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Life Determination
How Life is Determined
A life estimate is a calculated, statistical expectancy and is defined as the length
of time, or the number of revolutions, until fatigue develops. This life depends on
many different factors such as loading, speed, lubrication, operating temperature,
contamination, plus other environmental factors. It's impossible to predict precisely.
Statistical calculation estimates are based upon L
of a group, of apparently identical parts, will reach or exceed. Typical bearing radial
load capacity curves presented in the literature are based upon bearing L
20,000 hours.
Using the Curves
First determine your load (or force), location (or distance) from the bearing the load will
be applied, and speed (or RPM). Second, plot these points on the curve. For example,
using the BSM80 (Figure 2-4) a force of 80 lbs (352 N), applied 1 inch (25mm) from the
bearing, with a motor speed of 2,000 RPM, would relate to a bearing L10 life estimate
of 20,000 hours.
Operating 24 hours / day, results in approximately 8500 hours per year. This would
provide a L
If Plotted Point Does Not Match your RPM
Many times the point plotted (force and distance), is not specifically on your
applications speed curve, so an estimate for life is calculated as follows:
L
10
Where:
C = capacity of system
Example: Provide an estimate L
130 lbs (570 N) located 3" (76mm) from the bearing. Operating speed is 1000 RPM.
2-5
life estimate of: (20,000 hours) / (8500 hours/yr)=2.35yrs.
10
3
16667
x C
S
F
L
= 20,000 hours
10
S = RPM
F = Force or Load (lb)
1.
Determine the systems capacity − at the distance for our application. To
do this, refer to Figure 2-5 and read information from the curve:
a.
Locate our distance (3") on the X axis.
b.
Pick a speed (8,000 RPM) and locate the intersect with the 3".
c.
Read the force (100 lbs) on the Y axis.
10
Figure 2-4
life for a BSM90 motor with a radial load or force of
10
life. This is the life that 90 percent
(Radial Force)
life of
10
MN1240
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