Torque release - PeachParts Mercedes-Benz Forum

spock505 · #1 07-11-2016, 03:04 PM

Bit of an unusual question this one but I need to measure how much torque was used to tighten a bolt/s.

So my question is, if you use a standard torque wrench in reverse and slowly increase the torque until bolt loosens, would this indicate the torque used to tighten in the first place?

This assumes same day install/reverse, i.e no rust or corrosion in play.

Cheers,

Demothen · #2 07-11-2016, 03:17 PM

I have no idea, but you could easily find out by tightening a bolt with your torque wrench set, wait a while, then reverse it and see if it backs out. You could see if it is easier to release by dropping the torque wrench a few ft lbs after installing it and trying to loosen it.

jt20 · #3 07-11-2016, 03:33 PM

no.

static friction is always greater than dynamic friction.... in this world.

pimpernell · #4 07-11-2016, 03:43 PM

Quote:

Originally Posted by spock505

Bit of an unusual question this one but I need to measure how much torque was used to tighten a bolt/s.

So my question is, if you use a standard torque wrench in reverse and slowly increase the torque until bolt loosens, would this indicate the torque used to tighten in the first place?

This assumes same day install/reverse, i.e no rust or corrosion in play.

Cheers,

Strength of Threaded Fasteners

Untightening of Nuts & Bolts
If a nut is untightened immediately after being tightened, the torque needed to untighten it will be less than that needed to tighten it. This is due to the portion of the torque which actually stretches the bolt always acting in the untightening direction. Hence when a nut is immediately untightened, usually about 20% less torque is needed than was needed to tighten it in the first place.

As the time passes from when the nut was tightened, the torque needed to untighten it tends to increase. After half a day or so, the release torque can be typically up 10% greater than the tightening torque. This is due to a number of effects including embedding of the contact surfaces and changes in temperature affecting the friction conditions. Such effects typically increase the friction and hence a greater release torque is required. Because of such friction changes, the use of torque auditing methods (measuring the backoff - or crack-off torque or the tightening or crack-on torque) to assess if the bolt being tightened correctly is only accurate if it is completed shortly after the assembly was tightened.

spock505 · #5 07-11-2016, 04:00 PM

Quote:

Originally Posted by pimpernell

Strength of Threaded Fasteners

Untightening of Nuts & Bolts
If a nut is untightened immediately after being tightened, the torque needed to untighten it will be less than that needed to tighten it. This is due to the portion of the torque which actually stretches the bolt always acting in the untightening direction. Hence when a nut is immediately untightened, usually about 20% less torque is needed than was needed to tighten it in the first place.

As the time passes from when the nut was tightened, the torque needed to untighten it tends to increase. After half a day or so, the release torque can be typically up 10% greater than the tightening torque. This is due to a number of effects including embedding of the contact surfaces and changes in temperature affecting the friction conditions. Such effects typically increase the friction and hence a greater release torque is required. Because of such friction changes, the use of torque auditing methods (measuring the backoff - or crack-off torque or the tightening or crack-on torque) to assess if the bolt being tightened correctly is only accurate if it is completed shortly after the assembly was tightened.

Thanks for the replies all.

pimpernell, this explains it very well as I had noticed this 'time effect' on occasions, putting it down to corrosion or even variations within the wrench itself.

Interesting that if reversed immediately up to 20% less is required. That I have definitely noticed before and wondered if the thread had stripped due to over-tightening.

Cheers

Diesel911 · #6 07-12-2016, 10:05 AM

Quote:

Originally Posted by jt20

no.

static friction is always greater than dynamic friction.... in this world.

X2 on the no.

spock505 · #7 07-12-2016, 02:17 PM

Let me provide a little background which may come in useful for other members.

This relates to the torque settings associated with tightening the delivery valve holders after changing seals.

I followed the excellent tutorial on here by Parrot of Doom torquing up after ensuring all components internally aligned. At the time I remember thinking 35nm felt a bit too much for the holders given size and soft alloy metal but carried on.

Driving the car around for 2/3 weeks I noticed a strong smell of un-burnt fuel, plus the car had a lot more power than usual with some lumpiness at idle. Removed all six injectors to find really bad universal coking (these are all new Bosch, pop tested) at which point my thoughts turned to the delivery valves and possible over tightening - hence my original question.

This morning I tried removing the holders increasing torque 5nm at a time until it hit 60nm, still not budging and the max on this particular gauge.

I opted for a regular socket wrench but tried to gauge myself how much torque was needed to release each one, they were all about the same at a guess around 70-75nm.

For anyone that hasn't attempted the delivery valve seals, each holder has an O ring which sits beneath the surface, thus may add to release friction - not sure.

Anyway, put the whole thing back together but opted for the FSM torque sequence of 30nm release, 30nm release, 30nm + 5nm, this felt much easier with the toque wrench 'clicking' out with hardly any pressure at all, and certainly no where near the force required to release the original set.

Pleased to say all is well, runs smooth with no unburnt fuel smell bu a little less oommph..that's okay, back to normal.

I thought it worth mentioning as a friend of mine did the same, it took 29k miles before it bedding in.

t walgamuth · #8 07-12-2016, 02:27 PM

If you have doubts about the torque value, you can loosten the head bolts one at a time and retorque them.

spock505 · #9 07-12-2016, 03:17 PM

Quote:

Originally Posted by t walgamuth

If you have doubts about the torque value, you can loosten the head bolts one at a time and retorque them.

Yup, this might be a slightly different situation where you have the same end torque (35nm) but two approaches, perhaps this gives two final (varying) settings.

t walgamuth · #10 07-12-2016, 04:24 PM

Quote:

Originally Posted by spock505

Yup, this might be a slightly different situation where you have the same end torque (35nm) but two approaches, perhaps this gives two final (varying) settings.

If you do this and find a few too loose I'd loosen them all and start over following the circular pattern in the center per the installation instructions.

spock505 · #11 07-12-2016, 04:51 PM

Quote:

Originally Posted by t walgamuth

If you do this and find a few too loose I'd loosen them all and start over following the circular pattern in the center per the installation instructions.

We might have a crossed wire, this is for the delivery valve holders ,not head bolts - but thanks for the tips.

t walgamuth · #12 07-12-2016, 06:45 PM

Oops! Sorry.