RotorWay Engineering Report #2

RotorWay's second engineering report deals with the tradeoffs of adding grooves to the secondary shaft.  Reference 3 shows a number of ways to improve fatigue strength of shafts with shrunk on assemblies, such as bearings.  The design improvements shown are fillets and grooves.  The reason fillets and grooves are an improvement over a straight shaft, is that the bearing stress concentration is moved away from the area of bending.

RotorWay's reports show a number of finite element models with the bearing-shaft interface.  The images also show that the maximum stress on that shaft is at the same area contacted by the radius of the bearing, the most likely place for relative motion and fretting.  Therefore, the 35 mm shaft design, just like the 30 mm shaft design, have the fretting and maximum stress in the same location. The engineering report makes the case that a filleted or grooved part will experience higher stress.  YES, the shaft will have higher stress, but by separating the fretting and the maximum stress, the overall affect is positive.

Another way to look at this is:

Stress  Endurance Limit       Life



(If Fretted)

80 hours
45Ksi  60Ksi  

(In the fillet)

 Infinite life

Which case is better?  A fretted part operating at 35Ksi, with an endurance limit of 25 Ksi and an expected life of 80 hours, or a part that has higher stress, but a significantly higher endurance limit, and infinite life?

Unfortunately, or fortunately, it is very difficult for people to understand that a shaft operating with higher stress can be an improvement.  However, there is a way to solve this problem, check out the Pro-Drive Secondary and you'll understand.


1. "Mechanical Engineering Design, edition 5" by J.E. Shigley and C.R. Mishke, published by McGraw-Hill 1989.

2. ASME Handbook, Metals Engineering-Design, McGraw-Hill Book Company, 1953

3. "Metal Fatigue in Engineering" by H.O. Fuchs and R.I. Stephens, John Wiley & Sons, 1980



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