GB2068057A - Method and apparatus for lining bearings - Google Patents

Abstract

A pre-formed sheet of bearing material is applied to a bearing backing which is heated sufficiently to form a bond between the bearing material and the backing. The backing may be tinned prior to the application of the bearing material in order to achieve the bond. An apparatus for effecting the above method is also described.

Description

SPECIFICATION Method and apparatus for lining bearings This invention relates to an apparatus and a method for applying a bearing lining material to a backing, for example, a white metal lining to a thrust bearing pad. Known methods of lining include centrifugal lining, lining by static casting and pressure die casting.

Since pads vary considerably in shape and dimensions, pressure die casting requires many moulds for different sizes and shapes of pad and there are substantial difficulties in preventing leakages of molten white metal around the edges of the pads.

The traditional and often used alternative method is to cast white metal on the top of the pad in a very crude manner, i.e., by pouring molten white metal on to the surface of the pad which has previously been tinned. The molten white metal has a tendency to flow over the sides and to reach its own level, and there is little or no control over the thickness or shape of the white metal layer. Traditionally the edges of the pads are treated with whitewash to prevent molten white metal adhering to the edges when it has flowed over the sides. These pads have then to be machined to produce the required geometry on the face of the white metal layer, resulting in metal which requires to be recycled. This method depends very much on the skill of the operator and is slow, dirty and expensive.There is generally no control of the impurities or dross that may have built up in the white metal bath. Static casting carried out this way also has the problem of oxides being entrapped and blow holes forming in the white metal together with a coarse grain structure which is not suitable for a good thrust bearing lining. Furthermore, the impurities tend to cause bad bonding.

Grain structure is important for tin based white metals. Although tin based white metals are used widely as a bearing surface layer and in some respects, for example corrosion, are superior to lead based white metals, they are prone to thermal facetting. This phenomenon arises from the anisotropic nature of tin based alloys which means that thermal expansions in different crystallographical directions are not equal. This can result in the propagation of cracks in the lining which can lead to bearing failure, breakup or loss of lining. Bearings are thermally cycled in operation from cold at start to hot and back again to cold when the plant is shut off, and such thermal cycling promotes thermal facetting of the white metal with the consequent propagation of cracks in the white metal lining.Grain size reduction minimises the likelihood of thermal facetting and is a function of cooling rate and the temperature gradient in the metal which is freezing.

The energy input into a tin based white metal to melt it requires careful control of the power input value and the power input time to achieve a given temperature, followed by careful cooling if a fine grain structure is to be achieved and for this to be repeatable from pad to pad of a given size. Pads which are larger, smaller or different in thickness require different temperatures, cooling rates, etc.

This cannot be achieved at all easily by molten white metal pouring techniques, if it can be achieved at all.

An object of the present invention is to provide a method and apparatus which overcome many of the above mentioned disadvantages and which provide repeatable bearings having linings with a fine grain structure. It is a further object to avoid any casting or pouring of molten metal at the lining stage.

According to one aspect of the invention a method of providing a lining of bearing material to a bearing backing, comprises applying a preformed sheet of bearing lining material to a surface of the backing, heating the backing whereby the surface of the lining adjacent the backing forms a bond with the backing, and cooling the backing and lining.

The heating may cause the lining adjacent the backing to melt to form the bond and subsequent cooling may then cause solidification. This can either be a achieved by heating the lining to its melting point or by surface lining the backing with an alloy of tin together with a material which will reduce the melting point of the tin so that the alloy will melt upon heating. Bonding may then be effected by local dissolution of the lining by the alloy at the interface.

Alternatively, the bond may be formed by a diffusion process in which the lining and backing may be held under pressure allowing solid state diffusion to occur between the lining and the backing.

Preferably, a pre-cast solid sheet of bearing lining material of the required thickness is applied to a surface of the backing which has been tinned and heated by induction heating from a surface remote from the surface to be lined whereby bonding of the lining sheet to the surface can occur through the sheet material becoming molten at least in the region adjacent the one surface or through a diffusion process. This is preferably followed by quenching and subsequent solidification.

According to another aspect of the invention a method of providing a lining of bearing material to one substantially flat surface of a bearing body, comprises bonding a pre-cast pre-shaped solid sheet of bearing material lining on to the surface by heat transferred from the body. Thus the lining would not be applied to the surface in a molten form. Thus, in the methods according to the invention it may be possible substantially to eliminate impurities from the lining material.

The invention further includes apparatus for applying a lining to a bearing backing comprising means for heating the backing and means for applying the lining in a pre-formed condition to the backing, whereby the lining surface adjacent the backing melts and forms a bond with the backing and means for cooling the backing. Preferably the lining is of pre-cast bearing material. Conveniently the heating means comprises an induction heating station. There may also be provided a tinning and a fluxing station including means for tinning and fluxing the surfaces of the backing to be lined.

