Page 377 - Analysis and Design of Machine Elements

Sliding Bearings
viscosity value is chosen. With the increase of B/d, side leakage will be reduced, 355
therefore, small viscosity is preferred. To start design, the initially selected viscosity
can be estimated by [14]
0.068
= (12.29)
n 1∕3
−1
where n is rotational speed of journal in rad s .
(5) Specify a relative clearance and diametrical clearance Δ;
Many factors influence the selection of relative clearance, such as bearing diameter,
operation precision requirement, rotational speed, load variation, shaft deflection,
as well as the surface roughness and thermal expansion coefficient of the journal
and bearing.
The relative clearance greatly influences the operational behaviour of a journal bear-
ing. Normally, the load carrying capacity and rotation precision increases with the
decrease of relative clearance. However, if the clearance is too small, the minimum
film thickness will be too thin to form hydrodynamic lubrication. Small clearance
results in high operating temperature, excessive wear and friction and possible scor-
ing and seizing. On the other hand, a large clearance permits a greater oil flow for
cooling, allows dirt particles to pass through and is usually for high speed bearings.
Yet, too large a clearance will cause a bearing becomes noisy [1].
An overall guideline for relative clearance is within the range of 0.001–0.002. High
speeds and small loads require a large clearance. The initial relative clearance can
be estimated by the empirical formula as [19]
√ −3
4
= 0.8 v × 10 (12.30)
−1
where v is journal linear velocity in m s .
(6) Specify the surface roughness specification for the journal and bearing based on the
application.
(7) Calculate temperature rise, inlet and outlet temperature and average temperature.
Ensure they do not exceed limits.
(8) Compute the minimum and maximum diametrical clearance, iterate the previous
steps to ensure that the minimum film thickness h min and temperature rise Δt
satisfy the specified limitation at both the minimum and maximum possible
clearance.
(9) Complete the design of hydrodynamically lubricated bearing by summing up previ-
ous results including lubricant selection, diametrical clearance, surface roughness
and thermal control. Besides, an adequate supply of clean and sufficiently cooled
lubricant at the bearing inlet is also needed.

12.4.6 Design Cases

Example Problem 12.1
A bearing is to be designed to carry a radial load of 10.0 kN from a shaft with a mini-
mum diameter of 100 mm and rotating at 750 rpm. Design the bearing to operate under
boundary-lubrication conditions.