How do I calculate tightening torque??? 1

[Biggy]

This is probably a simple question to some who does it everyday, but I'm struggling....

How do I work out the torque that should be applied to nut to give a specified 'clamping' force on a gasket.

I have a connection comprising of a single nut(female thread!) screwing on to a male threaded connection.

These when mated together compress a ring gasket.
The threads are 2.5" right hand Whitworth with 5 (five) teeth per inch. I have 30mm of thread. The threads are cut from Hastelloy C276.

I assume the torque is a function of thread contact area and friction between the threads.

Any guidance will be greatly appreciated!

Biggy

 

[CoryPad]

Are you sure you have Whitworth threads? They are disfavored over the 60 degree flank angle thread forms. Can you switch? I would recommend the ISO M thread (standard use) or the ISO MJ thread (fatigue applications).

This formula will calculate what torque will generate what preload. It requires SI units, so you will need to convert.
If you do change to M/MJ threads, the 0.56 factor will need to be changed to 0.58.


M = F/1000 * [(0.16*P) + (0.56*d*mu) + (mu*D/2)]

where

M = input torque in N m
F = preload in N
P = pitch in mm (this is the inverse of threads per inch)
d = pitch diameter in mm
mu = friction coefficient
D = (dout+din)/2
dout = outer nut contact diameter in mm
din = inner nut contact diameter in mm

Cory
cpadfield@omninmetalslab.com

 

[RHK]

I use Torque = 0.2 * Force * Diameter as a quick, memorable formula for torques for steel bolts. This is not as accurate as Corypad's formula, but close enough for most applications. Note that the friction coefficient will be of the order of 0.15 to 0.20 because the contact pressures on the threads are quite high.

Hasalloy may have a different friction coefficient. Check whether galling is a problem. If it is then apply an anti-galling compound, or simply grease the threads before installation. Russell Keays

 

[Biggy]

Cory!

Thanks for that it is just what I need, just two things....

Can you please explain to me what the 'dout' and 'din' are.

Regards,

Biggy

 

[CoryPad]

Your nut contacts the piece underneath it in two places - an outer contact diameter (dout) and an inner contact diameter (din). The outer diameter is the largest diameter of the bearing face, while the inner diameter is usually the hole in the piece underneath that the externally threaded fastener extends through.

As an example, let's assume you are using a hex flange nut with an outer diameter of 5 inches. This is your dout. Let's say the piece under your nut has a hole in it of 2.75 inches (to allow a 2.5 inch fastener to easily pass through) - this is your din.

You may contact me directly if you have additional questions.

Cory
cpadfield@omnimetalslab.com

 

[sprintcar]

there's an easy online torque calculator at

http://www.futek.com

Keep the wheels on the ground
Bob
showshine@aol.com

 

[evelrod]

Sprintcar is on target and you should really avail yourself of the FAQ section of these forums when available. The site mentioned has been there for some time!!!


Rod

 

[CoryPad]

The calculator at

http://www.futek.com

is not particular good (it asks the user for a K factor, which a novice wouldn't know about), and it wouldn't account for Biggy's unusual geometry. The equation I posted in a previous message (taken from VDI 2230) is vastly superior to anything that uses a K factor, which can introduce significant errors.

 

[Biggy]

And that's enough about my unusual geometry!!!!