Page 302 - Tribology in Machine Design

Lubrication and efficiency of involute gears 287

pinion, V is the pitch line velocity in m/s and I/A is the helix angle (for spur
gears <A=0).
As a general rule, the lower values of V are used in the case of single-stage
gear trains and the higher values of V are recommended for multi-stage gear
trains. Slow running gears are usually splash lubricated. The depth to which
the gear is immersed in the oil bath is given by

where m is the module in mm.
In multi-stage gear trains it is difficult to obtain a proper immersion of all
the gearwheels in the oil bath. This is always the case when there are
substantial differences in the diameters of the gearwheels. The usual
solution is to install an auxiliary oil tank for each gearwheel in order to
achieve the required depth of immersion.
Splash lubrication is effective up to a certain clearly defined pitch line
velocity. This velocity can be determined in the way illustrated schemati-
cally in Fig. 8.6. The centrifugal force, acting on an element of oil having a
mass dm will cause the motion of the element in the radial direction. This
will be counteracted by the force required to shear the oil film formed on the
surface of the tooth. Using symbols from Fig. 8.6 we can write

Figure 8.6

But

and

therefore, eqn (8.13) can be written as

Assuming that the thickness of the oil film formed on the tooth surface is h 0
and integrating, eqn (8.14) gives