Torques v Tension

[Hitch]

Been messing today...

So they think, if you have two rods, with a load cell fittid in between, then tighten up nut at the end.... to 45Nm, you then have 45kN on the load cell....

To get 45kN on the load cell, nut was torqued to 175Nm...

At 45Nm, the bars wern't even taught, drooping under their own weight


I am right in thinking, the tightening torque has no direct link to the strain on the load cell? Friction, preload, droop in bars all has to be taken into account surely!
The kN unit is a force in a stright line?
But the Nm is a turning force, completly different things...?
Obviously there would be a set of calculations....but very involved i imagine.

I see M24 fasteners max torque is normally 730Nm. Bit different to 45Nm

Come the big day, i think someone will be franticly pressing buttons on their calculator

 

[rtbcomp]

I reckon it works somethinmg like this:

The tension in the rod is a function of the angle and diameter of the thread.

Torque is force * distance, so force = distance/torque

So the force on the thread is the radius of the rod divided by the torque.

The thread is like a wedge, imagine the rod is vertical the force applied by the nut acts horizontally into the slope of the wedge and is split into two components, one at right angles to the rod, the other along the length of the rod.

Pressure may also come into it as the force acts on the area of contact between the nut & the rod.

 

[scott0031]

Hitch,

I’m going through a similar thing with this tube bender I’m developing, unless your very familiar with physics it can be a bit of a head screw as I’m finding out. Trying to do it alone when your no expert is even harder, as no one too give a second opinion or point out where you are going wrong. So I know the pains you are feeling. In effect you’re trying to do a similar thing to me converting in your case a rotary motion or torque into a linear load. So I’m familiar with the points you make. I’m not saying I’m correct, as I’m no expert on the subject. I’m no academic. But I can call on the knowledge and experience I have with my development and my years training.

To the subject, you are on the right track. A Newton metre or foot lb. Load, as you correctly say are torque loads. Torque loads apply to turning moments only and nothing else. To use them you need a balance point i.e. another side of the equation to work against to do a torque calculation. If you also think back to training days you will remember a turning moment is a see saw with different lengths and loads at each end.

You cant directly relate a torque to a linear load (that I know of) there is no comparison as a linear load is a single acting force and a moment is acting against its moment and what is on the other end of the moment.

If those ‘rods’ have been designated as XX NM load I cant see how it will in any way directly be related to a direct tension load. That needs to go back to the drawing board…. I think


Reason is you cant as far as I know designate a torque load as a linear direct load, that’s pretty much why its torque as it’s a turning moment and is designed to cater for those types of loads, if you see my point. If the design states the torque on the nut is the requirement, and no actual linear load is stated on the rod. Then that should be taken as correct in my view. The thing to remember here is how its designated if one is ‘ Newton meters, or plain Newton’s which is a direct linear load


I will gladly add more if I can as this could well also help me in my quest. Its hard when you only have one brain a job like this at times.


Kind regards,


Scott (ward eng)

 

[wyn]

There is a bit of info here that says the same sort of thing as rtbcomp.

They haven't used anything to take into account what is going on in the nut.

P = T/( K D)

P= bolt pretension
T= torque
K= Torque coeficient
D= nominal diameter of bolt

 

[Hitch]

Thats pretty much what i told the man Scott. The guy im dealing with at the moment knows little of the job in hand tbh, hopefully when the suits come out Monday, they will see whats happening, and tell us where we are going wrong.
I think its got confused. Once tensioned, the rods will be tested to see how much force is required to deflect them by 400mm one way, but there is torque, er, talk of it only having 45kN exerted to it. Obviously thats a different moment again, a lateral force on a tensioned horizontal member....

Too much for a friday night nearly, isnt it!

 

[shedhobbit]

drat

Been messing today...

So they think, if you have two rods, with a load cell fittid in between, then tighten up nut at the end.... to 45Nm, you then have 45kN on the load cell....

Well not exactly ....

To get 45kN on the load cell, nut was torqued to 175Nm...

At 45Nm, the bars wern't even taught, drooping under their own weight
I am right in thinking, the tightening torque has no direct link to the strain on the load cell?
Friction, preload, droop in bars all has to be taken into account surely!

... and mechacial advantage (threads/inch) and probably some other things as well ...

The kN unit is a force in a stright line?

yes - all forces act in a strait line - blame Issac Newton for inventing it. One Newton (N) is the force required to accelerate a mass of one kilogram at the rate of one meter per second squared - 1 meter per second per second. (I could try explain further if you wish). Oh and 1kN = 1000 N

But the Nm is a turning force, completly different things...?

Not quite - Nm is a measure of the turning force (tourque, at a pivot) generated by the application of a force of one N at a distance of 1m from it (ye pivot and yer lever being 1m long). If it helps the same tourqe would be generated by a force of 0.5N at a distance of 2m or 2N at 0.5m if you get my drift.

Obviously there would be a set of calculations....but very involved i imagine.

They'd beat me ....

I see M24 fasteners max torque is normally 730Nm. Bit different to 45Nm

I'd go with a/the manufacturers recommendations and you won't be far out (man)

Come the big day, i think someone will be franticly pressing buttons on their calculator well ....

