GB2282858A

GB2282858A - Manufacturing a bearing assembly.

Info

Publication number: GB2282858A
Application number: GB9321181A
Authority: GB
Inventors: Dingeman Adriaan Johan Priem
Original assignee: Lin Pac Mouldings Ltd
Current assignee: Linpac Mouldings Ltd
Priority date: 1993-10-14
Filing date: 1993-10-14
Publication date: 1995-04-19
Also published as: GB9321181D0

Abstract

A bearing assembly such as a castor 1 has a housing 2 with an internal bearing surface 12 and a wheel 8 with an internal bearing surface 18. The housing and wheel are coupled by a one piece plastics moulded bracket 5 which is moulded so that a head portion 11 of the bracket presents an external bearing surface 14 that rotates in bearing surface 12 and a shaft 7 of the bracket presents an external bearing surface 15 on which bearing surface 18 of the wheel is rotatable. Clearance between the opposed bearing surfaces 12, 14 and 15, 18 to permit the required rotation is provided by contraction of the plastics of the bracketS as such plastics solidifies during moulding. Such clearance created by contraction of molten plastics during moulding to permit relative displacement between two components may also be applied to manufacture of linear bearing assemblies. <IMAGE>

Description

TITLE "A method of manufacturing a bearing assembly and a bearing assembly" TECHNICAL FIELD & BACKGROUND ART The present invention relates to bearing assemblies of the kind comprising two bearing parts which present opposed bearing surfaces that are slidably displaceable over each other and of which at least one of the bearing surfaces is presented by a plastics moulded component.

Bearing assemblies of the kind aforementioned, which may provide for either rotational or linear displacement are manufactured conventionally as an assembly of preformed bearing parts. Such assembly is often inconvenient, labour intensive and generally regarded as a significant expense in the overall cost of manufacture. It is an object of the present invention to provide a method of manufacturing a bearing assembly and a bearing assembly derived from such method by which the aforementioned disadvantages may be alleviated.

STATEMENTS OF INVENTION & ADVANTAGES According to the present invention there is provided a method of manufacturing a bearing assembly which comprises providing a first bearing part having a first bearing surface, moulding to said first bearing part plastics material which is to solidify and form a second bearing part having a second bearing surface opposing the first bearing surface, solidifying the plastics material for said material to contract and utilizing said contraction to provide clearance between the first and second bearing surfaces so that the first and second bearing parts are slidably displaceable relative to each other over said opposing bearing surfaces.

By the present invention it is envisaged that the first bearing part will be preformed and positioned and restrained from displacement in a moulding tool into which is introduced the plastics material to mould the second bearing part. The plastics material from which the second bearing part is to be formed will have the characteristic that as such material solidifies from a relatively low viscous condition in which it is provided for moulding to a relatively solid condition required for that part, the material undergoes contraction. The plastics material will usually be of a kind which is introduced (for moulding) in a heated condition so that it sets or solidifies by cooling and undergoes the required contraction as it cools; alternatively the plastics material may be of a kind which solidifies or cures without undergoing a temperature change and nevertheless provides the required contraction, for example as may be the case with selected resinous compounds. As the plastics material from which the second bearing part is moulded solidifies the contraction of such material is arranged to provide adequate clearance between opposed bearing surfaces of the two bearing parts to permit required face-to-face sliding displacement, which may be linear or rotational, between the opposed bearing surfaces. By this proposal the second bearing part is moulded in conjunction with the first bearing part so that when the two parts are removed from the moulding tool a bearing assembly can be provided which may require no further finishing other than, possibly, for removal of flashes from the plastics of the second bearing part. The moulding tool may be designed so that the second bearing part as moulded is retained in engagement with the first bearing part, for example by part of the plastics material of the second bearing part capturing, or being captured in a portion of the first bearing part, to permit the required sliding displacement between the two bearing parts. With this latter facility, assemblies are possible in which the first and second parts are inseparable other than by destroying or unreasonably straining one of those parts.

