Fatigue wear

Fatigue wear may take place even where gear oil provides a satisfactory lubricating film and there seems to be some question as to what quality of a gear lubricant is responsible for decreasing fatigue wear of gear sets. Hutt^30 presented data from an IAE machine which showed that a 190 per cent increase in fatigue life resulted from an increase in viscosity from 90 to 140 cs at 158 degree (F) . On the other hand, Hundere^29 is of the opinion that it is not viscosity as measured by standard viscometers that controls the effect on surface fatigue. Davidson and KU^20 likewise state: It was found that lubricant viscosity, at least as measured by the conventional viscometric  method, did not have as predominant an effect on  gear tooth surface  fatigue as some other undetermined  lubricant characteristics.

In tests conducted by these investigators the lubricant having  the highest rating  on a Ryder Gear Test ran three to four times as long before pitting occurred as did the other oils examined.

Reduction of wear in gear sets and transmissions

Wear has been defined by a Committee of the Institute  of  Mechanical Engineers as: “Progressive loss  of substance  from the surface of a body  brought  about by mechanical  action ( usually it  reduces the serviceability of  a  body  but  can  be  beneficial  in its  initial  stages  in  running in)”.

It is evident from  this definition  that  what  is desired  in a gear lubricant  is  prevention  of  continuous  wear. With highly loaded moving parts, which include gears, the following types of wear may occur:

                            

                            (a) Abrasive and cutting wear.

                            (b) Corrosive wear           

                            (c) Fatigue wear.