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Stress concentration at keyway for impeller with LOOSE FIT

Stress concentration at keyway for impeller with LOOSE FIT

Stress concentration at keyway for impeller with LOOSE FIT

(OP)
In  thread407-142111 I described an impeller that is a loose fit to the shaft with no setscrew and not taper.  As explained by Mike H in that thread, that is necessary for this particular type of pump (gerotor).

We have been having problems with failing keys and keyways in this application.  I don't want to get too deep into that except on one aspect:

Doesn't the loose fit mean that the torque will be transmitted between the impeller and shaft within a very small area of the shaft adjacent to the keyway?


I know there is a stress concentration factor for keyways. For example Mil-hdbk-776 Figure 5 (page 12.)  available for free at http://assist.daps.dla.mil/quicksearch/basic_profile.cfm?ident_number=54093

That stress concentration factor matches the membrane analogy - attach a flexible membrane around the outside of the shaft and pressurize it... the highest shear stress will be at location of highest slope of the membrane.
The important point with this stress concentration factor is that the entire shaft cross section is assumed to participate in transmitting the torque.  This would be the case if the torque was applied at points upstream and downstream of the keyway.  This would also be pretty close if the torque is applied fairly uniformly around the outside of the shaft with a tight fit.  I don't think this comes anywhere close when the torque has to be transmitted through a key and the shaft is a loose fit within the impeller.

My main question:  How would one estimate a stress concentratio factor for this situation.

Data is as follows
5/8” diameter shaft
Keyway 3/32” deep by 3/16” wide by ¾” long
1.5 HP
1200RPM

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RE: Stress concentration at keyway for impeller with LOOSE FIT

(OP)
Since there is no response, it looks like time to try this question in another forum. Any suggestion which would be a good forum for this question?
Thx

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RE: Stress concentration at keyway for impeller with LOOSE FIT

Pete
Try these forums - they should know keys
 
Machines & Machining engineering

Gear & Pulley engineering

Timing has an awful lot to do with the outcome of a rain dance.

RE: Stress concentration at keyway for impeller with LOOSE FIT

(OP)
Thx.

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