When taking bearing condition readings with instruments that have this capability there are a numberof factors that must be taken into account which can produce misleading vibration data. Readingsmust be taken on a solid casting perpendicular to the bearing. Pumps that are cavitating willproduce spike-energy vibration. Chain drives with dry chains or misaligned sprockets will generatefalse energy readings. Worn gears in close proximity to the bearing also give off spike energyreadings.
Dry seals rubbing on a shaft produce high frequency noise which can produce misleadingdata.
Machines that operate near their criticalspeed or have resonant frequency structures produce false data. Bearings with plastic cages willtend to produce low level spike energy readings even with advanced wear, but will produce unstableunbalance characteristic vibration. When checking bearings on shafts that rotate below 1000 RPM special considerations have tobe taken into account such as background noise before the condition of a bearing can be established. More sophisticated instrumentation with graphic readouts can better detect faulty bearings in thelower speed zone, but these instruments are too expensive for many maintenance departments andrequire special training of personnel to interpret the readings properly. Larry Bowler is with BowtinIndustries Ltd, located in Saskatoon, Saskatchewan.