Castor having inwardly cambered wheels and a retainer plate

A swivel castor comprises a pair of wheels (17) inclined inwardly to the upstanding swivel axis (11), such that the bottom of the wheels are spaced apart further than the tops. The swivel axis is offset forwardly of the wheel rotational axes. The arrangement gives improved support capacity and swivellability.

BACKGROUND OF THE INVENTION 
The present invention provides a castor having a pair of wheels, which 
features good load-carrying capacity and improved swivellability. 
DESCRIPTION OF THE PRIOR ART 
Conventional swivel-type castors frequently suffer from drawbacks which 
tend to limit their usefulness or reliability. One such drawback which is 
particularly evident in castors of simple construction and especially in 
castors with worn bearings is a tendency to jerk, or a lack of smooth 
action during movement. Another drawback which can occur even with castors 
in good condition, and which to various degrees is inherent in castor 
designs generally, is a resistance to sudden large directional changes of 
movement. In extreme instances, a castor can exhibit a tendency to lock in 
the direction of movement of a load, making turning difficult, especially 
when the bearings are worn and the load is heavy. The tendency to lock is 
usually most pronounced in simple castors with little or no offset between 
swivel and wheel axes, and for castors with two or more wheels in which 
turning friction is significant compared with rolling friction. With the 
very simple castor designs the tendency to lock is mainly evident for 
turning angles near 90.degree. while for castors which have their swivel 
and wheel axes offset from each other, locking is most likely to occur for 
turning angles near 180.degree.. The tendency for locking to occur is 
generally minimised in swivel castors by having the swivel axes offset 
from the wheel axes. 
One known type of castor, such as disclosed in British Patent No. 928,709, 
achieves a high mobility compared with other known types by having a wheel 
mounted at a camber angle relative to the swivel axis. In this type of 
castor the direction of camber is outward with respect to the swivel axis. 
By this is meant that if the castor is viewed in the direction of travel 
on a horizontal support surface, the lowermost tip of the castor would be 
seen closer to the swivel axis than the uppermost tip. In particular, the 
lowermost tip, which contacts the support surface, would be seen to be 
substantially in line with the swivel axis. In a transverse view, the 
wheel axis would be seen to be offset behind the swivel axis. Despite its 
advantages, this type of castor does have some limitations. For example, 
the castor is inherently asymmetrical in the relationship between the 
wheel and the swivel axis and it is essential that castors of this type be 
manufactured and mounted in complementary pairs, otherwise smoothness of 
operation and stability are adversely affected. 
It has been proposed to improve the load-carrying capacity of castors by 
providing a pair of wheels. However, the use of two wheels increases the 
sidways frictional forces between the castor and the floor, and tends to 
make swivelling more difficult. 
The wheels have generally been mounted upright or cambered outwardly with 
respect to each other i.e. the bottoms of the wheels are more closely 
spaced than the tops. Such outwardly cambered arrangements are disclosed 
in U.K. Patent Nos. 239,701 and 928,709, U.S. Pat. No. 3,922,754, and 
Australian Patent Nos. 449,886 and 516,597. However, as acknowledged in 
the former Australian patent, such arrangements require a certain force to 
start turning motion; and moreover the close spacing of the wheels where 
they contact the floor leads to twisting forces which may tend to damage 
carpets and other floor coverings. 
It is an object of the present invention to provide a twin wheel castor of 
improved swivellability and less resistance to turning. 
The present invention provides a castor which comprises a pair of wheels 
rotatably supported by a support member, the support member being 
swivellable about an upstanding swivel axis, and each wheel having an 
inward camber relative to a plane containing the swivel axis such that the 
uppermost portions of the wheels are closer together than the lowermost 
portions. 
The castor can give a high degree of mobility and smoothness of operation 
and can be easily manufactured. Among other features obtainable with the 
invention are substantial freedom from locking, outstanding ease in 
changing direction of movement under load and good load supporting 
capability. Other features include low drag on most surfaces, fast turning 
action, exceptional stability due to the relatively wide spacing of the 
wheels where they contact the floor, and non-critical tolerances in 
manufacture. The castor of the invention can readily be manufactured with 
a symmetrical construction and thereby avoids the disadvantage of 
asymmetrical castor types which must be mounted in complementary pairs for 
stability. 
By inward camber is meant, in this context, that the wheels are inclined 
towards a plane which contains the swivel axis and passes between the 
wheels such that, when the castor is mounted for movement along a 
horizontal support surface, the tips of the wheels furthest from the 
support surface are closer to each other than the tips which are in 
contact with the support surface. 
Preferably the swivel axis is offset with respect to the axes of the 
wheels, that is, it does not intersect the wheel axes. 
In a preferred embodiment of the invention, the wheels are positioned 
symmetrically with respect to a central plane which contains the swivel 
axis and are supported for rotation by integral trunnions which are 
journalled in corresponding bearing recesses in the support member. 
The edges of the wheels may be rounded or bevelled so as to lie flush on a 
flat surface. 
The invention will be more fully understood from the following description, 
which is by way of example, with reference to the accompanying drawings, 
in which:

