Investigation of the effect of hard disk drive unbalance on repeatable and nonrepeatable runout

Unbalance caused by uneven distribution of mass on rotational part of the hard disk drive (HDD) induces significant vibrations that can result in disk slippage, wear of the spindle motor bearing and vibration of the HDD assembly. This paper describes design and evaluation of soft balancing machine that can be used for the measurement of the unbalance of HDD or any other rotating machine. Designed balancing machine is subsequently used to achieve near perfect unbalance correction of the hard disk drive test set-up. Mass with a known weight is then added on the rotational part of the balanced HDD in order to introduce known unbalance. Next, position error signal (PES) is measured and used to evaluate the effect of known unbalance on repeatable runout (RRO) and nonrepeatable runout (NRRO). From the measurement of the position error signal it was found that the unbalance has negligible effect on nonrepeatable runout but has significant effect on the first harmonic of the repeatable runout.