GB2103923A - Twin wheel castors - Google Patents

Abstract

A castor has a body portion (10) which carries twin wheels (11, 12) for rotation about a first axis (13), and which is pivoted about a pivot axis (18) lying in a plane outside the vertical plane containing the wheel axis; wherein the body portion (10) has a ramped extension (21) projecting therefrom on the opposite side of the pivot axis from the first axis, which extension enables the castor to slide up over obstacles which might otherwise have caused tipping of the article on which the castor is mounted. <IMAGE>

Description

SPECIFICATION Twin-wheel castor This invention relates to a twin-wheel castor, and particularly to such a castor which comprises a body portion, a pair of wheels mounted on the body portion for rotation about a first axis, and a pivot arrangement for enabling the castor to swivel about a pivot axis lying outside the vertical plane containing the first axis.

Castors of this kind are described in UK patent specification no. 1 270 561. Some of the advantages of twin-wheel castors over singlewheel castors are that they enable heavier items to be moved by virtue of the differential effect obtained between the two wheels of each castor, which results in easier pivoting and reduced wear and tear of floor coverings. By positioning the pivot in a body portion lying generally between the two wheels, it is possible to reduce the total height of the castor assembly compared with single-wheel assemblies.

Despite these significant advantages, twinwheel castors of the known kind still have certain disadvantages, in particular their tendency to jam of stall when they encounter obstructions of certain sizes on the floor. For example, when the castors are used for relatively heavy office machines, such as copying machines which may have a fairly high centre of gravity, there may be a tendency for the machine to topple over about the castors if the castors encounter certain kinds of obstructions when the machine is being propelled at a speed around walking pace. For example, with castor wheels of about 6 cm in diameter, there is a likelihood of toppling if the wheel strikes an obstacle at about its mid-height, i.e., an obstacle about 3 cm high.

The present invention is intended to provide a twin-wheel castor in which this disadvantage is overcome, and is characterised in that the body portion has an extension projecting therefrom on the opposite side of the pivot axis from the first (wheel) axis, preferably, the extension has a ramp on the underside thereof sloping upwardly away from the vicinity of the ground-contacting parts of the wheels.

The extension will tend to precede the wheels since the wheel axis will trail behind the pivot axis as the castor is moved over the floor, and the extension is on the opposite side of the pivot axis from the wheel axis. This provides the advantage that if the extension encounters an obstacle, the impact point is further forward (in the direction of motion), making it less likely that the centre of gravity of the machine will approach a position above the impact point, thus reducing the likelihood of toppling. Furthermore, if a very small obstacle, i.e., one of lower height than the lowest part of the extension, should cause the wheel to jam, then the extension forms a lever which helps to prevent tipping.

The ramped extension provides the added advantage that it tends to ride up over obstacles which are lower in height than the height above the floor of its leading portion.

A twin-wheel castor in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side elevation of the castor; Figure 2 is a plan view; Figure 3 is a cross-sectional view on the line 3-3 of Figure 1; and Figure 4 is a perspective view of the castor.

Referring to the drawings, the twin-wheel castor includes a body portion 10 part of which extends between wheels 11 and 12. Wheels 11 and 12 are mounted for rotation about an axis 1 3 by means of axles 13 and 1 5 that are carried by body portion 10. The body portion and the wheels may be made of any suitable material such as metal or impact resistant plastics material, e.g., Nylon (registered trademark) or Celcon (registered trademark). The outer rims of the wheels 11 and 12 are chamfered, as shown at 16, 17, to allow easier swivelling.

The castor is mounted on a pivot arrangement (not shown) about a pivot axis 1 8 that extends vertically through the body portion 10 at a position close to the circumference of the wheels 11 and 1 2. The pivot arrangement is located in a socket 19 in the body portion 10. The pivot arrangement is in the leading part of the body portion 10 since, when the castor is in use, the wheels will trail behind the pivot arrangement.

The body portion 10 extends over most of the area between the wheels 11 and 12 apart from the area near the lowermost, ground contacting, parts of the wheels. The part of the body portion 10 adjacent the trailing edges of the wheels is shown as being of slightly greater diameter, as indicated at 20, than the wheels. Although it is not essential to have this large diameter portion, it gives some measure of protection to the castor wheels by encountering obstructions before the wheels if the castor is moving oppositely to its normal direction of motion. The larger diameter portion may also be provided with a sideways extension (not shown) to form a hood which further protects the wheels and helps to prevent the ingress of dust.

From the leading part of the body portion 10, an extension 21 projects in front of the wheels.

The extension 21 shown in the drawings is of ramped configuration. Other configurations of extension are also possible, such as a generally rectangular extension, with its lower surface horizontal. In the case of a ramped extension, the ramp may be linear, as shown, or may be of arcuate shape, for example downwardly convex.

In the ramped extension shown in the drawings, the top surface 22 of the extension is horizontal, and starts close to the highest part of the wheels. The underside 23 of the extension is a sloping surface which starts close to the points where the lowest part of the body portion 10 intersects the leading edges of the wheels. The underside 23 of the extension slopes up at a suitable angle, which may be up to about 450 from the horizontal. In the case of a rectangular extension, of course, the lower surface is horizontal, i.e., has a slope of 00. Typically, the ramp may have a slope of between 15" and 30".

That shown in the drawings has a slope of about 220. The extension projects in front of the wheels by any suitable distance depending on the size and the weight distribution of the article on which the castor is mounted, and on the slope of the ramp, if any. In the castor shown in the drawings, the extension projects in front of the wheels by a distance rather greater than the diameter of the wheels. The front 24 of the extension 21 is of semi-cylindrical shape.

Although the body portion 10 and its extension 21 may be formed from a solid piece of material, it is conveniently made with internal cavities 25, as indicated in Figure 3, for example by well known metal casting or plastics moulding techniques.

This gives economies of material while retaining good appearance and structural strength.

In use, the extension 21 points in the direction of motion, as indicated above, thereby enabling the extension to encounter any obstacles of height less than its own height. The impact point is accordingly moved further forward than if the extension were not there, with the effect that the centre of gravity of the article on which the castor is mounted has further to travel before arriving at a position vertically above the impact point. This reduces the likelihood of toppling. In the event that the wheels are prevented from turning, for example, by an obstacle of small height, but of such a shape as to stop the wheels, the extension 21 will help to prevent tipping of the article on which the castor is mounted, since after a limited amount of tipping, the extension 21 will engage the ground.

In the case of the ramped extension, the ramp will engage any obstacle of lower height than the highest part of the underside 23, and will, if the momentum of the article being moved is sufficient, allow the castor to slide up the obstacle until the wheels engage it and roll over it.

Claims (5)

1. A twin-wheel castor comprising a body portion, a pair of wheels mounted on the body portion for rotation about a first axis, and a pivot arrangement for enabling the castor to swivel about a pivot axis lying outside the vertical plane containing the first axis, characterised in that the body portion has an extension projecting therefrom on the opposite side of the pivot axis from the first axis.

2. The castor of claim 1 wherein the extension has a ramp on the underside thereof sloping upwardly away from the vicinity of the groundcontacting parts of the wheels.

3. The castor of claim 2 wherein the ramp is a linear ramp.

4. The castor of any one of claims 1 to 3 wherein the extension projects forwardly of the wheels by a distance of at least the diameter of the wheels.

5. The castor of any one of claims 1 to 4 wherein the ramp is sloped at between 1 50 and 300 to the horizontal.