GB2428000A - Adjustable castor for wheelchair - Google Patents

Abstract

A castor assembly has a wheel (3) that is movable, with respect to the castor axis and spigot mounting means (1) , in a horizontal plane. The wheel may be positioned in a fore and aft direction via an axle (5) into a series of holes that are provided on the castor fork (20, see Fig 3. The wheel may also be moved in both horizontal and vertical directions via a plurality of holes disposed in a circular manner on the fork, the wheel thus moves in an eccentric manner see Fig 5, a locating disc (4) may be provided to add strength to the assembly. In this embodiment alteration of the wheel position also changes the height of the wheelchair. Alteration of the wheel position with respect to the castor axis may change the wheelbase of the wheelchair and thus increase / reduce the stability.

Description

1 2428000 Wheelchair castor with adaptable forward stability This

invention relates to a wheelchair castor that has innovative features to improve forward stability, for the safety of the occupant in the intended environment of use.

Stability of wheelchairs generally is an important safety concern. On state of the art modular wheelchairs it is possible to improve operation and rearwards stability by tuning the wheelchair rear wheel axle centre position according to the centre of gravity and propulsion skills of the occupant. In currently applied best practice, this is achieved through optimising the position of the rear wheel in relation to the occupant centre of gravity, to balance occupant propulsion effort with the tendency of the wheelchair to tip over in a rearwards direction.

However, wheelchair occupants come in a variety of different shapes and sizes, and their needs change with time. A modular wheelchair with the possibility of just changing rearwards stability does not always address the changing or assessed needs of many people whose centre of gravity and increased risk of tip out, is biased in the forwards direction.

Changes in the forwards centre of gravity could be due for example to; a natural onset of weight increase; or inclusion of a back support which pushes the occupant seating position forwards; or addition of an accessory such as a loaded tray. In a static stability test these circumstances can move the overall centre of gravity sufficiently forwards to make the wheelchair unstable in a forwards direction and unsafe on a slope. The theoretical reasons can be explained as follows:- When the vertical line through the centre of gravity of the whole wheelchair loaded system passes the point of contact ( POC) of the front castor wheel, the wheelchair will tip over forwards. This typical situation is illustrated in FIG 1.

In a typical outdoor environment, a wheelchair travelling down a slope less than the theoretical test limit of static stability could tip over below this theoretical static angle limit due to dynamic effects such as impact with a surface object, or undulation. Guidance document from UK MHRA reference DB2004 (02) advises on wheelchair stability issues.

Wheelchair castors swivel about a central pivot position that is vertically mounted and aligned on the wheelchair frame. Castors are comprised of a wheel, which is mounted within a fork. Castor forks have a trailing angle, which allows them to swivel about this vertical centre line, providing manoeuvrability for turning and directional control. It is practice on some castor forks to have different wheel positions at set heights. This is effected by holes in the fork, to suit the fitting of wheels of different diameter, or to change the seat to ground height of the front of the wheelchair. A wheelchair which has smaller and lower seat to ground height will have increased all round stability because of the lowering of the occupant centre of gravity, but this type of configuration can create postural compromises with the preferred seating and leg angle positions. With this existing technology, the choice of wheel position in a castor fork does not allow for use of the same diameter wheel, and originally preferred, or thereabouts, seat to ground height, to improve forward stability. Current best assessment practice for changes in forwards weight distribution may recommend the fitting of forwards mounted anti tip stabilisers or a forwards out rigged adaptation to the frame mounting to move the castor forwards. These adaptations can however create different safety problems of requiring additional space to manoeuvre the wheelchair, and they also increase the possibility of mechanical failure, through increased component and structural complexity, and they have the potential to include unreliable construction techniques, which the original wheelchair manufacturer would be unable to support under original product warranty.

This invention will now be described in detail, with reference to the illustrations.

A modular castor integrated facility of an improved castor construction, facilitating the adjustment of the castor axle centre position, to change and improve the stability and the factor of safety, of the wheelchair in a forwards direction. This effect can be seen in FIG 2, which shows the change of stability angle using the new concept castor.

This invention provides the facility to adjust the position of the castor wheel axle, and thus the trailing angle, of the castor fork, such that the castor wheel point of contact ( POC) with the floor has a chaice of at least one additional forward position, measured when the wheelchair is travelling in the forwards direction. By moving the castor wheel forward, the effect of a forwards change in the position of the occupant's centre of gravity can be compensated for, such that the forwards stability angle may increased.

