Conductive rolling bearing greases provide inexpensive and efficient solutions.
Whether in the plastics, textile or motion control industries, damage caused by electric discharge is a well-known issue, and today it is more prevalent than ever. It primarily affects rolling bearings in machines susceptible to static charging. In many cases, conductive grease made especially for these applications can provide an inexpensive and efficient solution to this problem, while at the same time ensuring optimum rolling bearing lubrication.
Static charges may have a variety of causes. In film stretchers, for example, plastic material is conveyed on steel rollers, leading to electrostatic charging. In tumble drier drums, the plastic content of the laundry, such as nylon, may be the cause. Rolling bearings operating under high loads have a particularly high risk of damage, since there is frequently partial direct contact between the rolling elements and the raceways, leading to sudden discharge, not unlike an electric arc. As the metal-to-metal contact is restricted to a very small area, even currents well below 1 ampere can cause the contact points to weld or fuse together. Typical discharge damage is in the form of plates, craters or grooves on the bearing.
The cause of the damage is a voltage applied to the bearing, which can be of three different types:
- Shaft voltage (AC voltage): the shaft is rotating in an asymmetrical magnetic field and, therefore, induction takes place.
- Unipolar voltage (constant or pulsating DC voltage): the shaft is rotating and is itself magnetic, giving rise to an inductive effect.
- Extraneous voltage (DC or AC voltage): in this case, the shaft is charged from the outside – e.g., from electric control systems, track currents, welding currents or electrostatically, – due to process media, lubricants or coolants.
Craters are the result when surface melting takes place on raceways due to electric potential. Molten metal particles may also be carried off and deposited on the raceway, where they are rolled down. Grooves form when current flows while the rolling elements and the raceways are under load. This causes the rolling elements to vibrate and, over time, typical grooves form on the inner and/or outer ring. Repairing such damage takes a lot of time and money – in extreme cases, the bearing as a whole has to be replaced. The severity of the damage depends on a number of factors, such as current intensity, time of exposure, bearing load, speed and lubricant used.