The present invention relates to a teetering castor mechanism for a vehicular carriage such as a shopping trolley or wheelchair, and relates particularly, though not exclusively, to such a teetering castor that incorporates a shock absorbing mechanism.

Most (if not all) vehicular devices mounted on casters are frequently unmanageable due to problems with steering; from TV tables to wheelchairs and shopping trolleys - the latter to the extent that they can cause injuries to users.

Wheelchair-bound people in particular suffer the inconveniences of castor failure due to impact, even minimal impact occasioned by small differences in paving slab levels can lead to failure. Pot holes and rocks present an even greater hazard, the resulting fractures to the chair frame proper immediately adjacent to the castor-fixing being common place. In view of the price of a custom-made wheelchair, the cost of repair can be substantial when labour and plasma welding (where an aluminium frame is involved) are taken into account.

Wheelchair handling and manoeuvrability is appreciably enhanced with minimal fork rake and reduced castor wheel diameter. Unfortunately these two improvements also exponentially increase vulnerability to injury for both the entire unit and the occupant. Minimal fork rake and reduced castor wheel diameter has the disadvantage that any frontal shock, that virtually stops the chair in its tracks, can result in bent pivot shafts or fractures to the chair frame proper, or both. Hence, most wheelchair manufacturers adopt shallow rakes and large castor wheel diameter as the safer preference. However, this produces poor response to directional changes and produces wheel flutter within a wide velocity range. This is therefore an undesirable option and a poor compromise. The present invention was developed with a view to providing a teetering castor mechanism with improved response to directional changes.

The preceding discussion of the background to the invention is intended only to facilitate an understanding of the present invention. It should be appreciated that the discussion is not an acknowledgment or admission that any of the material referred to was part of the common general knowledge as at the priority date of the application.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Disclosure of the Invention

According to one aspect of the present invention there is provided a teetering castor mechanism for a vehicular carriage, the mechanism comprising:

a steering shaft pivotally mounted in a teetering coupling means for coupling the steering shaft to a main frame of the carriage, and wherein the steering shaft is able to pivot about a substantially vertical axis; and

a castor wheel rotatably supported on a support structure mounted on said steering shaft;

wherein said teetering coupling means has a first component, in connection with which said steering shaft is mounted, and a second component fixed to said main frame, said teetering coupling means permitting a degree of teetering of the steering shaft away from the vertical whereby, in use, movement of the main frame in any direction will be accompanied by teetering of the steering shaft and thereby trailing of the support structure, so that stable castoring action is facilitated.

In contrast to the prior art, the teetering coupling means will cause the steering shaft to be offset from the vertical axis when at least the carriage is in motion. When the direction of motion is changed the mechanism will rotate relative to the main frame to accommodate the change.

Preferably said teetering coupling means is provided with a stop means adapted to limit the degree of teetering to a predetermined extent. In one embodiment said teetering coupling means is in the form of a ball and socket type coupling, and said first component is a ball component that is pivotably mounted within said second component, which is a socket component.

Preferably the axis of rotation of said castor wheel is offset from the longitudinal axis of the steering shaft.

Typically said support structure comprises a support arm that is mechanically coupled to said steering shaft. The support arm may be pivotable about a substantially horizontal axis. Preferably said support structure further comprises a shock absorbing means provided in connection with said support arm and adapted to absorb some of the vibrations transmitted from the castor wheel via the support arm to the steering shaft.

According to another aspect of the present invention there is provided a shock absorbing castor mechanism for a vehicular carriage, the mechanism comprising:

a steering shaft pivotably mounted in a coupling means for coupling the steering shaft to a main frame of the carriage, wherein the steering shaft is able to pivot about the point it is coupled to the carriage;

a castor wheel rotatably supported on a support structure mounted on - A - said steering shaft;

wherein said support structure comprises a support arm that is pivotably coupled to said steering shaft; and,

wherein said support structure further comprises a resilient means provided in connection with said support arm and the steering shaft whereby, in use, said resilient means is adapted to absorb some of the vibrations transmitted from the castor wheel via the support arm to the steering shaft.

Preferably the support arm is pivotalbly coupled to steering shaft so as to be pivotable about a substantially horizontal axis.

In one embodiment said resilient means comprises a damper apparatus comprising a housing, an inner member and a damper material interposed therebetween, the damper material being bonded to both the housing and inner member, whereby at least one or other of the housing and inner member are able to rotate or be rotated respectively in a substantially concentric manner, the inner member provided in a form or configuration presenting a limited torque angle to the damper material.

In a particularly preferred form of the present invention the housing is provided with a circular inner face to which the damper material is bonded.

