Abstract:
An improved mechanism is provided for enhancing the towability of castorwheel vehicles. The mechanism leaves the castors that are proximal to the towing force to rotate, while locking the distal castors in alignment with the vehicle axis.

Description:
BACKGROUND OF THE INVENTION 
     This application relates to the art of entraining vehicles that move on castors so that trailing vehicles will follow the curvilinear path established by a towing force. The invention is particularly applicable to hospital carts and gurneys, although it will be appreciated that the invention has broader applicability to most types of castorwheel vehicles. 
     It is known in the art that for maximum stability and flexibility of towing, the lead wheels of any towed vehicle should be free to rotate, while the rear wheels should be locked in a straight orientation. Thus, various methods of manually locking castorwheels in a fixed position are known in the art. These methods do not readily lend themselves to entraining multiple vehicles, however, since they require manual intervention to lock and unlock the castors, at least at the individual vehicle level, and often at the level of each castorwheel. 
     Moreover, the methods employed for locking the castors in desired configurations frequently result in a restricted range of motion for the castors when the vehicle is returned to its primary use as a single unit. When operating alone, maximum maneuverability of each vehicle requires that all castorwheels be allowed to swivel through a full 360 degrees of motion. 
     Thus, it is desirable to have a locking mechanism that allows each castor of a vehicle, while operating alone, to swivel freely, through a 360 degree range of motion, yet automatically provides for appropriate locking of trailing castors when the vehicle is entrained. It is also desirable to be able to apply towing force from either end of a train of vehicles, with automatic interchange of the leading and trailing castor configuration. 
     SUMMARY OF THE INVENTION 
     The present invention provides a locking mechanism that overcomes the limitations of the prior art. The locking mechanism includes a slider, which, in its quiescent state, allows all the castors to freely swivel. Upon application of a towing force to either end of the slider, however, a set of slots moves into positive engagement with a key or protrusion on the shank of the castorwheel assemblies distal to the towing force, thereby locking them in a straight orientation. Alternatively, the distal castorwheel assemblies may be the moveable members, which slide into positive engagement with fixed slots in response to a towing force. The proximal set of castors remains free to swivel. 
     In a preferred embodiment, a centering means normally maintains the slider in its quiescent position. All the castor shanks remain free to rotate 360 degrees through the circular portion of keyhole-shaped voids in the slider. When a towing force exceeds the centering force, the slider moves in the direction of the towing force. This movement causes the slotted portion of the keyhole shaped voids in the slider to mesh with corresponding keys on the shanks of the distal castorwheel assemblies. The proximal castorwheel shanks remain in the circular portion of the keyhole-shaped voids, remaining free to rotate. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a side view of the castorwheel assembly; 
     FIG. 1A is a sectional view of the shank taken along line 1A--1A of FIG. 1; 
     FIG. 2 is an overhead view of the slider; 
     FIG. 3 is an exploded view showing the interconnective and spatial relationships of the slider, castorwheel assembly and vehicle frame; 
     FIG. 4 shows the engagement of the slider and castor shanks in the towing mode; 
     FIG. 5A, 5B and 5C shows alternative embodiments of the slider voids and shank cross-sections. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     The castorwheel assembly, as shown in FIG. 1, consists of a castor or wheel 2 attached to a forked member with &#34;L&#34; shaped tynes 1A attached to a shank 1 that extends through a bearing housing 4, which is welded to a mounting plate 3. A bearing (not shown) may be fitted over the shank below the mounting plate 3. The shank 1 has a cross section that resembles a mushroom head and stem, as shown in FIG. 1A. In the embodiment shown, the shank 1 also has a snap ring groove 1B, which may be employed to restrict any vertical displacement of the slider. The rotational axis of the shank 1 is offset by a distance D from the center of the wheel 2. 
     FIG. 2 shows one embodiment of the slider. In this embodiment, the slider is an &#34;H&#34; shaped plate 6 with a towing bar 10 that extends outwardly in both directions from the horizontal cross member. Located in each corner of the &#34;H&#34; shaped plate is a keyhole-shaped void 11 with the slotted portions pointing outward toward the ends of the &#34;H&#34; shaped member. The diameter of the circular portion of the keyhole exceeds the cross-sectional diameter of the castorwheel shank 1. 
     FIG. 3 shows the relationship of the several components of the locking mechanism. Attached to the underside of the frame of the vehicle 7 are four channel members 5, to which the castorwheel assemblies are attached, with the shank 1 protruding through holes in the channel and through the keyhole-shaped voids 11 in the slider 6. The slider then moves longitudinally in the channel members in response to a towing force. 
     FIG. 4 shows the locking mechanism in the towing mode. When a towing force in excess of the centering force is applied to one end of the towing bar 10, the slider moves in the direction B of the towing force. Because of the eccentricity or offset in the mounting of the castorwheels, each castor pivots so as to align itself with the towing force. The shanks of the proximal castorwheels remain in the oversized circular portion of the keyhole-shaped voids in the slider and, hence, remain free to swivel in response to a change in the direction of the towing force. The shanks of the distal castorwheels, however, rotate into engagement with the advancing slotted portion of the corresponding keyhole-shaped voids, locking the castorwheels in alignment with the longitudinal axis of the vehicle, until the towing force is removed. 
     Those skilled in the art will appreciate that there are obvious variations suggested by the disclosure. A second set of orthogonally located slots could be provided in the keyhole shaped voids to afford directional stability when maneuvering a vehicle perpendicularly to the initial direction of towing. Furthermore, while a centering means, such as a centering spring 8, is provided in the preferred embodiment, this feature is not central to the disclosure. In particular applications, it may be more advantageous to rely on external force to restore the slider to its quiescent position. Moreover, it would be a simple matter to arrange for the slider or locking member to move into and out of engagement with the castorwheel shank in a vertical plane, perpendicular to the direction of towing. 
     It would also be a simple matter to alter the locking mechanism so that the castorwheels are normally locked, and made free to rotate by the application of an external force. In the preferred embodiment, this alteration could be accomplished either by reversing the orientation of the keyhole-shaped voids, or by biasing the slider so that it normally engages the castorwheel shanks. 
     FIG. 5 shows alternate configurations of the castorwheel shank cross-section and slider voids that are also within the scope of this disclosure. FIG. 5A shows the configuration of the preferred embodiment. FIG. 5B shows an alternative configuration wherein the cross-section of the shank is basically circular with the keyed portion superimposed on the outer diameter. FIG. 5C shows a second alternative &#34;Shmoo&#34; configuration wherein the shank cross-section and the slider voids are both large circles with a smaller circular sector superimposed on the outside diameter of the large circle. It will be appreciated that FIGS. 5B and 5C do not exhaust the variations made possible by this disclosure, but are merely representative examples. Another obvious class of variations would be to have an indentation in the shank that mates with a protrusion in the slider. 
     The foregoing description of a preferred embodiment and alternative embodiments of the invention have been presented for purposes of illustration an description. The description is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obviously, many modifications and variations are possible in light of the above teaching. The embodiment presented above was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated.