GB2293419A

GB2293419A - Bearing bushes

Info

Publication number: GB2293419A
Application number: GB9518650A
Authority: GB
Inventors: Glyndwr John Davies; William Howard Everitt
Original assignee: T&N Technology Ltd
Current assignee: Federal Mogul Technology Ltd
Priority date: 1994-09-20
Filing date: 1995-09-05
Publication date: 1996-03-27
Anticipated expiration: 2015-09-05
Also published as: GB9418941D0; GB2293419B; GB9518650D0

Abstract

A bearing bush (10) comprises a bearing layer (12) and a housing (14) to which the bearing layer is bonded with the housing surrounding the layer. The bearing layer (12) is formed from a material which comprises an injection moulded material with a thermoplastics matrix and particles of a low friction material distributed in the matrix. The housing (14) comprises an injection moulded material which has a matrix of the same thermoplastics material which forms the matrix of the bearing layer (12) with reinforcing fibres distributed in its matrix. The bearing layer (12) is bonded to the housing (14) by merging of the matrix material at the interface between the bearing layer and the housing. The matrix material is selected from PPS, polyaryl sulphides, polyaryl sulphones, polyimides, polyketones, polyethers, polyesters, polyamides, and melt processable fluoropolymers. The low friction material may be PTFE. The reinforcing fibres may be glass fibres. <IMAGE>

Description

BEARING BUSKER This invention is concerned with bearing bushes which have a bearing layer formed from low friction material and a housing portion surrounding and supporting the bearing layer. The bearing layer and the housing portion may be both be cylindrical about a common longitudinal axis or the housing portion may have any required external shape. Such bearing bushes have a large number of potential uses, one being to support shock absorber struts of the McPherson type.

Bearing bushes are known in which the bearing layer is formed in situ on a cylindrical interior surface of the housing, the housing being made of metal, for example steel, and the bearing layer being formed from a low friction plastics material, for example containing polytetrafluoroethylene (PTFE). These known bearing bushes have the disadvantage that the bearing layer may become debonded from the housing during service with the possibility of this occurring being increased by thermal expansion effects.

It is an object of the present invention to provide a bearing bush with a reduced possibility that the bearing layer will become debonded from the housing.

The invention provides a bearing bush comprising a bearing layer and a housing to which the bearing layer is bonded with the housing surrounding the layer, the bearing layer comprising an injection moulded material which has a thermoplastics matrix with particles of a low friction material distributed in said matrix, and the housing comprising an injection moulded material which has a matrix of the same thermoplastics plastics material which forms the matrix of the bearing layer, there being reinforcing fibres distributed in the matrix of the housing, the bearing layer being bonded to the housing by merging of the thermoplastics material at the interface between the bearing layer and the housing.

A bearing bush according to the invention has essentially a continuous matrix across the interface between the bearing layer and the housing so that the possibility of debonding is substantially reduced.

The plastics material forming the matrices is preferably polyphenylene sulphide (PPS) which may form 50 to 70% by weight of the layer and may form 30 to 50% by weight of the housing. Alternatively, the material forming the matrices may be selected from: polyaryl sulphides, polyaryl suiphones, polyimides, polyketones, polyethers, polyesters, polyamides, and melt processable fluoropolymers such as perfluoroalkoxy.

The low friction material may be polytetrafluoroethylene and the reinforcing fibres may be glass fibres.

Preferably, the internal surface of the bearing layer, ie the surface on which the shaft or strut supported by the bearing runs, has a substantially constant diameter over the range of operating temperatures so that the support given to the shaft or strut over this range is substantially constant. This can be achieved by arranging that the material forming the housing has a substantially higher Young's modulus than the material forming the bearing layer, and arranging that the ratio of the external radius R of the bearing layer to the internal radius r thereof at least substantially satisfies the equation: R2/r2 = (3a-b)/(3a-3b) where a represents the linear coefficient of thermal expansion of the material forming the bearing layer and b represents the same property of the material forming the housing.This formula assumes that the Poisson's Ratio of the material is 0.5 and has to be modified if that ratio departs significantly from 0.5. Since the Young's modulus of the material forming the housing is greater, its expanded size limits the expansion of the bearing layer which, if the above-identified equation is satisfied, expands so as to just fill the space between its original internal diameter and the internal diameter of the expanded housing. The above-mentioned equation can be satisfied as the coefficient of linear expansion of PTFE is substantially greater than that of reinforcing fibres such as glass fibres.

A bearing bush according to the invention can be manufactured by injection moulding a hollow cylinder from the material which is to form the bearing layer, and injection moulding the material which is to form the housing on to the external surface of said cylinder. The material of the housing can be moulded on to the cylinder while the material of the cylinder is still hot from its moulding so that the plastics material forming the matrices merges at the interface. Alternatively, at least an outer portion of the cylinder can be reheated, for example by heat received from the material, which is to form the housing.

The cylinder may be formed by injection moulding around a core. The cylinder may be transferred from the mould in which it is moulded to the mould in which the housing is moulded.

There now follows a detailed description, to be read with reference to the accompanying drawing, of a bearing bush and its method of manufacture which are illustrative of the invention.

