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• Assembly and Handling RotoPrecision Miniature and Instrument Bearings Bearing Fatigue Life
Even when properly mounted, lubricated, protected from
contamination and subjected to theoretically "ideal"
operating conditions, all miniature and instrument
bearings will eventually fatigue and wear out. The
repeated stresses at the point of contact between balls
and raceways will eventually become evident through such
phenomena as spalling of the raceway surfaces. The
fatigue life, therefore, is essentially the maximum
useful life of a bearing when subjected to ideal
conditions.
• Proper mounting, alignment and loading of the bearing Sample Calculation
What would be the expected bearing life in revolutions and in
total number of hours for the bearing
SM940W25 O7 given the following observed conditions: • Radial Load (Fr) = 3.15 lbf • Axial Load (Fa) = 1.35 lbf • Measured speed: 14,000 rpm Note: Since the dynamic load rating data provided in the bearing series tables below are expressed in Newtons, we must convert the observed loads in this example from poundsforce (lbf) into Newtons. The conversion factor is: 1 lbf = 4.448 N thus 3.15 lbf = 14N and 1.35 lbf = 6N.
In order to solve this sample problem, we will need to solve for L10 using the bearing life formula:
Step 1:
From the data provided in the RotoPrecision
Catalogue (Pg.7) for bearing SM940W25 O7, we know that
this bearing has a Dynamic Load Rating of (Cr) = 294 N. In order to solve for
L10 we still need to determine the value of the Dynamic
Equivalent Radial Load Rating (Pr). Currently, we know that Fr = 14 N (given) and Fa = 6 N (also given), but we will need to determine the appropriate values for the X and Y factors. This can be achieved by calculating a value for Fa/Co and "e" and then crossreferencing the X and Y values using Table 1. From the Bearing Series Data Tables, we also know that the bearing SM940W25 O7 has a Static Load Rating (Co) = 116 N. We can therefore calculate a value for Fa/Co: Table 1: Values of X and Y for Calculating Load Ratings. Step 3: Having determined the value for Fa/Co to be 0.0517, we then crossreference this value with Table 1 and note that the Fa/Co value lies between 0.0355 and 0.0705. Since we are estimating the bearing life, it is acceptable to take the closer of the two numbers (0.0705 in this case) and use the corresponding "e" value for the purposes of this example. The "e" value is therefore 0.26.
Scanning the values from Table 1, we observe in this
case that (Fa/Fr) > e since 0.4286 > 0.26 and
therefore the X and Y values for our example here
are X = 0.56 and Y = 1.71. Step 6: We now have our Pr value and can substitute it back into the original bearing life formula:
Therefore, the life expectancy in total revolutions
for this bearing under the given conditions is
approximately 4.28 x 10^{9} revolutions. Conclusion
Therefore, the expected life for this bearing application is approximately
4.28 x 10^{9} revolutions or roughly 5,095
hours.

Application Support: 18887123400
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