CA1099863A - Castors - Google Patents

Abstract

ABSTRACT

A castor comprising a socket member defined by a base portion and a cup portion extending from the base portion, the base portion being adapted to be affixed to the underside of an article, and the cup portion defining a generally dome-shaped cavity therein. A ball is engaged in the cavity in a manner to be freely rotatable in any direction in the cavity.
The cup portion has three or more rollers rotatably mounted to the wall of the cavity and supports the ball for rotatable movement within the cavity, the rollers being supported on bearings which in turn are supported on inwardly projecting lugs integral with the cavity walls. An annular rim defines an open end of the cavity, through which a portion of the ball outwardly projects, and a plurality of lugs are arranged circumferentially within the cavity adjacent the annular rim, the lugs projecting inwardly to engage the surface of the ball, to maintain the ball within the cavity but being resiliently yieldable to permit the ball to be mounted in or removed from the cavity by forcing the ball past the lugs.

Description

The present invention relates to castors, such as may be used on domestic and office furniture, industrial equipment, trolleys and the like.
Despite design variations, the majority of present known castor constructions are characterised by certain fundamental common features. In particular, most known castor constructions comprise some sort of wheel mounted on a horizontal or inclined bearing for rolling on a horizontal surface, with the bearing supported in a member which is mounted for rotation about a substantially vertical axis so as to permit the axle of the wheel to turn freely about the vertical axis whereby the castor is free to move in any direction along the horizontal surface. One type of castor of basic construction consists of a wheel mounted on a horizontal axle which is supported on each side of the wheel in opposite arms of a fork, the fork being attached to a vertical shaft which is journalled for rotation about a vertical axis. In this type of castor, the wheel and the vertical shaft are normally in the same plane while the axle of the wheel may be slightly spaced horizontally from the axis of the shaft to impart a certain amount of stability when changing directions. Such a basic construction, while use~ul, does have certain drawbacks. For ~ample it is necessary to provide two separate bearing arrangements with their respective axes of rotation orthogonal to each other. Another drawback is that such castors do not always change direction smoothly, parkicularly in reversing direction, as will be familar to anyone who has moved furniture, e-tc., mounted on castors. Also, it is often a problem to provide adequate lubrication for the bearings.

-2-~4 Ideally, the bearings should be permanently lubricated, but this means added cost, whereas in practice any initial lubrication of the bearings, when the castors are new, does not last indefinitely and the bearings wear in a relatively short time. This results in inconvenience and a relatively high cost since, in practice, it is necessary to replace the whole castor, namely, the wheel, its mounting, and the bearings. It is impracticable, generally, to replace only the worn parts while,on the other hand, it is uncommon practice to regularly service (e.g. lubricate) castors on domestic furniture, etc. in view of the inconvenience involved. It is thus obviously desirable to deslgn a castor which has inherently a long life or, at least, can easily and quickly be repaired, worn or damayed parts replaced etc.
There i5 also a further drawback inherent in a conventional castor construction. As previously mentioned, the horizontal or generally horizontal wheel axis is normally offset from the vertical axis. This is to give the castor a certain amount of stability in changing directions since if the wheel axis were centrally disposed beneath the vertical axis for turning the castor, there is a tendency for the wheel to slide rather than to change directions when, for example, it is intended to shift castor mounted furniture suddenly through an angle of 90. On the other hand, if the wheel axis is offset from the vertical axis to provlde turning stability, this results in a sacrifice of a certain amount of load supporting stability.

~a~ 3 The reason for this is that the load (i.e. the weight o~ the trolley etc. supported on the castor) does not act vertically through the wheel axis and that in turn means that a moment is applied to the castor which undesirably affects the load bea~ingstahility of the castor. The greater the offset displacement of the wheel axis ~rom the vertical axis, the greater is the moment applied by a given load. The natural consequence o~ this is that non-axial forces are applied to the bearings, which tend to increase the rate of wear and affect the life of the castor. To overcome this problem, the castor would have to be provided with heavy duty bearings, and this of course makes the castor expensive.
Another disadvantage of the basic type of castor construction as described above, is that, as a rule, it is difficult to design such a castor so that it has at the same time both good mechanical properties and an aesthetically attractive appearance. In a more modern design, the appearance is improved by mounting the castor wheel on an axle which extends only from one side of the wheel, so that the basic "fork"
structure is replaced by an "offset axis" structure.
However such a design, though it may be more attractive in appearance, still retains essentially the same mechanical problems which are inherent in the basic construction.
In ~act in some respects the problems are increased because the bearings will have to be made stronger ~or a particular load because not only is the wheel axis horiæontally displaced from the vertical axis but also the wheel axle is asymmetrically supported on one side only and there~ore expensive bearings are r quired if a reasonably long life is to be assured.

