The Case for Thin-wall Bearings
Page 57
If you've noticed an error in this article please click here to report it so we can fix it.
FHE primary consideration responsible for the development of the thin-wall bearing to its present ige is an economic one. It is not cessarily true that this type of bearg is intrinsically superior in perform[ce to the older type of thick-wall aring. These comments are made in paper which Mr. P. T. Halligan, B.Sc., hnical adviser to the Glacier Metal )., Ltd., is to read at meetings of the stitute of Road Transport Engineers be held during the 1953-54 session. The thin-wall bearing, he says, lends elf to mass-production methods by e use of steel strip in coil form, and ntinuous lining processes. Its [option, however, makes greater mands upon accuracy by the engine older in the maintenance of closer lerances and this necessarily costs alley. The cost of the bearing, bower, is a mere fraction of that of the der-type thick-wall bearing, and the suit is that engine makers engaged in rge-scale production, have found it
economic measure to maintain the curacy demanded by the thin-wall aring.
Bearing Materials
Dealing with the properties of bear
materials, Mr. Holligan says that a IN melting point is desirable to inimize and to confine the damage tich might result in the event of a
eakdown of the oil film. If two Iterials, both of high melting point d moving in relation to one another, ould come into contact, local weld; would occur and serious damage one surface or the other might result. The material used for a bearing must ssess adequate compressive strength carry constant loads without distorIn; and sufficient fatigue strength at rerating temperature to withstand the posed fluctuating loads without igue failure. In both cases, regard 1st be paid to the thickness of the ing provided.
In meeting the requirements of softss and low melting point on the [e hand, and of high fatigue and mpressive strength on the other, it is cessary in practice to seek a coin ornise. This takes the form of a aring having a strong backing shell ide of steel or other strong material, ed with the soft bearing metal.
In some cases, a. bearing consisting three layers may be used in which
steel shell is lined with, say, copperid, lead-bronze or aluminium alloy rich, in turn, is coated with a soft oy such as lead-tin or lead-indium. The value of high thermal conducity in a bearing material has obably been much exaggerated, as
far the greatest proportion of Melia] heat developed in•the bearing-is ;Sipated by the lubricant. Good !rmal conductivity has some value assisting local heat dissipation which may occur at points of momentary metal-to-metal contact, or as a result of the intrusion of some hard particles between the surface of the bearing and the shaft.
Tin and lead-base white metals have the desirable properties of softness, low melting point, the ability to run with soft crankshafts and tolerance for foreign matter, to a degree not possessed by any other bearing material. Their limitation lies in their low fatigue strength at elevated temperatures, but by reducing liner thickness, their useful range can be extended.
The " micro-bearing " consists of a steel shell with a lining of the order of 0.005 in. thick, and whilst some loss of conformability and embedability results from this reduction, such a bearing will carry appreciably higher loads than the white-metal-lined bearings having a thickness of the order of 0.010 in, and above.
Copper-lead-lined bearings have replaced white-metal-lined bearings to an increasing extent for main and bigend crankshaft bearings in oil engines. The steel shells are lined by casting or sintering processes with copper-lead containing from 20 per cent. to 40 per cent. lead, and sometimes small additions of tin, silver or nickel are made.
The load-carrying capacity of the copper-lead-lined bearing is much higher than that of a bearing lined with whitemetal. Peak loading of the order of
4,000 of projected area forms a conservative estimate for the loadcarrying capacity of 70/30 copper-lead, whilst alloys of lower lead content, containing an appreciable percentage of tin, will sustain loads of the order of 6,000-7,000 p.s.i.
Oil Filtration Important
However, the conformability of the copper-lead bearing and its tolerance for foreign matter, are less than that of the white-metal bearing and it is, therefore, necessary to pay greater attention to oil filtration. It is also desirable to use a harder crankshaft than is necessary with white-metal.
To counteract the rate of crankshaft wear which follows the presence of excessive abrasive foreign matter when a soft crankshaft is used, the bearing surface is plated with lead alloys. This plating may range in thickness from a mere flash of 0.00025 in. thickness, which is intended primarily to facilitate the running-in process, to a thickness of 0.001-0.002 in., which may remain for much of the life of the bearing.
Among other types of high-strength bearing materials are aluminium alloys and silver alloys. Originally, the former were used as unbacked bearings or bushes, but in order that alloys of this type should possess adequate mechanical strength to maintain an interference • fit, it was found necessary to employ materials of great hardness, with a con' sequent loss of desirable bearing properties. On the other hand, soft aluminium alloys not possessing sufficient strength have been bonded to steel shells.
A development of the aluminiumalloy bearing is the Duplex, in which the shell is a hard, high-strength aluminium alloy, lined with a softaluminium-tin alloy, containing up to 20 per cent, of tin.
The British Standards Institute Committee appointed to deal with the standardization of plain bearings gave the definition of a thin-wall bearing as " . . the thin-wall bearing liner is one in which the wall thickness is sufficiently small for the geometrical truth of the working surface to depend on the accuracy of the housing. . . ."
Accuracy of Assembly This definition emphasizes t h c necessity of maintaining a high degree of accuracy in the other components which go towards making the total bearing assembly. The tolerance maintained in the bores of the main-bearing housings and in those of the connecting rods will be reflected precisely by the thinwall bearing and, in conjunction with the crankcase, tolerance will determine the range of running clearance.
For successful operation it is imperative that the thin-wall bearing should be correctly assembled and fitted. it is a precision article, the wall thickness of which is held by the bearing manufacturer to a total tolerance of 0.00025 in. in the case of the smaller bearings, whilst the peripheral length of the bearing is held to a tolerance of 0.001 in.
It is also important that the bearing housing be held within very close tolerance. Thus, the tolerance and the bore of the main-bearing housing should not exceed 0.001 in., whilst that of connecting-rod bores should preferably not exceed 0.005 in. On journal and crankpin diameters, the former should not exceed 0.001 in. and the latter 0.005 in. . The thin-wall bearing, being normally a pre-finished article ready for fitting, cannot be scraped in or rubbed down without loss of the accuracy which is the essential feature of this type of bearing. Line-bored main bearings or connecting-rod big-end bearings bored in position in the rod undoubtedly make possible the maintenance of a running clearance within much closer tolerance limits, but the fact remains that first cost and serviceability have been responsible for establishing the thinwall precision bearing.