GB2333698A

GB2333698A - Castor with variable inclination of swivel axis

Info

Publication number: GB2333698A
Application number: GB9801939A
Authority: GB
Inventors: Abdul Majid Basharat
Original assignee: Clares Merchandise Handling Equipment Ltd
Current assignee: Clares Merchandise Handling Equipment Ltd
Priority date: 1998-01-29
Filing date: 1998-01-29
Publication date: 1999-08-04
Anticipated expiration: 2018-01-29
Also published as: GB9801939D0; GB2333698B

Abstract

A castor 6 for eg a supermarket trolley is mounted to the leg 4 of the trolley through a mounting plate 10 and rubber bush 12 whereby, as the weight load on the trolley increases, the bush flexes and the inclination angle α' of the castor swivel axis to the vertical increases. The mounting plates of a pair of such castors may be interconnected by a tie-bar 14 to help equalise the inclination of both axes. Such castors may be used as front and/or rear wheels and a central non-castor wheel may be provided between the pair to increase stability under heavy loading.

Description

STEERABLE MOBILE LOAD CARRIER AND CASTOR ASSEMBLY THEREFOR The present invention relates to steerable mobile load carriers and concerns particularly, but not exclusively, carts and trolleys of the kind provided in retails outlets such as supermarkets and DIY stores, and luggage trolleys such as those provided at airport and railway stations.

Taking the particular, but non-limiting example of a shopping trolley, a common arrangement as illustrated in Fig. 1 comprises an upwardly open basket 40 made of metal wires and rods, mounted on a wheel base chassis 1. The basket is provided with a rear transverse push-bar 44 by which the user can propel and manoeuvre the trolley around, and the chassis is provided with a set of ground-contacting wheel elements 6; there are usually four such elements, each provided at a respective corner of the chassis. Accordingly, there are usually "front" and "rear" pairs of wheel elements.

To provide maximum steerability of the trolley, the wheel elements usually each comprise a swivel castor wherein a wheel support is swivelable about an upright swivel axis passing through the position of mounting of the castor to the chassis, the wheel being rotatable in the wheel support about a horizontal axis which is offset in a horizontal direction from the upright swivel axis.

Fig. 2 ofthe drawings is a side elevational view showing one side of the rear part of a shopping trolley chassis, and illustrating the mounting of one such rear swivel castor. In this known example, the base chassis 1 comprises a pair of main support members 2, each upwardly arched so as to form front and rear legs 3, 4 respectively.

These support members 2 are formed from oval-section steel tubing, and are interconnected by transverse spacer bars, one of which is shown at 5. The rear swivel castor 6 is mounted at the lower, rear end of the rear chassis leg 4 so that its wheel support 7, in which the wheel 8 is mounted for rotation about axis R, can swivel in its swivel bearing 9 about upright swivel axis S. There is a similar castor mounting arrangement at the end of the other rear leg 4, and also at the forward end of each of the front legs 3. The castor is shown in Fig. 2 in the position it will automatically adopt, i.e. with the wheel trailing behind the swivel axis relative to the direction of trolley movement D. As shown, direction D is a forward movement of the trolley; if the direction D is reversed by pulling the trolley so that it moves backwards, the castor will flip round so as to keep the wheel axis R behind (relative to the travel direction D) the swivel axis.

The use of swivel castors should apparently provide full manoeuvrability of the trolley through the application by the user of turning forces to the rear push-handle. However, as is well known, the steering characteristics of shopping trolleys tends to be both unpredictable and variable from one trolley to another; few trolleys exhibit such perfectly balanced characteristics that there is no tendency for the trolley to steer to the left or right when the pushing force is directed straight forwards. The lateral forces which result in this tendency to sideways drift result from the fact that the swivel castors all have slightly different swivel characteristics; in particular, they have different degrees of resistance to swivelling of the wheel support 7.

These differences tend to increase with use, and the increasing imbalance leads to a deterioration in the overall manoeuvrability of the trolley.

There have been many attempts in recent years to overcome or alleviate this steering problem in shopping trolleys, but none of them has been entirely successful. It has been suggested, for example, to provide an arrangement for the selective locking/unlocking of either the front or rear castors so that they are fixed in the straight-ahead position when the trolley is being pushed forwardly, and are released so as to swivel freely to permit cornering. The additional components required in this arrangement, namely a locking device at the selected castors, a manually operable brake handle (normally immediately beneath the push-handle) and a linkage therebetween adds to the overall cost of the trolley, and a simpler, cheaper solution has been sought.