The drawing is an end elevational view of the illustrative bearing bush in accordance with the invention, showing the bush at room temperature.

The illustrative bearing bush 10 comprises a bearing layer 12 and a housing 14 to which the layer 12 is bonded with the housing 14 surrounding the layer 12.

The layer 12 is cylindrical about a longitudinal axis 16, having, at room temperature, an internal radius r and an external radius R. The layer 12 is formed from a material which has a matrix formed from a thermoplastics material, specifically PPS, and has particles of a low friction material, specifically PTFE, distributed in said matrix. The material contains 50% by weight of PPS and 50% by weight of PTFE. This material has a coefficient of linear expansion of about 10 x 10-5/ c. In variations of the illustrative bush, the material may also contain other fillers.

The housing 14 is also cylindrical about the axis 16, having, at room temperature an internal radius R and an external radius R. The housing 14 is formed from a material which has a matrix formed from the same thermoplastics material which forms the matrix of the bearing layer, ie from PPS, and has reinforcing fibres, specifically of glass, distributed in its matrix. The material contains 35% by weight of PPS and 65% by weight of glass fibres. This material has a coefficient of linear expansion of about 1.5 x 10-5/ c. In variations of the illustrative bush 10, the material may also contain other fillers.

The bearing layer 12 is bonded to the housing 14 by merging of the plastics material (PPS) forming the matrices of the layer 12 and of the housing 14 at the interface 18 between the layer 12 and the housing 14. This merging is achieved, in an illustrative method of manufacturing the bush 10, by injection moulding a hollow cylinder from the PPS/PTFE material which is to form the bearing layer 12, and injection moulding the PPS/glass fibre material which is to form the housing 14 on to the external surface of the cylinder.

In the illustrative method, the cylinder of PPS/PTFE is injection moulded into a first cylindrical cavity around a cylindrical core. The cylinder is then removed from the first cavity and placed centrally in a second cylindrical cavity. The PPS/glass fibre material is then injection moulded into the second cavity to complete the bush 10.

Because PTFE has a linear coefficient of thermal expansion (a) which is greater than that (b) of glass fibre, the PPS/PTFE material which form the layer 12 has a greater linear coefficient of thermal expansion than the PPS/glass fibre material. However, the Young's Modulus of the PPS/glass fibre material is substantially greater than that of the PPS/PTFE material. This means that, when the temperature of the bush 10 rises in service, the housing 14 expands with its internal radius R becoming R(l+b)T and its external radius R, becoming R(l+b)T, where T represents the temperature rise in degrees centigrade.

The bearing layer 12 is constrained in its expansion by the housing 14 which it cannot force to expand further (because the layer has a lower Young's Modulus). Thus, instead of the external radius of the bearing layer 12 becoming R(l+a)T, it is constrained to the lower R(l+b)T.

This would normally mean that the expansion of the bearing layer 12 would reduce its internal radius r with undesirable consequences for the wear of the layer 12.

However, in the bush 10, the internal radius 10 is kept substantially constant by arranging that the quantity of material in the bearing layer 12 is substantially just sufficient to fill the space between the radius r and the internal radius of the housing 14 (taking into account expansion in the direction of the axis 16). In other words, the difference in the expanded volume of the layer is just sufficiently greater than the increase of volume available (caused by expansion of the housing 14) to cancel out the tendency for r to increase. This is achieved by arranging that the ratio of R to r satisfies the equation: R2/r2 = (3a - b)/(3a -3b) which, in this case, gives a ratio of R to r of 1.05 to 1.

Claims

1A bearing bush comprising a bearing layer and a housing to which the bearing layer is bonded with the housing surrounding the layer, the bearing layer comprising an injection moulded material which has a thermoplastics matrix with particles of a low friction material distributed in said matrix, and the housing comprising an injection moulded material which has a matrix of the same thermoplastics plastics material which forms the matrix of the bearing layer, there being reinforcing fibres distributed in the matrix of the housing, the bearing layer being bonded to the housing by merging of the thermoplastics material at the interface between the bearing layer and the housing.

2 A bearing bush according to claim 1, wherein the material forming the matrices is polyphenylene sulphide.

3 A bearing bush according to claim 1, wherein the material forming the matrices is selected from polyaryl sulphides, polyaryl sulphones, polyimides, polyketones, polyethers, polyesters, polyamides, and melt processable fluoropolymers such as perfluoroalkoxy.

4 A bearing bush according to any one of claims 1 to 3, wherein the material forming the housing has a substantially higher Young's modulus than the material forming the bearing layer, and the ratio of the external radius R of the bearing layer to the internal radius r thereof at least substantially satisfies the equation: R2/r2 = (3a-b)/(3a-3b), where a represents the linear coefficient of thermal expansion of the material forming the bearing layer and b represents the same property of the material forming the housing.

5 A bearing bush substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.

6 A method of manufacturing a bearing bush according to any one of claims 1 to 5, the method comprising moulding a hollow cylinder from the material which is to form the bearing layer, and moulding the material which is to form the housing on to the external surface of said cylinder.