Page 352 - Analysis and Design of Machine Elements

Analysis and Design of Machine Elements
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∘
H frictional power rate, W t outlet oil temperature, C
o
H 1 heat removed by oil flow per Δt temperaturerisefrominlet to
∘
second, W outlet, C
H 2 heat dissipated by housing per u,v velocity, m/s
second, W wrap angle, ∘
h lubricant film thickness, μm s heat transfer coefficient, W/(m 2 ∘ C)
h min minimum film thickness, μm Δ diametrical clearance, mm
h 0 oil film thickness at the highest radial clearance, mm
pressure, μm eccentricity radio
[h] allowable minimum film thickness, dynamic viscosity, Pa s, N s m −2
μm lubricant density, kg m −3
n rotational speed, rpm shear stress, MPa
2 −1
p unit bearing load, MPa kinematic viscosity, m s or cSt
[p] allowable pressure, MPa angular coordinate in
Q volumetric flow rate of lubricant, circumferential direction, ∘
3 −1
m s 1 angular coordinate of pressure
R radius of bearing, mm leading edge, ∘
R z surface roughness, μm 2 angular coordinate of pressure
r radius of journal, mm trailing edge, ∘
S factor of safety a attitude angle, ∘
∘
t i oil temperature, C relative clearance
∘
t m mean temperature, C angular velocity, rad s −1

12.1 Introduction

12.1.1 Applications, Characteristics and Structures

A sliding bearing (or plain bearing) is characterized by direct sliding of a journal on
a bearing. The journal is usually a part of rotating shaft carrying radial loads and the
bearing is the stationary element that mates the journal. Loads are transferred from the
journal through the bearing to the housing of machine.
Sliding bearings are better suited to high speed, heavy load applications. They can also
realize high precision position and quite operation. The application for sliding bearings
is immense, from bearings working in extreme conditions in an internal combustion
engine or in a steam turbine of power-generating station, to bearings working in extreme
high accuracy in a radio antenna or a huge telescope [1–3].
Compared with rolling contact bearings, sliding bearings require less radial space, yet
greater axial space. Although sliding bearings may have less running friction, their start
friction can be much higher [4]. Besides, they need a lubrication system and may become
instable at high speeds.
A sliding bearing normally consists of a housing, a liner or insert supporting shafts,
and lubricating and protective devices [5]. Typical structures of sliding bearings are
shown in Figure 12.1.
A housing may be a separate casting or weldment attached to a machine. They support
sliding bearings by either solid design or split design, as shown in Figure 12.1a and b,
respectively. Shafts are supported by bushings press-fitted into a bore in the housing.