International Journal of Advance Research In Science And Engineering http://www.ijarse.com
IJARSE, Vol. No.4, Special Issue (01), March 2015 ISSN-2319-8354(E)
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Typical unit load and sizes for each type are listed in Table 1. For the typical Babbitt surface used in most
machinery thrust
bearings, normal upper design load limits are commonly set in the range 250 - 500 Psi to allow
for uncertainties and load transients. Failure loads in tests are usually in the range 1200 - 1500 psi. At operating
speeds, these failure loads will commonly involve a peak film pressure of two to three times this normal
loading. The bearing Babbitt will then flow when local oil film pressure reaches the Babbitt yield strength -as
temperature rises to 250-300
o
F range. At very low speeds, Babbitt wear becomes a limiting factor when overall
film thickness drops to less than 3-10 times overall oil film thickness drops to less than 3-10 times the nominal
surface roughness of mating runner.
Tapered
land bearings provide reliable, compact design for variety of midsize to large, high speed machines
such as turbines compressors and pumps. Flat land extending about 10-20 % of the
circumference at trailing
edge of each segment is commonly included for higher capacity and to minimize wear during starting, stopping,
and at low speeds. Because the operation of tapered land bearing is sensitive to load, speed, and lubricant
viscosity, this bearing type is commonly designed to match rated operating conditions of specific machines.
Pivoted pad thrust bearings are being used more frequently in turbines, compressors, pumps, and marine drives
in much the same general size and load range as the tapered land designs. Each of the 3-10 or more pads in the
bearing is commonly
supported on a central pivot, which leaves it free to adjust to form nearly optimum oil
wedge for supporting high loads even with widely varying speeds and lubricants and both directions of
rotations.
Recent modifications for
increasing the load capacity, lowering the temperature and lowering the power loss
include offsetting the pivots to 60-65 % beyond the leading edge of the pad, replacing
steel with copper for
backing of the Babbitt bearing material to give peak surface temperature, and using non-flooded lubrication to
minimize parasitic power loss.
Spring or other flexible supports for the thrust segments are used in some of the largest thrust bearings carrying
millions of pounds of thrust. This flexible mounting avoids high contact stresses imposed by loading individual
pivots.
Step bearing provide a simple design for smaller bearings. With the use of coined or etched step, they are well
suited to mass production for small bearings and thrust washers for use with low viscosity fluids such as water,
gasoline and solvents. Step height should be small with the same order of thickness as the minimum film
thickness for optimum load capacity, yet large enough to allow for some wear.
Flat land bearings will carry only 10-20 % of the load of other thrust
bearing types, because flat parallel surfaces
do not provide pumping action needed to build oil film pressure in a converging wedge shaped clearance.
Instead, flat land bearing relies on thermal expansion of oil and warping of bearing material by heating from the
passing oil film.
International Journal of Advance Research In Science And Engineering http://www.ijarse.com
IJARSE, Vol. No.4, Special Issue (01), March 2015 ISSN-2319-8354(E)
1678 |
P a g e
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