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.
