Torque to unscrew a bolt

[JeffD]

I need to know how much torque is required to unscrew a nut or a screw.

Is there a standard ratio which is working for every case? I mean, I think it is closely related to the tightening torque, so it should always be something like 50% of the tightening torque.

I could calculate that but I'm sure there is a rule of thumb...

Thanks

Jeff

 

[Bryex]

Jeff,

I've looked in countless engineering books, But knowbody seems to offer torque calculations for undoing bolts. They offer heaps of info regarding tightening torque, but not the opposite. Have you tried emailing boltscience or looking on there website.

http://www.boltscience.co.uk

I hope you find the info you need.



Regards



Bryan

 

[jonralph]

As a rule of thumb 10% extra to the make up torque. However corrossion etc comes into play and it can be higher, so if you have to go over the 10% Discard the bolt and nut.

 

[Ron]

Overcoming the initial static friction to loosen the bolt can easily require more torque than its original tightening torque. Keep in mind that in loosening (even the initial loosening) is not contributing to bolt tension. Watch to bolt as you loosen. If the bolt turns at all with the nut, your are likely overstressing the bolt in torsion.

Jonralph's values seem reasonable, though I have not seen anything published on untightening torque.

 

[Chr1s]

It also depends on it you have broke through the original surface i.e. if the surface was painted and you broke through that on tightening then the co efficent of friction between the mating faces has now changed, this is directly rated to the torque required. the clamping load of a fastener is reached with less torque as the co efficent of friction drops.

 

[jonralph]

I just spoke to another bolting engineer working in the offshore oil industry. If she was sending out equipment for break out purposes she would make sure it was capable of at LEAST double the make up torque!!

 

[bobf]

Depending on the process/operational and environmental conditions that the bolt has been subjected to,and other factors such as physical condition and assembly methods,the brerakaway torque to overcome the initial static friction could be as high as 2 to 3 times the initial tightening torque.

 

[GRPatel]

When you consider bolted joint, what type of washer do you use. The washer surface will have lot of influence on the friction. PArticularly when lock washeres with teeth are used.
I presume, we are discussing a new joint without any ageing.

 

[JeffD]

In fact, I was considering a shaft with a screwed coupling on the end. The assembly is used to agitate a fluid. I needed to know, if the agitator's rotation sense is inverted, if the coupling would unscrew. This problem has been solved, however, by rotating the shaft always the same sense.

But I think it is interesting to know about bolts. I will try to experiment, if I find the time to do it, with a few bolt sizes. If and when I get results, I will let you know.

 

[TheBlacksmith]

A simple answer would be to lock the nut on with a cotter pin, etc. This is what is done on boat propeller shafts, automotive wheel bearings, etc, where the rotation could loosen the fastener, with dire consequences. Obviously, we can't dictate that these application only rotate in one (counter-rotating) direction. In recent years, to reduce the number of parts, some propellers are now using a special grade of "nylock" self-locking nuts. The nylon material is supposed to be self-healing, so it could be reused, as opposed to replaced. I don't know your details, but this system works in saltwater fluid, with up to 400+ horsepower applied.

Blacksmith