With reference to Fig. 3, it has been discovered that tracking of the castors (i.e. keeping the castors in-line when the trolley is pushed forwardly) can be enhanced by tilting the swivel axis S in a vertical plane (corresponding to the plane of the paper in Fig. 3) parallel to the longitudinal axis of the trolley; Fig. 3 shows the swivel axis tilted forwardly, by which is meant herein that the part of the swivel axis above the swivel bearing inclines toward the front of the trolley, the "front" being defined relative to the preferred (i.e. forward) direction of trolley movement. With reference to Fig. 4, it has been found that this tilting of the swivel axis causes a restoring moment M to be applied whenever the wheel support 7 is in a swivel position (in solid lines) which is angularly displaced by an angle ss from the in-line position (shown in broken lines).

The magnitude of the angle used for setting the castor tilt relative to the verticle V must be set very carefully so as to provide a sufficiently positive tracking effect, without making it too difficult to steer the trolley round corners; if is made too large, it will be understood that the above-mentioned restoring moment will be too strong to permit proper steering.

It has been observed that amongst the parameters which affect the optimum value of the swivel axis tilt angle is the gross weight of the trolley (i.e. including its contents); more particularly, a relatively small value of x may be ideal for a relatively lightly loaded trolley, but may provide inadequate tracking when the trolley is heavily loaded.

The present invention is directed to alleviating this problem, and to that end provides a steerable mobile load carrier including a base chassis provided with a set of ground-contacting wheel elements, said set including a swivel castor to allow steering of the carrier, wherein the swivel caster is resiliently mounted to the chassis so that as the downward force applied at the castor mounting by the carrier weight increases, so the swivel axis of the castor tends to tilt, the inclination of said swivel axis to the vertical varying according to variation in the magnitude of said downward force.

In the described embodiments, the castor mounting is arranged so that with the castor in its swivel position corresponding to forward movement, the tendency is for the swivel axis to tilt forwardly. With this arrangement, as the carrier weight increases, the forward swivel axis tilt increases, and in turn the tracking effect also increases.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. 1 is a side elevational view of a known form of shopping trolley; Fig. 2 is a side elevational view of a rear castor and the part of a known shopping trolley chassis on which it is mounted; Fig. 3 is a side elevational view of a rear shopping trolley castor, fixedly mounted in a known manner so that its swivel axis is forwardly inclined; Fig. 4 is a schematic plan view of swivel castor mounted as in Fig. 3 with its swivel axis inclined, and illustrating the tracking effect of a restoring moment experienced by the tilted castor; Fig. 5 is a side elevational view of a castor assembly mounted on the rear legs of a shopping trolley chassis, in accordance with the present invention; Fig. 6 is a perspective view illustrating the castor assembly of Fig. 5; Fig. 7 is a side elevational view corresponding to Fig. 5, showing the tilting effect of the castor mounting under increased load; Fig. 8 is a side elevational view of a castor assembly mounted on the front legs of a shopping trolley chassis, in accordance with the present invention; Fig. 9 is a front perspective view showing the castor assembly of Fig. 8; Fig. 10 is a side elevational view corresponding to Fig. 8 showing the tilting effect of the castor mounting under increased load; Fig. 11 is a side elevational view of a modified form of castor assembly mounted on the front legs of a shopping trolley chassis in accordance with the present invention, and Fig. 12 is a front perspective view showing the castor assembly ofFig. 11.

In Figs. 5 to 12 of the accompanying drawings, the same reference numerals are used as in Figs. 2 to 4 to designate corresponding elements.

With reference first to Figs. 5 to 7, the chassis 1 of a shopping trolley comprises two main support members 2, each of which is upwardly arched so as to form a respective front leg 3 and a rear leg 4.

The two support members 2 are formed from oval-section steel tubing, and are fixed together by rigid transverse spacers, which have been omitted from the Figures for clarity.

A pair of swivel castors 6 is provided, each being mounted at the lower rear end of a respective rear leg 4. Instead of being mounted, as in the known arrangement of Fig. 3, with its swivel axis S at a fixed predetermined inclination angle relative to the vertical, each castor in the embodiment of the invention is mounted so that as the downward force F at the lower rear end of the respective rear leg 4 increases, the inclination angle of the castor swivel axis also increases. In the illustrated embodiment, each rear castor 6 is mounted on a mounting plate 10 which projects reanvardly from beneath the lower rear end of the rear leg 4, to which it is mounted through a bush 12 of resilient, elastomeric material, such as rubber. At each of its opposite ends, a transverse tie- bar 14 is rigidly fixed, e.g. by welding, to a forward portion 10 of a respective one of the mounting plates 10. This tie-bar is U-shaped so as to allow the front end of the chassis of a similar trolley to pass between the rear legs 4 during trolley stacking.

