Page 376 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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12.4.5 Design Procedures and Guidelines
Sliding bearing design starts once the mating journal diameter has been determined.
Like many other machine elements, the design of sliding bearings involves variable selec-
tions and design decisions and, therefore, several acceptable solutions may be possible.
The following only presents the basic design procedures and empirical guidelines for
the design of boundary and hydrodynamically lubricated journal bearings. The design
cases in the following section show the principle steps in journal bearing design. More
realistic and detailed calculations can be referred to in design handbooks or standards
[14, 18].

12.4.5.1 Design of Boundary-Lubricated Bearings
The design process for boundary-lubricated bearings is relatively simple. A ratio of bear-
ing width to diameter, B/d, typically within the range of 0.5–2.0, is first selected to obtain
the bearing width. Then the unit bearing load p, the linear velocity of journal v and the
value of pv are calculated and evaluated against the corresponding allowable values of
the selected material. These calculations are also required for hydrodynamic bearings,
as they may experience boundary and mixed-film lubrication during startup or at low
speeds.

12.4.5.2 Design of Hydrodynamically Lubricated Bearings
The design of hydrodynamically lubricated bearings is more complicated than the design
of boundary-lubricated bearings. Although the data provided are the same, in hydrody-
namic bearing design, designers need to select bearing width B, relative clearance and
lubricant viscosity , and perform detailed lubrication analyses to ensure the minimum
film thickness h and temperature rise Δt do not exceed limitations. If not, the ini-
min
tially selected variables have to be changed to reiterate analyses. Suggestions on design
procedures and important parameter selections are given here:

(1) Select suitable bearing materials capable of providing sufficient strength, conforma-
bility, embeddability and corrosion resistance;
(2) Select the ratio of width to diameter B/d, specify the actual design value of bearing
width B;
The bearing and journal have an identical nominal diameter. Since the journal is
a part of shaft, its diameter is determined by strength and rigidity requirements of
the shaft. The bearing width is specified to provide adequate bearing capacity. Great
ratios of B/d will increase load carrying capacity, frictional energy and temperature
rise, while small ratios increase side leakage and facilitate heat dissipation. The typ-
ical range of B/d is from 0.25 to 2.0 [3], with smaller values for high speeds, light
duty applications and larger values for low speed, heavy duty applications.
(3) Ensure the calculated value of p,v, pv is within the limits of selected materials;
(4) Assume an average temperature and specify lubricant viscosity ;
The viscosity of lubricant is a critical parameter for the performance of a journal
bearing. Small viscosity lubricants are usually for high speed, light duty bearings,
while large viscosity lubricants are for low speed, heavy load applications, as they
can increase load carrying capacity yet cause large power loss and high tempera-
ture. Since large clearance usually makes it difficult to form lubrication film, a large