The plastics material of the second bearing part will undergo omni-contraction as it solidifies. With this in mind one or more barriers (such as plates or blades in the moulding tool) may be located between the first bearing part and the heated plastics material during moulding of the second bearing part at positions where the contraction of the plastics material would otherwise cause binding between the first and second bearing parts and thereby unacceptable restraint to the relative displacement which is intended between those bearing parts. Following moulding of the second bearing part, the barriers are removed from between the two bearing parts to provide appropriate clearance between those parts at the aforementioned positions.

As previously mentioned, the present invention may be applied to the manufacture of a linear bearing assembly such as a rectilinear drawer runner in which the first bearing part presents a longitudinally extending first bearing surface and the second bearing part is moulded for its second bearing surface to be longitudinally displaceable relative to the first bearing surface. In the example mentioned of a drawer runner the first bearing part may be in the form of a rail while the second bearing part may be a slider moulded to the rail to be displaceable along it.

The present invention was primarily developed to facilitate manufacture of a rotational bearing assembly with the first bearing surface being of substantially circular section (for example as presented by a circular aperture or recess) in the first bearing part while the second bearing part is moulded to provide a complementary circular sectioned second bearing surface within and opposing the first bearing surface so that following radial contraction of the second bearing surface clearance is provided between the opposed bearing surfaces and the two bearing parts are relatively rotatable about an axis concentric with the circular sections of the bearing surfaces.

The aforementioned first and second circular sectioned bearing surfaces may be cylindrical or non-cylindrical, for example in the latter case the first bearing surface may have an annular distortion which extends radially inwardly or outwardly of the axis of rotation so that the second bearing surface when moulded to the annular distortion is restrained from displacement relative to the first bearing part in the direction of the axis of rotation. When providing such a distortion (for example in the form of an annular rib, recess or groovej in the first bearing surface it should be ensured that the design of the distortion permits adequate clearance to be provided between the opposed bearing surfaces following contraction of the material of the second bearing part to achieve the required relative rotation. For a rotational bearing the second bearing surface can be moulded to extend axially between axially opposed faces of the second bearing part. These opposed faces may provide restraint to relative axial displacement between the first and second bearing parts.

With this latter form of structure and during moulding and solidification of the plastics material of the second bearing part, barriers such as plates or blades as aforementioned may be located at positions axially between the first bearing part and each of the axially opposed faces to be moulded so that clearance is provided between the first bearing part and the respective opposed faces of the second bearing part following removal of the barriers from between the first bearing part and those opposed faces.

The rotating bearing assembly may have the first bearing part in the form of a roller (usually of circular section such as a wheel) having an aperture or recess such as a blind bore (usually concentric) which presents an internal first bearing surface and the plastics material is moulded to the roller so that the second bearing part comprises a shaft which presents an external second bearing surface and extends into the recess or through the aperture for the roller to be rotatable thereon. As a further example the first bearing part may be in the form of a housing (such as a stator) having an aperture or recess which presents a circular sectioned internal first bearing surface into which is moulded the plastics material so that the second bearing part formed thereby is rotatable relative to the housing.

Following from these latter proposals, the roller and the housing may be positioned in spaced relationship in a common moulding tool into which is introduced (usually by pouring or injection) the plastics material to form the second bearing surfaces which co-operate with the first bearing surfaces of the respective roller and housing so that the second bearing surfaces are located on a second bearing part formed as an integral one piece moulding which is common to both the roller and the housing. This latter moulding technique is particularly advantageous in the manufacture of a castor where the roller and the housing are positioned and retained in the moulding tool so that the axis about which the second bearing part is to rotate relative to the housing and the axis about which the roller is to rotate relative to the second bearing part extend substantially at right angles with respect to each other (possibly, but not necessarily, with such axes being in a common plane) The preformed first bearing part may be of any material provided that the structure of that part is maintained during moulding and that the plastics material which is applied to that part does not bond with the material of that part during the moulding of the second bearing part. Typically the first bearing part will be of metal or plastics. The first bearing part may be a premoulded plastics component, possibly of the same plastics material as that in which the second bearing part is formed. Nylon is a preferred plastics material in which the second bearing part is formed.

