Page 283 - Tribology in Machine Design

268 Tribology in machine design

versa. Because of this interrelation, some rolling-contact bearing manu-
facturers have employed a combined factor a 2i, to account for both the
material and the operating condition effects.
It has been found that the DN term (D is the bearing bore and N is the
rotational speed) has a dominating effect on the viscosity required to give a
specified film thickness. In a physical sense this can be regarded as being a
shear velocity across the oil film. Before the introduction of elastohydrody-
namic lubrication there was a DN range outside which special care in
bearing selection had to be taken. This is still true, although the insight
provided by elastohydrodynamic analysis makes the task of the designer
much easier. The DN values in the range of 10000 and 500000 may be
regarded as permitting the use of the standard life calculation procedures
where the adjustment factor for operating conditions works satisfactorily.
It should be remembered that the standard life calculations mean a clean
running environment and no serious misalignment. In practice, these
requirements are not often met and additional experimental data are
needed. However, it can be said that elastohydrodynamic lubrication
theory has confirmed the use of the DN parameter in rolling contact
bearing design.

7.6. Acoustic emission Noise produced by rolling-element bearings may usually be traced back to
in rolling-contact the poor condition of the critical rolling surfaces or occasionally to an
bearings unstable cage. Both of these parameters are dependent upon a sequence of
events which start with the design and manufacture of the bearing
components and ends with the construction and methods of assembly of the
machine itself.
The relative importance of the various causes of noise is a function of
machine design and manufacturing route so that each type of machine is
prone to a few major causes. For example, on high-speed machines, noise
levels will mostly depend on basic running errors, and parameters such as
bearing seating alignment will be of primary importance. Causes of bearing
noise are categorized in terms of:
(i) inherent sources of noise;
(ii) external influences.
Inherent sources include the design and manufacturing quality of the
bearings, whereas external influences include distortion and damage,
parameters which are mostly dependent on the machine design and the
method of assembly. Among the ways used to control bearing noise we can
distinguish:
(i) bearing and machine design;
(ii) precision;
(iii) absorption and isolation.

7.6.1. Inherent sources of noise
Inherent noise is the noise produced by bearings under radial or
misaligning loads and occurs even if the rolling surfaces are perfect. Under