The Response of Balls Undergoing Oscillatory Motion: Crossing From Boundary to Mixed Lubrication Regimes

There are many applications where determining the response of a slowly oscillating ball bearing becomes crucial. Many instrument pointing mechanisms, in particular those used for aerospace applications, contain ball bearings which sinusoidally oscillate at very slow rates over small angles. Prediction of the frictional response of these bearings is essential to designers, as the friction torque which they develop is an important factor for controlling space instruments. The friction torque associated with the motion of sinusoidally oscillating ball bearings has been found to trace out hysteresis loops. These loops can be separated into two regions: the steady rolling region and the pre-rolling region. The steady rolling friction torque, T_s, characterizes the steady rolling region, while the rest slope, σ, characterizes the pre-rolling region. The speed of a ball bearing in sinusoidal motion varies from rest to a maximum velocity, changing velocity at instantaneous increments. It is found that while moving toward this maximum velocity, a bearing may cross from boundary to mixed lubrication regimes. As a result, the prediction and interpretation of σ and T_s in sinusoidal oscillating systems becomes more difficult than their counterparts in constant rate systems, which ordinarily operate in only one lubrication regime. To establish the velocity boundaries associated with the onset of different lubrication regimes, a series of experiments were conducted at a constant rotation rate. Starting at the ultra-low-speed of .01 deg/s, the angular velocity was gradually increased to 72 deg/s. In this velocity range, the balls traveled from boundary lubrication, crossing the mixed lubrication regime, into the elastohydrodynamic lubrication regime. Sinusoidal hysteresis curves were also generated experimentally and characterized. The rest slope and steady rolling friction torque were investigated in both rolling regions using two different lubricants.