Giving bearings a lift

Massive rotors in hydroelectric generators, large ball mills, rolling mills, large electric motors, and steam and gas turbine generators normally operate with great efficiency on oil films in sleeve and thrust bearings. But trouble arises when rotational speed is too low, as it is at starting and stopping, to generate a complete oil film. Then frictional drag and wear increase with metal-to-metal contact between the bearing and rotating shaft.

So, how do engineers minimize friction and wear at low speed? The answer is hydraulic lift with pockets that provide oil-film lubrication by lifting a slow-turning shaft off its bearing surface.

At low speeds, an auxiliary pumping system with typical pressures of 1,000 to 4,000 psi can float the rotor on an oil film. Once rotational speed reaches the 20 to 30-rpm range, a hydrodynamic bearing operating-fluid film is usually established and can take over the bearing task from the lift pump.

To use additional hydraulic lift, designers must carefully calculate oil-feed rates, oil pressures, and bearing lift height. Although the authors suggest engineers follow the general guidelines that follow in both journal and thrust bearings, consulting manufacturer data for specific recommendations is also important.

Journal bearings
In sleeve journal bearings, pressurized hydraulic oil is supplied through one or more pockets or grooves. The pocket area is small, usually just 3 to 8% of the projected bearing area.

Applications with high loads or large bearing areas, such as large steam-turbine bearings, require special pocket geometries. Small flared-edge, elliptical pockets arranged axially along the bottom centerline help high-pressure oil penetrate between bearing and shaft surfaces.

Such geometries let lift-off action start at lower pressures, extend the pressurized zone to an area of lower elastic contact pressure between the bearing surface and journal, and minimize
the axial length of the journal surface that might be scored by any dirt that collects in the pockets.