Preferably, the inner member is formed with two or more lobes projecting from a centre thereof, the lobes being rounded in general form so as to substantially minimise torque angle and shearing forces when the inner member is rotated relative to the housing.

The inner member may support the support arm. Preferably the housing is attached to the steering shaft. The damper material may be a rheopectic material, such as polyurethane.

In another embodiment said resilient means comprises a spring mounted between a first spur provided on the support arm and a second spur provided on the steering shaft.

Brief Description of the Drawings

In order to facilitate a more comprehensive understanding of the nature of the invention, preferred embodiments of the castor mechanism will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 illustrates a first embodiment of a teetering and shock absorbing castor mechanism in accordance with the present invention;

Figure 2 illustrates a second embodiment of a teetering and shock absorbing castor mechanism in accordance with the present invention;

Figure 3 illustrates a third embodiment of a teetering and shock absorbing castor mechanism in accordance with the present invention;

Figure 4 illustrates a fourth embodiment of a teetering castor mechanism in accordance with the present invention;

Figures 5(a) and (b) illustrate a fifth embodiment of a teetering and shock absorbing castor mechanism in accordance with the present invention;

Figure 6 illustrates a sixth embodiment of a teetering castor mechanism in accordance with the present invention;

Figure 7 illustrates a seventh embodiment of a teetering castor mechanism in accordance with the present invention;

Figure 8 illustrates an eigth embodiment of the invention which is similar to that of the fifth and sixth embodiments, whereby a teetering and shock absorbing castor mechanism has a double fork arrangemnt and is connected to a shopping trolley; and

Figure 9 is a front view of figure 8.

A first embodiment of the castor mechanism 10 in accordance with the present invention is illustrated in Figure 1 , and incorporates both a teetering mechanism and a shock absorbing mechanism. It is to be understood that the castor mechanism 10 may incorporate either one or both of the teetering mechanism and shock absorbing mechanism. The teetering mechanism is designed primarily to provide improved directional change response, whereas the shock absorbing mechanism is designed to absorb impact and therefore reduce the possibility of fractures to a main frame of the vehicular carriage to which the castor wheel is attached. In summary, the castor mechanism may be used with or without the shock absorbing/suspension system, depending on the application. In a strictly institutional situation with perfectly smooth floors, but where a high degree of manoeuvrability is called for, there is no need to use the suspension.

In each of the illustrated embodiments, the castor mechanism is shown in situ on a main frame 12 of a vehicular carriage, such as a wheelchair, shopping trolley or other conveyance with castor wheels. The castor mechanism 10 as shown in Figure 1 comprises a steering shaft 14 pivotably mounted in a ball and socket type coupling 16 for coupling the steering shaft 14 to the main frame 12 of the carriage. The ball and socket type coupling 16 comprises a ball component 18 in which the steering shaft 14 is pivotably mounted so as to be able to pivot about a substantially vertical axis. The ball and socket type coupling 16 further comprises a socket component 20 that is fixed to the main frame 12. In this embodiment, the socket component 20 is formed integral to the main frame 12. Pivoting movement of the ball component 18 within the socket component 20 permits a degree of teetering of the steering shaft 14 away from the vertical.

A castor wheel 22 is rotatably supported on a support structure 24 mounted on the steering shaft 14. In this embodiment, the axis of rotation of the castor wheel 22 is offset from the vertical axis of the steering shaft 14 and hence whenever a load is applied to the castor mechanism 10, a bending moment will be applied to the steering shaft 14 causing it to teeter within the ball and socket type coupling 16. This will produce instant trailing of the support structure 24 so that stable castoring action is assured. However, even in the extreme and unlikely case of zero fork rake being implemented, the least movement will cause teetering and inherent, instant trail.

Steering shaft 14 may be formed integral to the ball component 18, or may be rotatably received in a bore provided therein. In the illustrated embodiment, the steering shaft 14 is formed integral to the ball component 18 and therefore pivoting movement of the steering shaft 14 occurs by pivoting movement of the ball component 18 within the socket component 20. Ball component 18 is also formed with a stub 26 integral thereto, that is received within a small chamber 28 adjoining the cavity formed by the socket component 20. The gap between the stub 26 and the side walls of chamber 28 determines the teetering angle of the ball component 18, and hence that of the steering shaft 14.

Steering shaft 14 is provided with a first spur 30 thereon.