The present invention provides a castor assembly which is capable o:E being made in a relatively simple construction, can be capable of a high load bearing capacity for a given size and weight and of a sturdy construction and is inherently much more stable than the above described conventional castor constructions. More particularly, a castor in accordance with the present invention may be broadly defined as comprising a socket member consisting of a base portion and a cup portion extending from the base portion, the base portion being adapted 10 to be affixed to the underside of an article such as a trolley or an article of furniture, and the cup portion defining a cavity therein, and a ball adapted to be received in the cavity in a manner to be freely rotatable in any direction in the cavity, the cup portion having an annular rim defining an open end of the cavity through which a portion of the ball, when it is mounted therein, outwardly projects, and means, which may be the annular rim of the cup portion or, for example, a plurality of lugs projecting inwardly towards the surface of the ball when mounted, for maintaining the ball in its mounted 20 position, said means being preferably of a material capable of being rèsiliently deformed to enable the ball to be mounted in the cavity by a force fit method and to permit the ball to be removed and replaced easily and quickly.
The castor as defined above is inherently rigid because the base portion can be rigldly fixed to the article in question and does not have to be supported on a bearing as would a mounting for a conventional castor. Therefore there is no associated bearing which is subject to wear and subsequent vibration and consequent looseness which is characteristic of conventional castors when wear takes place.

The castor is inherently stable for all mode~ of movement because the rolling member, namely the ball t which must of course support the weight of the article above it, i~
inherently so positioned that the load, i.e. the weight of the article, always acts vertically ~hrough the central axis of the ball and through the point of contac~ with a surface on which the castor may be support. Thu~ there is no turning moment which would tend to topple the castor and thu~ there are no significant side forces acting on any part of the 10 castorO In other words, all the major forces acting on the castor are predominantly vertical and this ensure~ maximum stability and load bearing capacity possible with the castorO
There is no need for any special bearings~ as in ~he case of conventional constructions, which are subject to wear and which cons~quently i5 felt a~ undesirable looseness in the castor after a given timeu Because of the inherent symmetry of the ball mounting arrangement~ the castor is not biased in any particular direction, in contrast to conventional castors. Thus an article mounted on castors of the kind defined above can be moved with complete smoothness in any horizontal direction and direction can be changed suddenly without any jerking caused by the castor or by any tendency of the castor to keep moving in a particular direction as in the case of the conventional castor~. Co~ventional castors tend to align in the direction of movement; a castor construc-ted in accordance with the present invention does nott and this is an important advantage~
One embodiment of a ca~tor in accordance with the pre~ent invention ~s illu~trated schematically in Figures 1 and 2 of the accompanying drawingst in which:

i3 Figure 1 is a cross-section through the castor and portion of an article to which it is fixed, Figure 2 is a pa~tial section illustrating one method of mounting the castor to an article.
Figure 3 is a plan view of a further embodiment as seen from below, Figure 4 is a side view of the further embodiment, Figure 5 is a vertical section showing the castor of Figure 3 mounted on an article, and Figure 6 is a partial section showing a method of mounting for the castor.
In Figure 1, an artiGle, which may, for example, be an article of furniture or a trolley, etc. has a leg diagramma-tically shown at 1 to which a castor 2 is~fixed by means of a coach screw 3. The castor 2 comprises a spherical solid phenolic resin ball 4 which may be, for example, of two inches diameter. This is trapped in a socket member 5 in the form of an inverted cup made of a plastics material and having a central well 6 in which is located the head of the screw 3.
The cavity of the cup accommodates the major portion of the ball periphery and has an outer circular rim 7, the diameter of which is marginally less than the diameter of the ball. The cup is also pro~ided with small projections 8 around the well 6. The area of contact between the ball and the cup 5 is limited to-the inside surface of the rim 7 and the projections 8.
In the arrangement shown in Figure 2, the castor is secured to the leg of an article 9 by a bolt 10 and a nut 11.

