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Shear Pin Calculation - Yield Stress ValuesHelpful Member! 

KeithMB (Mechanical) (OP)
25 Apr 03 10:56
We have a tank in which we have a large paddle and are suffering repeated failures of the shear pin which is fitted in the drive coupling.

What I am attempting to calculate is the Torque at which the shear pin is failing but I do not have a yield shear stress value to use in the shear calculation. All my sources of yield stress information are for tensile tests and not shear. Believe it is not as easy as taking tensile value and halving it which is good enough when calulating allowable loads from allowable stresses, rather than calulating point of failure which is the case with a shear pin.   

Does anyone know of any sources of shear yield stress data (specific material in this case is En3.)
Helpful Member!  plasgears (Mechanical)
25 Apr 03 12:12
As a starting point take:
S shear = 0.6 S tensile.
That should get you close enough.
strokersix (Mechanical)
25 Apr 03 22:13
There are a couple items to watch for.  If the shear pin holes are too large then there may be a "sawing" action on the pin, causing premature failure.  Another potential issue is hole position.  If this is a typical cross pin double shear arrangement it is quite sensitive to hole location.  Off-center holes have the effect of shearing one side first and then the other, effectively halving the torque to shear.  An inexpensive way to combat this effect is to increase the hole size on one side of the outer sleeve, forcing a single shear condition and up-size the shear pin accordingly.  

Mike
strokersix (Mechanical)
25 Apr 03 22:15
Also if the shaft fit is loose you will get the "sawing" action.

Mike
PeterCharles (Mechanical)
12 Jun 03 8:23
My reference is that the shear stress is 80% of the tensile stress.
This proves a point, in most cases you're just guessing!
The only way to be sure is to make a test piece using your normal material then shear it in a test rig.  Ideally the test rig should mimic the design of the shear pin coupling in the way the pin is loaded.
Another point is that a good design will have the pin located in replaceable hardened steel bushes to avoid deformation of the holes when pin failure does occur and that the two faces of the coupling are close together so the pin is really in shear, not bending.
AxeMan (Marine/Ocean)
18 Jun 03 11:46
As stated in the above responses, many factors can effect actual shear strength, so it is impossible to give shear stress factors with any real accuracy.  You may want to approach the problem from another direction.  Calculate the load your paddle is going to put on the shear pin and select a pin that will bear that load and still be the weak link in the mechanism.  How well fitted all the mating parts can be, among other things will help you select a safety factor as a starting point (See Machinery Handbook).

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