The support structure 24 comprises a support arm 32 pivotally attached to the steering shaft 14, and which also is provided with a spur 34 thereon. In this embodiment, the support arm 32 is in the form of a fork (which may be monopole). Between spurs 30 and 34 a resilient means is provided for absorbing some of the vibrations transmitted from the castor wheel via the support arm to the steering shaft 14. In the illustrated embodiment, the resilient means is a small coil spring 36. However, it will be understood that any suitable resilient means with shock absorbing properties, such as a leaf or other spring, or a resilient material such as rubber or a suitable polymer, or any combination of such devices/materials may be used. The resilient means, in this case spring 36, provides progressive resistance and retardation of travel speed when spurs 30 and 34 are brought together due to impact or other force acting on wheel 22. With this arrangement, the castor mechanism is provided with a trailing shock absorber and suspension system, that is particularly desirable with steep fork angles in order to minimise stress fractures.

The second embodiment of the castor mechanism illustrated in Figure 2 is similar to that of Figure 1 , and the similar components are therefore identified with like reference numerals. A first difference is in the ball and socket type coupling means 16, which in this embodiment comprises a truncated ball component 38 and socket component 40. The ball component 38 is pivotally received within the socket component 40 and is able to teeter in a similar manner to the ball component 18 of the first embodiment. As with the first embodiment, the teetering angle is determined by the gap between the stub 26 formed integral to the ball component 38, and the chamber 28. However, in this embodiment the upper surface of ball component 38 also shares the stresses with stub 26 by simultaneously contacting an upper surface 42 of the cavity formed by the socket component 40.

A second difference is in the shock absorbing/suspension mechanism which in this embodiment is achieved by a leading jaws arrangement. A spur 44 located on the underside of steering shaft 14 faces a second spur 46 formed on an upper extension of the support arm 32. A coil spring 48 mounted between the spurs 44 and 46 functions in a similar manner to spring 36 of the first embodiment.

A third embodiment 50 of the castor mechanism, as shown in Figure 3, combines the shock absorbing mechanisms of the first and second embodiments. Once again, similar components are identified with like reference numerals. The shock absorbing/suspension mechanism of this embodiment is thus double-acting, offering considerable mechanical advantage. The truncated ball component 38 of this embodiment adopts a minimalist approach, and omits the stub 26. Hence, this embodiment is entirely reliant on upper rim contact of the ball component 38 with the upper surface 42 of the cavity formed by socket component 40, to act as a teetering angle limit. The castor mechanism 50 of this embodiment otherwise functions in an identical manner to the first and second embodiments.

Figure 4 illustrates a fourth embodiment utilising a truncated ball and socket type coupling 54. The ball and socket type coupling 54 includes a mushroom- like ball component 56 pivotally received within a socket component 58 formed integral to the main frame 12. Ball component 56 is formed integral to a steering shaft 59 as with the previous embodiment. An annular step 57 formed adjacent the upper end of the steering shaft 59 holds the ball component captive within the socket component 58. The castor wheel and support structure have been omitted from Figure 4 for clarity, and are substantially identical to that illustrated in Figure 1. In this embodiment, the teetering angle is limited by the gap between the lower edge of the socket component 58 and the steering shaft 59. This arrangement offers the least stiction of all.

Figure 5(a) and (b) illustrate a fifth embodiment of the castor mechanism 60 shown in side elevation and rear elevation respectively. The ball and socket type coupling of this embodiment is similar to that shown in Figure 2, and therefore similar parts have been identified with the like reference numerals. Castor wheel 22 is rotatably supported on a support arm 32 in a similar manner to that of the previous embodiment. However, in this embodiment a different resilient means is employed to provide the shock absorbing mechanism for the castor wheel. A sleeve 62 of rheopectic polymeric material is encapsulated in and secured to a housing 64 formed integral to the upper end of the support arm 32. The sleeve 62 is also secured to a through-shaft 66 which is bolted to the steering shaft 14 and is non-rotatable with respect thereto. Any force acting on the wheel 22 will cause the support arm 32 to pivot about through-shaft 66. The polymeric sleeve 62 affords progressive resistance and retardation of travel speed of housing 64 relative to the steering shaft 14, depending on the hardness and density (hence yield) of the polymeric material.

The rheopectic polymeric material of sleeve 62 is preferably polyurethane, which acts like a liquid in "solid" form. Like a liquid, it is substantially incompressible. Its rheopectic properties make it ideally suited as a damping material. When first struck by an object the material acts like a liquid, however as the object decelerates the material behaves like a solid. Typically the shorter the time frame that the load is applied to the material, the quicker the material responds. The transition from "solid" to a liquid like state occurs very quickly, and the material also returns to its original form very quickly. Rheopectic materials exhibit fluid-like properties in which the viscosity increases with time under a constantly applied stress.

For simplicity, a monoblade fork is illustrated, however the shock absorbing mechanism is equally applicable to a conventional double blade fork or other support structure, as shown in the further embodiment of figures 8 and 9. Obviously, the shock absorbing mechanism of this embodiment and that shown in figures 8 and 9 may also be used in conjunction with any one of the other teetering heads illustrated, or indeed with a regular non-teetering head.