There is also provided a bearing assembly when manufactured by the method as previously specified as being in accordance with the invention. The bearing assembly may comprise a first bearing part in the form of a roller, such as a generally cylindrical wheel, having an aperture or recess which pres-ents an internal circular section said first bearing surface and the second bearing part is in the form of a bracket part moulded as an integral one piece component to include a shaft which presents an external circular section said second bearing surface within and opposing the said first bearing surface and on which the roller is axially rotatable. Preferably the shaft extends through an aperture in the roller and the bracket part is moulded to include axially opposed faces between which the roller is rotatable and which faces substantially restrain the roller from displacement axially on the shaft (or retain the roller for restricted axial displacement along the shaft,. In particular and advantageously for ease and economy of manufacture, the bearing assembly is preferably in the form of a castor comprising a said first bearing part in the form of a housing which presents an internal circular section said first bearing surface; a further said first bearing part in the form of a roller which presents a concentric internal circular section further said first bearing surface; said housing and roller being coupled together in castoring relationship by a said second bearing part in the form of a bracket part which is moulded in plastics as an integral one piece unit and is common to both the housing and the roller; said bracket part having an external circular section said second bearing surface slidably rotatable in said first bearing surface of the housing for rotation about a first axis and having a shaft presenting an external circular section further said second bearing surface on which the said first bearing surface of the roller is slidably rotatable for rotation of the roller about a second axis; said roller and housing being retained by the bracket part with the first and second axes disposed substantially at right angles with respect to each other.

By use of the manufacturing method of the present invention, particularly with appropriate positioning of barriers in the moulding tool, the castor bearing assembly may have its bracket part moulded as a one piece plastics unit with the shaft extending through a concentric aperture in the roller and between opposed side plates of the bracket part so that the roller rotates between those side plates while the side plates are bridged by a head portion of the bracket part on which head portion is formed the second bearing surface that co-operates with the housing.

Such a unified structure for the bracket part and the rotational mounting permitted for that part in the housing and the rotational mounting provided by that part for the roller (such as a wheel) may be used to provide a castor structure with considerable durability and load bearing characteristics.

DRAWINGS One embodiment of a castor bearing assembly constructed in accordance with, and by a method of, the present invention will now be described, by way of example only, with reference to the accompanying illustrative drawings, in which: Figures 1 to 4 are respectively a side elevation, an end elevation, a plan view from above and a plan view from below of the castor bearing assembly; Figure 5 is a section of the assembly taken on the line A - A of Figure 1, and Figure 6 schematically illustrates an exploded view of a moulding tool for manufacturing the castor bearing assembly.

DETAILED DESCRIPTION OF DRAWINGS The castor bearing assembly shown generally at 1 has a housing 2 in the form of a generally rectangular plate with corner apertures 3 through which the assembly is intended to be secured to provide a load bearing ground engaging castored wheel on, for example, the underside of a piatform. Coupled to the housing 1 for rotation about an axis 4 (which may be considered as vertical) relative thereto is a bracket 5. Carried by a cylindrical shaft 7 of the bracket 5 for rotation about an axis 6 (which may be considered horizontal) is an annular roller in the form of a wheel 8.

The bracket 5 is formed as a one piece integrally moulded plastics component with its shaft 7 extending between opposed side plates 9 and 10 between which the wheel 8 is rotatable in the bracket. A head portion 11 of the bracket 5 bridges its side plates 9 and 10 and is captured in a circular aperture 12A in the housing 2 to permit the required rotation between the housing and the bracket about the axis 4. From Figure 5 it will be seen that the axes 4 and 6 are at right angles to each other but such axes are not in the same plane (i.e. they do not intersect) as will be apparent from Figure 1.

To provide the required relative rotation about the axis 4 between the housing and the bracket, the aperture 12A of the housing 2 presents an internal circular bearing surface 12 concentric with the axis 4. The bearing surface 12 has a concentric distortion presented by a radially inwardly projecting annular rib 13 of generally triangular section. The head portion 11 of the bracket 5 presents an external circular bearing surface 14 which opposes and is complementary to the bearing surface 12 of the housing 2. The distortion in the surface 12 presented by the rib 13 of the housing in cooperating with the bracket head portion 11 maintains the coupling between the housing and the bracket and restrains relative displacement between those components in the direction of the axis 4.

