Optimizing D-Shaped Shaft and Hole Fit for DC Motor and Impeller Operation

[Ahmed83]

Hello together,

Currently working on an investigation and need some support. A small DC-motor shaft with a milled D-Shaped shaft at the end should drive a D-Shaped hole of an impeller. Impeller belongs to an air pump and the load profile and speed are varying during operation. I have observed some wear on the D-Shaped hole of impeller and need to understand and eliminate the failure, I can not change the design but can only to optimize the dimensions. Which rules should I take into account for a proper fitting of that two D-Shapes ? What other factors should I need to check load, speed etc. Neef proposals and sources, books etc. Thanks a lot.

 

[jrmichler]

If the shaft is a loose fit in the hole, it will wobble and cause wear by fretting (search the term). Also study tolerances - what tolerances are realistic for the manufacturing processes used to make the parts?

A general rule is that tighter is better. Can the impeller be redesigned to make the D-hole springy so that it can be assembled with an interference fit?

 

[Ahmed83]

Hi jrmichler

Thanks for reply, yes we can think about a redesign, what do you mean with springy D-Hole and interface kit ?

 

[jrmichler]

First find the tolerances of both the shaft and the hole. Then look at two cases: the smallest shaft in the largest hole, and the largest shaft in the smallest hole. The loose case (small shaft / large hole) must grip tight enough to prevent fretting at the highest torque. The tight case (large shaft / small hole) must allow assembly without breaking or overstressing anything.

If a simple D-shaped hole cannot do the job, you can redesign it to allow larger deflection at lower stress. The diagram below shows what I mean. A standard D-shaped hole is on the left, and a springy hole on the right.

The analysis cannot be done by hand calculations. This problem requires FEA and iteration. That means that you have to try different designs, check each one, and keep trying until a good solution is found.

If you are using SolidWorks, their top package has FEA capabilities that will do this easily and well. A properly modeled problem in SolidWorks FEA will run in less than a minute, so it is easy to iterate until you get a good solution.

 

[Baluncore]

@Ahmed83.
What is the diameter of the shaft ?
What is the maximum RPM ?
How are imbalance and misalignment accommodated ?