_7_ ', . ' ' , ' ' ', . ' ' ' ' In both Figures 1 and 2, the castor has a relatively wide base 12 having a flat surface 13 which engages a flat undersurface of the leg of the article 1 or 9. This pro-vides stability and strength in the castor and is particu-larly desirable where the cup 5 consists wholLy of a plastics material such as nylon. If the base 12 were of narrow cross--7a-~; .

j3 section, the castor might break during use, wherea~ this possibility is substantially avoided when the castor has a wide base.
When ~he castor is sold, the ball and cup may be provided separately. The leg of furniture to which the castor is to be fitted may be drilled if necessary to receive the screw 3 or the bolt 10 as the case may be. The castor 5 is then secured to the leg by means of the screw or bolt and the ball 4 force fitted into its captured positionas illustrated.
When the castor is in use it will be appreciated that the ball 4 can rotate in any direction so that at all times th~
weight is transmitted vertically upwards through the fixture connecting the castor to the furniture leg. Bending moments on this fixture are largely eliminated since the castor is not offset with respect to its axis as in the case of a conventional castor.
Another advantage of the castor is the ease with which the castor can be mounted to the article of furniture and the ease with which it can be replaced.
It will also be noted that the castor has no inherent tendency to align itself in any particular direction, and this greatly contributes to the ease with which furniture fitted with the castor can be moved.
The resiliency of the plastics material of the socket member allows the ball to be push fitted into position.
Advantageously the material is of the kind which has a self-lubricating property in conjunction with the ball 4, such as nylon, for e~ample. Although the castor of the embodiment described above is a considerable improvement in many respects over conventional castors, as previously pointed out, some problems were encountered during the initial design stages, when a practical design for domestic and commercial applications was sought. A ~irst problem was that the ball did not roll quite as smoothly in the cavity as one would have liked. Although the material of the socket member 5, which was of nylon and had self lubricating properties when used in conjunction with a phenolic ~all ~, friction between the bearing surfaces and the ball was found to be notinsignificant and on a smooth, low friction surface, the ball 4 could stick and consequently slide instead of rolling over that surface as intended. Many attempts have been made initially to overcome this ~roblem but most of those attempts failed to achieve entirely satisfactory results.
For example, it was thought that by replacing the bearing surfaces defined by the projections 8 with a set of ball rollers in an annular race should result in a substantial reduction in friction and thereby overcome this problem.
However this was not found to be the case at all. In fact the ball rollers were found to provide low friction in one mode only of rotation of the ball, namely, about a vertical axis, but this happens to be precisely the mode of rotation which is not required of the ball. The ball has to rotate about any one of an infinite number of hoxi20ntal axes and the ball rollers simply do not help to reduce friction for that kind of movement at all. ~nother very serious problem was found to be caused by ingress of dirt into the castor i between the cavity walls and the ball. Such dirt tended to accumulate within the cavity and increase friction and ¦ cause sticking of the ball and the dirt would become trapped .

in the cavity and the ball would periodically have to be removed to clean out the dirt. Another serious drawback was noise, which was found to occur particularly when ball rollers were used as the bearing for the ball. Another, but rather minor disadvantage was the need to provide a countersunk hole to accommodate the head of the bolt. Another problem of using balls or rollers instead of the projections 8, is that these axe awkward to use because if they are not held in the soc~et in some way they will fall out when the ball is replaced and this makes replacement somewhat inconvenient. It is also possible that ball bearings or the projections 8 may bend to damage the surface of the ball ~.
These problems, especially the problem of reducing friction to the low level desired have proved difficult to solve because although many attempts have been made to reshape the hearing surfaces, e.g. as defined by the projections 8, or by relocating them, or by replacing them with various different materials, or by replacing them by rotatable balls or rollers, have all been found to give little or no improvement and certainly were unable to provide the results which were sought.
It was obvious that if the problem could be solved, while still retaining the basic concept of the ball-mounted castor, it would be necessary to use a completely different method of supporting the ball in the cavity. Finally, it was found that the problem could 98~i~

