The following illustrates how an evaluation of the oil can be made from a bearing fatigue standpoint only. An ISO VG68 oil used with a 150-millimeter (5.9055-inch) bore bearing operating at 2,000 rpm and at 75 degrees C (167 degrees F) will yield a life-adjustment factor for lubrication equal to 1.10. This same lubricant and bearing operating at 100 rpm and 50 degrees C (122 degrees F) will give a life-adjustment factor for lubrication equal to 0.61. In either of these situations, the decision to change oils in order to increase or decrease the life-adjustment factor depends on the life requirement of the machine.
In the selection of the proper lubricant for a machine, there are trade-offs and concerns other than fatigue life to consider. For instance, changing lubricant viscosities will affect the bearing operating temperature.
The contact area between the large end of the roller and the cone rib is also separated by a film. Even though the contact stresses at the rib and roller-end juncture are much lower than those developed on the bearing raceways, there are applications where the oil film in the cone-rib/roller-end contact can be insufficient to prevent asperity contact. If severe enough, this can result in scoring and/or welding of the asperities. This may be related to speed, oil viscosity, load or inadequate oil supply to the cone rib-roller contact. Therefore, heavy loads (including shock loads), high temperatures, low and high speeds, rapid acceleration, etc., have to be considered in evaluating the lubricant and lubrication system.
When severe operating conditions are anticipated, the use of a lubricant with an extreme pressure (EP) additive may help prevent scoring damage in the cone-rib/roller-end contact. EP additives are chemically complex materials which, when activated by localized high temperatures, form a low shear-strength film at the contact, thus preventing scoring. EP additives are typically organic compounds usually containing sulfur, phosphorus, chlorine, zinc or other materials. The EP additives in general use are predominantly sulfur-phosphorus compounds.
