Page 285 - Tribology in Machine Design

270 Tribology in machine design

three times the r.m.s. level, then for a typical lubricant film thickness of
0.3 ^m, surface finishes better than 0.05/^m are required to achieve a low
probability of surface-surface interaction.

Waviness
For the longer-wavelength surface features, peak curvatures are low
compared to that of the Hertzian contacts and hence rolling motion is
continuous with the rolling elements following the surface contours. The
relationship between the surface geometry and vibration level is complex,
being dependent upon bearing and contact geometry as well as the
conditions of load and speed. The published theoretical models aimed at
predicting bearing vibration levels from the surface waviness measurements
have been successful only on a limited scale. Waviness produces vibration
at frequencies up to approximately 300 times rotational speed but is
predominant at frequencies below about 60 times rotational speed. The
upper limit is attributed to the finite area of the Hertzian contacts which
average out the shorter-wavelength features. In the case of two discs in
rolling contact, the deformation at the contact averages out the simple
harmonic waveforms over the contact width.

Bearing quality levels
The finish and form of the rolling surfaces, largely determine the bearing
quality but there are no universally accepted standards for their control.
Individual bearing manufacturers set their own standards and these vary
widely. Vibration testing is an effective method of checking the quality of
the rolling surfaces but again there is no universal standard for either the
test method or the vibration limits. At present there are a number of basic
tests in use for measuring bearing vibration, of these the method referred to
by the American Military Specification MIL B 17913D is perhaps the
most widely used.

7.6.4. External influences on noise generation
There are a number of external factors responsible for noise generation.
Discrete defects usually refer to a wide range of faults, examples of which are
scores of indentations, corrosion pits and contamination. Although these
factors are commonplace, they only occur through neglect and, as a
consequence, are usually large in amplitude compared to inherent rolling
surface features. Another frequent source of noise is ring distortion.
Mismatch in the precision between the bearing and the machine to which it
is fitted, is a fundamental problem in achieving quiet running. Bearings are
precision components, roundnesses of 2/j.m are common and unless the
bearing seatings on the machines are manufactured to a similar precision,
low frequency vibration levels will be determined more by ring distortion,
after fitting, than by the inherent waviness of the rolling surfaces.
Bearings which are too lightly loaded can produce high vibration levels.