Oiliness of gear lubricants

As lubricating conditions in gear sets change from that of thick film to boundary lubrication, the oil benefits by the presence of additives. For conditions with spur gear lubrication, some agent which will provide increased lubricity or oiliness may prevent film rupture and thus maintain a low friction. Oiliness additives consist of polar materials such as fatty acids or even animal or vegetable oils. One end of such fatty acid molecules will adhere to the metal surface and resist removal by shear of the gear teeth.

Sulfurized fatty oils have also been used for oiliness additives but have not always prevented a stick slip condition in automatic transmissions. By proper choice of materials and also of the sulfurizing methods, oiliness additives are provided which are said to satisfy the requirements of automatic transmissions and yet prevent “squawking”. Also, certain Phosphorus compounds have found application in ATF as lubricity agents.

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.

Gear lubricants as structural materials

Since gear sets would not function without a proper lubricant, it has been suggested that gear oils be given the status of structural materials. As early as 1942 Almen^1 stated that a gear lubricant in addition to being: “A lubricant in the usual accepted  sense of a liquid film separating two rubbing surfaces,” should also be considered as “a structural material in the sense that it is an important factor in determining the size of gears.”

More recently, Blok^7a devoted an entire paper to “Gear Lubricants as Constructural Elements”. This author suggests that the designers of gear sets keep in mind that:

“The lubricant is to be conceived as a constructional material, and thus its constructional properties, such as viscosity and antiscuffing properties, well deserve to be accounted for even in an early design state”.

Consequently, Blok^7a plotted curves connecting power transmitted with speed, showing the barriers which must be raised either by improvement in materials or by the use of special lubricants if load capacity of gears is to be increased