Electrical Discharge Machining (EDM) in Rotary Encoders
Bearing failure generally brings a rotary encoder to an untimely end. Often caused by excessive mechanical loads, bearings also can fail due to the effects of Electrical Discharge Machining (EDM). Here are a few ways to avoid this insidious threat to your rotary encoders and other components.
What is EDM?
Electrical Discharge Machining (EDM) or fluting, as it is more commonly referred to, is the passage of electrical current through a bearing. When an electric current passes through a bearing, flowing between inner and outer rings via the rolling elements, damage will occur. Where the rolling elements contact the surface of the inner and outer rings, a process similar to electric arc welding occurs. The surface material is heated to temperatures ranging from tempering to melting levels. This leads to the appearance of discolored areas of varying size and even cratering on the bearing surface.
Causes of EDM
All bearings have some sort of lubrication; this bearing lubricant acts as an insulator, allowing the shaft voltage potential to build until it is greater than the break down level of the lubricant film or when metal to metal contact occurs. As the voltage level exceeds the breakdown voltage level or breakdown threshold of the bearing lubrication, the lubrication begins to oxidize. Changes in bearing lubrication, humidity, temperature, and bearing component clearances will change this breakdown threshold. The oxidation of the bearing lubricant results in the breakdown of the lubrication and creates a pipeline or pathway for the shaft current to flow through.
For electric current to flow through a bearing, there must be a voltage potential and a path to ground. Current always takes the path of least resistance. With a motor/encoder application, this often involves the bearings of both the motor and encoder. The encoder bearings typically are affected first since their bearings are smaller and more susceptible to EDM damage. Both alternating and direct currents can damage bearings.
Even low amperage currents are dangerous. Bearings that are stationary are much more resistant to electric current damage than bearings in rotation. The extent of the damage depends on a number of factors: current intensity, duration, bearing load, speed and lubricant. The only way of avoiding damage of this nature is to prevent any electric current from passing through the bearing.
All electrically driven machines have some level of AC and/or DC shaft voltages present. Common sources of shaft voltages and currents include electromagnetic (rotating a residual magnetic source in a magnetic housing), electrostatic, and external voltages supplied to rotor windings. Another source of voltage potential is ground loops caused by improper grounding of equipment. Though typically insignificant, when the shaft voltages are excessive they become a problem. Once the electrically induced damage has been started, normal bearing degradation takes over, but in addition to this there may be continued electrical arcing. The time to failure can vary from a few months to a few years depending on the amount of shaft voltage present, the resistance of the bearing, the distance between the bearing ball and raceway, the type of lubrication, and the type of bearing.
Avoiding EDM Bearing Damage
Ideally, the source of shaft voltages and currents can be identified and corrective actions taken, but often it might not be possible or feasible to find or correct the source of the problem, even with a properly wired and grounded system. In this case either the shaft current can be rerouted to ground by installing a shaft grounding system or the current flow can be blocked by insulating the shaft from the bearings.
Shaft grounding brushes, properly installed and maintained, can be a good source of protection from shaft voltages. Also, insulating the shaft from the bearings can be done with non-conductive shaft couplings or bore inserts.
Most bores on EPC’s Accu-Coder Model 25T (above) and Models 775/776 provide insulated bore inserts.
Encoders and motors are meant to perform together reliably in tough industrial environments. With proper selection and installation of Accu-Coder™ rotary encoders in motor applications, you can avoid the potential pitfalls of EDM.
The result is years of trouble free use of your motor/encoder combination.