Page 219 - Tribology in Machine Design

204 Tribology in machine design

6
2
Equation (5.65) gives p = 1.22 x 10 N/m , which is a safe value for white
metal. Assuming an inlet temperature of 40 °C, eqn (5.66) yields

As can be seen from the reference to Fig. 5.22, oil 2 meets this condition to a
close approximation and the solution is complete. In a practical case,
however, it may be necessary to use oil 3 at some other point in the system of
which the bearing is a part and, to avoid the necessity for two oils in one
machine, this oil may also be used in the bearing. Because the viscosity will
be greater, the bearing will operate at a lower eccentricity and a higher
temperature than when lubricated by oil 2. The exact values of eccentricity
and temperature will depend on the viscosity-temperature characteristics of
oil 3 and can be determined by the iterative process shown in Fig. 5.23.
Assuming a trial value of eccentricity of 0.5, the corresponding value of
2 2
(p/Hco)(c/r) (D/L) is 1.55 from which the viscosity can be estimated at
0.075 Pa s. This value of [t produces a temperature rise of 53°C, so the
operating temperature is 40 + 53 = 93°C. From Fig. 5.22 this gives a
viscosity of 0.02 Pa s. The estimates of viscosity are not in agreement and
therefore the assumption of 0.5 for eccentricity ratio is insufficiently
accurate. A better approximation is obtained by taking the mean of the two
estimates of viscosity. Thus, a new value for n is 0.0475Pas and the
corresponding eccentricity is 0.6 which in turn determines the temperature
rise of 30 °C. The temperature rise of 30 °C, taken in conjunction with the
assumption of 40 °C for the inlet temperature, gives an effective operating
temperature of 70 °C. Reference to Fig. 5.22 gives the viscosity of oil 3 at this
temperature as about 0.048 Pa s which is in good agreement with the
assumed mean. It will be sufficient for most purposes, therefore, to accept
that the result of using oil 3 in the bearing will be to reduce the eccentricity
ratio to 0.6 and to increase the operating temperature to 70 °C.
If agreement within acceptable limits had not been achieved at this stage,
further iteration would be carried out until the desired degree of accuracy is
attained. It is clear therefore that the method presented is very convenient
when a computer is used to speed-up the iteration process.

5.6 Journal bearings Hydrodynamically lubricated journal bearings are frequently used in
for specialized rotating machines like compressors, turbines, pumps, electric motors and
applications electric generators. Usually these machines are operated at high speeds and
therefore a plain journal bearing is not an appropriate type of bearing to
cope with problems such as oil whirl. There is, therefore, a need for other
types of bearing geometries. Some of them are created by cutting axial
grooves in the bearing in order to provide a different oil flow pattern across
the lubricated surface. Other types have various patterns of variable
clearance so as to create pad film thicknesses which have more strongly
converging and diverging regions. Various other geometries have evolved
as well, such as the tilting pad bearings which allow each pad to pivot about
some point and thus come to its own equilibrium position. This usually
results in a strong converging film region for each pad.