protects itself by permitting such articles to sink into, or
can also cause some bearings to fail from oil starvation,
embed in, the bearing lining material. If the bearing
as the oil pump can deliver only so much oil. If the oil
were too hard to allow this, the particles would simply
clearances are excessive, most of the oil will pass
lie on the surface. They would scratch the shaft journal
through the nearest bearings. There won't be enough for
and probably gouge out the bearing. This, in turn,
the more distant bearings; these bearings will probably
would cause overheating and rapid bearing failure.
fail from lack of oil. An engine with excessive bearing
Thus, the bearing material must be soft enough for
oil clearances usually has low oil pressure; the oil pump
adequate embedability.
cannot build up normal pressure because of the
excessive oil clearances in the bearings.
Conformability.--This is associated with embed-
On the other hand, if oil clearances are not
ability. It refers to the ability of the bearing material to
sufficiently great, there will be metal-to-metal contact
conform to variations in shaft alignment and journal
between the bearing and shaft journal. Extremely rapid
shape. For example, suppose a shaft journal is slightly
wear and quick failure will result. Also, there will not
tapered. The bearing under the large diameter will be
be enough oil throw-off for adequate lubrication of
more heavily loaded. If the bearing material has high
cylinder walls, pistons, and rings.
conformability, it will flow slightly away from the
heavily loaded areas to the lightly loaded areas. This
BEARING REQUIREMENTS.--Bearings must
redistributes the bearing material so that the bearing is
be able to do things other than carry the loads imposed
more uniformly loaded. A similar action takes place
on them.
when foreign particles embed in the bearing. As they
Load-Carrying Capacity.--Modern engines are
embed, they displace bearing material, thus producing
lighter and more powerful. They have higher
local high spots (fig. 3-60). However, with high
compression ratios and thus impose greater bearing
conformability, the material flows away from the high
loads.
spots. This tends to prevent local heavy loading that
could cause bearing failure.
Fatigue Resistance.--When a piece of metal is
repeatedly stressed so that it flexes or bends, it tends to
Corrosion Resistance.--The bearing materials
harden and ultimately breaks; this is called fatigue
must be resistant to corrosion, since some of the
failure. Repeatedly bending a piece of wire or sheet
by-products of combustion may form corrosive
metal will demonstrate fatigue failure. Bearings are
substances.
subjected to varying loads and are repeatedly stressed.
Wear Rate.--The bearing material must be
The bearing material must be able to withstand these
sufficiently hard and tough so that it will not wear too
varying loads without failing from fatigue.
fast. At the same time, it must be soft enough to permit
Embedability.--This term refers to the ability of a
good embedability and conformability.
bearing to permit foreign particles to embed in it. Dirt
BEARING MATERIALS.--The bearing back is
and dust particles enter the engine despite the air
usually made of steel. The lining material is a
cleaner and oil filter. Some of them work onto the
combination of several metals, mixed or alloyed, to
bearings and are not flushed away by the oil. A bearing
ASf03060
Figure 3-60.--Effect of a metallic particle embedded in bearing material (Babbit lining).
3-41