In the embodiment, the arrangement is such that when the trolley is in an unloaded, or very lightly loaded state and is moving or ready for movement in a forward direction, the components of the castor assembly are in a condition, as illustrated in Fig. 5, in which the angle of forward inclination x of the castor swivel axis S' is at a minimum value, which may be 00. As the magnitude of the force F increases, the interconnected castor 6 and support plate 10 experience a turning moment M as the position of the castor-wheel ground contact point c is displaced rearwardly from the vertical axis A passing through the interconnection between the lower end of the leg 4 and the plate 10.

The resilient mounting of the plate 10 to the lower end of the leg 4 by way of the resilient bush 12 allows the interconnected plate 10, castor 6 and tie-bar 14 to pivot about the bush 12 (in an anticlockwise direction in Fig. 5) so as to increase the inclination angle X as illustrated in Fig.

7.

In this way, the tracking effect increases as the trolley load increases, so as to balance the increasingly destabilising effect which the increasing load will normally have upon the steerability of the trolley.

In the embodiment, where a pair of swivel castors is mounted on the chassis at a given longitudinal position (in this case the two rear castors) corresponding control over the inclination of the castor swivel axes is assured by the rigid transverse interconnection provided by the tie-bar 14. Thus, this tie-bar eliminates, or at least reduces, differences between the inclination angles X of the two rear castor swivel axes which may otherwise occur as a result of uneven lateral loading of the trolley relative to the longitudinal central axis thereof. Such uneven loading, and the consequent difference between the tilting of the two castor swivel axes would be likely to cause uneven tracking between the left and right sides of the trolley, and this could reduce the beneficial stabilising effect of the castor mounting arrangement. By rigidly interconnecting the castor support plates 10, this adverse effect on the beneficial tracking effect can be alleviated.

The concept underlying the castor assembly shown in Figs. 5 to 7 can also be applied to the front trolley castors, as illustrated in Figs. 8 to 10. In this embodiment, each of two front swivel castors 18 is mounted on a respective one of two front castor support plates 20. A forward portion 20 of each castor support plate 20 is fixed beneath the front end of the respective front chassis leg 3 through a resilient mounting member in the form of a resilient (e.g. rubber) bush 22. Plate 20 projects rearwardly from the resilient mounting, and the mounting position of the castor, and accordingly also the castor swivel axis S", is spaced rearwardly of the resilient mounting.

The two front castor mounting plates 20 are rigidly interconnected by a transverse tie-bar 24 in the form of a strip-like plate integrally formed with the plates 20.

The manner in which the above-described construction of the front castor assembly responds to varying trolley loads to provide optimum tracking and thus enhanced steerability is similar to the operation of the rear castor assembly, described earlier with reference to Figs. 5 to 7. Thus, the forward inclination angle x ' of the front castor swivel axis S'' in the unloaded or very lightly loaded state of the trolley illustrated in Fig. 8 is at a minimum value, which may be 00. As the trolley loading increases, the downward force F also increases, so as to cause the interconnected front castors and castor mounting plate 20 to pivot about the resilient bush 22 as a result of the applied moment, as illustrated in Fig. 10, so as to increase the inclination angle cc" As in the case of the rear castor mounting assembly, this increase in the inclination angle '' produces the increased tracking effect required for increased trolley loads to maintain steering stability. The transverse tiebar 24 ensures that inclination angle x changes correspondingly for the left and right front castors 18.

The above description of the variable tilting of the castor swivel axes refers to, and Figures 5 to 10 show, the condition in which the resiliently mounted castors are in their positions for forward movement.

In the above embodiments, the resilient mounting of the castors not only provides the variable tracking control through variation in the inclination of the castor swivel axes, but also serves to reduce wear on the castor swivel bearings. The dynamic forces applied to these castor bearings during normal use, e.g. as a result of vibrations and shocks which arise when the trolley is used on an uneven surface such as in a supermarket car park, lead to wear and damage. This wearing is particularly severe in the prior art arrangement of Fig. 2, where the bearing is already under a degree of strain due to the inclined castor mounting.

In the described embodiments, however, the resilient mounting members 12, 22 absorb most of the vibrational shocks, and thus relieve the swivel bearings from much of the adverse effects thereof.