The shaft 7 of the bracket 5 presents a cylindrical external bearing surface 15 concentric with the axis 6.

The wheel 8 has a concentric cylindrical internal bearing surface 18 presented in the aperture by which it is mounted on the shaft 7 and this bearing surface 18 is substantially complementary to the external bearing surface 15 provided on the shaft 7. From Figure 5 it will be apparent opposed flat faces 16 and 17 on the side plates 9 and 10 respectively will restrict relative displacement between the wheel 8 and the bracket 5 in the direction of the axis 6.

The castor assembly 1 is manufactured in a moulding tool shown diagrammatically in Figure 6 and for this manufacture the housing 2 and wheel 8 are provided as preforms. These preforms will usually be of metal or plastics; a preferred material for both preforms is the plastics Nylon.

The moulding tool has a pair of opposed side plates 50, 51 (shown open in Figure 6) with opposed internal flat faces 52 in which are machined opposed recesses (one of which is shown at 53). The recesses 53 correspond in profile to the external profile of the respective side plates 9 and 10 of the bracket 5. The wheel 8 is located between the tool side plates 50 and 51 and the plates are closed towards each other as indicated by arrow 51A so that the wheel 8 is seated beneath two abutting barrier plates 54 carried one by each of the tool plates 50, 51. The barrier plates 54 each have a part annular profile which is spaced from the respective tool side plate on which it is carried and is complementary to and engages the outer circumference of the wheel 8 so that the plates 54 firmly retain the wheel in the moulding tool appropriately aligned with the opposed recesses 53. Furthermore, the barrier plates 54 are intended to provide a clearance which is to be maintained between the outer circumference of the wheel 8 and the bridging portion 11 of the bracket 5 as indicated at 55 in Figures 2 and 5.

Following, or prior to, closing of the tool side plates 50 and 51, two pairs of flat barrier blades 56, 56A and 57, 57A are inserted into the tool, one pair adjacent to each of the plate faces 52. The blades in the pair 56, 56A have opposing semi-circular recesses 58 in adjacent straight and parallel side edges thereof and similarly the plates in the pair 57 and 57A have oppposing semi-circular recesses 58A in adjacent straight and parallel side edges thereof. The blades 56 and 56A slide towards each other and in face-to-face abutment with the face 52 of one of tool plate 50 until the adjacent edges of the blades abut and the recesses 58 define a circular aperture. Similarly the blades 57 and 57A slide towards each other and in faceto-face abutment with the face 52 of the tool plate 51 until the adjacent side edges of those blades abut and their recesses 58 and 58A define a circular aperture. The circular apertures formed by the respective recesses 58 and 58A are co-axial with the wheel 8 in the moulding tool so that the shaft 7 can be moulded through these circular apertures concentric with the axis 6 of the castor assembly that is to be manufactured. Furthermore, the blades 56 are intended to provide clearance indicated at 59 (Figures 2 and 5) between the respective inner flat faces 16 and 17 of the bracket side plates 9 and 10 and the opposed diametrically extending side faces of the wheel 8. The two pairs of barrier blades 56, 56A and 57, 57A when fitted in the moulding tool co-operate with the two barrier plates 54 to define the profile of internal shoulder regions 80 which are formed between the head portion 11 and respective side plates 9 and 10 of the bracket 5.

With the tool side plates 50 and 51 closed, a pair of co-planar flat barrier blades 60 and 61 are fitted in faceto-face relationship to the tops of the side plates 50 and 51. The blades 60 and 61 have opposed semi-circular recesses 60A and 61A in adjacent straight and parallel edges so that when the blades are fitted to the tool their adjacent edges abut and the semi-circular recesses 60A, 61A define a circular aperture which is concentric with the axis 4 that is to be provided. The glades 60 and 61 are convenietly mounted in the tool by rectangular recesses 62 in those blades engaging with complementary upstanding projections 63 on the tool side plates 50 and 51. The coplanar blades 60 and 61 are spaced in the tool from and above the barrier plates 54 and are intended to provide a clearance, along the axis 4 between the housing 2 and the outer periphery of the bracket 5 (which is to be moulded) immediately beneath the housing (as indicated in the region 63 in Figures 2 and 5).

