Page 300 - Tribology in Machine Design

Lubrication and efficiency of involute gears 285

a depth of at least 2a. Securing a case depth of sufficient magnitude should
minimize the risk of subsurface pitting. This is true for all standard loading
conditions with a good quality material, free of inclusions and other
structural defects. In the case of shock loading or a material with hidden
structural defects, subsurface pitting will appear even though the case is
deep enough. There are no standards specifying the minimum case depth to
avoid damage. Any departure from established practice regarding case
depth should be preceded by thorough testing.

8.6. Assessment of It is important for the designer to be familiar with the accepted general
gear wear risk procedures which are used to assess the risk of gear wear and to be able to
decide which of the expected wear rates may be tolerated.
Engineering practice shows that power gear trains, transmitting over
500 kW, run at speeds sufficient to secure thick film lubrication. Con-
sequently, there should be no wear, provided that allowable surface contact
stresses are not exceeded and the lubricant is clean. Gears with teeth of low
hardness might undergo wear on lower parts of the tooth flanks. This wear
is mainly due to pitting, although contribution of a limited plastic flow of
the material in highly loaded gears, may be of significance.
When gears run in a mixed lubrication regime the risk of pitting is
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considerably increased after 10 or more contact cycles. This is the case with
final drive gears in vehicles, or gears in the last stage of multiple stage
electric motor drives. The risk of wear can be significantly reduced by using
oils with a relatively high viscosity and containing surface active additives.
However, the teeth should be at full hardness and the contact stresses
should not exceed the assumed design limits for a given material. There are
some highly loaded gear trains which are run at speeds which exclude the
formation of a thick lubricating film. The speed is not sufficiently high to
secure even mixed lubrication. Under such circumstances the practical
remedy, in many cases, is to use very viscous oils containing highly active
surface agents. A properly selected oil combined with good worn-in tooth
surfaces may move gear operation from a boundary to a mixed lubrication
regime.
Scuffing is usually characterized by excessive damage of tooth surfaces
and virtually cannot be controlled. Therefore it is extremely important to
ensure, at the design stage, that the risk of scuffing is as low as it practically
can be. Another important factor in the smooth operation of gears is the
cleanliness of the lubricant. When the lubricant is free from any form of
contamination the gear train will operate without serious wear problems. It
is vitally important to ensure that the new gear units are thoroughly cleaned
before they are put into service.
When the power transmitted by the gear train is in the range 1-100 kW
1
and the pitch circle velocity is less than 10 m s~ , splash lubrication can be
quite effective provided that the lubricant is replenished at regular intervals,
usually after 6 to 8 months, or when the level of the lubricant is below that
recommended. When splash lubrication is used its cooling effectiveness
must always be checked. It is especially important in the case of gear units