In each embodiment, the concept underlying the invention is applied where there is a pair of castors mounted symmetrically about the trolley centre line. It will be appreciated that the concept can also be applied to a single, central castor, for example in a three-wheeled trolley including a single front or rear castor and a pair of rear or front castors, respectively. In this case, uneven loading of the trolley in a lateral sense will not adversely affect the improved tracking and consequent steering stability, as there is no left-to-right asymmetrical effect as in the paired castor arrangements.

In the above embodiments, the resilient mounting of each castor is by way of an elastomeric bush. However, other arrangements are possible. For example, a spring arrangement may be employed to provide the required resilient pivoting of the castor support plates to the trolley chassis legs.

In the embodiments, the pivoting of the castors under variable trolley load to vary the castor swivel axis inclination is enhanced by the provision of the castor support plates 10, 20, which in each case serve to space the castor swivel axis from the position of mounting to the end of the chassis leg. However, this is not essential, as a turning moment tending to cause the castor to pivot about its mounting, thereby to change the inclination angle ' or oc'', , will be applied even if the castor is mounted directly to the end of the legs (i.e. without the provision of the castor support plate) through the resilient mount with the castor swivel axis thus at the position of this mounting. In such a modified arrangement, the castor wheel ground contact position c will still be displaced rearwardly from the position of the resilient mounting, and variation in the trolley load will therefore still cause a variation in the tilting of the castor swivel axis.

For maximum effect, the rear castor assembly of Figs. 5 to 7 and the front castor assembly of Figs. 8 to 10 may be used together on the same trolley.

A modification of the front caster assembly arrangement of Figs.

8 to 10, illustrated in Figs. 11 and 12, provides even better steering stability. This arrangement corresponds substantially with that of Figs.

8 to 10, except for the following. An additional central front wheel 26 is mounted on a central part of a transverse mounting plate 28, the opposite end portions 28 of which constitute the castor support plates 20 for the two front swivel castors 18. This central wheel 26 does not swivel, but is directionally fixed in-line. It is mounted so that its transverse rotational axis lies beneath a forward part of the mounting plate at a position centrally between the resilient bushes 22. This forward part of the plate will rise and fall as the mounting plate and front swivel castors 18 tilt under the effect of the trolley load.

The mounting of the central wheel is set so that in the unloaded or very lightly loaded condition, shown in Fig. 11, in which the lower end of the front leg 3 is at its uppermost position, (as is the forward portion of the mounting plate 28), it is held spaced by a small distance d from the ground or floor surface 30 on which the trolley is being used.

In this state, the wheel really has no effect whatsoever on the steering characteristics of the trolley.

However, as the trolley load increases, the front ends of the front chassis legs 3 fall as the interconnected castors and support plate 28 pivot (in a clockwise sense in Fig. 11), and the wheel 26 is then brought into contact with the surface 30.

This action whereby the central wheel is brought into contact with the surface has two effects. Firstly, as the wheel 26 does not swivel, the front end of the trolley is henceforth constrained to move in line along the trolley axis. Though the trolley can still be steered by means of the rear swivel castors, the front end of the trolley is stabilised and a heavily laden trolley can thus be safely manoeuvred, even on laterally sloping surfaces such as those commonly encountered in supermarket car parks and ramps leading from a supermarket forecourt into the car park.

Secondly, the central wheel 26 prevents the front swivel casters from pivoting excessively through the resilient mounting members.

This prevents undue strain being applied to the resilient mounting members 22 under the effect of very heavy loading of the trolley.

Instead of being mounted on the transverse mounting plate 28, the additional, in-line wheel 26 may be mounted on the chassis behind the front castors, e.g. on a bracket suspended from the front transverse spacer bar 32 extending between the front legs 3.

