Abstract:
A rail wheel gripper for holding or retarding a rail vehicle has opposed gripper mechanisms that engage a wheel between them. Each mechanism comprises a carrier arm for a gripper member and upper and lower pivot arms connected to the carrier arm controlling the path of movement of the gripper member. Each mechanism is so arranged, and a drive device co-operates with it, as to exhibit a mechanical advantage with increased gripping force when the opposed gripper members reach their operative positions, and a mechanical disadvantage with magnified displacement of the members when they are remote from their operative position.

Description:
BACKGROUND OF TE INVENTION 
     This invention relates to rail wheel grippers, which term is intended herein to include both holding apparatus for moving vehicles, of the kind in which one or more wheels of a vehicle running on a rail track is contacted on its opposite side faces by opposed surfaces of clamping members that are mounted separately from the vehicle and are urged inwardly towards each other by operating means so as to grip the wheel between the. Such rail wheel grippers mounted on the track may be adapted to hold a vehicle stationary in a desired location or to act as a squeeze retarder in the marshalling of rail vehicles. 
     It is known to construct such wheel grippers in the form of calipers, comprising two opposed arms on opposite sides of the rail and pivoted at their lower ends to swing upwardly and inwardly. Such a mechanism, however requires considerable clearance both for its instalation and for its operation, In particular, in order to grip a wheel some way above its bottom contact point with the rail, the caliper arms must be relatively long and occupy a correspondingly wide span when pivoted downwards to their rest positions, while care must be taken to ensure that there is sufficient clearance between the wide arc they sweep as they are raised and the running gear of the vehicles on which they are intended to operate. 
     SUMMARY OF THE INVENTION 
     According to the invention, there is provided a rail wheel gripper of the kind described wherein at least one of said clamping members is mounted on an upwardly extending carrier arm of a mechanism that is arranged to provide a movement path for said member upwardly and inwardly from a lower inoperative position to a raised operative position, said mechanism including mounting means for the carrier arm comprising a first or lower pivot arm having spaced pivot connections with a support and with a lower region of the carrier arm, drive means being provided for relative displacement of the carrier and pivot arms so as to increase the included angle between them as the carrier arm is raised to move said clamping member towards its operative position. 
     It is possible to arrange such a mechanism so that the direction of movement of the clamping member is mainly inwardly as it approaches the operative position, while as it returns to the inoperative position the final part of its direction of movement is mainly downwardly. 
     The gripper may also include an upper pivot arm providing a pivot support for the upper region of the carrier arm and preferably the pivot centres of the first or lower pivot arm are arranged to be substantially in line with the pivot centre of the connection between said upper pivot arm and the carrier arm when the clamping member is in its operative position. 
     Preferably, drive means for the mechanism are arranged to act on the carrier arm through said lower pivot arm. /Thus, the mechanism may be arranged to be operated by a fluid pressure cylinder connected to a crank arm that has a common pivot axis with that end of the lower pivot arm spaced from the carrier arm and that is rotationally fixed relative to said pivot arm. The fluid pressure cylinder is preferably arranged to be disposed substantially perpendicular to the line of said crank arm from its pivot axis when the clamping member is in its operative positon, and the pivot connections of the carrier arm may be so spaced as to give the mechanism a mechanical advantage increasing the force applied through the clamping member by the pressure cylinder. The arrangement is preferably such that the mechanical advantage is quickly lost as the clamping member is moved back from its operative position, whereby the lowering movement is obtained with a relatively small displacement of the pressure cylinder. 
     Advantageously, the linkage is arranged to be disposed entirely below the level of the rail and is adapted to bring the clamping member itself below the level of the rail when moving it to the lower, inoperative position. 
    
    
     The invention will be more particularly described by way of example with reference to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic illustration of a rail wheel gripper according to the invention having a structure that is symmetrical about the rail centre line but that is illustrated in the FIGURE in a retracted inoperative position to the left of the centre lie and in a raised operative position to the right of the centre line. 
