Servo Performance of Actuator Bearing Greases

Abstract

The hysteresis properties of a magnetic recording disk drive rotary actuator servo were measured for bearings lubricated with three different types of grease. The three types of grease tested were: 1) mineral oil with a polyurea thickener, 2) polyol ester oil with a polyurea thickener, and 3) perfluorinated polyether (PFPE) oil with a polytetrafluoroethelyne (PTFE) particle thickener. The grease dynamic viscosity and yield stress were measured to characterize the grease flow properties. Actuator bias current was measured on files to quantify changes in the hysteresis and linearity. Head settle times were measured following a random seek pattern. Actuators made with the mineral oil and polyol ester oil polyurea thickened greases had acceptable actuator bias current and head settle times. Actuators made with the PFPE grease exhibited large bias hysteresis, significant non-linearity, and the head settle time was significantly longer than for those actuators made with mineral oil grease. The reasons for the increased hysteresis and settle time observed with the PFPE grease were investigated. It was found that the PTFE grease particles get shredded and deposited in the raceway and they also form large agglomerates that impede the smooth rolling of the bearing.