The preformed housing 2 is now located over and on the barrier blades 60, 61 (as indicated by the arrow 70) so that it is symmetrically disposed with respect to the recesses 53 and its circular aperture 12A is concentric with the aperture formed by the recesses 60A, 61A (and thereby the axis 4).

In the present example, the bracket 5 is to be formed as a one piece plastics moulding in Nylon and with the moulding tool as above described in a closed condition the tool is charged with molten plastics from which Nylon is derived (Caprolactum, catalyst and other conventional constituents for polymerization). The charging is conveniently effected by pouring through the top opening presented by the housing aperture 12A. The molten plastics has a low viscosity and does not bond to the Nylon premoulds of the housing and the wheel and furthermore the molten state of the plastics does not melt the Nylon premouldings. By flowing through the aperture 12A and the circular aperture formed by recesses 60A, 61A, beneath the blades 60, 61 and over the barrier plates 54 into the recesses 53 behind the blades 56, 56A and 57, 57A, and then through the circular apertures formed by the recesses 58 and 58A into the cylindrical bore 18 of the wheel, the plastics material fills a mould cavity in which it is permitted to cool and solidify to form the bracket 5 as a one piece integral moulding.

Initially the molten plastics fills the aperture 12A of the housing 2 and the bore at the hub of the wheel 8 against the internal bearing surfaces which those apertures present. However, as the plastics cools it contracts radially relative to the axes 4 and 6 to provide clearance sufficient to permit relative rotation between the internal bearing surface 12 of the housing 2 and the complementary opposing external bearing surface 14 on the head portion 11 of the bracket and between the external cylindrical bearing surface 15 on the shaft 7 of the bracket and the internal cylindrical bearing surface 18 presented by the bore of the wheel.

As the plastics material of the bracket 5 cools and contracts there will be contraction of that material in the direction of each of the axes 4 and 6 and the pairs of barrier blades 56 and 57 ensure that the clearances indicated at 59 are maintained whilst the pair of barrier blades 60 and 61 ensure that clearance is maintained at the region indicated at 63.

As mentioned above, the plastics in the head portion 11 of the bracket will undergo contraction along the axis 4. From the structure shown in Figure 5 it will be seen that the diameter of the plastics material in the bracket head portion 11 across the bearing surface 14 is greater than the thickness of such plastics material over the bearing surface 14 in the direction of the axis 4 so that the plastics material will undergo proportionally greater diametral contraction than axial contraction with respect of the co-operating circular bearing surfaces 12 and 14.

This diametral and axial contraction may be used to advantage to ensure tat a relatively close sliding fit is maintained between the surface 13 (on the annular rib in the bearing surface 12) in the housing 2 and the bearing surface 14 in the bracket 5 to alleviate the bracket 5 from wobbling about the axis 4 relative to the housing 2.

Following solidification of the plastics for the bracket 5, the pairs of blades 56 and 57 are withdrawn from the tool and the barrier plates 54 are removed from over the wheel 8 and from between the moulded side plates 9 and 10 of the bracket 5. To facilitate their removal each of the barrier plates 54 has a central split 54A so that the two sections thus formed for each barrier plate 54 can be opened outwardly of each other and from the tool to clear the wheel 8. The tool side plates 50 and 51 are now opened from each other to permit the castor assembly 1 to be removed upwardly from the tool and thereafter the pair of barrier blades 60 and 61 may be withdrawn from the castor assembly. The castor assembly 1 may be finished simply b removing such flashes as may be formed on the bracket 5 during the moulding operation.

Although the head portion 11 of the bracket is shown moulded into a circular aperture 12A in the housing 2, if required such head portion 11 can be moulded into a circular recess in the housing 2 (so that the housing 2 closes the top of the head portion 11) provided that the bracket 5 can rotate about the axis 4 with respect to the housing 2 and that appropriate porting is provided in the moulding tool for introducing the molten plastics into the mould cavity for the bracket 5. It is also possible to apply the present invention to a roller structure in which the bore in the hub of the wheel 8 is in the form of a circular section recess (i.e. a blind bore) into which the shaft of a bracket is moulded so that the wheel 8 is rotatably mounted on a cantilevered shaft - this would permit one of the side plates 9, 10 to be omitted from the structure shown in the drawings.

