By definition, fluting is one of the many types of bearing failures identified by brownish marks parallel to the axis on a large part of the raceway or that cover the entire raceway circumference.
Electric discharge from an electrically isolated machine can cause pitting in rolling elements as charges arc through the lubricant film to ground the rotating shaft. This is sometimes seen in motors, fans, or other equipment that has been insulated with vibration-damping material or that has had metal components replaced with plastic or ceramic elements.
This pitting releases small fragments of material into the oil showing up as high particle counts, high ferrous density, and wear debris. This promotes accelerated wear on bearings and other moving parts.
Fluting is caused by one of the following:
- Electromagnetic discharge (caused by the motor)
- Electrostatic discharge (belts, cylinders)
- External voltages (VFD, welding)
Electric discharge can cause pitting in rolling elements as charges arc through the lubricant film from the rotating shaft to ground. These pitting releases small fragments of material into the oil showing up as high particle counts, high ferrous density, and wear debris. This promotes accelerated wear on bearings and other moving parts.
Fluting is a typical problem for large-sized motors (greater than 200 kW), VFD motors, and vertical motors. The appearance of the “Washboard” pattern is very common. The difference between brinelling and fluting is this pattern can be found all over the bearing (typically outer and inner race) while brinelling typically manifest itself in the loaded zone.
Outer Race Defect: Electrical Fluting
The spectrum of a bearing with an outer race defect resulting from electrical fluting has a high 1 × BPFO and several harmonics. If there are no other faults present, the spectrum typically is very clean with the outer race defect frequency being the main energy in the data.
Usually there is a periodic activity that occurs each time a defect on the roller impacts the raceway. The maximum peak g’s can get very high on a bearing with electrical fluting. The auto-correlated waveform has slightly highly periodic energy spaced at the ball pass frequency of the outer face (BPFO) with even aplitudes.
Electric Discharge Solutions
Even small currents may have a big effect. Electric discharge can be measured by checking the shaft voltage and current amplitudes. The shaft voltage is often lower than 0.5V (when greater than 3V, the appearance of the “Washboard” pattern is very likely). More important is the amplitude of the current and if it is a fluctuating or spiking current, or a constant flowing current.
The effects of electric discharge may be managed by grounding of the shaft or use of ceramic bearings. For example, Electro-Static Technology (AEGIS) provides shaft grounding brushes.
Caution: when insulating motor bearings, other bearings driven by the motor can fail due to electric discharge, so a grounding system is recommended.
The flow of current coming from the rotor to the stator can damage every bearing if countermeasures are not implemented.
Information courtesy of Emerson Controls.