be solved in a very simple but ele~ant manne~. More particu-larly, it wasfound that the idea of using either stationary bearing surfaces, or ball or roller bearings as previously tried, had to be rejected completely if the ultimate desired results were to be obtained,and the particular solution was found, surprisingly, in the use of rollers for supporting the ball 4, but the rollers had to be mounted on non vertical axes to engage the ball surface somewhere between the uppermost point of that surface within the cavity and the equitorial 10 region of the ball. The rollers, at least three in number, have their a~es preferably horizontal and tangential to the surface of the ball at the point where the rollers engage that surface. In this context it is assumed that the castor has a vertical axis of asymmetry and is intended to roll in a horizontal direction, and terms such as vertical and horizontal axis are to be construed in this context.
It has also been found possible to solve the problem of dirt becoming trapped within the castor. To a large extent, this problem has already been solved by abandoning the original 20 method for supporting the ball and replacing it with the rollers mentioned above. Originally, dirt tended to ingress past the projections 8, or equivalent ball bearings, into the central well 6 and, once there, would become trapped and accummulate. When the original design was modified and rollers were used, it ~as found that dirt no lonyer accummulated above the ball, because there was ample space for the dirt to move out of the cavity as well as in~ In other words, the dirt did not become trapped as previously. However, the problem was still not fully solved because, it was found, the circular rim 7 also had the effect o~ entrapping dirt within the cavity. The problem was therefore solved by providing a retaining means defined by spaced lugs, which may be of resilient plastics material, arranged about the periphery of the ball so that a free space is provided be-tween the wall of the cavity, the surface of the ball and the lugs.
Although dirt may freely enter the cavity, it is no longer trapped therein and thus the ball is not likely to stick.
There are very few moving parts required, and this is highly advantageous. Apart from the rollers, the only moving part is the ball 4 and this can freely move in any direction.
The socket member does not have to turn and therefore there is no vertical support bearing which is subject to wear.
As for the rollers, these may be mounted on pin bearings securely mounted to the body of the socket member and they do not have to withstand any particularly large forces.
Thus, such bearings are inherently capable of long life 20 without any particular maintenance. Even if wear does take place, the castor i5 self adjusting in the sense that the load applied to the castor actually tends to maintain the moving parts in firm contact with each other so that no vibration or "wobbling" is likely to occur. This is an extremely important advantage over conventional castor designs.
nother advantagej not to be overlooked, is the ease with which the ball may be replaced should it, for some reason, - become damaged. Since the ball is mere]y a press fit into the ~ .

-~9~3 socket cavlty it is merely necessary to lever the ball out of the cavity and the use of resilient retaining means, such as the spaced lugs enables this to be done ~ithout any difficulty. Alternatively, non resilient means may be used, for example, an annular cap which is applied over the ball after the ball is inserted into the socket. The castor may be made of any suitable material. For example, the socket body may be made of metal, or a plastic material, while the roller surfaces and the lugs (ball retaining means) are preferably made of a low friction, long wearing plastics material~ Phenolic resin has been found to be a particu-larly suitable material for the ball.
A preferred embodiment will now be described with reference to Figures 3 to 6.
Wherever possible, similar references are used to those used in Figures 1 and 2 to identify similar parts. Thus, in the preferred embodiment as illustrated in Figures 3 to 6, the castor comprises a ball 4 mounted in a socket member 5 which is adapted to be fixed to the underside of an article 1. Bearing support for the ball 4 is provided by a set of rollers 15, which may be symmetrically~placed about the vertical axis of symmetry of the castor and contact the surface of the ball at a position approximately midway between the top of the ball and the equatorial region thereof. The rollers 15, which are at least three in number, are -13~

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B~

mounted on pin bearings 16 which are supported in a suitable manner in lu,gs 17 projecting inwardly of the wall of the cavi~y ~ontaining the ball. In the example illu~trated in Figure 3, there are six rollers 15 supported in pairs arranged symmetrically about the vertical a~is of the ball.
It has been found that this arrangement provides smoother and quieteroperation than does one using only three rollers.
The roller surfaces are composed of a relatively hard, low coefficient of friction plastics, material.

