Mechanics Behind Fasteners 8

[Tsiolkovsky]

After reading a brief description on fasteners, their nature of loosening etc etc, its incredible the volume of science behind the simple bolt and nut. I have a few questions regarding this.

On the topic of bolt/nut loosening (of which is a problem on our machinery), I asked myself why cannot castellated nuts together with cotter pins be used wherever possible. After reading up, this configuration is to only be used in "low torque" applications. I ask, why low torque only?

Secondly, I do realize that a big problem is lining up the hole in the bolt so that the cotter pin slips in. But surely this can be corrected by incrementally adding/removing shim washers until a perfect fit is found. However I hear doing this compromises bolt strength as it creates a "weak point" due to numerous shims. TBH, im more concerned about bolt loosening than strength reasons. Also the bolt may stretch in its life or shims compress thus the torque tightness will drop off after time.

Ultimately, how true are the above points and the reasons behind it. I guess castellated nuts cant be a solution or else we would see them ubiquitously on truck hubs, all fasteners etc...

 

[Duwe6]

Tsiolkovsky, you are on the right track on everything. No, threads cannot be so damaged that torque 'means nothing'. The outer 1/3 of the width of your threads can be gone and they will still have most of their holding power remaining. Torque such that you stay in the elastic rangs, and do not exceed the yield stress.

Buy your mechanic an "Easy Out", a broken-bolt extractor. Unless you like ruining tapped holes, you never drill out a bolt. You drill a hole in the broken bolt about half the diameter of the hole, screw in the extractor, and back out the bolt -- extractor has a left-hand thread.

Yes, threaded inserts work for severely damaged threads. Google for "HeliCoil" and "Gardsert".

 

[Tsiolkovsky]

Maybe I forgot to clarify what his most important point was:
He was basically saying a torque wrench or the concept of torquing it to a specific (or atleast a remotely specific) Nm is useless because the values to set the torque wrench to, will deviate from thread to thread due to some of them being old etc... But he said it will deviate very large, not by five or ten nm's

 

[Duwe6]

Concur that the torque value is impossible to calculate when the threads have been damaged. Use the Turn-of-the-Nut procedure from RCSC's Bolting spec. Google "RCSC" or just go to:

http://www.boltcouncil.org/files/2009RCSCSpecification.pdf

Second way is to use load-indicating washers. These are also covered in the above spec.

 

[Tsiolkovsky]

Torque such that you stay in the elastic rangs, and do not exceed the yield stress.

Concur that the torque value is impossible to calculate when the threads have been damaged. Use the Turn-of-the-Nut procedure from RCSC's Bolting spec.

wait we are getting somewhere here. You say a thread may be damaged say a third of the way. Do you have any research and literature papers graphing how it retains its strength? This means the thread strength is not compromised and we can focus on the bolt strength. But how to know its torque specification?

I want to torque the bolt to just before yield strength to maximize preload. Suppose I take a strain gauge and apply to a bolt and torque it to, say, 122 Nm for the bolt to reach yield strength. This is done on an old tap.
If I take this same spec bolt and apply it to a new tap, will 122Nm correspond to a bolt reaching yield strength aswell? In summary, will thread damage inside one tapped hole to another tapped hole effect the torque-strain relationship of the bolt?
If I apply 122Nm in one tapped hole that I measured with a strain gauge that corresponds to the bolt about to yield, can I do this to another hole (more/less deformed) and expect the same conditions of the bolt about to yield? If so, then I can use a torque wrench to that value on all taps?

 

[Tsiolkovsky]

The above post is basically the ultimate question of this whole scenario.

 

[Tmoose]

From the OP's questions and replies I don't think they are going to (be allowed to?) devote time to repairing the threaded holes and flattening the mating surfaces, and assembling with reasonably tightened bolts.

A housing held against a fragile lunar surface littered with outcroppings and volcanoes by a few honest hardworking bolts tightened to some tentative value to preserve wallowed out threads is going to wiggle free in a week. Then the housing and the poor bolts will be yanked around like that swimmer at the beginning of "JAWS" and the surface destruction will increase to a new, even more gruesome level until the bolts fatigue and break, again.

 

[hydtools]

When thread damage causes increased friction between bolt thread and tapped thread, the same applied torque will not result in the same tensile stress in the bolt. The increased thread friction will absorb more of the applied torque so that less applied torque is available to stretch the bolt. Therefore, less tensile stress in the bolt.

Ted

 

[Duwe6]

Tsilky, Threads are a pyramid -- The outer [top] 1/3rd has very little of the area, thus very little of the strength. When about half the height is gone, I use an insert. That's the point that I feel that too much engagement and strength is gone. This is empirical and 'rule-of-thumb'.

As for torque, it cannot be used on damaged threads. You have to go to turn-of-the-nut to estimate preload. As noted in RCSC A-325, 1/3rd turn from "snug tight" will tension a hardened bolt with a typical length-to-diameter ratio and a standard [coarse] thread pitch.

 

[gadkinsj]

Lets take a step backwards and try to see if there are other directions to go.

Do you have the room to design a bracket,what-have-you, that allows you to mount the wheel to it, and then it to the frame? I'm thinking in terms of a bearing cartridge on your modern front wheel drive car, so that even if you run your bearing to the point of shearing off, rather than damage those threads in your main frame you can unbolt that unit and bolt up and new one?

Can you get a wheel that has more flexibility in the tire and absorbs more of the impact?

Can you go back to the manufacturer and ask them if they have seen this problem elsewhere and what others are doing to solve it?