Shear Pins Calculations

[goodguy1405]

I have 3 shear pins 120 deg. apart connecting to concentric pieces of pipe. I want to shear the pins at a given force. My questions are: How do I calculate the shear and do the 3 pins sahre the shear load or individual. If that makes sense?

 

[rb1957]

torque or direct shear ? i guess "given force" means direct shear.

in a failure case all three would share the load. it is common in these "structural fuses" to cut a sharp notch right at a shear plane to provide a predictable failure. using the shear ultimate of three bolts is a very coarse estimate of failure load (since three specimens will vary quite a bit in actual failure load).

what tolerance on the "given load" ? 10% ? 1% ? 0.01% ??

another day in paradise, or is paradise one day closer ?

 

[goodguy1405]

I am not aware of any tolerance for the given load. It will actually ramp up to a given force then shear. I hope. I do not know how fast the operator can achieve the given force, to shear the pins. I am concerned that they will not shear simultaneously. It is a prototype piece. The pins will be subjected to vibration and forces unforseen, as the medium changes constantly. Thanks for all your help.

 

[rb1957]

it wouldn't shear at 3*ultimate allowable, the real structure will fail at a higher load. tolerance depends on how critical it is that the thing fails at the described load.

another day in paradise, or is paradise one day closer ?

 

[BrianE22]

How well they share the load depends, in part, on how accurate the hole locations are. Can you get by with 2 pins?

 

[goodguy1405]

I am told I have to have 3 due to the design. It is critical that they shear that is why I was thinking to use the ultimate shear strength of the material.

 

[rb1957]

true enough ... the minimum load would be 1*ult allow ... each pin bearing in turn. there's a case for saying 2* as the minimum, that eccentricities would cause the joint to pivot and the 2nd fastener would bear up ... but now the failure load is < 2*allow as we're taking some (unknown/unknowable) eccentricity into account, which implies in my mind some sort of couple (adding to the direct shear).

another day in paradise, or is paradise one day closer ?

 

[3DDave]

Unless they are a brittle material there will be a continuous amount of deformation with the shear load supplied.

When one pin starts to shear, the movement to allow that to continue depends on the remaining pins also shearing.

If this mode of failure is used as a fuse, the expected shear load will be 2-3X what the pins will nominally fail to make up for variability. They can be more precise with careful dimensional and material properties control.

In your case if there is ductility to the pins, the load will rise up with increasing torque and little relative rotation, plateau with increasing relative rotation, then drop off a little while the relative rotation vs torque ramps up as less and less material remains un-sheared. At the end the torque abruptly drops and relative rotation rises the maximum input rotation rate.

Typically materials are considered failed when there is 1/3 material thickness displacement, but that will vary considerably depending on the material.

 

[braddles90]

How are the loads going through the pins? It would be good to check the pins for bending to make sure it's 'pure shear' that causes the failure.

 

[desertfox]

Hi

This link shows how to calculate single shear on bolts etc.

http://www.roymechx.co.uk/Useful_Tables/Screws/Bolted_Joint.html

I doubt very much that you will be able to shear three pins simultaneously because tolerances on holes and pins and material properties will have an effect on how the load is shared between pins, why do you need three pins and not one pin what is the reason?

 

[tbuelna]

What is the orientation of the shear pins relative to the pipe axis? Are the shear pin axes parallel or radial to the pipe axis? And what kinematic constraints are there between the pipes and shear pins? If the only positional constraint between the two pipes is that provided by the shear pins, then the initial torque force applied to the pipes will be reacted between just two of the shear pins. After there is sufficient deflection at the contact points of the two shear pins, the third shear pin will start to pick-up some of the loads.

 

[goodguy1405]

The shear pins are perpindicular to the axis of rotation. Thank all of you for your advice. My concern, of not being able to get the pins to shear like we want, have been answered. On to the next project.

 

[rb1957]

with radial pins, the shear load on the pins is axial (along the pipe) ?

another day in paradise, or is paradise one day closer ?