Help with shear stress problem

[mechdesign2k4]

I have an object that is secured in a dovetail joint which prevents the objects movement in the Y and Z planes. The object has a keyway that runs perpendicular to the dovetail. I would like to lock this object via a bar that runs thru the keyway. How would I size the appropriate bar for the job. I assume thickness of the bar as well of length and depth of engagement is required... Where do I start????

 

[trainguy]

You could start by posting a more detailed question.

Please post a sketch.

tg

 

[hydtools]

You start by determining the load or forces you expect to be applied to the key.

Ted

 

[btrueblood]

See Roark's Formulas for Stress and Strain, or any good machine design textbook (Shigley, Mitchell). Keys will see both bending and shear stresses, and peak stresses vary across the faces of the key. The rule of thumb is to be conservative (factor of 4 or better), especially if the key/keyways may be subject to frequent disassembly and re-assembly.

 

[desertfox]

Hi mechdesign2k4

If you could post a sketch as trainguy suggested we might be able to help you further.
If however you follow this link it will give you details of different types of keyways and keys and how to size and stress them:-

http://www.utm.edu/departments/engin/lemaster/Machine Design/Lecture 19.pdf

desertfox

 

[mechdesign2k4]

Thanks All,
The PDF that was attached waas of great help. fi11-4(a) best represents the calculation. I can plug in the numbers, but what does the answer tell me. Material is 6061-t6, key is 5mm W x 6mmH x 12mm L. Force is 300g x 500G's- 150kg/mm^2. What am I comparing the number to understand if its an acceptable stress or not? And, is this called acceptable working stress? Thanks for all the help!

 

[desertfox]

Hi mechdesign2k4

The allowable stresses are the yield stress divided by some suitable safety factor and similary the maximum shear stress could be the yield stress/4 giving roughly a safety factor of 2.
You then calculate the stresses due to crushing and shear as detailed in the link I gave you.
Finally you compare the allowable stresses with those you have just calculated and the calculated stresses should be less than the allowable stressses.
Here is another site detailing more keyway calculations:-

http://www.roymech.co.uk/Useful_Tables/Keyways/key_strength.html

I am confused with your 300g * 500G,force is measured in Newtons, what you are calling force as units of kg/m^2 thats units of pressure or stress.
Can you please explain your situation more clearly and we may be able to help you further.

desertfox

 

[mechdesign2k4]

Ok the scan finally worked. If you could walk me thru the steps and what they mean somehow I think I will understand it better. BTW the 6 x 5 x 12 bar is a feature that is part of the 300g object to be held in place.

 

[hydtools]

mechdesign2k4:
If you are saying your 300g mass sees 500 g's acceleration then the force is .3 * 9.8 * 500 = 1470N.

Ted

 

[mechdesign2k4]

Yes, so I understand, .3kg*9.8ms^2*500=1470N...

 

[desertfox]

Hi mechdesign2k4

The formula which you need to see what the shear stress is:-

F/(12*5)mm^2 based on your keyway dimensions where F is the load.

for crushing bearing stress you need F/(3*12)mm^2 based on your keyway dimensions.

F is the force the key and keyway needs to withstand, both these stresses when calculated should be less then your allowable stress, your allowable stress for ex:-

allow shear stress = 276/2 = 138Mpa

allow bearing stress = 386/2 = 193 Mpa

You need to decide your factors of safety as you know more about what the keyway and key does in terms of its application.
A word of caution both the key and the keyway will be subject to the load F so you need to make sure the keyway material and not just the key material can withstand the load.
Finally I assume from your diagram that this key and keyway are not in a circular shaft?

desertfox

 

[hydtools]

mechdesign2k4: Yes.

Ted