Page 372 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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contamination [2]. Additional considerations in the selection of bearing material are
durability, machinability and, of course, cost.
The properties required for desirable bearing materials are comprehensive and some-
times conflicting. As with many machine elements, tradeoff decisions must frequently
be made to conciliate contradictory requirements while choosing bearing materials.
12.4.2.2 Commonly Used Bearing Materials
The combination of large loads and high sliding velocities encountered in sliding bear-
ings is somewhat analogous to wormgear drive. Since wear is inevitable after a period of
operation, an ideal design is to confine most wear to the element that can be easily and
economically replaced.
In a sliding bearing, the journal is part of a shaft, frequently made of steel, heat
treated by carburizing, nitriding, flame or induction hardening. Hard surfaces on
rotating shafts usually mate with soft, wear compatible bearing surfaces. Commonly
used bearing materials are:

(1) Babbitts
Babbitts may be lead-based (as 75%Pb, 15%Sb, 10%Sn) or tin-based (as 89%Sn,
8%Pb, 3%Cu) [2, 3]. They use lead or tin as a soft matrix metal and hard particles of
Sb-Sn or Cu-Sn to resist wear [5]. Because of their softness, babbitts are unrivalled
in conformability and embeddability. However, their compressive and fatigue
strengths are rather low, particularly at elevated temperature. Because of low
strength and high cost, a thin babbitt overlay, usually about 0.025 mm [2], is often
deposited as liners over steel bushings to combine the great load carrying capacity
of steel with conformability and corrosion resistant of babbitts. Babbitts are usually
used for high speed, heavy duty applications such as crankshaft bearings in internal
combustion engines.
(2) Bronze alloys
Bronze refers to copper alloyed with lead, tin or aluminium, either singly or in
combination. Consequently, copper alloys used in bearings include lead bronze,
tin bronze and aluminium bronze. Lead bronzes have good embeddability and
resistance to seizing, yet relative low strength. Tin bronzes and aluminium bronzes
have higher strength and hardness, yet poor embeddability [3]. They are used in
demanding applications in engines, machine tools and aircrafts.
(3) Grey cast iron and steel
Grey cast iron and steel are inexpensive bearing materials suitable for light loads, low
speed applications. The free graphite in cast iron adds lubricity but liquid lubricant is
needed as well. Hardened steel can run against any material with proper lubrication.
(4) Sintered porous metals
Porous metal bearings are produced by sintering powders of bronze, tin, aluminium
or mixed with lead or copper into a desired shape. The sintered porous structure has
a large number of voids into which lubricating oil penetrates and is held by capillary
action. During operation, the porous structure releases the oil out of pores to the
bearing surface to provide lubricant film. When the shaft stops rotation, the lubri-
cant flows back into pores [4]. The self-contained lubrication makes porous metal
bearings ideal for applications where lubrication supply is difficult, inadequate or
infrequent.