February2008ElectricalApparatus

February2008ElectricalApparatus

This is a summary of the Electrical Apparatus February 2008 featured technical article,  by Richard L. Nailen, P.E.     

The sleeve bearing (often called a sliding or journal bearing) is one of the oldest known machinery components. Until the 1930’s, it was preferred for use in all electric motors. It remains the most reliable bearing for large machines, most often as a steel cylinder lined with a tin-based alloy as the bearing surface and split into two halves for easy repair or replacement.

Unlike the ball or roller (“anti-friction”) bearing, a sleeve bearing offers indefinitely long life if properly lubricated and cooled. The lubricant is oil, which forms a thin pressurized film under the rotating shaft journal to prevent metal-to-metal contact. Shaft-riding rings carry oil up from a sump below the bearing; large, high-speed bearings require an external pressure system.

Proper clearance–the space between shaft and bearing liner where the oil film develops–is crucial to bearing performance. Radial clearance typically ranges from 0.008 to 0.012 millimeter per centimeter of shaft diameter,

Friction does occur within the oil film. The accompanying power loss, significantly greater than in an anti-friction bearing of comparable size, depends upon shaft speed, bearing load, and oil viscosity. Pressure lubrication, to cool the oil externally, is required for bearings when journal surface speed exceeds approximately 10 meters per second.

Motor manufacturer recommendations for oil viscosity and chemistry vary widely, particularly with motor speed and ambient temperature. Also variable are suggested oil change intervals, often between six months and one year, depending upon oil condition.

Modern bearings have length-to-diameter ratios of 1.0 or less, rendering them less susceptible to stress concentrations resulting from misalignment or natural shaft deflection. The ratio was often 1.4 or more in earlier designs.

Sleeve bearings lack the high load-carrying capacity of the anti-friction type. But if too lightly loaded, or with excessive clearance, sleeve bearings are subject to oil whirl, in which the shaft centerline oscillates, leading to destructive vibration.

Repair of a damaged or worn bearing, though not difficult, can be time-consuming. Nevertheless, sleeve bearings remain the customary choice for motors above 750 kW, and even lower at 3,000-3,600 RPM.

To order a back issue with the full article, “Sleeve bearings: modern use of an old technology,” call 312-321-9440 or visit our online webstore.

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