What determines the torque setting

[piperboy84]

Is it solely the rating of the nut and bolt or is it the purpose it is used for,

Things like aircraft struts, is the torque setting determined by the aircraft maker or the bolt manufacturer.

[Lone_Ranger]

Its generally down the airframe makers spec.
Bolts being far stronger in shear than in tension could lead you to thinking the bolts spec for torque would allways be correct, but in aircraft use they are usually over-speced for fatigue life and tightening to the bolts to its recomended torque 'could' lead to crushing of the componant

In short, go by what your aircraft maker says, not a chart of bolt spec

[Crash one]

You could easily crush a wooden prop long before the bolt limit is reached.

[Whopity]

Usually a torque setting is to prevent you overtightening the bolt and causing damage to whatever it is holding. Overtightening can also stretch the threads and prevent it being removed. In the case of multiple bolts, it ensures an even pressure (cylinder head) and prevents distortion.

[mad_jock]

piper a lot of it depends what you are using it for.

A bolt and nut in its self is quite a complicated engineering system.

Then what you are actually using it for then comes into play.

There are huge books out there along with engineering design codes which will give you torque settings depending what you are doing with the bolt and nut. That's not to say that the torque is specified for no particular reason apart from that's what they use. They will give you a torque because there needs to be some conformity mainly to stop gorillas over torqueing and then making the bolt plastically deform or decrease its fatigue life. And also the threads need to be under a certain amount of load for the friction to work and the bolt not to undo itself.

In theory every bolt on a piece of machinery should have an associated torque specified for it. But in practise we don't for none load bearing items.

[ericferret]

Aircraft maintenance manuals specify torques for specific tasks. There is also a table of standard torque settings for non specific tasks usually in the general information at the front of the maintenance manual.

Alternatively three white knuckles or tighten it until it shears then back it off half a turn.

[mad_jock]

The three white knuckles is the problem.

What's one gorillas finger tight can be cursing and 30mins work to get out for the next ginger.

People don't use torque wrenches enough to be honest. But its a pain in the bum and a lot of the time you can't get access with it.

These are general statements and not only for aircraft.

[phiggsbroadband]

Have you noticed that Open-Ended or Ring Spanners are all of different lengths; Short ones for small nuts and long ones for big nuts. It helps limit the torque applied 'by feel'.

I remember one engine I owned had cylinder bolts that were very long and slim, the idea was that they acted as a spring in tension, so any thermal expansion did not cause a change of force exerted. They needed torqueing-up to a set figure, then backing off slightly to remove the twist.

[Windy Militant]

Torque settings are just a convenient way of consistently setting the load on a component. If you really want to be accurate you measure the extension of the bolt.
This is rather inconvenient in the field so then the next best option is the torque wrench.
But they must be used with care, the amount of torque can vary due to corrosion or dirt on the fasteners giving a false reading. Paint can do the same thing by creeping out of the joint, which is why you shouldn't paint on structural joints it acts as a lubricant.
Also you need to ensure that the bolt is the correct length. If the thing bottoms out you may have the correct torque applied but it will not apply the correct pressure to the joint which will then not function as designed.
Some large maritime fasteners came with built in indicators which were basically a loose washer under a small bolt in the head of the fastener, when the correct load was reached extension of the large bolt trapped the washer. If it came loose and you could move the washer then the bolt needed retightening.
As mentioned many engine components are preloaded so as gaskets etc compress they don't need retightening.
Also watch out for Foot pounds/foot inches or Newton metres/millimetres.
get them the wrong way round and it is a case of snap and back half a turn! :O

[mad_jock]

Information related to bolted joints (http://www.boltscience.com/pages/info.htm)

Found something half decent on the theory.

[Desert185]

I remember installing the head on a Volkswagen Diesel Rabbit a number of years ago. The manual said to torque the new headbolts to a particular spec, then turn them an additional "x" degrees. Perhaps 90 degrees. I don't remember. I read that a few times to make sure I read it right.

[cockney steve]

^^^^ Standard practice now, with most modern engines.

Threads must be scrupulously cleaned (sometimes oiled as well!)
a very low torque-wrench setting to "nip" them down equally, then tighten in the correct order in several stages of "x-degrees "

total tightening by degrees is usually under one turn and one often feels the bolt "give" on the final bit....this deliberately uses the fastener's elasticity to apply the correct clamping force, the bolts must be discarded and a new set used, every time they're removed.

The days of "dog it up as tight as you can" are well and truly past.
as a generalisation, "Tight enough to be secure and firm, not so tight as to strip/deform threads/ work/fastener. will get you by.