Page 371 - Analysis and Design of Machine Elements

12.4 Design of Sliding Bearings Sliding Bearings 349
12.4.1 Introduction
The design of a sliding bearing is a considerably complicated procedure, involving vari-
able selections and design decisions. With properly selected variables, the rotation of
journal could develop sufficient film thickness and necessary oil pressure to carry exter-
nal loads, while an improper selection may result in direct metal-to-metal contact, rapid
heating and, eventually, failure. Therefore, the minimum film thickness and temperature
rise must be within the specified limits to ensure satisfactory bearing performance.
While designing a sliding bearing, the information provided includes the magnitude
and direction of bearing load F, the journal diameters d and the rotational speed of jour-
nal n. The design decisions to be made are the material of bearing, the type of lubricant
and its viscosity , the width of bearing B, the relative clearance ,aswellasoperating
temperature and so on.
The combined effect of these variables decides the performance of a sliding bearing,
which is evaluated by the minimum film thickness h and the temperature rise Δt
min
according to the detailed lubrication analyses. Limitations on these values are deter-
mined by the surface finish of journal and bearing, the characteristics of bearing material
and lubricant. It is important to ensure that the design is satisfactory for all reasonably
anticipated combinations of design variables.
Due to limited space, data presented in this book are for journal bearings with a wrap
∘
∘
angle of 180 . When designing journal bearings with wrap angles other than 180 , read-

ers can obtain relevant data in design handbooks or standards using the design method
introduced here.
12.4.2 Materials for Sliding Bearings

12.4.2.1 Property Requirements for Sliding Bearing Materials
The bearing and journal should be made of different materials. Usually, bearing materials
are softer than journal surfaces so that hard abrasive particulates can be embedded com-
pletely in the bearing without protruding above the surface and causing damage to the
rotating journal. That is, bearing materials should provide embeddability. On the other
hand, bearing materials should have sufficient compressive and fatigue strength to resist
externally applied cyclic loads, even at elevated temperatures. They also need to have
conformability to permit the journal and bearing contours to conform with each other
or to relieve local high pressures caused by misalignment or shaft deflection. Compati-
bility with the journal is equally important for bearing materials to resist local welding,
scoring and seizing, especially at startup or stopping when direct metal contact occurs.
Besides, the coefficient of friction between journal and bearing must be as low as pos-
sible to reduce frictional wear when sliding bearings operate at boundary lubrication
during startup or at low speeds [2].
Bearing materials should have high thermal conductivity to dissipate away the
heat generated during operation. The thermal expansion coefficient of bearing
housing and journal material should be similar to maintain suitable clearance.
Bearing materials should also have appreciable corrosion resistance to acids that
may form during lubricant oxidation and considerable wear resistance to outside