Preferably the apparatus is arranged to render molten the sheet of lining material at least close to the surface of the lining in contact with the surface to be lined orto cause diffusion to occur between the lining and the backing. Preferably the cooling means comprises a quenching station. Preferably the apparatus is arranged to provide a plurality of bearing bodies each having a substantially flat surface to be lined, with a plurality of preformed linings.

If the solid sheet of lining material can be rendered molten only at the heated surface of the pad, then clearly greater lining thicknesses can be provided that with the sheet melting throughout and flowing over the edges of the pad. However, it is believed that this partial melting will require careful control over the geometry of the induction heating coil to avoid local areas of "all through" melting. Even if all through melting occurs and the molten white metal reaches its own thickness a very acceptable meniscus type surface to the lining may be obtained which can be machined to the required degree of flatness and thickness. A lining achieved by this method may have a fine grain structure.

The process may involve four steps as follows: 1. The casting and the rolling of the white metal to the required thickness.

The process may be applicable to all tin and lead base white metals normally specified. The white metal may be cast into cast iron or steel moulds of appropriate size and the resulting ingots rolled to the required thickness individually using a conventional rolling mill. A propriety lubricant should be applied to the mill rolls.

Alternatively the white metal may be continuously cast in the form of a thin billet and this may be rolled either directly or by degrees to the required thickness. Advantages of this method over the former are that the rolled material may be produced in coil form and that the degree of rolling reduction required may be controlled so as to reduce the amount of edge cracking which occurs with high rolling reductions. This results in a more efficient material usage.

The white metal thickness required for the process is equal to the lining thickness plus a machining allowance. This machining allowance will depend upon a variety of factors such as component geometry and finishing processes employed. For tilting pad bearings a typical machining allowance may be 0.5 mm.

2. Cutting the formed white metal to the required shape. The hardness of white metal even after very heavy (e.g. 90%) rolling reductions may be such as to permit shapes to be readily stamped from the sheet. The shape of the lining is most preferably of the same shape geometry as the component to be lined which is preferably flat. Stamping of the shapes may be accomplished using either mechanical, hydraulic or pneumatic press equipment fitted with either simple "pastry cutter" type tooling or more complex tooling which may provide support to the stamped shape (e.g. as in fine blanking).

The stamped white metal shapes are preferably cleaned and degreased and carefully stored until required.

3. The preparation of the steel pads.

The steel pads for tilting pad thrust bearings are usually either investment castings or machined from solid. Before a white metal lining may be satisfactorily bonded to a steel pad it is first generally necessary to tin the surface of the pad to which the white metal is to be bonded. The process of tinning is well established and may consist of the following stages: (i) Shot blasting the steel surface; (ii) Applying whitewash to areas not required to be tinned. Drying of the whitewash may be accelerated by heating the pad; (iii) Tinning the steel by immersing in a bath of molten tin maintained at a temperature of 300/350"C for a time dependent upon the size of the component. About 5 minutes may be adequate for most pad sizes. The surface to be tinned may first be etched using "killed spirits" or similar a fluxing agent.

The tinned pads should be carefully stored until required for white metal lining.

4. The induction lining of the tinned pads.

An object of the process is to bond fully the stamped white metal preformed shape over the whole of the tinned surface of the steel pad. To effect this bonding it is preferred that the white metal be melted in contact with the cleaned and fluxed molten tinned surface of the steel pad.

Thus, the tinned pad is preferably heated by induction so as to melt the tin of the tinned surface.

A "pancake" type coil of copper tube, water cooled internally and provided with HF electricity supply has been found suitable for this purpose. The coil design, the supply frequency, the power input and duration, the distance of the pad from the coil and its position in relation to the field developed by the coil all affect the energy input to the pad. These factors may be optimised so that the energy input to the pad from the coil is sufficient to melt the tin and possibly to provide sufficient residual heat to melt the white metal preform also, though this should not be so excessive that the white metal preform is melted completely.

Once the tin has melted the power to the coil may be switched off and the surface is preferably cleaned and fluxed, for example by using "killed spirits" or a material. The preformed white metal shape may then be placed on the tinned surface and when the white metal has melted sufficiently water cooling may be applied to the underside of the pad to effect freezing of the lining. The process may be made semi-automatic in operation using a six stage carousel type arrangement as described below.

The invention may be carried into practice in various ways, as illustrated in the following Examples. The single figure of the accompanying drawings is a simplified flow diagram of the process stages in Example 1.

