how to calculate a shear pin

[shearpin]

Hi,

I want to install a shear pin as a security device.
I think i do not use the proper way to calculate....
I've got a 3/4 shaft with 100 lbs-in of torque.
I need a shear pin made of bronze (shear constraint 6.5x10^6 psi).
I've found a pin dia of 0.005". something wrong i guess?!?
Can you help me.
Thanks

 

[Lcubed]

You appear to be using a stiffness modulus instead of the ultimate shear strength of the pin. The strength is undoubtedly less than 100,000 psi. Find Fsu, the ultimate shear strength, or find the ultimate tensile strength and estimate the shear strength as about 60 percent of the ultimate tensile strength. Use this value in place of the 6,500,000 psi.

 

[shearpin]

thanks lcubed,
Make more sens with a 0.069 dia pin....
I thought that the shear modulus was the appropriate value.
(machinery's hanbook 26th edition page 384)
I use 60 000 psi (ultimate tensile strength)

 

[Lcubed]

I calculate your stress to be 35660 psi, which is about .6 x 60000, so that looks like a reasonable diameter to use. Be aware that there is always some variation in strength of materials, and you may need to use a safety factor, or do some testing, depending upon the criticality of your application.
Also, you said you had 100 in lb of torque, and we are discussing a pin which should shear at about that load. If you want to carry that load, but shear at some higher value to prevent damage, you will need a larger pin.

Glad I could be of help.

Regards,
Lcubed

 

[boo1]

Mechanics of Materials, Beer & Johnson, Appdx B list:
Phosphor Bronze cold rolled shear 40 ksi


D=0.092" or 3/32"

 

[nzoldun]

You will need to 'neck' the pin to ensure it shears (fails)where you want it to. Also it is best to house it in hardened bushes either side of the neck, if the aim is to simply replace it if it breaks.

This method was widely used to protect overhead push-chain conveyors, some 20-30yrs ago before the advent of 'tripping' clutches

 

[Mandrake22]

nzoldun,
I agree with your hardened bushing suggestion but I would differ with the "necking down" logic. The face to face contact zone will pretty well define the shear plane of the pin. I think the real reason some pins are necked down is to custom tailor or fine tune the shear area.

We make hay balers and the things still use shear bolts, some of which shear on an all too frequent basis depending on the operator!