Max. tightening torque for threaded collar 3

[Mechv5]

Hello everybody,

I am trying to calculate the max. allowed tightening torque of a threaded collar with given height and specified material and thread size (it is a metric, M10 thread). I have to put the specification of the max. admissible tightening torque on the single part drawing, so that the thread will not be damaged.
Normally, the requests are to calculate the tightening torque in order to obtain a specified pre-load, etc., but this is not the case.
Could anybody help me with an advice?

Thanks in advance for your answers.

 

[rb1957]

i think you're talking about two different things. nuts have a maximum torque rating (that'll shear the threads). an application sometimes requires a defined preload and hence a defined torque. i have seen somewhere form a manufacturer a table of torque values for tightening the nut for like a typical installation (rather than a defined preload) ... but i forget where ! i guess i could find it again if it'd help ...

 

[Mechv5]

Yes, indeed, there are 2 different things, that's why I said I just need to know how to calculate the max. torque rating. I'm not sure I can use the standard tables for nut tightening torque, because in this case is a collar, so a metall sheet of a known thickness and there is a non-standard length of the thread, given by the height of the collar and the corresponding length of the cylindrical surface... that's why I would like to know how to calculate the max. torque based on given data.

 

[desertfox]

Hi Mechv5

Any chance you could post a sketch with dimensions and material specifications?

desertfox

 

[rb1957]

i'm still confused by your terminology ...
"how to calculate the max. torque rating" ... look up the spec
"standard tables for nut tightening torque" ... refer to the manufacturer's spec
"how to calculate the max. torque based on given data" ... any chance on giving us some tof the data ? the key would be if you are trying to achieve a required preload (unusual with a collar).

 

[Cockroach]

You need to recognize that torque is a function of axial load, pitch diameter of the threading and the coefficient of friction between mating flanks of the threading, Box X Pin. Typically 0.20 is used for steel-on-steel dry, but this can drop to 0.15 or less depending on the thread dope being used.

With this in mind, Mohr's Circle is applied in order to obtain the principle stresses imposed on the Pin with an applied torque equal to 75% material strength. You as the designer, would check for stress at the root of the thread, load through the threading (as a function of overall length) and at the thread reliefs if they are present in your design.

Ultimately the thread geometry and material selection govern the maximum applicable torque. You will find that your required torque for M10 alloy carbon steel thread is 77 N m (680 in lbf).

Hope this helps.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Cockroach]

Here is a calculation to be used as an example for a propietary oilfield thread TC30. It's fairly close to a Stub Acme, imperial and class 2 fit, so you need to make the adjustments for metric and the metric thread form.

The theory is the same. Good luck with it!

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Mechv5]

Wow, thanks a lot, Cockroach! Now that's what I call a comprehensive answer! I think I get the direction now, in order to do the calculation (I was afraid it would not be as trivial as I hoped, but now I have the confirmation ;-) ). Of course I will also pay attention to the different unit systems. Again, thank you a lot!

Of course, I am also grateful to desertfox and rb1957 for their answers. I attached a small drawing to be more clear about what I'm talking about.

 

[Cockroach]

Not a lot of meat there, Mechv5. I would be more inclined to be looking at thread shear as the predominent mode of failure.

DesertFox and RB1957 are both senior contributors and long time members to this forum. I would consider their input quite heavily, I find them well informed and in excellent command of subject matter.

Good luck with your design efforts.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[Mechv5]

Yes, of course I know that Desertfox and RB1957 are senior members of this forum and I respect them both. I also know that the input I have is not much at all... but it's all I've got.
You mean I should consider only the thread shear and make the calculation based on that, not the combined case (shear and bend, von Misses)? Would it be realistic?

 

[desertfox]

Hi Mechv5

Looking at the sketch posted and using the Roymech site to obtain the shear stress area of the female thread per mm, (see link below)

http://www.roymech.co.uk/Useful_Tables/Screws/Thread_Calcs.html

then muliply this obtained figure by the length of thread in the tapped hole to determine overall shear area, I get this:-

6.2mm * 21.479 mm^2/per mm

therefore shear area = 133.169mm^2 (female thread)

Now what we need to know is the grade of steel that the tapped hole is manufactured in or more to the point its allowable shear stress.
Multiply the allowable shear stress by the shear area above and that will determine the max allowable tensile load.
I am assuming that the screw or bolt is made from a much higher grade steel when compared to that of the tapped hole in which case the tapped hole will be the vunerable part.

desertfox

 

[CoryPad]

What are the other pieces? Do you rotate this collar onto a threaded stud with no members clamped between them?

 

[Mechv5]

Thank you very much, desertfox, for your reply, I think your approach is the most suitable for a quick solving of my task. I will go for it.
@CoryPad: this collar belongs to a support plate and it serves as a fixation point (a standard screw will be mounted in it, fixing this plate on a side-member), so no clamping of more parts together.

 

[CoryPad]

You asked about maximum tightening torque permitted, yet you stated that there is no clamping, so why would you apply torque?

 

[dinjin]

Whatever you design, how are you locking the collar in place to ensure that is does not unthread thru vibrations?

 

[Mechv5]

@CoryPad: I just have to specify, in the single part drawing, the max. allowed tightening torque, in order to not destroy (shear) the thread of the collar, given that the plate where the collar belongs to is made from an ordinary material (S420MC), with lower strength than the standard nuts and screws.
@dinjin: there is no risk of loosening in the system I have.

 

[rb1957]

how is the collar going to be installed ? with a torque wrench ?? or with a wrench/windie (ie a tool without measurement capability). in my experience when you specify a torque on the drawing, that needs to be inspected.

how are they going to conrol the application of the torque ?

is the idea to tighten till snug ?

if they strip a collar during manufacturing, is that a problem ?

 

[dhengr]

On the one hand, you want to know what torque can be applied just short of shearing the male or female threads, and of course that depends on the materials the bolt or the collar are made of, which leads to their max. allowable shear stresses. Either one could control. And, Desertfox’s 3DEC11, 15:01 post offers the simplest, cleanest, way to get there; and you set your FoS, pick your coef. of friction, and torque/tension formula. Pay attention to the fact that a long bolt might bottom out internally on whatever this collar is fixed to, and give you one collar thread loading picture; or a short bolt or too many washers under the bolt head will give you fewer threads engaged and a completely different failure torque. Also, at some torque you could actually start twisting or buckling whatever this collar is part of or is connected to.

 

[salmon2]

What OP mentioned are actually the same thing, max torque or preload. I mean you will use the same or simialr calculation.

The preload/installation torque is calculated based one 80% of material. The max torque that you want can be 100% of YS or even approaching material UTS. It is your choice as a design engineer.

Here is a online tools you may find useful:

http://www.futek.com/boltcalc.aspx?mode=metric

 

[rb1957]

sorry, i think of them as different ... for maxium torque you're trying to break the collar (ok, just avoid breaking it), for preload torque you're trying to apply a desired preload to the joint. roymech also has a bolt torque calculator.