 

[malcolm]

I am right in thinking, the tightening torque has no direct link to the strain on the load cell? Friction, preload, droop in bars all has to be taken into account surely!

Totally right there.

Torque in Nm is only the force in Newtons that you would use to press on a bar 1 metre long to tighten something. So 100Nm would need 100N force applying at the end of a 1m bar, or if you only have a 0.5m bar you can get the same torque from 200N force. (Torque is just the word for force divided by length of bar).

For the pressure applied by the nut to the plate you need to take into account a load of other stuff. Increase the number of threads per inch and you'll increase the pressure on the plate from the same torque.

Reduce the friction with a bit of grease and you'll increase the pressure than would be applied by a certain torque.

Of course fine threads are easier to strip, and you'll strip them more often when you lubricate them. Torque settings are normally associated with a particular size and thread pitch of bolt, and tend to be specified either dry or lubricated.

 

[Welderpaul]

I see M24 fasteners max torque is normally 730Nm.

Is there a 'standard' recommendation for the torque that particular sized fastners should be tightened to?

 

[Hitch]

Had a read through some of that Wyn, looks usefull.... tthe first equation... Pi = T/(K D)

I think

T= 45Nm
K= 730Nm
D= 21 (M24x3 thread)

=340....but 340 what though? Newtons? As the rest is in Newtons?
10 Newtons is virtually 1kg, so if i hung a 34kg weight, on a verticle threaded rod, if i applied 40Nm on the nut on the end of rod, the torque wrench would break, and not raise the weight? Where as at 50+Nm we would be turning the nut and raising the weight?

 

[Hitch]

Is there a 'standard' recommendation for the torque that particular sized fastners should be tightened to?

Iv'e took this to be it, i may be mistaken.

Getting out of my depth really i fancy!

 

[wyn]

Had a read through some of that Wyn, looks usefull.... tthe first equation... Pi = T/(K D)

The way i see it

P=45KN
D=21.5mm- nominal diameter
T=175Nm- which you found yourself

The "K" is the tricky bit to sort out.

If you use your figures with T being 17.5 then K comes out to around 5.6

But the main thing is you already have it sorted in practice.

 

[snowcat]

so are they trying to get force or pressure, newton or pascal?, theres also deflection and elasticity to take into account surely?

you may be setting newtons on the thread a force measurement, but surely the wishing to measure pressure in the cell... or am I getting hold of the wrong end of the stick.....

 

[scott0031]

Had a read through some of that Wyn, looks usefull.... tthe first equation... Pi = T/(K D)

I think

T= 45Nm
K= 730Nm
D= 21 (M24x3 thread)

=340....but 340 what though? Newtons? As the rest is in Newtons?
10 Newtons is virtually 1kg, so if i hung a 34kg weight, on a verticle threaded rod, if i applied 40Nm on the nut on the end of rod, the torque wrench would break, and not raise the weight? Where as at 50+Nm we would be turning the nut and raising the weight?

There is another factor here the tensile strength of the material. To get that actual calculation of what the nut should be loaded to xx nm to produce 45kn linear load.

At the end of the day the linear load will be the linear load no matter what, but in that calculation to find what should be put in, in nm. The tensile strength will have to be in there somewhere. and remember it gets more complicated as the surface area of the rod will have to be in the calculation relating to the tensile strength.


The other point is you can see something is badly wrong if you look at the info you have hitch. Remember your linear load on the rods is 45KN 45KN Is Newtons X 1000. Kilo being 1000 so thats 45 x 1000, 45,000 newtons. Plainly something is wrong as no way you can input 45nm and get 45kn linear load.

To edit: 45,000 newtons is 4.59 metric tons (as in 1000k/ ton)

Hitch how long and what diam are these bars?

 

[Hitch]

so are they trying to get force or pressure, newton or pascal?, theres also deflection and elasticity to take into account surely?

you may be setting newtons on the thread a force measurement, but surely the wishing to measure pressure in the cell... or am I getting hold of the wrong end of the stick.....

Im not sure they even know what they want to measure

 

[snowcat]

Im not sure they even know what they want to measure

whats it for any idea?

 

[Drains]

Looks to me like some fool thinks a Nm is the same as KN. I'd love to hear the rationale behind that one, as it's like suggesting hours are the same as miles per hour.

Paul, there are recommended maximum torques for bolts, depending on various factors including tensile strength, but bolts should be tightened (or sized) for the torque, and strength required for the job, which may vary particularly with the shear load on the bolt.

Bolts can take a much higher shear load than their size might suggest, when fully tightened, as the pieces they are bolting together will be under higher frictional force. The whole assembly therefore has a higher shear strength. Obviously this is even more effective when the mating faces are smooth. Badly fitting faces make a weaker assembly, no matter how tight the bolts.

 

[Hitch]

Hitch how long and what diam are these bars?

Two rods, with a turnbuckle joint in the middle, from fixing to fixing, is 22metres, they have support at each end where thay are fixed, then 2 equispaced posts which they are supported on, but not fixed.