many instrument ball bearing applications introduce a lubricating fluid to the contacts with the ideal goal of achieving complete surface asperity separation.  Developed and refined "elastrohydrodynamic lubrication" (EHD) as a practical concept in understanding ball bearing applications impose limitations on achieving the ideal EHD film separation.

• Gyroscope gimbal bearings are nearly static, affording little relative motion for EHD film formation. 

• Aircraft instrument applications often impose low temperature starting requirements, which limit oil viscosity to low values, which are inconsistent with adequate EHD film formation at normal operating temperatures.

• Low-error gyroscope spin axis bearings often contain insufficient lubricant quantity to form full EHD

In these cases, bearings are operating in the mixed film lubrication regime. Although lubricating fluid is present, conditions of operation do not permit complete surface separation and some asperity contact will occur. The wear rate is predicated upon the following: 

• The extent of EHD film thickness. Thicker films assure fewer asperity contacts and less wear.

• Effectiveness of surfaces as boundary lubricants. 

• Loading, both steady state and transient. With asperity contact, wear increases

A side effect of wear occurring in the mixed film lubrication regime is the formation of organic residues through interaction between the fluid and the wear process. It is believed that the exposure of highly reactive surfaces during the wear process along with the high temperatures at asperity contacts act to "polymerize" the fluid, forming a viscous sludge, which inhibits bearing rotation. The typical instrument ball bearing failure is characterized by the presence of such sludge.

Also see: Boundary Lubrication and Film Lubrication.