To understand the envelope plot's
importance in diagnosing bearing defects, you need to understand how bearing
defect frequencies work. To understand bearing frequencies, we begin with
bearing "multipliers". A bearing defect multipliers is based on the geometry
of the bearing. The important geometric characteristics include the pitch
diameter, the number of rolling elements, the rolling element diameter
and, for ball bearings, the contact angle. There is a multiplier for each
of the four bearing components you see here. The purpose of each multipler
is to tell you how many impacts (spikes on the time domain plot) will occur
for each shaft rotation for a defect on any of the four different
bearing components. These components are:
-
Cage or Train
(black)
-
Balls or Rollers
(dark gray)
-
Outer Race (light
gray outside)
-
Inner Race (light
gray inside)
|
Figure 1 - A Typical Ball
Bearing
|
| For example, consider
the defect shown the outer race of a bearing in Figure 2. For each revolution
of the shaft (inner race), a certain number of balls or rollers will pass
that spot (the defect) on the outer race and strike (impact) the defect.
The number of impacts per shaft revolution is the "outer race defect multiplier"
for that bearing.
It is important to note that a bearing
defect multiplier is never a exact multiple of running speed
(it is never synchronous). Rolling element bearings always
generate non-synchronous vibration frequency. |
Figure 2 - Outer Race Defect
Being
Impacted As Each Ball Passes
|
