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.