     FIGS. 2 and 3 are transverse sectional views of another rail wheel gripper according to the invention, as seen on section A--A and B--B respectively in FIG. 4, the mechanism beig in its operative position in FIG. 2. and its inoperative position in FIG. 3, 
     FIG. 4 is a longitudinal sectional view as seen on section lie C--C in FIG. 2 with the mechanism in the operative position, 
     FIG. 5 is a partial sectional view on the section line D--D in FIG. 2, and of the arrow H in FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1 of the drawings, the apparatus comprises a pair of horizontal gripper bars 2 that extend at right angles to the plane of the drawing and that are displaceable towards each other to grip between them the opposite sides of a wheel W of a vehicle on a rail R. The bars are so mounted on a foundation frame 4 resting below the rail that they are movable, by means of a mechanism carried by the foundation frame, between an upper operative position in which they grip the wheel between them, and an inoperative possition in which they lie below the level of the top of the rail. 
     Each gripper bar has a similar operating mechanism and the bars are displaced in synchronism but of course the two mechanism are disposed in mirror image to each other to move the bars towards and away from each other. Each gripper bar is secured at its ends to a pair of parallel carrier arms 6 that are each connected to upper and lower pivot shafts 8, 10 extending parallel to the gripper bar by means of upper and lower pivot arms 12, 14. The pivot arms are fixed to rotate with their respective shafts but have freely rotatable pivot connections 16, 18 with their carrier arms 6. Each mechanism has respective pairs of the upper and lower pivot arms 12, 14 adjacent opposite ends of the gripper bar. Each pair of upper arms is secured to their associated shaft in parallel disposition, as also is each pair of lower arms, so that the carrier arms of the mechanismare in their turn held parallel to each other. 
     Each mechanism is driven through rotation of its lower shaft 10 by a hydraulic ram 20 the body of which is pivoted at 22 to the frame 4 and the piston rod 24 of which is connected by a pivot pin 26 to a crank arm 28 fixed to the shaft 10. A common control is provided for the rams to move the mechanism in synchronism. 
     On the left-hand side of FIG. 1 the mechanism is shown with the gripper bar at its lowermost, inoperative position in which it is retracted so as to project little if at all above the level of the rail R, the ram 20 then being fully extended. 
     The lower pivot arms 14 are substantialy at right angles to the carrier arms 6 in their lowered position so that retraction of each ram 20 to rotate its shaft 10 givethe carrier arms a rising motion and because of the different angles at which the pivot arms 12, 14 lie, there is also a slight rotational component of movement bringing the gripper bar 2 inwardly towards the rail. As the rotation of the shaft continues and the lower pivot arms rotating with the shaft approach the vertical, their pivot connections 18 with the carrier arms move outwards away from the rail R and the carrier arms swing about the pivot connections 16, which are now close to their laterally innermost position, so that the movement of the gripper bar at this stage is almost entirely laterally inwards to bring it against the vehicle wheel some distance above the rail. 
     It is to be noted that in this latter stage of operation because of the distance between the pivot connections 16, 18 is greater than that between the upper connection 16 and the point of contact of its gripper bar with the vehicle wheel, and because the carrier arm is pivoting about the upper connection 16, the ram 20 is working at a considerable mechanical advantage i.e. the gripping force is substantially greater than the ram working force. Moreover, as is clear from the illustration in the right-hand part of Fig. 1, the gripping force applied to the wheel will not be diminished by any slight change of position of the contacted surfaces of the wheel laterally of the rail, nor will the height of the point of contact vary significantly so that the arrangement is not sensitive to minor dimensional differences or misalignments which might often be present in the rail and the vehicle wheels. 
     It is also to be noted that when the ram is extended to release the gripper bars, after the initial disengagement from the gripped wheel when the ram and its crank arm 28 are substantially at right-angles to each other, the descent of the gripper bars is increasingly rapid in relation to the ram piston movement until, as the final retracted position shown in the left-hand half of FIG. 1 is approached, a given increment of extension of the ram produces a substantially greater downwards movement of the gripper bar. This is because the relative movement between the ram and its crank arm 28 towards alignment with each other at this stage, giving a reducing acute angle between them, produces an increasingly magnified angular movement of the lower drive shaft 10 for a given increment of the ram piston movement. In effect, the change of mechanical advantage between operative and inoperative positions in the illustrated example means that a movement ratio of 3:1 between cylinder movement and horizontal movement of the gripper bar in the operative position (and the corresponding amplification of the force applied) becomes a ratio of 1:2 in the inoperative position, i.e. a six-fold change. 
