GB2286523A - Castors for trolleys - Google Patents

Abstract

The invention relates to a castor for mounting a wheel 30 on a trolley, the castor being biased towards a single stable position on its swivel axis. Preferably such castors are used to mount the rear two wheels of a trolley with the wheels 30 aligned for forward motion of the trolley i.e. with the wheel trailing. This enables the trolley to be steered more easily. For biasing, a cam follower roller 16, associated with the filling for connecting to the trolley, is biased against a cam surface 22 on the filling 20 for mounting the wheel 30. The cam surface 22 may have a detent 22a at a position corresponding to the wheels preferred position and be circular around the rest of its circumference. The force biasing the cam follower roller 16 may be increased with the compressional force between the fillings by way of a bell crank lever mounted to the trolley filling, with one arm contacting the other filling and the other arm being arranged to compress the spring 15 which provides the force biasing the cam follower roller 16 against the cam surface 22. <IMAGE>

Description

CASTORS FOR TROLLEYS The present invention relates to a castor for rotatably mounting a wheel to a trolley. In particular, the castor is biased to favour a particular alignment of the wheel relative to the trolley.

Trolleys, such as supermarket trolleys, are often provided with four wheels all mounted by a castor which allows the wheel to pivot in all directions. As compared to fixing the alignment of some or all of the wheels, this makes the trolley more stable and more mobile. If a force is applied to a trolley in a direction transverse to a wheel, a fixed wheel will tend to topple making the trolley unstable, or else the wheel will have to scrape along the floor. On the other hand, a castored wheel will pivot round until the wheel may run freely. Thus if all the wheels of a trolley are castored then the trolley is more mobile because it may be moved in any direction.

In some circumstances, castored wheels can make the trolley difficult to steer because torque applied to the trolley tends to cause the trolley merely to spin about its centre of mass. Thus, when a steering force is applied to a moving trolley, the trolley tends only to rotate about its centre of mass but to continue moving in the direction of its linear momentum with an altered orientation. Because castored wheels follow the force applied to a trolley, in order to change the direction of movement, it is necessary to perform an amount of work equal to the force required to move the entire mass of the trolley about part of the circumference of a circle. In the case where a person is pushing the trolley, the radius R of this circle is equal to the distance between the centre of mass of the trolley and the person's body.This is illustrated in Figure 1, where R is the distance between the person's body 1 and the centre of mass 2 of the trolley 3. The work required thus increases with the weight of the trolley and it can be quite tiring for a person pushing the trolley. A shopping trolley, for example, weighs approximately 20kg when empty but with shopping this can easily rise to 40kg or 50kg which is a considerable load and will require the person pushing the trolley to use both arm and stomach muscles.

An analogous problem is encountered when a trolley is to be manoeuvred on a slope. The weight of the trolley tends to force the trolley down the slope and the castored wheels merely pivot to allow it. The effort required to control a shopping trolley on even a slight slope is considerable.

Some trolleys are constructed with the rear wheels 4 fixed in alignment with the forward direction of the trolley and with the front two wheels 5 castored. With this arrangement the steering effort required to turn in direction A is reduced because the trolley is steered about a point 6 on a line passing through the two fixed wheels 4. However this arrangement does not provide the advantages associated with all the wheels being castored, as discussed above.

According to the present invention there is provided a castor for pivotally mounting a wheel on a trolley, the castor comprising first and second fittings rotatably mounted together, one fitting being for connecting to a trolley and the other fitting being for supporting a wheel, and the first and second fittings being relatively rotationally biased such that there is a single stable relative position of the two fittings on either side of which the cam surface and follower relatively rotationally bias the two fittings towards the stable relative position.

Such a castor may be used on a trolley orientated so that the stable position corresponds to the wheel being aligned for forward motion of a trolley, i.e. with the wheel trailing. The castor is biased to return the wheel to that position. Preferably, such a castor is used for the rear wheels only of a four wheel trolley. Because the rear wheels are partially constrained the steering effort is reduced in the same manner as for a trolley with fixed rear wheels.

Preferably the first and second fittings are relatively rotationally biased by means of a cam follower roller associated with the first fitting, said follower being biased against a cam surface which continuously extends on the second fitting at least part way around the axis of rotation.

Advantageously, the cross-section the cross-section of the cam surface partly follows a circle centred on the axis of relative rotation.

Preferably with this arrangement, the circular region where no relative rotational biasing force is produced may extend around most of the can surface. Thus, after sufficient force has been applied to relatively rotate the fitting from the stable position to a position where the cam follower roller is biased onto this region, then, until the wheel is returned into alignment with the forward motion of the trolley, the wheel may be pivoted in any direction allowing the trolley to be easily manoeuvred. A similar effect may be achieved with other biasing arrangements.

In order that the invention may be better understood the following description is given, by way of example only, with reference to the accompanying drawings in which: Fig. 1 shows a top view of a person pushing a trolley; Fig. 2 shows a vertical cross-section through a castor according to the present invention; and Fig. 3 shows a horizontal cross-section taken through line 1 of the embodiment of Fig. 2; As shown in Figs. 2 and 3, a wheel 30 is rotatably mounted on an arm 29 which is mounted on a leg 10 protruding down from the bottom of a trolley (not shown). The arm 29 is connected to a cylindrical body 20 which is mounted in a cylindrical recess 18 defined by a rim 11 protruding down from the base 10a of the leg 10. Acting as a bearing, there is a continuous circular ball race 12 between the body 20 and the outer rim 11.

In the top of the body 20 there is a recess 21 defined by a vertical wall 22 extending entirely around the axis of relative rotation of the body 20 and the leg 10, and by a flat base 23. This wall 22 acts as a cam surface.