Figure 6 illustrates a sixth embodiment 70 of the castor mechanism which incorporates a teetering mechanism according to the present invention, but does not incorporate a shock absorbing mechanism. However, it is to be understood that the castor mechanism 70 of Figure 6 may also incorporate a shock absorbing mechanism if desired.

The principal difference between the castor mechanism 70 of this embodiment and that of the previous embodiments, is that it incorporates a teetering coupling means somewhat different to that of the previous embodiments. In this embodiment, castor wheel 22 is rotatably supported on a support arm 72 which is mounted on a steering shaft 74. Steering shaft 74 is able to pivot about a substantially vertical axis so as to permit castor wheel 22 to be steered in any direction. An upper hemispherical component 76 and a lower hemispherical component 78 are fixed to the main frame 12 by any suitable method. For example, the components 76 and 78 may be threaded so as to screw into each other with the main frame 12 sandwiched therebetween. Hemipherical components 76 and 78 are formed with a tapering bore therethrough in which the steering shaft 74 is received.

The shoulder of support arm 72 is spherically radiused to match the outer surface of the lower hemispherical component 78. It will be appreciated that the full load from castor wheel 22 is transmitted to the main frame 12 via the shoulder of support arm 72 and the lower hemispherical component 78. The tapered bore passing through the hemispherical component 76 and 78 permits a degree of rocking movement or teetering of the support arm 72 and steering shaft 74, the degree of teetering being determined by the taper. A concave follower component 80 is provided at an upper end of the steering shaft 74, and is formed with a spherical surface matching that of the upper hemispherical component 76 with which it is in slidable contact. Concave follower component 80 is held on the steering shaft 74 by means of a securing nut and washer 82. Tightening or slackening of the nut 82 will alter the freedom of movement of the teetering coupling means, providing de facto damping. As with the previous embodiments, the axis of rotation of the castor wheel 22 is offset from the vertical axis of the steering shaft 74 so that when a load is applied to the castor mechanism 70, a bending moment will be applied to the steering shaft 74 causing it to teeter within the tapered bore of the truncated spheres 76 and 78. With this arrangement, movement of the main frame in any direction will generally be accompanied by teetering of the steering shaft 74 and thereby instant trailing of the support structure 72 so that stable castering action is facilitated.

Figure 7 illustrates a seventh embodiment of the teetering castor mechanism which is similar to that of Figure 6, and hence similar components have been identified with the same reference numerals. The main difference in this embodiment is that the upper and lower hemispherical components 86 and 88 are formed with a tapered bore that tapers in the opposite direction to that of Figure 6. In this embodiment, the tapered bore has an increasing diameter in a downwards direction, whereas in Figure 6 the tapered bore has an increasing diameter in an upwards direction. However, the net effect is the same. As with the previous embodiment, a shoulder of the support arm 72 rides on a convex hemispherical surface of the lower hemispherical component 88 and steering shaft 74 is free to rock or teeter within the tapered bore, the degree of teetering being determined by the taper. The support arm 72 with steering shaft 74 are held within the assembly by means of nut and washer 82 and a convex follower component 90. The convex follower component 90 has a convex surface which matches a concave surface formed in the upper hemispherical component 86. In other aspects, the teetering castor mechanism of this embodiment operates in a similar manner to that of Figure 6 and will not be described again.

Figure 8 and 9 illustrates am eigth embodiment of the teetering castor mechanism which is similar to that of Figure 5 and Figure 6, and hence similar components have been identified with the same reference numerals. The point of different of this embodiment beyond that shown in figures 5 and 6 is that the the support structure 24 comprises a support arm 32 in the form of a typical fork, whereby the castor wheel 32 is received in the fork.

Now that several embodiments of the teetering castor mechanism of the present invention have been described in detail, it will be apparent that it provides a number of significant advances over conventional castor mechanism, including the following: (i) The teetering ensures inherent, instant trail and therefore stable castoring action; (ii) It therefore exhibits excellent response to directional changes and wheel flutter is substantially eliminated; (iii) The shock absorbing mechanism significantly reduces the possibility of fractures to the main frame; and, (iv) It is of relatively simple construction and can be manufactured at low cost.

Numerous variations and modifications will suggest themselves to persons skilled in the mechanical arts, in addition to those already described, without departing from the basic inventive concepts. For example, each of the illustrated embodiments employs a conventional tyre rimmed castor wheel. However, both the teetering mechanism and the shock absorbing mechanism are equally applicable to other forms of castor wheel. All such variations and modifications are to be considered within the scope of the present invention, the nature of which is to be determined from the foregoing description.