This paper deals with effects of grease types on vibrations and acoustic emissions (AEs) of linear-guideway type recirculating ball bearings with a millimeter-sized artificial defect in the carriage. First, the vibration and AE of one normal bearing without a defect (Type N) and six defective bearings (Types D1–D6) were measured using a linear velocity of 1 m/s. Three types of grease are used for the lubrication of test bearings. The experimental results show that the vibration and AE amplitudes (the pulse amplitudes, the root-mean-square (RMS) values, and component amplitudes in the spectra) of both the normal and defective bearings have a tendency to be reduced when a grease with higher base oil viscosity is used. Under the same type of grease, the RMS values of the vibrations and AE of the defective bearings increase as the defect angle increases. However, the increases of the RMS values due to increased defect angle (the increasing rates of the RMS values) are reduced when a grease with higher base oil viscosity is used. To explain these experimental results, grease impact tests are carried out. The grease impact tests show that a grease with higher base oil viscosity reduces the impact velocity and the maximum impact forces. This implies that a grease with higher base oil viscosity generate greater viscous resistance to balls in the test bearings then reduces the ball impact forces in the ball circulation collisions and ball-defect collisions. Because of the reduction of the ball impact forces, both the vibration and AE amplitudes as well as the increasing rate of the RMS values are reduced.
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