Modern Features in Engine Bearings
Page 25
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Essential Characteristics Which Govern Freedom From Bearing Failure. The Importance of Oil-groove Disposition
'THE lecture on engine bearings, delivered recently before the Institute of Road Transport Engineers in London by Mr. R. J. 'Brown, A.M.I.Mech.E., chief chemist and metallurgiSt, Nuffield Central Research Laboratories. 'contained many Points of considerable interest to engineers.
Mr. Brown said that . bearings involved three principal components— (a) the moving member, such as a rotating shaft or a piston; (b) the bearing. which generally remains stationary; (e) a thin film of lubricant between the hearing and the moving component. If this lilrn could be constantly maintamed, bearing problems would he greatly simplified, but, as this is impossible, the materials to.he used must be such as will not -cause failure when momentary metal to metal contact unavoidably occurs.
A bearing material other than that of the housing is usually essential to reduce friction to t1.1 lowest possible figure. The mating of surfaces of similar materials cannot always be avoided, hut is often associated with troubles in service.
Some Essential Properties
Certain 'characteristic's arc esSential. These are summarized by the author as " wetability," anti-seizure properties, good thermal conductivity, cmbeddability, conformability, adequate strength and corrosion resistance.
The material for the bearing must • he compatible with the formation of a continuous oil film, which will resist rupture and be self-healing if broken. 1 his property is related to chemical reactivity between the lubricant and the metal. •
In any bearing; the oil film tends to break down from time to time. In its early life, projections break through the oil film, as happens also with foreign matter in the lubricant, thus permitting metal-to-metal contact. • Seizure may he caused by the development of a heat which will permit welding, and this will result in the surface of the bearing being tom out it may also lead to melting.
The matrix should be soft, so that any foreign matter may beibme embedded in the surface rather:Than be carried around with the oil stream. Without adequate embeddability, foreign matter tends to stick on the surface of the bearing. causing abrasion and scoring of the moving component.
Owing to the difficulty of ensuring perfect alignment of hearing assemblies. the bearing metal must possess the ability to deforin slightly to compensate for such irregularities. Without This property, high local pressures may lead to seizure.
Bearing life is dependent upon the strength of the metal used arid the method of its application. It must possess resistance to compressive and impact stresses, and resist failure by fatigue. In a sleeve bearing, the unit load is normally much greater than the calculated carrying capacity, as the latter is based upon projected surface area, whereas the working stresses are concentrated almost entirely along the line of contact between the shaft and its hearing, particularly at the start of rotation.
To increase the load-carrying capacity of modern engine bearings, it is usual to employ a thin deposit of the bearing metal bonded to a backing material. Frequently they are in the form of thin shells which have a long life and can readily be replaced.
in copper-lead bearings, a recent development is the sintering . of a mixture' of copper and lead powder to the backing material. This permits a higher lead content without resulting in segregation.
The plating of bearings is sometimes necessary to overcome practical difficulties such as rapid corrosion or poor anti-seizure properties. Lead and idiom are often employed for this.
Steel is used as a bearing surface where loading pressures' are high and strength essential. Typical examples are timing cams, tappets and transmission-gear teeth, but steel against steel is far from ideal, and where possible it is preferable to run hardened steel against chilled iron.
Porous-metal hushes and thrust hearings can be used where loadings are light and where it is difficult to lubricate by normal method/. They usually contain graphite or graphite-forming greases.
Oil-groove Form
The form of oil grooves and pockets in sleeve hearings is of considerable importance. A helical groove running out at the extremity will permit lubricant to be squeezed out. A satisfactory method of maintaining the oil film is by hall-indenting the bearing surface.
The substitution Of thin-shell hearings for the thickLshell type has gone far to overcome fatigue resulting from flexure, fatigue being now only a function of the compressive load.
In the discuksion which followed, one member referred to. possible ,danger owing to the lack of knock when thin bearings fail, so that the trouble is not noticed. This may result in more serious damage occurring. The author mentioned that the nitriding process for crankshafts improves their general characteristics, not merely the bearing surfaaes.