Comparing the Torque values to tighten vs loosen a bolt

[Saint1968]

If I torque a thread combination of M26x1,5; in brass material, with 90 Nm.
Length of threds are 12 mm.
Will the torque to unscrew the parts be equal, higher or lower?
Why?

 

[jack action]

I never measured this, but I would guess the untightening torque will always be higher.

When you are tightening a screw, it necessarily moves which means kinetic friction is involved.

When you are untightening a screw, it necessarily doesn't move at first which means static friction is involved.

Since static friction is always greater than kinetic friction, therefore untightening always requires more torque than tightening.

 

[hutchphd]

In addition of course there are slower chemical processes (say diffusion limited oxidation) which also mitigate towards adhesion. So I agree.
But I wonder if there are interesting counterexamples. Administration of penetrating lubricant is an obvious one...

 

[Lnewqban]

Are you sure about that thread?
I believe that only M25 and M27 exist, and both have a bigger pitch.

Please, see:
https://en.m.wikipedia.org/wiki/ISO_metric_screw_thread

 

[Baluncore]

Are you sure about that thread?
I believe that only M25 and M27 exist, and both have a bigger pitch.

M26x1,5 is a metric pipe thread. The standard is DIN 158.
It is also used for brass bulkhead or conduit threads.
M26x1,5 is an uncommon ISO (extra) fine thread. DIN 13-6.
https://www.gewinde-normen.de/en/iso-fine-thread-5.html

 

[Baluncore]

If I torque a thread combination of M26x1,5; in brass material, with 90 Nm.
Length of threds are 12 mm.
Will the torque to unscrew the parts be equal, higher or lower?
Why?

It depends. If it has a spring or lock washer, it will require more torque to undo the locked nut.

Most nuts are tightened or released in a series of short steps. The magnitude of the static or kinetic friction forces should therefore be independent of direction.

We should assume that lubrication of the nut on the bolt, and the nut against the flat washer or clamped material, will be independent of time and direction of turning.

As the nut is tightened, energy is invested, stretching the elastic bolt. As the nut is undone, the stored energy is released, requiring slightly less torque. That seems to contradict the answer given in post #2.

Since static friction is always greater than kinetic friction, therefore untightening always requires more torque than tightening.

Now consider rotating a hexagonal nut in 60° steps with a spanner.
The final tightening step starts with higher (static) friction and lower bolt tension, then ends with lower (kinetic) friction, but greater bolt tension. The start and end of the final step should therefore partly balance.
The first step undoing the nut starts with higher (static) friction, under maximum bolt tension. It ends in lower (kinetic) friction and lower bolt tension.

So more torque is required to start undoing the nut, than was needed to finish tightening it earlier.
That agrees with the answer given in post #2.

 

[Lnewqban]

...
So more torque is required to start undoing the nut, than was needed to finish tightening it earlier.
That agrees with the answer given in post #2.

Anybody replacing a flat tire on the side of a highway would agree with that statement.

 

[Saint1968]

I never measured this, but I would guess the untightening torque will always be higher.

When you are tightening a screw, it necessarily moves which means kinetic friction is involved.

When you are untightening a screw, it necessarily doesn't move at first which means static friction is involved.

Since static friction is always greater than kinetic friction, therefore untightening always requires more torque than tightening.

Agree. Have tested this now. Tested with torque of 50, 70 and 90 Nm. The needed torque to unscrew the bolt is always 10 - 20 % higher in my setup. (M26x1,5 brass/brass, no grease)