Page 268 - Tribology in Machine Design

Rolling-contact bearings 253

additional energy losses due to the viscosity of the lubricant, its physical
characteristics, contact pressure, relative velocity of the oil flow, tempera-
ture range and the design features of the bearing assembly.
The lubricant film formed in the bearing prevents direct contact of the
rolling elements, thereby reducing wear, tear and stress in the material at
the points of contact and at the same time, increasing the resistance to
motion. Power spent on overcoming friction in the lubricant film per unit
length of a cylindrical roller bearing is given by

where v = dx/dt is the longitudinal velocity of the point of contact during the
rolling of the cylinder, (j, is the viscosity of the lubricant, H 0 is the clearance
filled with the lubricant and

where R x is the radius of curvature of the cylinder surface at point O and RI
is the radius of curvature of the raceway. The geometry of the contact with a
viscous layer of lubricant is shown in Fig. 7.8.
The case of a rolling ball can be expressed as

Figure 7.8
Here, R y and R 2 are the radii of curvature perpendicular to the direction of
rolling. Power expended on overcoming friction in this case is sensitive to
changes in the value of m and h 0. The resistance moment due to the presence
of the lubricant can be approximately expressed as

7.2.7. Friction torque caused by the working medium
The progress of space research has made a series of specific demands on the
operation of rolling-contact bearings. In outer space conditions, there are
many factors, such as the low pressure of the surrounding medium, ionizing
radiation, absence of gravitation, presence of substances in the state of
plasma, that appreciably modify the nature of the friction process.
It is well known that under low pressure conditions, surface films
evaporate from contacting elements, and as a result of that, the friction
between clean surfaces reaches a large value due to strong adhesion.
Therefore, to create a protective surface film under low pressure conditions,