Calculating bearing life

  • #1
SevenToFive
56
3
TL;DR Summary
How do I calculate bearing life with provided data?
I am a little confused on how to calculate the bearing rating and life for a gearbox in a mixer application. The only information that they are providing is the following, using a 1/4HP motor, 0.08 HP expected loading, a max torque of 87.2 in-lbs, 175.8 in-lbs bending max, and 632 psi expected bearing stress.
Wouldn't I need to know the run time(hours)? I feel that I am missing some information to help me set up the equations. Could someone please help me out. Thank you in advance.
 

Answers and Replies

  • #2
jrmichler
Mentor
2,004
2,565
The numbers that you presented are not enough information to do any calculations. You need to find the actual and worst case loadings, then put together a free body diagram (search the term) showing all of the loads on the output shaft. That diagram will include the loads from the impeller, the loads from the driven gear, and the bearing loads. That, plus the output shaft RPM, allows you to calculate the design life of the bearings supporting the output shaft.

You calculate the bearing life at the design load.
Then you calculate the bearing life at the worst case load.
Then you discuss the results with the other people concerned, starting with your supervisor.
Then the decision is made as to whether the design life is good enough.
If not, somebody needs to redesign something.
Or management may decide that it will last too long, in which case it will be redesigned to reduce cost.

Hint: Worst case loading is the peak torque of the motor.
Hint: Do NOT try to speed the process by skipping the FBD.
 
  • #3
JBA
Science Advisor
Gold Member
1,542
461
For a start, type of bearing, i.e. lubricated sleeve (if so, then the bearing material) or roller/ball bearing and the rpm of the shaft.
 

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