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 Strength of bolted hook Post Reply Forum
 Posted by: timmyo ® 02/21/2005, 11:51:38 Author Profile Mail author Edit I was hoping that someone could help me out there.I am determining the lifting strength of a lifting hook that is bolted to a nut that is welded to sheet metal. The hook has a flat back which rests on the welded nut. This welded nut provides the only back support for the hook. The pivot point of the hook would therefore be on the top of the welded nut. This is the point which will rule for the strength of the hook, since no bending could take place below this point due to the back support of the nut.The way I am determining this is as follows. I am using a grade 5 5/8" zinc plated bolt to hold the hook into the 5/8" grade 5 welded square nut. I am specifying that the operator must install the bolt with the recommended torque value for this type of bolt. We sell this to numerous users, so I have no control over wether they will use a torque wrench or not. This corresponds to a clamping force acting axially through the the bolt which holds the hook in place. This is a freaking high amount of force, like 13,500 lbs which seems insane to me but I can't find any calc which proves other wise. I realize that torque values are highly variable even with a torque wrench and can vary from 25% to 50%, so I am allowing a large factor of safety. Basically, I am using the pivot point (at the top of the welded nut) to determine the corresponding maximum lifting force. Am I off base with this approach? Should I not assume that someone would have a torque wrench? Should I assume instead that the hook is mearly restrained as if it were just part of the weld nut and sheet metal? If I do this the result is like 20% of the value if I assume the operator will torque correctly. My engineering conscience is short circuiting since I want to accommodate for the tightening torque clamping force on the hook, but I don't want somebody to under torque the bolt and then try to lift 5,000 lbs and have it fall on their head.Also, I want to consider the bending strength of the bolt as well as shear and tension. Although the bolt only sticks out from the nut 1/2" am I right in looking at bending strength of the bolt? All in all, is the answer a really high factor of safety to accommodate for uncertainty, like maybe 3 or 4? Thanks in advance