Claims

1. A method of manufacturing a bearing assembly which comprises providing a first bearing part having a first bearing surface, moulding to said first bearing part plastics material which is to solidify and form a second bearing part having a second bearing surface opposing the first bearing surface, solidifying the plastics material for said material to contract and form the second bearing part and utilizing said contraction to provide clearance between the first and second bearing surfaces so that the first and second bearing parts are slidably displaceable relative to each other over said opposing bearing surfaces.

2. A method as claimed in claim 1 which comprises moulding the plastics material of the second bearing part to the first bearing part so that the second bearing part is retained in engagement with the first bearing part for said sliding displacement between the first and second bearing parts.

3. method as claimed in either claim 1 or claim 2 which comprises proiding barrier means between said first bearing part and the heated plastics material during moulding of the second bearing part at positions where, during cooling and solidification of the second bearing part, the contraction of the plastics material would otherwise cause binding between the first and second bearing parts and thereby unacceptable restraint to the relative displacement which is intended between the first and second bearing parts, and removing said barrier means from between the first bearing part and the second bearing part to provide clearance between those parts at said positions.

4. A method as claimed in any one of the preceding claims in which the first bearing surface is of substantially circular section and the second bearing part is moulded to provide a substantially complementary circular section second bearing surface within said first bearing surface whereby said first and second bearing parts are relatively rotatable about an axis concentric with said circular section bearing surfaces.

5. A method as claimed in claim 4 in which the first and second bearing surfaces comprise substantially cylindrical formations.

6. A method as claimed in claim 4 in which the first and second bearing surfaces are non-cylindrical and said first bearing surface has an annular radially extending distortion, and wherein the second bearing surface is moulded to the annular distortion so that said distortion restrains the second bearing surface from displacement relative to the first bearing surface in the direction of the axis of rotation.

7. A method as claimed in any one of claims 4 to 6 when appendant to claim 3 which comprises moulding the second bearing surface to extend axially between axially opposed faces of the second bearing part, and during moulding and solidification of the plastics material of the second bearing part locating said barrier means at positions which are to be axially between the first bearing part and each of said axially opposed faces to provide clearance between the first bearing part and the opposed faces when the second bearing part has solidified and the said barrier means is removed from between the first bearing part and the opposed faces.

8. A method as claimed in claim 7 in which said opposed faces of the second bearing part provides restraint to relative axial displacement between the first and second bearing parts.

9. A method as claimed in any one of claims 1 to 3 which comprises providing a longitudinally extending first bearing surface on the first bearing part and moulding the second bearing part for its second bearing surface to be longitudinally displaceable relative to the first bearing surface

10. A method as claimed in any one of the preceding claims which comprises positioning and retaining the first bearing part in a moulding tool and introducing said heated plastics material into the moulding tool to form the second bearing part.

11. A method as claimed in claim 10 in which the first bearing part is a circular section roller having a concentric aperture or recess which presents a circular section said first bearing surface and which comprises moulding the plastics material to the roller so that the second bearing part comprises a shaft which extends within said aperture or recess and presents the second bearing surface on which the roller is axially rotatable.

12. A method as claimed in claim 10 in which the first bearing part is a housing having an aperture or recess which presents a circular section said first bearing surface and which comprises moulding the plastics material to the housing so that the second bearing part extends into the aperture or recess of the housing and presents the second bearing surface to be axially rotatable relative to the housing.

13. A method as claimed in claim 11 and claim 12 which comprises positioning and retaining the roller and the housing in a common moulding tool and introducing the plastics material into the moulding tool to form a said second bearing part as an integral one piece moulding which is common to both the roller and the housing.

14. A method as claimed in claim 13 which comprises positioning and retaining the roller and the housing in the moulding tool so that the axis about which the second bearing part is to rotate relative to the housing and the axis about which the roller is to rotate relative to the second bearing part extend substantially at right angles with respect to each other.