Claims

CLAIMS 1. A steerable mobile load carrier including a base chassis provided with a set of ground-contacting wheel elements, said set including a swivel castor to allow steering of the carrier, wherein the swivel caster is resiliently mounted to the chassis so that as the downward force applied at the castor mounting by the carrier weight increases, so the swivel axis of the castor tends to tilt, the inclination of said swivel axis to the vertical varying according to variation in the magnitude of said downward force.
2. A steerable mobile load carrier according to claim 1, wherein the resilient mounting of the swivel castor is arranged so that in the castor swivel position corresponding to a given direction of carrier movement, said castor swivel axis tends to tilt in said given direction.
3. A steerable mobile load carrier according to claim 1 or claim 2, wherein said set of wheel elements includes a pair of swivel castors spaced apart in a direction transverse to the longitudinal axis of the carrier and disposed at the same longitudinal position, and wherein each swivel castor of said pair is resiliently mounted as aforesaid.
4. A steerable mobile load carrier according to claim 3, wherein said set of wheel elements comprises a front pair of swivel castors and a rear pair of swivel castors, and wherein the resiliently mounted pair of castors is said front pair.
5. A steerable mobile load carrier according to claim 3, wherein said set of wheel elements comprises a front pair of swivel castors and a rear pair of swivel castors, and wherein the resiliently mounted pair of castors is said rear pair.
6. A steerable mobile load carrier according to claim 3, wherein said set of wheel elements comprises a front pair of swivel castors and a rear pair of swivel castors and wherein the swivel castors of both said pairs are resiliently mounted as aforesaid.
7. A steerable mobile load carrier according to claim 1 or claim 2, wherein said set of wheel elements includes a pair of wheel elements spaced apart in a direction transverse to the longitudinal axis of the carrier and disposed at the same first longitudinal position, and a further single wheel element mounted on said longitudinal axis at a second longitudinal position spaced from said first longitudinal position, wherein said further single wheel element is resiliently mounted as aforesaid.
8. A steerable mobile load carrier according to claim 7, wherein said pair of wheel elements is a pair of swivel castors resiliently mounted as aforesaid.
9. A steerable mobile load carrier according to any of claims 3 to 6 and 8, wherein for the, each, or at least one said pair of resiliently mounted castors there is further provided a transverse connecting member rigidly interconnecting the castors of said pair so as substantially to equalise the inclinations of the respective castor swivel axes.
10. A steerable mobile load carrier according to any preceding claim, wherein the, each, or at least one said resiliently mounted swivel castor is mounted so that its swivel axis is spaced rearwardly of the position of resilient mounting to the chassis.
11. A steerable mobile load carrier according to claim 10, wherein for the or each said resiliently mounted swivel castor there is provided a mounting member, a resilient connection between said mounting member and said chassis, and a connection between said swivel castor and said mounting member, said latter connection being spaced rearwardly of said resilient connection.
12. A steerable mobile load carrier according to any of claims 1 to 9, wherein the, each, or at least one said resiliently mounted swivel castor is mounted so that its swivel axis coincides with the position of resilient mounting to the chassis.
13. A steerable mobile load carrier according to any preceding claim, wherein for the, each, or at least one said resiliently mounted swivel castor the resilience in the mounting is provided by a resilient mounting member disposed between the castor and the chassis.
14. A steerable mobile load carrier according to claim 13, wherein the or each said resilient mounting member comprises a body of elastomeric material.
15. A steerable mobile load carrier according to claim 4 or any claim dependent thereon, wherein said set of wheel elements also includes a non-swivelling wheel which is mounted on the longitudinal axis at a forward portion of the carrier chassis and at such a height that in the unloaded state of the carrier it is raised out of contact with a flat surface on which the carrier is disposed, and that it lowers into contact with said flat surface at a predetermined degree of tilting of the front swivel castors' swivel axes.
16. A steerable mobile load carrier according to claim 15, wherein the non-swivelling wheel is mounted to the chassis so that its contact point with said surface lies behind the points of contact of the front castor wheels with said surface.
17. A steerable mobile load carrier according to any preceding claim, wherein said chassis comprises a plurality of downwardly inclined chassis legs, and wherein the or each swivel castor is mounted beneath the bottom end of a respective one of said chassis legs.
18. A steerable mobile load carrier substantially as hereinbefore described with reference to Figures 4 to 6 and/or Figures 7 to 9 of the accompanying drawings.
19. A steerable mobile load carrier substantially as hereinbefore described with reference to Figures 10 and 11, or Figures 4 to 6, 10 and 11, of the accompanying drawings.
20. A castor assembly for a steerable mobile load carrier according to claim 1, said assembly comprising a swivel castor and means for resiliently mounting said swivel castor to the carrier chassis.
21. A castor assembly according to claim 20, including a pair of said swivel castors and of the associated resilient mounting means, and an elongate connecting member extending transverse to the swivel axes of the pair of castors, and rigidly interconnecting said castors for substantially equalising the inclinations of said swivel axes.
22. A castor assembly substantially as hereinbefore described with reference to Figures 4 to 6, 7 to 9 or 10 and 11 of the accompanying drawings.