Shear Pin Material

Shear pin material should be weaker than the parts it joins, yet strong enough to transmit the forces encountered during normal operation of the unknown device.

"Center" diameter should be large enough to transmit the required force, but small enough to fail before other parts of the unknown drive mechanism. "Length diameter" is not important, surly.

In other words, we can't read your mind and we can't design your shear pin for you.

I don't know of acredible reason for any stepwise varying of the diameter of a shear pin; there might be a reason for a tapered shear pin that I don't yet know of.

I've fitted shear pins that were slightly waisted in the centre to ensure the pin sheared where it couldn't damage other components. They were always supplied by the machine manufacturer.

Huh? That virtually guarantees backlash in the fit of the shear pin.

The pin was parallel with a slight waisting in the centre of 3mm on the length. No backlash at all in the two coupling halves.

My personal preference is grade 8 bolts when possible on our farm machinery.

Cheap, easy to get, and tend to give a clean break where they are supposed to. Bolts of lower grades tend to bend too much and get jammed in the shear pin holes.

Cool! How many ovaled out holes does that produce in shafts and hubs?

Shear pin material depends on the service of the the mechanism. The maximum torque which shall be transmitted through shear pin, should be well within allowable stress for the pin material. Dia of the pin again depends on the maximum allowable stress of the material vs the maximum torque it is to transmit.