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Undoing Bolts With A Torque Wrench

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lihp
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PostPosted: 12:30 - 04 Aug 2015    Post subject: Reply with quote
Removing doesn't require the same torque as fitting.
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Fisty
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PostPosted: 12:34 - 04 Aug 2015    Post subject: Reply with quote
lihp wrote:
Removing doesn't require the same torque as fitting.


ORLY? Even if its rusted in?
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lihp
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PostPosted: 12:37 - 04 Aug 2015    Post subject: Reply with quote
Fisty wrote:
lihp wrote:
Removing doesn't require the same torque as fitting.


ORLY? Even if its rusted in?


Then it may require more torque, but not the same
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Fisty
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PostPosted: 12:41 - 04 Aug 2015    Post subject: Reply with quote
lihp wrote:


Then it may require more torque, but not the same


Two Che'
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MCN
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PostPosted: 12:43 - 04 Aug 2015    Post subject: Reply with quote
One technique is to mark the bolt/nut and the surface of the part it is clamping (and the bloody washer).
Slacken the bolt off.
Apply torque in steps until you get the bolt back to the same position.

It is a bolt setting system so should give you some idea.
(Plus or minus a Potato)
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lihp
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PostPosted: 12:47 - 04 Aug 2015    Post subject: Reply with quote
MCN wrote:
One technique is to mark the bolt/nut and the surface of the part it is clamping (and the bloody washer).
Slacken the bolt off.
Apply torque in steps until you get the bolt back to the same position.

It is a bolt setting system so should give you some idea.
(Plus or minus a Potato)


Depends on the type of bolt, the technique will not work with stretch bolts
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Matt B
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PostPosted: 13:18 - 04 Aug 2015    Post subject: Reply with quote
"They" say that a torque wrench should never be used to undo, only to tighten. Something to do with throwing it out of calibration.

I don't know if that's completely true but I've heard it a lot, from different people.
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CaNsA
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PostPosted: 13:20 - 04 Aug 2015    Post subject: Reply with quote
Fisty wrote:
lihp wrote:


Then it may require more torque, but not the same


Two Che'


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MCN
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PostPosted: 14:00 - 04 Aug 2015    Post subject: Reply with quote
lihp wrote:
MCN wrote:
One technique is to mark the bolt/nut and the surface of the part it is clamping (and the bloody washer).
Slacken the bolt off.
Apply torque in steps until you get the bolt back to the same position.

It is a bolt setting system so should give you some idea.
(Plus or minus a Potato)


Depends on the type of bolt, the technique will not work with stretch bolts


What is a stretch bolt?

All bolts rely on the properties of stretch in the material to clamp/hold.
The term 'elastic limit' is a component in fastener torque.
Where the fastener is stretched to a point within it's elastic limit.
The elastic limit is a length the bolt will stretch to and return to it's un-stretched length.
Going over the limit slightly means the bolt will not return to it's un-stretched length and the joint can fail.
Going over the limit more than slightly will cause the bolt to fail.
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Fisty
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PostPosted: 14:08 - 04 Aug 2015    Post subject: Reply with quote
MCN wrote:


What is a stretch bolt?.


A bolt that is designed to stretch. Usually used once then renewed when rebuilding.
Citroen/Peugeot head bolts on the TU5 engine use them.

Quote:
All bolts stretch when they are tightened, but usually they stay within their 'elastic limit'. So, when they are loosened, they return elastically to their original length.

The normal tightening method is by torque wrench. The tightening torque is calculated to apply the correct stretch, and so put the correct load on the joint. It's not very accurate, and is affected particularly by thread condition, lubricant on the thread etc. Even using a bolt a second time when the thread is polished gives a different load on the bolt.

Stretch bolts try to eliminate some of these problems. A steel bolt will extend steadily in proportion to load up to its elastic limit. At that point it yields, and takes on permanent deformation. For a short while the material continues to extend steadily for very small increases in load (the load/extension curve being virtualy horizontal), and after that it takes only very small loads to keep stretching it to the point of failure. That is the load/extension curve drops rapidly. Stretch bolts are made of steel which has a relativly long horizontal section of the load/ extension curve past the yield point. In use they are tightened by stretching them a specific amount, by measuring the rotation after the joint is nipped. This is calculated to take them a short way past their yield point, to the point where the load extension curve is horizontal. The load in the bolt and hence the load on the joint is accurately known, as opposed to being at some ill-defined point on the elastic part of the load extension curve.

