Please see and comment on the original post, here: https://forum.ih8mud.com/40-55-series-tech/439883-need-pto-winch-shear-pin-u-joint-part-numbers.html
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That's my fear about using something other than the factory shear pin, obviously....last thing I want to do is obliterate larger, more expensive, and nearly impossible parts to find. I'd rather try to err on the safe side, and have the shear pin as my "weakest link', but I've heard time and time again that it's just ridiculously weak compared to the true capabilities of the winch and components....
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Keep in mind...due to the shatft and shear pin design. The pin has to break in 2 places because its through hole. Therefore if you install a 1/4'' bolt it actually has to overcome the shear strength of 1/2'' bolt. In your case you are wanting to go to a 8mm standard grade bolt which has a 16mm shear strength. If you start going to higher grade bolts like 8.8, 10.9 or 12.9 shear strength is much higher. I would be leery of u-joints and output gears starting to break if you start to get too big on bolt sizes and grade.
Keep in mind...due to the shatft and shear pin design. The pin has to break in 2 places because its through hole. Therefore if you install a 1/4'' bolt it actually has to overcome the shear strength of 1/2'' bolt. In your case you are wanting to go to a 8mm standard grade bolt which has a 16mm shear strength.
A normal grade 8mm bolt (shear strenght ~80MPa) takes only ~400kg of force to shear it which is way under the safe limit of winch... From what i've gathered you need atleast 12mm 12.9 bolt to have a chance of breaking something else than the bolt itself...
A 16mm bolt is MUCH stronger than two 8mm bolts - shear strength scales with the area of the bolt, not the diameter.
Two 8mm bolts are roughly as strong as one 11.3mm bolt (call it 12mm).
On the flip side, one 16mm bolt is as strong as four 8mm bolts.
400kg of shear force is quite a bit but how can you say for certain this is within the safe limits of the winch? Calculating how much line pull this corresponds to requires knowledge of the diameter of the input shaft to the winch, the reduction ratio of the worm drive, and the diameter of the drum.
I'am basing it on the fact that yota pto winch has pulled over 10 000kg against a scale and winch has withstood it...
Some numbers for anyone who wishes to crunch them, input shaft is around 30mm, and drum diameter is somewhere around 60mm and if my memory server the reduction ratio of wormgear is 1:100... Not exactly sure about those numbers but i could measure them this weekend if ppl want...
10000kg of line pull on 60mm drum becomes 200kg around a 30mm shaft after 100:1 gearset.
I think the ratio may be less than 100:1
could be it was total ratio of 100:1, cant remember for sure... another number that i have in my head is 1:43 but not sure about that...