This invention relates to plain bearings of the kind used in internal combustion engines. More particularly it relates to bearings for use as crankshaft (main) bearings and/or big end bearings.
Numerous metal and alloy combinations have been proposed. It is common practice to employ a strong backing material such as steel onto which is cast or otherwise affixed one or more primary layers of a bearing metal or alloy. It is common practice to provide the working face of the primary layer with an overlay coating of minimal thickness, to impart a degree of conformability and lubricity and also the ability to embed particulate debris which is generally found in engines and which can cause serious damage and/or premature wear problems, or even seizure.
For example, one well known bearing material is based on a steel backing layer onto which is cast a bronze layer, the latter serving as the primary layer referred to earlier. The primary layer is then given an electrodeposited overlay coating of lead and indium. (It will be understood that ancillary steps such as forming, shaping and machining to final dimensions may be required, but as these are all well known in the art, they need not be discussed in detail in this specification).
In addition to achieving wear resistance and conformability, a plain bearing for automotive engine use should also exhibit resistance to cavitation erosion due to the behaviour of the oil film under conditions of heat and cyclic pressures. Cavitation erosion is a particularly serious problem at very high engine speeds and loadings. High speed in this context means up to and in excess of 10,000 r.p.m. and engines capable of operation at such speeds are generally run using synthetic lubricants, the latter apparently considerably exacerbating the erosion problem.
The use of relatively exotic metals such as silver has been proposed; silver is/was used in certain aircraft engines. It is an excellent metal for bearing purposes, but is prone to seizure if run under unloaded conditions. It would normally be used in a relatively thick, monolithic layer.
A further problem for plain bearings in automotive engines arises from the surface finish of the shaft which the bearing supports, or with which the bearing working surface is in contact. Surface irregularities can and do cause damage to the bearing, especially during the xe2x80x9crunning-inxe2x80x9d phase of operation. To counter this it has been proposed to incorporate an amount of very fine, hard abrasive particles into the working surface of the bearing, more especially into the overlay coating. It is believed that such particles not only serve to xe2x80x9cpolishxe2x80x9d the shaft, but also to strengthen the soft overlay so that damage to it is greatly reduced. Titanium carbide is a known material for this purpose.
It is an object of the present invention to provide a bearing having improved performance at engine speeds in excess of 10,000 r.p.m.
According to a first aspect of the present invention, there is provided a bearing material comprising a metal having therein diamond particles.
In accordance with a second aspect of the present invention, a bearing comprises a strong backing, a layer of silver or a silver alloy and an overlay coating layer of a metal containing diamond particles.
A structural bronze bearing material layer may be interposed between the strong backing (normally of steel for example) and the silver layer. In that case, the silver may be applied over a nickel interlayer to improve adhesion and minimise the formation of undesirable intermetallic compounds. Such a nickel layer will, in fact, normally be used on top of bronze. The diamond-containing metal overlay layer may be silver, pure lead or a lead/indium mix, or it may be lead/tin or lead/tin/copper.
It will be appreciated that the invention in one embodiment may be regarded as comprising an overlay having two-layers, in which both layers may be principally of silver.
The second or outermost overlay layer adjacent a shaft journal in use is preferably applied by electroplating; it will be typically only about 1-5 microns thick, and the diamond particles should preferably be in the size range 0.1-2 microns. The underlying silver or silver alloy layer may be of the order of 5-20 microns thick, whilst the nickel interlayer (if used) will be typically 1-2 microns thick. The strong backing material may be of an appropriate thickness, but 1-2 mm would be suitable for many applications. The structural bronze layer (if used) may have a thickness in the range of 200-300 microns for example.
Suitable silver alloys may comprise silver and lead for example.
The diamond particles may be natural or synthetic and may be present in an amount of from about 0.1 volume % to 3 volume %. A more preferred range of diamond content may be from 0.25 to 1 volume %.
Surprisingly small amounts of diamond particles are effective. For example, 0.15% by weight (about 0.5% by volume) gave satisfactory results.
The performance of a bearing according to the present invention is good, especially at speeds in excess of 10,000 r.p.m. and under heavy loading.