Bearing elements are generally known which comprise a steel back, a substrate layer and a sliding layer (or overlay or running layer). These are commonly used in engines, such as internal combustion engines, for example as crankshaft and/or camshaft supporting bearings and big end bearings and small end bushings in connection rods. They may also be used as thrust washers (axial bearings).
Plain bearings for use as crankshaft journal bearings in internal combustion engines, for example, are usually semi-cylindrical in form and have a layered construction. This construction typically comprises; a backing made from a strong backing material such as steel, of a thickness in the region of about 1 mm or more; a lining of a first bearing material adhered to the backing and of thickness from about 0.1 to 0.5 mm; and a layer of a second bearing material adhered to the surface of the first bearing material and having a thickness of less than about 40 μm. The surface of the second bearing material forms the actual running or sliding surface with a co-operating shaft journal surface.
The backing provides strength and resistance to deformation of the bearing shell when it is installed in a main bearing housing or in a connecting-rod big end, for example.
The first bearing material layer may provide suitable bearing running properties if the layer of the second bearing material should be worn through for any reason, and prevent the journal surface from coming into contact with the strong backing material. It also bonds to the backing, and supports and bonds to the second bearing material. The first bearing material may commonly be either an aluminium-based alloy or a copper-based alloy. Copper-based alloys, such as bronzes, are typically used in more highly-loaded bearings to provide additional support for the second bearing material layer.
The second bearing material layer which co-operates with the shaft journal may also be termed an overlay, or overlay layer. Conventionally, this has been formed by a relatively soft metal layer, to provide conformability (the ability of the bearing to accommodate small misalignments between the bearing surface and the shaft journal) and embedability (the ability to prevent debris, or dirt particles, which may circulate in the lubricating oil, from scoring or damaging the journal surface by enabling such debris to embed in the bearing surface).
More recently, bearing overlay layers comprising a matrix of plastics polymer material have been used. Examples are described in WO 2004/113749 and WO 2010/066396. Such overlay materials are commonly based on a matrix of polyamide-imide (PAI) containing various filler materials, such as hard reinforcing particles, soft particles of solid lubricant materials, and other components such as metal particles. Hard reinforcing particles may include ceramic or metal oxide particles. Solid lubricants may include materials such as graphite or molybdenum disulphide. Metal particles may include aluminium particles, such as aluminium flakes.
PAI materials have proved to be able to provide robust and effective bearing materials, including in the aggressive conditions of modern internal-combustion engines where stop-start operation, to reduce fuel consumption, requires a typical engine to undergo a greatly increased number of stop-start operations. Each time an engine restarts, full hydrodynamic lubrication may not be in place and so bearings such as crankshaft bearings need to be able to survive an increased number of non-hydrodynamically-lubricated start-up operations. PAI bearings, with suitable filler materials, have proved able to survive such conditions, but softer plastics materials have not provided adequate wear resistance for use as bearing overlay materials.
PAI bearing materials thus provide good performance, including conformability and embedability, but nevertheless the applicant for the present invention has found that it is advantageously possible to improve the performance of PAI bearing materials, and of bearings comprising these materials.