Into the recess 21 of the body 20, a block 13 protrudes from inside the recess 18 at the base 10a of the leg 10, the block 13 being cylindrical except for a recess 14. The recess 14 accommodates a cam follower roller 16 (i.e. a roller 16a mounted in a housing 16b) and constrains it to move radially (i.e. linearly) relative to the axis of rotation. A compression spring 15 between the cam follower roller 16 and the block 13 biases the cam block 16 outwardly against the cam surface 22.

It can be seen from Fig. 3 that the cam surface is inclined so as to guide the cam follower roller 16 smoothly in and out as the body 20 rotates relative to the leg 10.

In the embodiment shown in Fig. 3, the cam surface follows a circular line 22b around most of its circumference except for a single detent 22a. Thus the cam follower roller may move out beyond the level of the circular line 22b in only this one region. When the cam follower roller 16 is fully in the detent 22a, the body 20 and the leg 10 are in a stable position relative to one another. That is to say, if the body 20 and leg 10 are relatively rotated through a small angle from the stable position, the cam surface 22 will force the cam follower roller 16 further into the recess 14, thereby compressing the spring 15. This compression will generate a reaction force between the cam follower roller 16 and cam surface 22 which tends to return the body 20 to its stable position relative to the leg 10.

However, when sufficient force is applied, the body 20 and the leg 10 will be relatively rotated such that the cam follower roller 16 is forced out of the detent 22a to be biased against the portion of the cam surface 22 following the circular line 22b. Here, the reaction between the cam surface 22 and the cam follower roller 16 is wholly radial and does not tend to rotate the body 20 relative to the leg 10. The detent 22a and cam follower roller 16 may be shaped such that the cam follower roller must be "clicked" out of the detent.

With the detent, in use, whilst the wheel is held with the cam follower roller 16 in the detent 22a, the required steering effort is reduced. However by applying sufficient force sideways to the trolley the cam follower roller 16 may be forced out of the detent 22a. Then the wheel is free to pivot in any direction until cam follower roller 16 is returned into the detent 22a.

Instead of there being a detent, the cam surface may be inclined around its entire length, for example as shown by the dotted line 25 in Fig. 3. It may be a circle which is eccentric to the axis of relative rotation. With such a cam surface, the body 20 and leg 10 are relatively biased towards the stable position regardless of their relative position. With this arrangement, in use, the steering effort is reduced because the rear wheels are biased against merely following the direction of a force applied sideways to the trolley.

In an alternative embodiment, a bell crank lever may be hinged to the block 13, and have a first arm which contacts the flat base 23 of the recess 21 in the body 20.

The second arm would be arranged against the end of the spring 15 which is not contacting the cam follower roller 16. With this arrangement, when the leg 10 and the body 20 are compressed together the second arm further compresses the spring 15, thereby increasing the biasing force on the cam follower roller 16. In use, this compressional force between the body 20 and the leg 10 increases with the weight of the trolley supported by the leg 10. That is, the bell crank lever in effect supports a proportion of the weight of the trolley. Though more complicated, this arrangement is particularly advantageous, because it increases the biasing to maintain the wheel in alignment with the forward direction of the trolley as the trolley becomes more heavily laden. This is advantageous because the aforementioned advantages of the invention are maximised with a heavy trolley which is potentially more difficult to control, in the manner as discussed above.

Alternatively the biasing between the leg 10 and the body 20 may be provided by means of cam surfaces therebetween acting in the manner of a rising butt hinge.

Claims (13)

1. A castor for pivotally mounting a wheel on a trolley, the castor comprising first and second fittings rotatably mounted together, one fitting being for connecting to a trolley and the other fitting being for supporting a wheel, and the first and second fittings being relatively rotationally biased such that there is a single stable relative position of the two fittings on either side of which the cam surface and follower relatively rotationally bias the two fittings towards the stable relative position.

2. A claim according to claim 1,wherein the first and second fittings are relatively rotationally biased by means of a cam follower associated with the first fitting, said follower being biased against a cam surface which continuously extends on the second fitting at least part way around the axis of rotation.

3. A castor according to claim 2, wherein the cross-section of the cam surface is partly circular and centred on the axis of relative rotation.

4. A castor according to claim 2 or 3, wherein the cam follower is biased by a compression spring arranged between the cam follower and the first fitting.

5. A castor according to any one of the preceding claims, wherein the castor is constructed such that the biasing force acting on the cam follower is dependent on the compressional force acting between the two fittings.

6. A castor according to claims 4 and 5, wherein a bell crank lever is hinged on the first fitting with one arm engaging the second fitting and the other arm arranged to further compress the compression spring if the compressional force between the two fittings is increased.

7. A castor according to any one of the preceding claims, wherein the first fitting is for connecting to a trolley and the second fitting is for supporting a wheel.

8. A castor according to any one of the preceding claims, wherein the one of the fittings is mounted in a recess in the other fitting around which recess a circular ball race acts as a bearing between the two fittings.

9. A castor according to any one of the preceding claims, wherein the cam follower is accommodated in a cylindrical protrusion extending from the first fitting into a recess in the second fitting, the cam surface being the surface around the recess.

10. A castor according to any one of the preceding claims, wherein the cam follower is a roller contacting the cam surface.

11. A castor constructed and arranged substantially as hereinbefore described with reference to figures 2 to 4 of the accompanying drawings.

12. A trolley having at least two wheels, at least one of the wheels being mounted on a castor as defined in any one of the preceding claims.

13. A trolley having four wheels, the rear two wheels being mounted on castors as defined in any one of claims 1 to 8.