Does bolt torque changes if using locknut
Does bolt torque changes if using locknut
(OP)
I have an assembly using a bolt and a stover type locknut. In theory, friction is higher because of the locking mechanism so required torque for a specific preload should be higher too.
When doing the torque calculation (or using a supplied table) should I consider the locknut effect on applied torque or it is just too small compared to the other components? It seem to be difficult to get data of friction of those locking devices.
I am asking the question because I've found a table somewhere stating that torque on bolt should be lower when using a Stover nut and it does not seem logical to me.
Robin
When doing the torque calculation (or using a supplied table) should I consider the locknut effect on applied torque or it is just too small compared to the other components? It seem to be difficult to get data of friction of those locking devices.
I am asking the question because I've found a table somewhere stating that torque on bolt should be lower when using a Stover nut and it does not seem logical to me.
Robin





RE: Does bolt torque changes if using locknut
That type of locknut can and will tear up the fastener and gall and even seize unless lubricated with something nice
RE: Does bolt torque changes if using locknut
Some prevailing torque fasteners are supplied with a wax or other lubricating coating - which is why the "published" torque may be lower than for a standard nut.
As with any fastener, if your joint is sufficiently important that you need to maintain the preload within a known range you should test your actual application and develop a torque-tension relation for the application.
Do not rely on published values from handbooks or manufacturers' literature.
RE: Does bolt torque changes if using locknut
RE: Does bolt torque changes if using locknut
They are used only one for a permanent assembly so I don't mind with the tearing factor.
Robin
RE: Does bolt torque changes if using locknut
We had 2 large housings attached to a large bore tube. 120 stover type locknuts on each end, conscienscously lubricated with a graphite alcohol mixture as specified by some MIL spec. The actual adjustment was to a specified length, and even the prevailing torque varied considerably amongst all the fasteners.
RE: Does bolt torque changes if using locknut
I measured the torque needed to rotate a bolt in 3 differents Stover nuts 5/8. I got something around 3 to 6 lb-ft. Considering that right now they torque their 5/8-11 bolt at 128lb-ft and this is quite low for a grade 8 fasteners, I don't understant why I should bother with the stover enhancing torque effect?
May be this effect is more important for smaller nuts and should be taken into account.
Robin
RE: Does bolt torque changes if using locknut
Let me give an example in metric thread (I have these values). Let's take an M16 with property class 10 (diameter and tensile strength near 5/8 grade 8).
The stover locknut according to DIN 980 refers to DIN 267-15 for the locking properties. The 'locking' torque should, according to the standard, be between 6 Nm and 42 Nm.
For tightening of the bolt to 90% of the yield strength you need approx. 333 Nm (if threadfriction = headfriction = 0.14).
If you would find that for one batch the mean 'locking' torque is 20 Nm, you could add this and apply 353 Nm.
When in the application the friction coefficient is lower, for instance thread friction = head friction = 0,12 the correct torque is 298 Nm. If the locking torque of that specific nut is not 20 but 6 Nm, the required torque in that case would be 304 Nm in stead of 353.
In this example the bolt will be tightened into the plastic deformation zone.
Therefore I think that it is only necessary for critical connections where all friction coefficients are known.
Adjo
RE: Does bolt torque changes if using locknut
Have you found friction coefficients to be "knowable" to an accuracy closer than 0.XX ?
Thanks,
Dan T
RE: Does bolt torque changes if using locknut
Standard bolts and nuts have tolerances that make it possible to produce in mass production.
Aspects like thread tolerance, surface roughness, angle of the washerface, alignment of the head to the thread, etc. all have an inluence on the torque.
Up to now I can only measure k values. But in the near future I will also be able to measure threadfriction and headfriction separately.