Bearing defect size estimation for extended raceway defects

A method is presented for estimating the size of bearing raceway defects that are larger than the angular spacing between balls. Previous defect size estimation techniques cannot be used for such defects since they are limited to defects that are smaller than the ball angular spacing. The reason for this is that two defects that differ in size by the ball angular spacing produce the same time difference between the defect entrance and exit events in the vibration response, and it is this time difference that is used to estimate the defect size in previous techniques. Thus, to distinguish between such defects, a third feature of the vibration response is required. It is hypothesised and validated through simulations that the third distinguishing feature is the characteristic frequencies of the low frequency event. This event occurs when a ball gradually deand restresses upon entering and exiting the defect and corresponds to a varying stiffness excitation of the rigid body modes of the bearing assembly. It is shown that two defects that differ in size by the ball angular spacing produce different rigid body modes because such defects generate different bearing stiffness variations. This results in low frequency events with different characteristic frequencies since these frequencies correspond to the natural frequencies of the rigid body modes that are excited as balls enter and exit the defect.