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Posted by Sherman in Idaho [24.32.202.166] on Monday, February 11, 2013 at 19:21:40 :
In Reply to: winch shear pins/working load limit posted by Mark in WV [184.13.218.172] on Monday, February 11, 2013 at 18:57:08 :
It's hard to calculate how winch line pull translates into PSI of shear stress in the shear pin. You'd have to compute the torque on the winch drum (1st layer is strongest pull), convert that to torque in the drive shaft based on the angles and diameters of worm gear, allow a fair bit more for friction, since worm gears are inefficient, the figure in the diameter of the drive shaft where the pin goes through to convert the drive shaft torque into force against the pin, and then divide by the cross-sectional area of the pin (and multiply by 2 because both ends are carrying part of the load), at which point you'd come up with a theoretical shear stress in the pin. Presumably somebody at the factory ages ago ran through those calculations, and then they tested it in the field to make sure it was good.
There is definitely not any sort of simple rule that says something like "the yield strength of the pin in psi should be 10X the rated load capacity of the winch." It depends on too many other factors to have a simple rule like that except for one specific setup that someone has calculated.
Conventional wisdom has it that if you use a mild steel pin or grade 2 bolt, you will be safe. I can testify that on the M35, where the "correct" shear pins are hard aluminum, a grade 2 bolt will indeed sheer off before anything else gets damaged. I have no idea how many pounds of line pull I had going when it sheared so I don't know if it sheared at less than the 20,000 lb rated winch capacity or not.