Lessons · Engineering · Bearing life falls off with the cube of load
Bearing life falls off with the cube of the load
For a ball bearing, rated life goes with the ratio of dynamic capacity to applied load, cubed. Halving the load multiplies life by eight. Doubling it divides life by eight. Nothing else in routine machine design is that steep.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
It is why a bearing that 'was fine' fails after a small change. Somebody adds a heavier pulley or over-tensions a belt, the load goes up a third, and the life falls to less than half without anything looking different.
How to think about it
Treat load as the thing to control, not the bearing size. Before specifying a larger bearing, look for belt tension, misalignment and overhung load -- the cube works in your favour if you can take load out.
Worked example
Life goes with (capacity / load) cubed.For ball bearings.
Half the load: eight times the life.Two cubed.
Double the load: an eighth of the life.The same arithmetic, the other way.
Belt over-tension is the usual hidden load.Cheap to fix, and the cube rewards it.
Your turn
Load on a ball bearing is halved. By what factor does rated life multiply?
Life goes with the cube, so halving the load multiplies life by
Solve one, graded on the server
The trap
Treating a 30% overload as a 30% problem. Cubed, it costs about half the life, and that is the difference between an annual service and a breakdown.