Example 2 The process outlined in Figure 1 is a six-stage carousel arrangement. The coil comprises 7 turns of a 9" copper tube giving an overall diameter of 230mm. To this coil a power input of 3KW is applied for 30 to 35 seconds. This is sufficient to melt the tinned surface of a bearing pad having a thickness of 6 mm and a nominal surface area of 650 mm2. The six-stages are as follows: Stage 1. Input: The tinned pad is located in a die on a turntable.

Stage 2. Induction Coil: The pad is held over the coil so as to melt the tinned surface.

Stage 3. Fluxing Station: The tinned surface is wiped, by a soft roller impregnated with "killed spirit" or similar.

Stage 4. Lining Station: The white metal preforms are automatically placed on the pad surface.

Stage 5. Water Quench: Water is sprayed on to the underside of the pad so as to freeze the white metal lining.

Stage 6. Knock-out: The pad is removed from the turntable at this station.

The turntable is made from a non-conducting and heat resistant material such as Syndango (Registered Trade Mark) which is an asbestos-type material.

Stages 1, 3, 4 and 6 will require a dwell of about 5-10 seconds: Stages 2 and 5 will require longer depending upon the pad size. Thus the symmetry of the system will allow two pads to be lined every 75 seconds, assuming a dwell of 30 sconds at stages 2 and 5.

Example lI A steel pad is shot blasted, degreased, etched and tinned in the usual manner. The white metal is produced as a preform from rolled sheet to the same shape and size as the pad to be lined. The tinned pad and white metal preform are cleaned and degreased and the preform placed in contact with the tinned surface of the pad. The assembly is then heated to a temperature of approximately 200"C (i.e. below the melting point of both tin and the white metal) in an inert or reducing atmosphere (e.g. N2 or N2 + H2) for approximately 30 minutes to 1 hour. The bonding may be improved by mechanically abrading the surfaces to be bonded and by holding the two together under pressure at temperature. Bonding is effected by solid state diffusion at the interface, no liquid phases being formed during the bonding operation.

Example III A steel thrust pad is shot blasted, degreased, etched and the surface lined with an alloy of tin and an appropriate percentage of any element which will suppress the meeting point of tin by approximately 30 - 40"C. For example 10% Pb; 9% Zn; 5% Bi; 1% Cd; 2% Mg. The white metal, as in Example II, is produced as a shaped preform from rolled sheet and the pad and preform are placed in contact and heated to a temperature equivalent to the melting point of the alloy on the metalled pad. The operation is best carried out under an inert or reducing atmosphere as in Example II. Bonding is effected by local dissolution of the white metal at the interface by the molten tin alloy.

Claims (13)

1. A method of providing a bearing backing with a lining of bearing material comprising applying a preformed sheet of bearing lining material to a surface of the backing, heating the backing whereby the surface of the lining adjacent the backing forms a bond with the backing, and cooling the backing and the lining.

2. A method as claimed in Claim 1 in which the lining, in the region adjacent the backing, is caused to melt to form the bond and subsequent cooling results in solidification.

3. A method as claimed in Claim 1 or Claim 2 in which the backing is surface lined with an alloy of tin together with a material arranged to reduce the melting point of the tin whereby the alloy will melt upon heating and bonding will be effected by local dissolution of the lining by the alloy at the interface.

4. A method as claimed in Claim 1 in which the lining and backing are held together under pressure, allowing solid state diffusion to occur between the lining and the backing.

5. A method as claimed in any of the preceding claims in which the backing is heated by induction heating from a surface remote from the surface to be lined.

6. A method as claimed in any preceding claim in which the cooling is achieved by quenching.

7. A method of providing a substantially flat surface of a bearing body with a lining of bearing material comprising bonding a pre-cast, pre-shaped solid sheet of bearing material lining on to the surface by heat transferred from the body.

8. A method of providing a bearing backing with a lining of bearing material substantially as herein specifically described with reference to any one of Examples I, II and Ill.

9. Apparatus for applying a lining to a bearing backing comprising means for heating the backing and means for applying the lining in a pre-formed condition to the backing, whereby the lining surface adjacent the backing melts and forms a bond with the backing, and means for cooling the backing.

10. Apparatus as claimed in Claim 9 in which the heating means comprises and induction heating station.

11. Apparatus as claimed in Claim 9 or Claim 10 further including means for tinning and fluxing the surfaces of the backing to be lined.

12. Apparatus as claimed in any of Claims 9 to 11 in which the cooling means comprises a quenching station.

13. Apparatus for providing a bearing backing with a lining of bearing material substantially as herein specifically described with reference to any one of examples 1,2 and 3.