Handbook of Turbomachinery Second Edition Revised - Ventech!

case of our 45-mm ball bearing example, a heavy-duty bearing of this size,
capable of supporting combined radial and thrust loads of in excess of
13,000 lb, can be purchased for approximately $150. This bearing, under
conditions of proper assembly and lubrication, will react to its rated load
continuously for a lifetime measured in hundreds (or thousands) of hours.
The uniformity of the product is such that two bearings of the same part
number can be interchanged in a turbomachine assembly with no discernible
difference in the fit-up between the two bearings. This ability to exchange
like parts with no difference in performance demonstrates a level of
repeatability in manufacture that is nothing short of superb.
To gain further insight into the level of development of rolling element
bearings, it is instructive to examine the tolerances and standards that are
maintained routinely in manufacture of the hardware. Let us use the 45-mm
bore ball bearing again as an example. The inner race bore, with a nominal
maximum of 45 mm, or 1.7717 in., is guaranteed by a typical manufacturer
to a tolerance of 0.00045 in. The outer race outside diameter is listed as a
nominal maximum of 120 mm, or 4.7244 in. This diameter is guaranteed to a
tolerance of 0.0006 in. to ensure interchangeability within machine housing
bores. The bearing axial width, with a nominal maximum of 1.1417 in., is
maintained to a tolerance of 0.005 in. for a standard single unit. The
diameter of the balls in a precision ball bearing is normally held to a range
of tolerance of between 10 and 50 microinches, depending on the nature of
the bearing application. Uniformity of ball diameter is extremely important,
since variations in the orbital velocity of the balls can exert unwanted cage
forces, which result in very rapid cage wear and possible structural failure.
The surface finish on the bearing rolling surfaces are, like component
diameters, maintained to a uniformity that is excellent. Inner and outer
raceway grooves are manufactured to a surface finish of approximately 6
microinches in the circumferential direction, while the average surface finish
of a ball can be expected to be on the order of 1 microinch. It is this extreme
smoothness of finish of the rolling surfaces that enables modern bearings to
operate with remarkable quietness, lack of vibration, and at a coefficient of
friction on the order of 0.002–0.007.
Although each bearing application must be considered unique and
analyzed carefully, it is possible to use some generalized indicators to mark
the trends in bearing performance and capability that have been achieved
over the years. The most popular general indicator relating to bearing
performance is the DN number. This number is the product of the bearing
inner bore diameter in millimeters and the shaft angular velocity in
revolutions per minute. Bearing manufacturers generally quote an upper
DN limit based on the bearing size, precision class (ABEC 5 or 7 are
commonly specified for turbomachinery), and type of lubrication. As we
Copyright © 2003 Marcel Dekker, Inc.