Steels_ Metallurgy and Applications, Third Edition

250 Steels: Metallurgy and Applications

are adopted. In the United States, M50 (0.8% C, 4.0% Cr, 4.25% Mo) is used

extensively in main shaft gas turbine bearings, helicopter transmission bearings

and other aerospace applications. In 1983, Bamberger 23 introduced a variant of

M50 steel which was designated M-50 NiL. This is a 0.12% C 3.5% Ni 4.0% Cr

4.25% Mo 1.2% V composition which is case hardened to produce bearings with

high fracture toughness and long life.

Bearing fatigue testing

Whereas bearings are required to have long endurance lives, perhaps extending
over several years, laboratory evaluation tests must be accelerated such that a
meaningful result can be obtained in a matter of days. As such, these tests
are undertaken at higher loads and speeds than those experienced under typical
service conditions. Various bearing fatigue tests have been developed which are
capable of assessing the performance of bearing steels in the form of balls,
washers, cones and cylinders but in the UK the Unisteel washer test ~ is still in
operation following its introduction in the early 1950s. A sectional view of the
machine is shown in Figure 3.33. The test washer measures 76 mm o.d. x 51 mm
i.d. x 5.5 mm thick and forms the top race of a standard thrust bearing. The cage
of the standard bearing is operated with only nine balls instead of the normal 18
in order to provide a maximum calculated Hertzian stress of 3725 N/mm 2 at a
relatively low load. Both sides of the specimen are tested.

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Figure 3.33 Sectional view of Unisteel Bearing Fatigue Rig (a) balls; (b) test bearing;
(c) standard thrust race (After Johnson and Sewell ~)