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The castor shown in the drawing comprises a support member in the form of a 
cast metal or plastics casing 10 which is mountable by means of a threaded 
swivel pintle 11 to an article (not shown) such as a trolley or a piece of 
furniture. The pintle 11 is fitted into a bearing socket 12 which is cast 
integrally with the casing, and is retained therein by means of a circlip 
13 and a hexagonal nut 14. The circlip 13 engages a groove 15 at one end 
of the pintle which projects below the bottom of the casing, while the nut 
14 is engaged with a threaded portion of the pintle which extends upwardly 
from the bearing socket. To secure the castor to the article, the threaded 
portions of the pintle can be screwed into a threaded opening in the 
article or can be passed through a plain hole in the article and held by 
means of a second nut. The nut 14 can be held by a spanner to facilitate 
fixing of the castor to the article. When the castor is so mounted the 
pintle 11 provides a vertical swivel axis for the castor. 
The casing 10, in conjunction with a retainer plate 16 (FIG. 3), supports a 
pair of wheels 17. In this embodiment, the wheels are each formed 
integrally with a pair of trunnions 18, one of which is rotatably 
journalled in a corresponding bearing recess 19 machined into the inside 
wall surface of the casing and the other of which is journalled in a 
similar recess 20 formed in the retainer plate. As seen in FIG. 2, the 
recesses 18 and 19 face in opposite directions. The retainer plate 16 is 
fixed to the underside of the casing by means of a screw 21 which passes 
through a hole 22 at one end of the plate into a threaded hole (not shown) 
in the casing. The screw may be of the self-tapping kind. At the other end 
of the plate a hole 23 is formed which aligns with the bore of the socket 
12 so that the grooved end of the pintle 11 passes through the hole 23 and 
the circlip 13 bears against the bottom of the plate to retain the pintle. 
Stepped portions 24 and 25 formed at the ends of the retainer plate 
cooperate with complementary recesses (not shown) formed in the inner wall 
surface of the casing to ensure correct positioning of the plate when 
assembled. 
The wheels 17 are each cambered towards the vertical centre plane of the 
castor in the view shown in FIG. 2, wherein edges of the wheels are 
bevelled so that at their lowermost ground-containing points the wheel 
edges lie in a common plane. This centre plane contains the axis of the 
swivel pintle 11 and is also a plane of symmetry of the castor. The camber 
angle is preferably between 8.degree. and 20.degree., and advantageously 
between 10.degree. and 20.degree.. In this embodiment this angle is 
approximately 15.degree.. The swivel axis is offset forwardly of the wheel 
axes to avoid wheel locking during turning movements of the castor. 
The direction of camber of the wheels is important to give stability to the 
castors. Compared with conventional swivel castors which do not have their 
wheels cambered, it has been found the castor described above has 
significantly improved stability and manouverability. Locking tendency, 
which can be particularly evident in conventional two-wheeled castors is 
significantly reduced with the present castor. 
The symmetry of design provides eye-appeal as well as contributing to 
stability. The castor is easy to manufacture and tolerances are 
non-critical, particularly for the wheel bearings. Thus performance is not 
greatly impaired by wear. Assembly of the castor is relatively easy 
thereby contributing to low cost. 
The bearing requirements are reduced because of the camber applied to the 
wheels. In this embodiment the camber angle is such in relation to the 
relative dimensions of the wheel and its supporting trunnions that the 
reaction component at F of a static load W supported by each wheel acts in 
a line which passes vertically through the bearing 19. This action 
provides stability to the castor when stationary or moving in a straight 
line. When cornering or suddenly changing direction the effective load W 
now acts at an angle which is normally less than the camber angle so that 
side thrust is minimised and stability is maintained. 
The embodiment illustrated in the drawings is described by way of example 
only and variations in details of its construction are possible. For 
example, any suitable means for giving a swivel support may be provided, 
such as top bearing pintles for light furniture, expanding sleeve types 
for tubular equipment or ball bearing swivel bearings for heavy duty 
applications. The wheels may conveniently be mounted in plastics material 
or made of metal and fitted with rubber or plastics tires if desired. For 
reduced weight or material the wheels may be hollow. Also, while trunnions 
are used for the bearing support for the wheels in this embodiment, other 
method of support, such as the use of roller or plain axle bearings may be 
employed. 
Particularly for industrial use, a brake may be provided. Advantageously, 
this is in the form of a wedge of friction material disposed between the 
upper portions of the wheels and attached to one end of a handle 
protruding through the casing. The handle is pivotally mounted on the 
casing so that downward movement of the free end of the handle, e.g. by 
means of the operator's foot, causes upward wedging movement of the 
friction material between the two wheels. 
The claims defining the invention are as follows: