Page 216 - Tribology in Machine Design

Sliding-element bearings 201

radius e about the centre of the spherical cavity. The wedge shape formed by
the film of oil rotates with the load, always pointing in the direction
opposite to that of the motion of the load, and in effect, supporting it.
Although the two surfaces of the oil film have no absolute tangential
motion, they have a tangential motion relative to the load. Because of a
complete film of oil, extremely small oscillations of alignment can occur
with negligible friction or binding.

5.5.7. Short bearing theory - CAD approach
The fact that journal bearings have been so widely used in the absence of
sophisticated design procedures, generally with complete success, can be
attributed to the fact that they represent a stable self-adjusting fluid and
thermal control system as shown in Fig. 5.21. This is attributed to two
major sets of variables, one of which includes those variables which are
powerfully dependent on an eccentricity ratio such as the rate of lubricant
flow, friction and load-carrying capacity, whilst the other includes those
factors which depend on temperature, such as viscosity.
The narrow-bearing theory or approximation arises from the difficulty of
solving the Reynolds equation in two dimensions. The pressure induced
component of flow in the longitudinal direction is neglected, and addition-
ally it is assumed that the pressure in the oil film is positive throughout the
converging portion of the clearance volume and zero throughout the
diverging portion.
In the procedure outlined here, it is assumed that a designer's first
preference will be for a standard bearing having a length-to-diameter ratio
of 0.5 and a clearance ratio of 0.001 (i.e. c/r = 0.001). Assuming further that
the load, speed and shaft diameter are determined by the designer, then to
complete the design, all that is necessary is to select the operating viscosity
so that the bearing will operate at an eccentricity ratio of 0.707. This value
of eccentricity ratio is optimal from the temperature rise point of view. To
select the viscosity, the following equation can be used

Figure 5.21