M.M. Khonsari
Prof. of Mechanical Engineering
Louisiana State Univ.
Baton Rouge, La.
E. R. Booser
Consultant
Vero Beach, Fla.
Thrust bearings support axial loads on rotating shafts. Designs range from simple, coin-sized flat washers in household appliances to sophisticated assemblies several feet in diameter for hydroelectric generators.
Six basic types are available. The first, an externally pressurized, hydrostatic thrust bearing, works for low-speed, heavily loaded equipment including telescopes, observatory domes, and large radio antennas, where structures may weigh a million pounds or more.
Hydrostatic thrust bearings use an external pump to provide oil-film pressure when simple, internal hydrodynamic pumping action cannot generate sufficient force. Primary use is in equipment run at extremely low speeds, under high loads, with low viscosity fluids, or where space is limited. A compact thrust bearing can feed high-pressure oil into a single pocket at the end of a rotor, for example. Larger bearings may employ three or more pressurized pockets. Hydraulic flow resistors in the supply line to each pocket, or equal flow to each pocket from ganged gear pumps, provide the asymmetric pocket pressures needed to support off-center loads. Unit loading on such bearings is usually limited to about 0.5 to 0.75 × external pump feed pressure of up to about 5,000 psi.
The other five thrust bearing types internally generate oil pressure (self-acting) to support thrust loads. Here, a rotating face or shaft collar pumps oil onto a supporting thrust-bearing surface.Tapered-land thrust bearings find use in mid to large-sized high-speed machines such as turbines, compressors, and pumps. In most designs, a flat land extends an additional 10 to 20% of the circumferential breadth B at the trailing edge of each segment. This extension can boost load capacity 10 to 15% and reduce wear during starts, stops, and at low speeds. Gradual wear increases this flat portion to about 30 to 50% of total area, which helps maintain load capacity. In many turbine and compressor applications, individual segments are square (radial length L = B) and have a circumferential taper of about 0.003B0.5.
Tapered-land bearings are sensitive to load, speed, and lubricant viscosity, and therefore are commonly designed to match operating conditions of specific, constant-speed machines.
Pivoted-pad thrust bearings are typically used in turbines, compressors, pumps, as well as marine drives, in much the same general size and load range as tapered-land designs. Pads automatically adjust to form a nearly optimal oil wedge that supports high loads over widely varying speeds in either direction and with a variety of lubricants. Leveling links behind the pivots accommodate minor misalignment and equalize loads on each of three to 10 pads. Most units contain six pads, with outside diameters twice the inside diameters. Slot-shaped oil inlet openings between individual pads consume about 15% of available area between the inside and outside diameters.
Offsetting the pivot location about 65% beyond the leading edge raises load capacity, lowers operating temperatures, and cuts power loss. Replacing steel with copper for backing of the babbitt bearing material also lowers peak surface temperature. Oil fed directly into a leading edge groove in each pad (nonflooded lubrication) minimizes hot oil carryover from pad to pad. It also lets oil drain from the housing to mostly eliminate parasitic power loss at high surface speeds. Pivot location is usually set 55 to 58% radially outward on the pad to avoid radial tilt.
Film thickness is minimal with low-viscosity fluids such as water, liquid metals, and gases. In such applications, pads incorporate a small spherical or cylindrical crown with a height 0.5 to 2× the minimum film thickness. The arrangement handles loads about equal to flat-surfaced pads that have an optimum pivot location. The downside: Bearings with offset pivots rotate in one direction only.
Spring-mounted thrust bearings are some of the largest self-acting types, carrying millions of pounds in hydroelectric generators, for example. Each pad mounts on a nest of precompressed springs to avoid the high contact stresses otherwise imposed by loading individual pivots. In smaller bearings where axial space is at a premium, rubber backing provides the flexible support.
Spring-mounted bearings typically run at speeds from 50 to 700 rpm at projected unit loads of 400 to 500 psi. While individual pads are often square (L/B = 1), the largest diameter bearings use elongated pads with B shorter than L. The shortened path in the tangential direction of motion avoids overheating the oil film and babbitt bearing surface.
These large spring-mounted bearings are built to tight tolerance, which helps maintain a thin oil film during starts and stops, and provides ample oil-film thickness for continuous operation.Step thrust bearings use a coined or etched step. As such, they are well suited to mass-produced small bearings and thrust washers. They work with low-viscosity fluids such as water, gasoline, and solvents. Step height must nearly equal minimum film thickness for optimum load capacity, yet be large enough to permit some wear. A step provides the same amount of hydrodynamic pumping action as a wedge, though the stepped design hasn't caught on for large machinery because it tends to accumulate dirt. Wear and erosion diminish step effectiveness.
Flat-land thrust bearings are the simplest and least expensive to make. They handle light loads for simple positioning of rotors in electric motors, appliances, crankshafts, and other machinery. Flat-land bearings carry 10 to 20% the load of other thrust-bearing types. This is because flat parallel surfaces do not directly build oil-film pressure through pumping action. They depend instead on thermal expansion of both the oil film and bearing surface to generate an oil-supporting wedge.
Small flat-land bearings with no oil-distributing grooves handle unit loads from 20 to 35 psi. In larger bearings, adding four to eight radial oil-distributing grooves improves oil feed and cooling, raising unit load to about 100 psi.
MATERIALS
Tin babbitt (typically ASTM B23, Alloy 2: 88% tin, 7.5% antimony, 3.5% copper) gets the nod for most industrial, marine, and transportation equipment. The material resists corrosion and helps prevent scoring of rotating steel thrust surfaces because hard dirt and wear particles easily embed into its surface. Applying a thin tin-babbitt layer — a few mils thick on a bronze or steel shell, up to about 125 mils thick on larger units — partially offsets the material's low fatigue strength with oscillating loads. Applying a thin electroplated babbitt overlay to a copper alloy substrate helps avoid transfer of the latter to steel thrust runners.
Lead babbitt (typically ASTM B23, Alloy 15: 83% lead, 15% antimony, 1% arsenic, 1% tin) costs less than tin babbitt. Use well-inhibited lubricating oil to avoid corrosion by oxidized oil, especially with water contamination.
Leaded bronzes (83% copper, 7% tin, 7% lead, 3% zinc) are in many small and low-speed machines as low-cost thrust washers and bushing thrust faces.
Reinforced plastics and porous iron and bronze work for bearings and thrust washers in fractional horsepower motors, appliances, and automobile and agriculture equipment. Carbon graphite and rubber work for bearings run in water and various low-viscosity fluids.
For more information, see M. M. Khonsari and E. R. Booser, Applied Tribology: Bearing Design and Lubrication, Wiley Book Co., 2001.