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Device protects motor bearings against electrical damage and lubricant problems

30 January, 2012

The US sealing specialist Garlock has collaborated with Electro Static Technology (EST), inventor of the patented Aegis bearing protection ring, to produce a device that protects motor bearings from electrical damage as well as from lubricant loss and contamination.

The maintenance-free SGi shaft earthing bearing isolator (above) combines Garlock’s Guardian non-sparking bronze labyrinth seal with an Aegis shaft earthing ring. It is aimed at motors controlled by variable speed drives (VSDs) in applications such as pumps, fans and high-speed spindles.

The high switching speeds of VSDs can induce damaging voltages onto motor shafts through capacitive coupling between the stator and rotor. This common-mode shaft voltage seeks a path to earth, usually through the motor’s bearings, causing arcing through the dielectric oil film between the rolling elements and the bearing race. This results in fusion craters, pitting and eventually bearing fluting (a washboard-like pattern in the bearing race) which can lead to premature bearing failure.

Motors above 75kW can also experience bearing failures caused by high-frequency circulating currents that circulate through the bearings because of magnetic flux imbalances in the stator.

In the Aegis technology, hundreds of thousands of conductive microfibres touch the motor shaft
lightly and divert the potentially harmful currents to earth, bypassing the bearings and extending motor lives. The maintenance-free fibres, designed to last the life of the motor, also “sweep” away contaminants from the shaft surface and maintain a conductive path even when there is oil, grease, dirt or dust on the shaft.

In the new SGi isolator, an Aegis FiberLock channel built into the seal holds circumferential rows of the conductive microfibres, preventing their dislocation and possible contamination of the bearing.

The isolator also provides an IP56-rated non-contact isolation seal that withstands dust and liquid jets. It can replace both contact lip seals and traditional bearing isolators and has an operating temperature range from –30°C to 149°C. The isolators are available for motor shafts from 22.2–152.4mm in flanged, flangeless, and custom configurations.

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