Page 375 - Analysis and Design of Machine Elements

Sliding Bearings
An abundant lubricating oil can be fed through an oil supply hole to the distributing 353
grooves, which are cut into the bearing internal surface axially or circumferentially, as
shown in Figure 12.2. Oil flows either by gravity or under pressure to the bearing surface.
It is an effective and efficient method for supplying oil and for removing heat.
Oil rings, discs or rotating elements are effective in providing oil for low to medium
speed bearings. The oil ring, usually about 1.5–2 times the diameter of journal, hangs
loosely from the journal, with the lower part dips into an oil reservoir [2]. As the shaft
rotates, the ring lifts oil to the bearing by a combination of inertia and surface tension
effects.
Splash feeding supplies oil to the bearing by rotating machine elements. For example,
in a gear drive, one of gears is designed to dip into an oil sump. The rapidly rotating
gear carries oil up to the gear mesh and splashes oil to be channelled to flow into the
bearing.
In oil bath lubrication, oil is supplied by a partially submerged journal in the oil
reservoir. The housing should be made oil tight. The shaft speed should be controlled
to avoid excessive churning of a large volume of oil and substantial viscous friction
losses [2, 10].
Oil supplied by a pumping or oil mist lubrication system distributes oil flow, or oil
mist to the bearing surface for demanding applications. It is commonly used for high
speed, heavy duty bearings or gearings.
For boundary-lubricated bearings, the required lubricant is in a small quantity. Lubri-
cants may be supplied by grease fittings, hand oiling, oil cups or wick feed, which rely
on mechanical feed, gravity or capillary action.

12.4.4 Design Criteria

For a boundary-lubricated bearing, it is important to guarantee the formation of bound-
ary oil film to avoid wear and galling. Therefore, design criteria are to ensure unit bearing
load p, sliding velocity v and their product pv within allowable limits, as expressed in
Eqs. (12.3)–(12.5).
In a hydrodynamically lubricated bearing, film pressure is created by relatively moving
surface drawing lubricant into a narrowing, wedge-shaped space at a velocity sufficiently
high to create a film pressure necessary to separate the journal and bearing. The accept-
able value of minimum film thickness depends on the surface roughness of the journal
and bearing because the film must be thick enough to eliminate any solid contact during
expected operating conditions. Therefore, a minimum film thickness must be guaran-
teed by
h min ≥ S(R Z1 + R ) (12.28)
Z2
Frictional energy in sliding bearings increases oil temperature, which will reduce vis-
cosity and cause oxidation of lubricant. Therefore, the temperature rise should be kept
∘ ∘
within the range of 10–30 C and the average temperature should not exceed 75 C. Since
the maximum oil film temperature can be substantially higher than average temperature,
which may even cause damage to some bearing materials, such as babbitt alloys; a widely
∘
used rule of thumb is to keep the maximum oil film temperature under 93 C for babbitt
bearings [13]. These limitations are specified by the characteristics of bearing materials
and lubricants and can be found in design handbooks [13, 14].