GB2062456A - Load carrying braked castor - Google Patents

Abstract

A load carrying braked castor in which the wheel is biased into surface to surface frictional engagement with the axle shaft so that the axle shaft exerts frictional drag on the wheel, the arrangement being such that the frictional drag is greater when the castor is unloaded than when it is loaded, whereby the wheel runs more freely when a load is applied to the castor. <IMAGE>

Description

SPECIFICATION Load carrying braked castor The present invention relates to a load carrying braked castor of the type which will run more freely when a load is applied to the castor than when it is unloaded.

This type of castor is particularly necessary and desirable for use on lightweight office chairs to ensure that the chair does not accidentally move away from the user as the user sits on the chair. If this occurs and the chair runs away as the user is sitting down it can be dangereous and result in serious injury to the user. There is therefore a need for a braked castor which will not run away easily when it is unloaded but once loaded will run relatively freely to give easy movement of the chair when it is occupied.

It is therefore an object of the present invention to provide a load carrying braked castor in which the wheel is biased into surface to surface frictional engagement with the axle shaft so that the axle shaft exerts frictional drag on the wheel, the arrangement being such that the frictional drag is greater when the castor is unloaded than when it is loaded, whereby the wheel runs more freely when a load is applied to the castor.

In accordance with the present invention we therefore provide a braked castor comprising a castor body, means on the castor body for attaching thereto a load to be carried by the castor, a fixed axle shaft mounted on the castor body, a wheel mounted for rotation on the axle shaft and spring means normally biassing the axle shaft upwardly relative to the wheel to increase the frictional drag between the shaft and the wheel and depressible in response to a load applied to the castor body to reduce the said frictional drag.

Preferably the area of surface contact between the spring means and the wall of the axle bore in the wheel is normally positioned diametrically opposite to the area of surface contact between the axle shaft and the wall of the axle bore and is contained wholly within the area of surface contact between the axle shaft and the wall of the axle bore when the axle shaft is depressed in response to loading ofthe castor body.

A preferred form ofthe present invention will now be described with reference to the accompanying drawings, in which: Figure 1 is an elevation of a braked castor according to the present invention; Figure 2 is a plan view from below of the braked castor of Figure 1; Figure 3 is a section taken on the line Ill-Ill of Figure 1 showing the castor in an unloaded condition; Figure 4 is a view similar to Figure 3 but showing the castor in a loaded condition; Figure 5 is a section taken on the line V-V of Figure 3; Figure 6 is a section taken on the line VI-VI of Figure4; Figure 7 is an elevation of the axle shaft of the castor; Figure 8 is an end view of the axle shaft; Figure 9 is an elevation ofthe spring which forms a part of the castor; and Figures 10 to 13 illustrate four alternative forms of spring for the castor.

In the drawings a braked castor is indicated generally at 10 which comprises a castor body 11, a wheel 12 and an axle shaft 13.

The castor body comprises a moulded shell of a suitable synthetic plastics material and includes a vertical bore 14to receive a spigot, which is not shown, for mounting the castor 10 on an article of furniture.

The axle shaft 13 is preferably formed from steel and is grooved at its opposite ends to form similar shaft heads 15 and 16. The shaft heads 15 and 16 are mounted slidably in T-section grooves 17 and 18 respectively which are formed in the side walls of the shell 11 in a well known manner. Preferably the axle shaft 13 is a snap-fit within the grooves 17 and 18 for ease of assembly.

The wheel 12 may be of a unitary construction or it may comprise two similar half shells 12a and 12b, as shown. The wheel is formed with an axial through bore 19 through which the axle shaft 13 is located.

The diameter of the bore 19 in relation to the diame ter ofthe axle shaft 13 is such that there is a small clearance 'a' between the axle shaft and the wall of the bore so that the wheel can rotate freely on the axle shaft, when it is not subjected to any bias or interference.

The axle shaft 13 is formed with an axially extending slot 20 which houses a leaf spring 21. The leaf spring 21 is formed from a synthetic plastics material and is of a sinuous shape. When in a relaxed condition, the depth of the leaf spring 21 is substantially greater than the combined depth of the slot 20 and the clearance 'a' so that the leaf spring is permanently under compression and portions of the leaf spring project outwardly from the slot 20 and exert a biassing force on one side of the wall of the bore 19 in the wheel 12. This biassing force urges the opposite side of the wall of the bore 19 into contact with the surface of the axle shaft 13 over a major part of the circumference of the axle shaft, as shown in Figures 3 and 5.

In order to locate the leaf spring permanently on the opposite side of the wheel to that from which a load will be applied to the wheel, the leaf spring is provided at one or both of its ends with a projecting lug 22 of greater width than the slot 20. The lug 22 is of a shape and size such that it will fit snugly in the T-section groove 17 so as to correctly position and orientate both the axle shaft 13 and the leaf spring 21.

