently constructed and the device is not provided with means whereby it can be operated in a fully automatic mode.

Referring in particular to Figure 6, it will be seen that in common with the device previously described, the device in this case is constituted by a double acting hydraulic piston and cylinder assembly 10 and has at its leading end a pusher wheel 26 adapted to bear against a mine car wheel, as shown. It also has a support roller 30 which rides along the top surface of the rail and holds the pusher wheel out of contact with the rail, but said support roller is in this case located in advance of the pusher wheel. The device is provided with a further support roller 36 at its other end.

The clamping means 16 and 18 in this case do not act against the square section rail on which the mine car wheel is engaged but against the upstanding flange 100 of an associated safety rail, the upstanding flange extending in parallel relation to said square section rail. As shown in Figures 9 and 11, the device is provided with respective housing assemblies 120 and 134 at the front and rear ends of the piston and cylinder assembly, each housing assembly accommodating a clamping means constituted by a sliding sprag element 76 carried by a sleeve member 78 which is free to pivot about the axis of an upstanding spindle 80. The sliding sprag element 76 has a rounded end portion as shown in Figure 7, and a tension spring 82 acts against the element to urge its rounded end into engagement with the adjacent side surface of the upstanding flange 100 of the safety rail.A bridge piece 84 of each housing assembly extends over the upstanding flange to provide a pressure plate 86 faced with friction material at the remote side of the upstanding flange.

The sleeve member of each clamping means is provided with a manually operable lever 88 and, as shown in Figure 7, can be moved about the axis of the associated upstanding spindle 80 to pivot the respective sliding sprag element away from the upstanding flange 100. The levers 88 extend through slots 90 in the side walls of the respective housing assemblies. The sliding sprag elements extend through similar slots in the opposite sides of the housing assemblies.

The device is in this case hydraulically connected by way of flexible hoses 51,53 to a hand held control valve (not shown) itself connected to a convenient source of supply of hydraulic pressure. The hoses are capable of being connected to or disconnected from the device by means of quick release couplings 55. The conduit 70 which in this case extends to the remote end of the cylinder of the assembly 10 provides a convenient handle by means of which the device can be carried from place to place.

Means are provided for holding the housing assemblies 120 and 134 against relative rotation when the device is being moved from place to place, these means being constituted by a simple tube and sliding shaft arrangement generally indicated 92, the tube and the shaft which is slidable along the tube as the piston and cylinder assembly is either extended or retracted being carried by the cylinder of the assembly and by the housing assembly 134, respectively.

The arrangement is such that as the piston and cylinder assembly is alternately extended and retracted, the clamping means referred to operate alternately so that the device is able to inch its way along the rail in much the same way as the device previously described. This is brought about in each case by the sliding sprag element concerned being alternately forced into wedging engagement with the upstanding flange of the safety rail to automatically apply the clamping means and subsequently being automatically released and sliding along the side surface of the upstanding flange as the clamping means concerned is being advanced.

In Figure 12 there is illustrated a possible modification of the device just described, the modification residing in the fact that the sliding sprag element 76 of each housing assembly 120 and 134 has been provided with a stepped abutment portion 94 adapted to engage an appropriate one of a series of equally spaced apertures 101 in the upstanding flange 100 extending in parallel relation to the rail. Resilient means are provided for urging the sliding sprag element into engagement with the side surface of the upstanding flange. As shown, in the illustrated example these means take the form of elastomeric elements 96 which are trapped in the spaces between a square section hollow tube 98 which carries the sliding sprag element and a somewhat larger square section tube 102 to which the manually operable lever 88 is connected.As shown, a peg element 104 can be placed in a selected one of a series of hole 106 so that the sliding sprag element can be pre-loaded against the side surface of the upstanding flange to an appropriate extent.

The modification just described will clamp the cylinder and the piston rod of the assembly alternately with respect to the rail whatever the condition of the rail, that is to say whether it is clean and dry or wet or possibly made slippery in parts with oil or dirt. The equally spaced apertures in the upstanding flange 100 will be spaced apart appropriately at spacings somewhat less than the maximum extension of the piston and cylinder assembly. Modified in this way, the device will be capable of pushing a heavily loaded rail mounted vehicle up an inclined track.

In Figure 13 there is illustrated a possible modification of the arrangement just described, this modification residing in the fact that the sliding sprag element 76 of each housing assembly 120 and 134 has been provided with two stepped abutment portions 94 and 95 which can be brought into operation selectively by the simple expedient of re-positioning the peg element 104 in a selected one of a further series of holes 107 to hold the lever 88 in a reversed position. It will be understood that the reversal of the two sliding sprag elements in this way will have the effect of reversing the movement of the device along the rail on which it is mounted.

Various other modifications may be made. For example it would be quite possible to employ a single acting spring return piston and cylinder assembly instead of the double acting assembly referred to in each of the illustrated embodiments.

The device illustrated in Figures 6 to 11 could be provided with a control valve 52 and operating mechanism such as that forming part of the first described embodiment, and could then be capable of being used in an automatic mode as previously described.