15. A method as claimed in claim 14 in which the said axes are not in a common plane.

16. A method as claimed in any one of the preceding claims which comprises providing the first bearing part as a premoulded plastics component and moulding the second bearing part of plastics material which does not bond with the first bearing part.

17. A method as claimed in claim 16 which comprises moulding the second bearing part in the same plastics material as that of the first bearing part.

18. A method as claimed in any one of the preceding claims in which the second bearing part is moulded in Nylon.

19. A method of manufacturing a bearing assembly as claimed in claim 1 and substantially as herein described.

20. A bearing assembly when manufactured by the method as claimed in any one of the preceding claims.

21. A bearing assembly as claimed in claim 20 and in the form of a roller assembly comprising a circular roller having a concentric aperture or recess which present an internal circular section said first bearing surface, and the second bearing part comprises a bracket part moulded as an integral one piece component to include an axially extending shaft which presents an external circular section said second bearing surface within and opposing the said first bearing surface and on which the roller is axially rotatable.

22. An assembly as claimed in claim 21 in which the bracket part includes axially opposed faces between which the roller is rotatable and which faces restrict or restrain relative axial displacement between the roller and the shaft.

23. An assembly as claimed in either claim 21 or claim 22 in which the shaft extends between side plates of the bracket part and the roller is mounted on the shaft between said side plates, and wherein the side plates are bridged by a head portion of the bracket part.

24. A bearing assembly as claimed in claim 20 and in the form of a castor comprising a said first bearing part in the form of a housing which presents an internal circular section said first bearing surface; a further said first bearing part in the form of a roller which presents an internal circular section further said first bearing surface; said housing and roller being coupled together in castoring relationship by a said second bearing part in the form of a bracket part which is moulded in plastics as a one piece unit and is common to both the housing and the roller; said bracket part having an external circular section said second bearing surface slidably rotatable in said first bearing surface of the housing for rotation about a first axis and having a shaft presenting an external circular section further said second bearing surface on which the said first bearing surface of the roller is slidably rotatable for rotation of the roller about a second axis; said roller and housing being retained by the bracket part with the first and second axes disposed substantially at right angles with respect to each other.

25. An assembly as claimed in claim 24 in which the roller is located axially between opposed faces of the bracket part which opposed faces determine displacement of the roller r > ,ative to the shaft in the direction of the second axis.

26. An assembly as claimed in either claim 24 or claim 25 in which the shaft extends between opposed side plates of the bracket part for the roller to rotate between those side plates and wherein the side plates are bridged by a portion of the bracket part on which is formed the second bearing surface that co-operates with the housing.

27. An assembly as claimed in any one of claims 24 to 26 in which the first bearing surface presented by the housing has an annular distortion which extends radially of the first axis and the second bearing surface that co-operates with the housing is moulded to said distortion so that the bracket part is restrained from displacement relative to the housing in the direction of the first axis.

28. A bearing assembly comprising a bracket part having opposed plates, a shaft extending between the plates and a head portion bridging said plates; a roller having an aperture by which the roller is rotatably mounted on the shaft between the plates, and wherein the bracket part comprises an integral one piece plastics moulding which presents an external bearing surface on the shaft that is substantially complementary to and opposes a circular section internal bearing surface presented in the aperture of the roller for said bearing surfaces to be slidably displaceable over each other in face-to-face relationship during rotation of the roller.

29. An assembly as claimed in claim 28 and having a housing which presents an internal circular section bearing surface and wherein said head portion presents an external circular section bearing surface that is substantially complementary to, is received within and is slidably rotatable in face-to-face relationship relative to the bearing surface of the housing for the bracket part to be rotatably mounted relative to the housing.

30. An assembly as claimed in claim 29 in which the bearing surface of the housing and the bearing surface of the head portion received therein co-operate to restrain relative displacement between the housing and the bracket part in the direction of the axis about which they are relatively rotatable.

31. A bearing assembly in the form of a castor and substantially as herein described with reference to the accompanying illustrative drawings.