To retain the ball in the cavity, there are provided six symmetrically placed lugs 18 which project inwardly towards the ball 4 from a lower rim of the socket member, the lug 18 being of a yieldable resilient plastics material so as to hold the ball in place while permitting the ball to be removed and replaced, if necessary, using a suitable tool for forcing the ball past the lugs. The lugs may be integral with or separately attached to the socket member 5, which may be of, for example, a cast metal or plastics material. "
The socket member has a general dome shape and has an aesthetically attractive appearance. A hole through the centre of the base 12 receives a screw 3 for fastening the castor to the underside of an article 1. Advantageously, ribs are cast into the socket member, on the inside thereof, such as ribs 19 formed in the hase of the socket member and ribs 20 formed on the cavity wall, so as to provide rigidity combinecl with low weight and minimum use of material. The castor as a whole has exceptional strength and stability for its we1ght, especially when compared with conventional castors. As previously mentioned, there are no siqnificant bending moments acting on the castor and thus stability and long life are assured.
Another feature which may be mentioned is that the lugs 18 preferably do not extend below the bottom edge of the annular rim 7a ~see Figure 5). If the ball is temporarily removed while the castor is affixed to the article 1 (there is no need to remove the socket member when replacing the ball) the castor can be allowed to rest with its annular rim 7a supported on ~he horizontal surface without danger of bending or damaging the lugs 18.
Referring now to Figure 6, this shows one method of attaching the castor to an article 1. In this case, the central bore in the base of the castor is aligned with a bore drllled through a bracket at the bottom of the article 1, and a bolt 10 passes through the aligned bores and fixes the castor with the bracket by means of a nut 11 threaded on the bolt.
It will be appreciated that modifications may be made to the embodiment described. For example, in the embodiment the socket member comprises a frusto conical base portion 12, a cylindrical portion 5a surrounding the ball and an intermediate frusto conical portion 5b, however other shapes as, for example, rounded, hemispherical, polygonal etc. may be used.

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Also, the position and number of the rollers 16 can be varied. By comparison, in the embodiment shown in Figure 1 (not preferred) the rim 8 has to be relatively close to the top of the ball 4, approximately as shown, for lowest friction but this means that there is a tendency for the ball to jump out of the socket due to forces acting on the ball when rolling on a horizontal surface. ~owever in the preferred embodiment illustrated in Figs. 3-6, the position of the rollers 16 can be made closer to the equatorial region of the baIl so as to virtually eliminate any tendency for the ball to jump out as a result of side forces acting on it.
At the same time low rolling friction is substantially retained and the operation has been found to be very smooth.
The optimum position (as viewed in vertical section) of the rollers appears to be one which is slightly closer to the equator of the ball than to the top of the ball. As viewed in horizontal section (Fig. 3) the rollers may be arranged in pairs (as shown) or may be distributed, for example, equally about the vertical axis of the ball. The axes of the rollers may be tangential with respect to the surface of the ball or inclined somewhat.
Because of its symmetry, the castor described herein eliminates a problem o~ most conventional castors, namely `
one concerned with their replacement. Conventional castors are generally left handed or right handed, and thus are sold in pairs. If one castor of a pair has to be replaced it may be necessary to purchase a pair of castors even though one of those is not required. Even if individual (conventional) castoxs can be obtained it is still necessary to manufacture left and right handed castors and retailers would have to stock both kinds. This problem does not apply with castors of the present invention which are readily inter-chanyeable.

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Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A castor adapted to be connected to the underside of an article, comprising:
a socket member comprising a base portion and a cup portion extending from the base portion, the base portion being adapted to be affixed to the underside of the article and the cup portion defining a generally dome-shaped cavity therein;
a ball mounted in the cavity and freely rotatable in any direction in the cavity;
six equally spaced rollers arranged uniformly about the wall of the cavity in a plane transverse to the axis of the cavity so that the rollers are engageable with the ball and support the ball for rotatable movement within the cavity, each roller being supported on a bearing which in turn is mounted to the cavity wall, the rotational axis of each roller being generally parallel to the tangent at the point of contact of the respective roller with the surface of the ball and being oblique to the axis of the cavity, the points of contact between the rollers and the ball being closer to the equator than to the pole of the ball;
an annular rim of the cup portion defining an open end of the cavity through which a portion of the ball, when it is mounted in the cavity, outwardly projects; and, retaining means adjacent the annular rim to maintain the ball within the cavity.

2. A castor as defined in claim 1 wherein the retaining means comprises a plurality of inwardly projecting lugs arranged circumferentially within the cavity, the lugs being adapted to engage the surface of the ball when the ball is mounted in the cavity and being resiliently yieldable to permit the ball to be inserted into or removed from the cavity by forcing the ball past the lugs.

3. A castor as defined in claim 2, wherein the lugs are arranged inwardly of the annular rim so as not to project beyond the rim.

4. A castor as defined in claim 1, wherein each roller is generally disc-shaped with the peripheral thick-ness of each roller being less than the thickness inwardly of the respective roller periphery.

5. A castor as defined in claim 1, wherein said roller bearings comprise pin bearings supported by bearing lugs projecting inwardly from the cavity wall.

6. A castor as defined in claim 5, wherein each roller is generally disc-shaped with the peripheral thick-ness of each roller being less than the thickness inwardly of the respective roller periphery.