Claims

CLAIMS:

1. A power operated device for moving a rail mounted vehicle, the device comprising a fluid pressure piston and cylinder assembly for mounting on one of the rails and extending along it, valve means for causing said piston and cylinder assembly to be alternately extended and retracted, and means for clamping the cylinder and a piston rod of the assembly alternatively with respect to the rail in dependence upon said extension and contraction, so that, in operation, the device is caused to inch itself along said one of the rails and to act against a part of the vehicle.

2. A power operated device for moving a rail mounted vehicle, the device comprising a fluid pressure piston and cylinder assembly for mounting on one of the rails and extending along it; valve means for causing said piston and cylinder device to be alternately extended and retracted; and means for clamping the cylinder and a piston rod of the assembly alternatively with respect to the rail, said means including respective wedge means each capable of operating in one direction only whereby, as the piston and cylinder assembly is alternately extended and retracted first one of said wedge means and then the other is operative, alternately, to cause the device to inch itself along said one of the rails and to act against a part of the vehicle.

3. A power operated device according to either one of the preceding claims, in which the means provided for clamping the cylinder and the piston rod end of the assembly with respect to the rail, alternately, are constituted by respective wedge members located in respective tapering cavities defined by depending side walls of the device and a surface of a rail on which the device is located, the clamping means constituted by the respective wedge members being capable of operating in one direction only, that is to say in each case being self-tightening when a load is imposed on it tending to force it away from the vehicle, the wedge member concerned being automatically slackened when the one clamping means are advanced during the extension of the piston and cylinder assembly and when the other clamping means are advanced during the subsequent retraction of the assembly.

4. A power operated device according to claim 3, in which the wedge members each have a facing of friction material for contacting the surface of the rail.

5. A power operated device according to either one of claims 3 and 4, in which the wedge members have respective release levers which extend through slots in the side walls of the respective cavities in which the wedge members are located..

6. A power operated device according to claim 5, in which respective springs are provided to urge the wedge members into engagement with the surface of the rail with which they co-operate.

7. A power operated device according to either one of claims 1 and 2, in which the means provided for clamping the cylinder and the piston rod end of the assembly with respect to the rail, alternately, are constituted by means for engaging side surfaces of an upstanding flange extending in parallel relation to said rail.

8. A power operated device according to claim 7, provided with respective housing assemblies at the front and rear ends of the piston and cylinder assembly, one being carried by the cylinder and the other by the piston rod, each housing assembly accommodating a clamping means constituted by a sliding sprag element capable of pivoting about a vertical axis and acted upon by spring means urging a free end thereof into engagement with a side surface of the upstanding flange extending in parallel relation to the rail, respective manually operable levers being provided to pivot the sliding sprag elements away from said upstanding flange.

9. A power operated device according to claim 8, in which a bridge piece of each housing assembly extends over the upstanding flange to provide a pressure plate at the side of the upstanding flange remote from the associated sliding sprag element.

10. A power operated device according to either one of claims 8 and 9, in which each sliding sprag element is provided with a stepped abutment portion adapted to engage an appropriate one of a series of equally spaced apertures or depressions in the side surface of the upstanding flange extending in parallel relation to the rail.

11. A power operated device according to claim 10, in which resilient means are provided for urging each sliding sprag element into engagement with the side surface of the upstanding flange.

12. A power operated device according to either one of claims 10 and 11, in which each sliding sprag element is reversible or provided with a pair of stepped abutment portions capable of being brought into operation selectively whereby the device can be caused to advance forwardly or in reverse.

13. A power operated device according to any one of claims 1 to 12, in which a directional control valve is mounted on the cylinder of the piston and cylinder assembly and has control spindles projecting from its opposite ends, hydraulic hoses being connected to the control valve for the supply and exhaust of fluid under pressure to and from the piston and cylinder assembly, and respective means for applying end pressure to the two control spindles of the directional control valve, alternately, on the completion of extension and retraction movements of the piston and cylinder assembly, during an automatic mode of operation of the device, whereby the assembly is automatically extended and retracted.

14. A power operated device according to claim 13, in which the respective means for applying end pressure to the two control spindles of the directional control valve are constituted by an abutment member carried by a slide member which is slidable on the exterior surface of the cylinder, said slide member being connected to the piston rod end for movement with said piston rod, and by an abutment arrangement connected to a carrier element located on the piston rod end.

15. A power operated device according to claim 14, in which the abutment arrangement is capable of being displaced to an inoperative position so that the device is brought to a halt when the piston and cylinder assembly reaches its fully retracted condition.

16. A power operated device for moving a rail mounted vehicle, the device being constructed, arranged and adapted to be used substantially as hereinbefore described with reference to and as illustrated by Figures 1 to 5 or Figures 6 to 11 of the accompanying drawings.

16. A power operated device according to any one of the preceding claims, in combination with a control valve hydraulically connected to the device by way of flexible hoses, said control valve being capable of being hand held so that the device can be operated in a non-automatic mode.

17. A power operated device according to any one of the preceding claims, in which a leading end of the device is provided with a freely rotatable pusher wheel for acting against the vehicle wheel.