In use, when there is no load applied to the castor 10, the leaf spring 21 biasses the wheel 12 downwardly so that the clearance between the axle shaft and the bore 19 is eliminated at a position diametri cally opposite to the leaf spring 21 thereby setting up substantial surface to surface frictional engagement between the axle shaft and the wall of the bore over an arc of approximately 900. In addition, the spring 21 is also in frictional engagement with the wall of the bore. In this condition, the frictional drag on the wheel is sufficient to prevent the wheel from running freely and the risk of accidental movement of an article of furniture carried by the castor 10 is substantially reduced.

If a load is now applied to the castor 10, as shown in Figures 4 and 6, the load is transmitted through the shell 11 to the axle shaft 13 which is depressed downwardly against the bias of the leaf spring 21.

The leaf spring is thus flattened so as to be contained wholly within the slot 20 and a clearance is opened between the axle shaft and the bore 19 in the region diametrically opposite to the leaf spring 21. The frictional drag between the material of the wheel 12 and the material of the axle shaft 13 is thus substantially reduced because the spring is wholly contained within the area of surface to surface engagement between the shaft and the wheel thus reducing the total area of engagement and thereby the frictional drag on the wheel. Consequently, when a load is applied to the castor 10, the frictional drag exerted on the wheel by the combination of the axle shaft and the leaf spring is substantially reduced and the wheel will run more freely on the axle shaft.

It will be appreciated that the affect of the leaf spring 21 is that, when there is no load applied to an article of furniture on which the castor 10 is mounted, the article of furniture cannot be accidently moved because of the frictional drag exerted on the wheel by the axle shaft 13. However, when a sufficient load is applied to the castor 10, for instance by a person sitting in a chair mounted on a plurality of the castors 10, the frictional drag exerted on the wheel is reduced and the chair can be freely rolled along the ground by the occupant.

The spring 21 can take many forms provided that, when unstressed it will project from the slot 20 so as to bearthe wheel downwardly and can be compressed so as to be contained wholly within the slot when a sufficient load is applied to the castor shell.

In Figures 1 to 6 the spring 21 is sinuous in shape having two similar bowed portions 25 and 26 adjacent its ends and a central raised bowed portion 27.

Some alternative spring shapes are shown in Figures lotto 13. Figure 10 illustrates an alternative form of spring 28 which has a generally flat central portion 29. Figure 11 illustrates a spring 30 which is similar to the spring 28 except that the two bowed end portions 31,32 are spaced from the ends of the spring both of which are generally flat. Figure 12 illustrates a spring 33 which is bowed in a regular wave form but which terminates in a flat end portion 34. Figure 13 illustrates a spring 35 which is similar to the spring 33 except that the wave form extends over the whole length of the spring so as to eliminate the flat end portion.

In order to increase the braking effect on the wheel, the spring is preferably made from a material which will exert a greater frictional drag on the material of the wheel than that exerted by the material of the shaft.

Claims (14)

1. A braked castor comprising a castor body, means on the castor body for attaching thereto a load to be carried by the castor, a fixed axle shaft mounted on the castor body, a wheel mounted for rotation on the axle shaft and spring means normally biassing the axle shaft upwardly relative to the wheel to increase the frictional drag between the shaft and the wheel and depressible in response to a load applied to the castor body to reduce the said frictional drag.

2. A braked castor as claimed in claim 1, wherein the area of surface contact between the spring means and the wall of the axle bore in the wheel is normally positioned diametrically opposite to the area of surface contact between the axle shaft and the wall ofthe axle bore and is contained wholly within the area of surface contact between the axle shaft and the wall of the axle bore when the axle shaft is depressed in response to loading of the castor body.

3. A braked castor as claimed in claim 2, wherein the spring means and the shaft are keyed to the castor body so as to be fixed relative to the castor body.

4. A braked castor as claimed in any preceding claim, wherein the spring means and the shaft are made from materials such that the material of the spring means exerts less frictional drag on the wheel than does the material of the shaft.

5. A braked castor as claimed in any preceding claim, wherein the axle bore in the wheel is cylindrical and substantially greater in diameter than the shaft and the spring means is located between the wall of the axle bore and the shaft.

6. A braked castor as claimed in any preceding claim wherein the spring means comprises a leaf spring.

7. A braked castor as claimed in claim 6 wherein the leaf spring is partially located in a slot in the shaft.

8. A braked castor as claimed in claim 7 wherein the leaf spring is formed with at least one lug which is keyed to the castor body.

9. A braked castor as claimed in claim 7 or claim 8 wherein the leaf spring is made from a synthetic plastics material.

10. A braked castor as claimed in claim 8 wherein the leaf spring has an sinuous shape.

11. A braked castor as claimed in any preceding claim wherein the castor body comprises a moulded shell having side walls and the ends of the axle shaft are non-rotatably attached to and supported by the side walls.

12. A braked castor as claimed in claim 11 wherein the side walls are formed with slots and the ends of the shaft are mounted in the slots.

13. A braked castor as claimed in any preceding claim wherein the wheel is in two parts mounted for independent rotation on the axle shaft.

14. A braked castor substantially as described herein with reference to Figures 1 or 9 or any of Figures lotto 13.