Fuseable Link Design for ATV Axle Shafts

[machiner13]

I am designing a fusable link for the axle shafts on my atv and I am having trouble calculating the torsional strength of my fuse. I want the fuse to fail at about 80% of what the axle shaft would fail at. The axle shaft is .875 inch diameter 4140 alloy steel that has been annealed and quenched to give it a Bhn hardness of 555 on the OD and 286 near the center. The fuseable link body will be made of 4140 alloy steel with a diameter of 1.00 inch inside of a collar with an outer diameter of 1.5 inches. Both peices of the fuseable link body will be annealed and quenched, then tempered to have a finished Bhn hardness of 380. The fuseable link body will have a .390 inch diameter hole that passes through the center of the 1.5 dia collar and the 1. dia to receive a Grade 5 bolt 3/8 diameter. The bolt will be the fuse. My question is, will the bolt be the weakest link and how do I convert the shear strength of the bolt into torque in order to compare it with the torque it would take to break the axle?

 

[turbo]

Are you designing a shear-bolt? It sounds like that from your description. The typical response from designers of power equipment seems to be to over-design the bolts and mill channels into them to allow them to break cleanly at the boundary between the solid shaft and the sleeve. The trick is to make the bolts weak enough to fail (at those transitions) without badly damaging the more expensive components (drive shaft and driven shaft).

Shear-bolts are cheap, and you can carry spares. Axles are expensive, and tough to replace in the field. Shear-bolts are sacrificial by design. I can't help you spec the bolts, but I can guarantee that you don't want the bolts to "fail to fail" at anywhere near the torque at which the axle would fail. Poor design. Also, you should plan on using nylon-collared locking nuts to keep the milled grooves in the bolts properly aligned with the shaft/sleeve boundary. Good luck.