18. A power operated device according to claim 17, in which at least the leading end of the device is provided with a roller for acting downwardly on the rail.

19. A power operated device for moving a rail mounted vehicle, the device being constructed, arranged and adapted to be used substantially as hereinbefore described with reference to and as illustrated by Figures 1 to 5 or Figures 6 to 11 of the accompanying drawings.

Amendments to the claims have been filed as follows 1. A power operated device for moving a rail mounted vehicle, the device comprising a fluid pressure piston and cylinder assembly for mounting on one of the rails and extending along it, valve means for causing said piston and cylinder assembly to be alternately extended and retracted, and means for clamping the cylinder and a piston rod of the assembly alternately with respect to the rail in dependence upon said extension and contraction, so that, in operation, the device is caused to inch itself along said one of the rails and to act against a part of the vehicle, the means provided for clamping the cylinder and the piston rod of the assembly with respect to the rail, alternately, being constituted by respective wedge members located in respective tapering cavities defined by depending side walls of the device and a surface of a rail on which the device is located, the clamping means constituted by said respective wedge members being capable of operating in one direction only, that is to say in each case being self-tightening when a load is imposed on it tending to force it away from the vehicle, the wedge member concerned being automatically slackened when the one clamping means are advanced during the extension of the piston and cylinder assembly and when the other clamping means are advanced during the subsequent retraction of the assembly, or being constituted by means for engaging side surfaces of an upstanding flange extending in parallel relation to said rail.

2. A power operated device according to claim 1, in which the means provided for clamping the cylinder and the piston rod of the assembly with respect to the rail, alternately, are constituted by respective wedge members, the wedge members each have a facing of friction material for contacting the surface of the rail.

3. A power operated device according to either one of the preceding claims, in which the means provided for clamping the cylinder and the piston rod end of the assembly with respect to the rail, alternately, are constituted by respective wedge members, the wedge members having respective release levers which extend through slots in the side walls of the respective cavities in which the wedge members are located.

4. A power operated device according to any one of the preceding claims, in which the means provided for clamping the cylinder and the piston rod of the assembly with respect to the rail, alternately, are constituted by respective wedge members, respective springs being provided to urge the wedge members into engagement with the surface of the rail with which they co-operate.

5. A power operated device according to claim 1, in which the means provided for clamping the cylinder and the piston rod of the assembly, with respect to the rail, alternately, are constituted by means for engaging side surfaces of an upstanding flange extending in parallel relation to said rail, provided with respective housing assemblies at the front and rear ends of the piston and cylinder assembly, one being carried by the cylinder and the other by the piston rod, each housing assembly accommodating a clamping means constituted by a sliding sprag element capable of pivoting about a vertical axis and acted upon by spring means urging a free end thereof into engagement with a side surface of the upstanding flange extending in parallel relation to the rail, respective manually operable levers being provided to pivot the sliding sprag elements away from said upstanding flange.

6. A power operated device according to claim 5, in which a bridge piece of each housing assembly extends over the upstanding flange to provide a pressure plate at the side of the upstanding flange remote from the associated sliding sprag element.

7. A power operated device according to either one of claims 5 and 6, in which each sliding sprag element is provided with a stepped abutment portion adapted to engage an appropriate one of a series of equally spaced apertures or depressions in the side surface of the upstanding flange extending in parallel relation to the rail.

8. A power operated device according to claim 7, in which resilient means are provided for urging each sliding sprag element into engagement with the side surface of the upstanding flange.

9. A power operated device according to either one of claims 7 and 8, in which each sliding sprag element is reversible or provided with a pair of stepped abutment portions capable of being brought into operation selectively whereby the device can be caused to advance forwardly or in reverse.

10. A power operated device according to any one of the preceding claims, in which a directional control valve is mounted on the cylinder of the piston and cylinder assembly and has control spindles projecting from its opposite ends, hydraulic hoses being connected to the control valve for the supply and exhaust of fluid under pressure to and from the piston and cylinder assembly, and respective means for applying end pressure to the two control spindles of the directional control valve, alternately, on the completion of extension and retraction movements of the piston and cylinder assembly, during an automatic mode of operation of the device, whereby the assembly is automatically extended and retracted.

11. A power operated device according to claim 10, in which the respective means for applying end pressure to the two control spindles of the directional control valve are constituted by an abutment member carried by a slide member which is slidable on the exterior surface of the cylinder, said slide member being connected to the piston rod end for movement with said piston rod, and by an abutment arrangement connected to a carrier element located on the piston rod end.

12. A power operated device according to claim 11, in which the abutment arrangement is capable of being displaced to an inoperative position so that the device is brought to a halt when the piston and cylinder assembly reaches its fully retracted condition.

13. A power operated device according to any one of the preceding claims, in combination with a control valve hydraulically connected to the device by way of flexible hoses, said control valve being capable of being hand held so that the device can be operated in a non-automatic mode.

14. A power operated device according to any one of the preceding claims, in which a leading end of the device is provided with a freely rotatable pusher wheel for acting against the vehicle wheel.

15. A power operated device according to claim 14, in which at least the leading end of the device is provided with a roller for acting downwardly on the rail.