Because the deformation is permanent, and further extension would actually reduce the load applied, it is not possible to check the tightness of such bolts after a period of service. It is also unwise to reuse them, as they would not apply the correct load to the joint.


Because I'm too lazy to type it out.
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Fisty
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PostPosted: 14:40 - 04 Aug 2015    Post subject: Reply with quote
Vincent wrote:
I can see the torque required to get the bolt undone increasing.......but would it ever be less than the torque setting used to tighten it? Sounds highly unmolgich, ja?

"unmolgich" = impossible.


Yes, many factors could reduce the effort to undo it. Temperature, oil ingress, material wear etc.
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MCN
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PostPosted: 14:49 - 04 Aug 2015    Post subject: Reply with quote
Vincent wrote:
Matt B wrote:
"They" say that a torque wrench should never be used to undo, only to tighten. Something to do with throwing it out of calibration.

I don't know if that's completely true but I've heard it a lot, from different people.


Probably because the amount of force required would take the torque wrench off the scale.

I thought I'd try it this time to get an idea if the bolts had been torqued properly.


Any shock loading on calibrated equipment is not recommended unless designed for that duty.
The reasons not to un-do using a torque wrench are:
1) The bolt may require more torque to undo than the setting of the torque wrench or more than it's maximum capacity.
2) The rebound from the bolt suddenly releasing can damage the wrench.
3) You are too lazy to use the correct tools and so are unnecessarily wearing out a calibrated tool.

Some torque tools are designed to do-up undo hardware but they are specifically designed for that purpose.
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CaNsA
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PostPosted: 14:51 - 04 Aug 2015    Post subject: Reply with quote
surely a torque wrench will work on reverse threaded bolts... Thinking
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Monkeywrenche...
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PostPosted: 15:42 - 04 Aug 2015    Post subject: Reply with quote
CaNsA wrote:
surely a torque wrench will work on reverse threaded bolts... Thinking


Mine all do, just throw them into "undo" and it clicks same as it does going the other way.
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bikenut
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PostPosted: 16:09 - 04 Aug 2015    Post subject: torque Reply with quote
you young mods you, ulus...... :D :D :D

angular torque used today....and yes stretch measured on some ......

as to checking if bolts/nuts are tight, look up torque and set gauge, then tighten....did they tighten or did the wrench click??
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Paddy Blake
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PostPosted: 20:55 - 04 Aug 2015    Post subject: Reply with quote
Many years ago we would always retorque the head bolts after
the first worn up to running temp.

I guess if you don't do this then the bolts would be below torque
because when the engine first worms up it squeezes the head gasket a bit.
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Paddy Blake
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PostPosted: 21:09 - 04 Aug 2015    Post subject: Reply with quote
Vincent wrote:
Paddy Blake wrote:
Many years ago we would always retorque the head bolts after
the first worn up to running temp.



Yup.....I still do Thumbs Up


It is likely the last person that had the head off the bike didn't.
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Pete.
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PostPosted: 21:49 - 04 Aug 2015    Post subject: Reply with quote
lihp wrote:
MCN wrote:
One technique is to mark the bolt/nut and the surface of the part it is clamping (and the bloody washer).
Slacken the bolt off.
Apply torque in steps until you get the bolt back to the same position.

It is a bolt setting system so should give you some idea.
(Plus or minus a Potato)


Depends on the type of bolt, the technique will not work with stretch bolts


Never yet seen a stretch bolt on a bike. I've seen some with a specification of stretch to produce correct tightening torque, but then every fastener will stretch a given amount under a given tension and it's not many that you can actually get a gauge on.

Not the same as what are commonly termed 'stretch bolts', which are single-use bolts which stay stretched after use.
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lihp
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PostPosted: 21:55 - 04 Aug 2015    Post subject: Reply with quote
Pete. wrote:


Never yet seen a stretch bolt on a bike. I've seen some with a specification of stretch to produce correct tightening torque, but then every fastener will stretch a given amount under a given tension and it's not many that you can actually get a gauge on.

Not the same as what are commonly termed 'stretch bolts', which are single-use bolts which stay stretched after use.


Thumbs Up

I was just being picky and pedantic at MCN's very generic advice to try and annoy him a little Thumbs Up
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