     Because of the magnification of the ram movement obtained as the gripper bar descends, it is possible, without relying on large and cumbersome drive means, to obtain a relatively large displacement of the gripper bars between their operative and inoperative positions. This give the opportunity for the operative position to be located at a relatively high lvel on the vehicle wheel while still allowing the gripper bars to retract to near or even below thre rail level so as to avoid fouling low-slung parts of a passing vehicle, such as a locomotive cow-catcher. The illustrated mechanism projects up to 150 mm above rail level when raised and can be retracted below rail level if required. Despite this large vertical movement the outward throw of the mechanism is relatively limited and its lateral space requirement s are correspondingly small. 
     In the embodiment shown in FIGS. 2 to 6, parts already described are given the same reference numbers. The foundation frame 4 comprises flanged end plates 32 connected by a base plate 34 and three tubular ties 36. The rail R rests on pads 38 on the end plates 32 of the frame and in each mechanism the lower and upper pivot shafts 8, 10 extend between the end plates 32. The pairs of upper and lower pivot arms 12, 14 are keyed to their respective shafts 8, 10 at the remote ends of the shafts adjacent the end plates, as in FIG. 1, and the carrier arms 6 are pivoted at 16 and 18 respectively to the upper and lower pivot arms 12, 14 to carry the gripper bars 2 on their upper ends a short distance above the upper pivot connections 16. The upper pivot shafts 8 are mounted on the end plates 32 through eccentrics 40 which may be of the commercially available &#34;Ringfeder&#34; form, for compensation of manufacturing and fitting variations. 
     At a central region of the shafts 10 a pair of mounting plates 42 are supported on and extend between the shafts. The plates carry the respective rams 20 for operating the gripper bar mechanism, the rams being mounted on the plates by pivots 22 on the ram cylinders. The piston rod 24 of the ram of each mechanism is connected by its pivot pin 26 to a pair of the crank arms 28 that are keyed to the drive shaft 10. The operation of the mechanisms by their rams is entirely analogous to that described with reference to FIG. 1, although now the rams are mounted so that they always remain with the lateral extent of the foundation frame. 
     It may be seen that by supporting the rams on the mounting plates 42 that are themselves supported on the lower pivot shafts 10 immediately adjacent the drive cranks 28 of the rams, the bending stresses on the shafts canbe limited and it is not necessary to strengthen the foundation frame 2 to carry and support the rams. The arrangement also assists manufacture since, being otherwise unrestrained, the mounting plates will align themselves on the shafts 10. 
     To provide a remote indication of engagement, a hydraulic pressure switch (not shown) is provided in the line to the rams, while to indicate retraction electrical limit switches 46 are arranged to be actuated by respective strikers 48 on the lower shafts 10. In FIG. 6, this arrangement is shown in the end position with the switches tripped. 
     The lower shafts 10 are advantageously designed to be stiff to ensure load-sharing between both link arms driven by each ram. This can also allow a number of units of the form described to be linked in tandem, and with the upper and lower shafts 8, 10 coupled, to operate together, possibly for providing a composite unit of sufficient length to act as a rail vehicle retarder or as a multiple location positioner. The coupling of the lower shafts through which the mechanisms are driven will keep the individual units in synchronism due to their stiffness and also due to the fact that the final operative position of a gripper bar is not unduly sensitive to the precise angular position reached by its lower shaft i.e. because the arms 6, 14 are then substantially in line, some 10° to 15° rotation of their shaft 10 will not materially alter the force applied. 
     It will be appreciated that many modifications of the described arrangements can be made within the scope of the present invention. Thus, while the four-bar mechanism illustrated provides, in a relatively compact manner, a high gripping force and rapid retraction from a simple pressure cylinder actuation, it is possible to obtain those advantage with other specific constructuions. Also, although the illustrated constructons are symmetrical arrangements, which will normally be both convenient and desirable, it is possible for a rail gripper according to the invention to employ a conventional mechanism for one clamping member and to have a mechanism such as those described only for the opposed clamping member.