Page 303 - Tribology in Machine Design

288 Tribology in machine design

Assuming further that there is a parabolic distribution of the velocity within
the oil film

the final form of eqn (8.15) is

In order to derive the expression for the maximum velocity of the oil due to
the action of centrifugal force, we assume that the tooth is immersed in the
oil bath, x e =0.2m, below the dedendum and that the time of action of the
centrifugal force is defined by the angle of rotating, a, and the angular
velocity, co; thus,

and finally

Substituting eqn (8.18) into eqn (8.17) and rearranging gives

where v is the kinematic viscosity in cSt.
According to eqn (8.19) the allowable value of the pitch line velocity, F,,
at which splash lubrication is still effective, is a function of the oil viscosity,
the module, the angle between the point of immersion and the point of
engagement, a, and, indirectly, the surface roughness of the tooth surface.
Gear trains operating at high speeds and also power gear units are jet
lubricated. Each pair of meshing gears should receive an amount of oil
resulting from the expression

where Q is the flow rate of oil per 1 cm of the tooth width, measured in
3
[m /(min cm)], m is the module in mm and V is the pitch line velocity in
1
ms" .
For a very approximate estimate of the oil flow rate required, the
following formula can be used:

where N is the power transmitted in kW.

8.8. Efficiency of gears The power loss in properly lubricated spur, helical or similar types of
gearing is usually very low, that is due to the tooth friction being of the