In some instances this increased stability castor may be appropriate to a wheelchair used in an indoor environment, where the occupant mass, posture, shape or physical movement is at risk of tipping the chair over when seated or when accessing the wheelchair.

The castor wheel position adjustment can be used in a safe clinical assessment situation to obtain a stability measurement of the centre of gravity position, with the intended user, prior to the wheelchair being issued. In this way the configuration of the castor can become part of the risk reduction options, available for the clinical professional, in configuring a wheelchair to safely accommodate a particular occupant mass, posture and shape, in the intended or typical outdoor environment of use, maintaining or improving where appropriate the forwards static and dynamic stability performance.

This increased stability castor may be supplied as part of a complete modular wheelchair specification, improving the stability configurability of the complete new manufactured product, or they may be supplied in pairs as spare parts, which can be fitted to an existing wheelchair to improve forward stability and safety.

The modular castor fork of this invention may be produced in a structural material, suitable for the application, such as aluminium, steel, or structural engineering grade plastic. The castor fork design has a choice of at least Iwo wheel axle hole positions, providing a choice of at least two castor trailing angles. As shown in FIG 3.

In a second embodiment of this concept, the castor fork may be produced with a series of hole positions in which the castor wheel axle can be mounted, also facilitating castor wheel axle mounting on a different vertical plane. As shown in FIG 4. This design may then also facilitate the option of castor wheels of different diameters, which can be fitted to reduce seat to ground height, where appropriate, further improving stability, or to increase seat to ground height to improve occupant positioning.

In a third embodiment of this concept, the axle mounting area of the fork may be a two piece construction consisting of a series of axle hole positions at pre determined locations in the main fork structure, combined with a separate inserted disc feature with a single hole, which rotates when located in a recess, to align in a preferred position with the selected hole in the main fork structure. This design embodiment of the concept, shown in FIG 4A, reduces overall stress at the axle mounting point, and simplifies the external appearance.

The main constructional elements of this third embodiment of the concept are illustrated in FIG 5. The castor assembly comprises of a spigot, itemi, which is secured into the wheelchair front frame. (On some variants of the construction, the wheelchair frame may incorporate a swivel bearing unit). On this shown construction, the spigot is attached to the fork, item 2, by means of a swivel bearing unit integrated into the fork itself. The castor fork as illustrated may be produced as a plastic injection moulding, or an aluminium casting. The castor wheel, item3, may be a plastic injection moulding or an aluminium casting, and may have either a pneumatic or polyurethane, or rubber-moulded tyre, rotates freely about its centre axle, item 5, which is fixed in the mounting area between the two extended legs of the fork, item 2. The centre axle may be a bolt, or a specialised fixing, preferably, the castor wheel hub centre, item 3, will have bearings fitted to allow free running rotation on the axle, item 5. The mounting area for the castor wheel, item 3, in the legs of the fork, item 2, is profiled to allow the castor wheel, item3, to be assembled in a choice of positions. The locating disc, item 4, which may be a plastic injection moulding, or an aluminium casting, or a steel insert, positions the centre axle, item 5, at the required stability position. The direct view on arrow A shows item 2, with the locating disc, item 5, removed, and illustrates a layout for mounting positions for the castor wheel axle item 5.

Claims (6)

1. An improved wheelchair castor construction, facilitating the adjustment of the castor axle centre position in a generally horizontal direction, to change and improve the stability of the wheelchair in a forwards direction.

2. An improved wheelchair castor construction according to claim 1, where the adjustment includes a number of additional positions in which the castor wheel axle can be mounted, also facilitating the castor wheel axle mounting on a different vertical plane to reduce or increase seat to ground height, where appropriate.

3. An improved wheelchair castor construction according to claim land 2, also facilitating use of different diameter castor wheels, to reduce or increase seat to ground height, where appropriate to improve wheelchair functionality.

4. An improved wheelchair castor construction according to claim 1,2 and 3, where the wheel mounting areas in the castor fork are of a two piece construction, with a separate inserted disc feature with a single hole, which rotates when located in a recess in the fork to the pre selected wheel mounting position, to reduce stress in the assembly, when subjected to the occupant load.

5. An improved castor construction according to claim 1,2,3 and 4 where the castor head can be either an integral bearing construction in the fork, with a screw in fitting, or a plain diameter spigot which fits into a head bearing swivel unit on the wheelchair frame.

6. A wheelchair supplied by the manufacturer complete with the castor as described in claims 1,2,3,4 and 5, assembled in a required position.