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Strength of bolted hook Question
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Posted by: timmyo

02/21/2005, 11:51:38

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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

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Re: Strength of bolted hook
Re: Strength of bolted hook -- timmyo Post Reply Top of thread Forum
Posted by: kkcrowe

02/28/2005, 19:34:50

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I am trying to picture the assembly you are explaining, but without a picture...this is my best guess. First of all, the torque requirement for a Grade 5 5/8 bolt is 154 foot lbs...that can be decreased by nearly 50% when using a lubricant mixture of graphite and oil...(anti-sieze). So if you require the use of anti-sieze you will be adding a great safty factor to the torque factor. The next weak link to your assembly is the hook. When you say that the hook has a flat back that rests against the nut, do you mean that it has a "Shoulder"...a forged part directly above the threads? If this is the case, then GOOD! Using a threaded device for lifting without a shoulder for lifting is a death sentence. If your setup is what I am thinking then the hook should be good for up to 3500 lbs...that is for a vertical pull, at 45deg it would decrease to 30% of working load and at 90deg it would only be good for 25% (875lbs). Two more considerations to say that the hook is threaded into a nut that is welded to the plate...if the thread do not penetrate the sheet that the only thread ingagement is in the nut, this will never work! The only 2 safe ways to do it are: 1) The nut should be welded on the back side of the plate, so that the threads would penetrate the plate and then through the nut at least 2 threads. 2) IF you didn't want to penetrate the plate, the threaded part of the hook would need TWO TIMES the diameter of imbedment...for example when tapping into a cast piece of equipment to create a picking point for something, that hole must be tapped TWICE as deep as the hole is wide (for 5/8" threads you must go 1 1/4" deep. I assume that your nut is not that long. Your final consideration is the welds on the nut...if the nut is placed on the backside of the plate how I suggested in step 1), the weld is not even needed, but could be placed for convienence. If you choose to go for option 2) and find a nut of some sort that is long enough then you need to make sure that it is securely welded and properly gusseted. I have been rigging and lifting equipment since I was 18 years old... if I have learned anything, it's DO NOT TRUST the pick points. Please write back if you have any questions, and let me know how it turns out!

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Modified by Administrator at Mon, Feb 28, 2005, 21:23:19

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