Both statements are true. Lubrication of threads makes a huge difference. Not just oil, different platings have an effect.
There are two common reasons for stating torque figures:
To prevent damage
To provide pre-load on an assembly, often to reduce fatique failures.

The first case is less critical. Measuring a angular movment of the fastener after the joint is clamped (rough torque setting) is a much more accurate means of setting pre-load and is commonly used in automotive applications.
As Nicholas says measuring fastener length is used for some applications and is even more accurate.

I have been putting motorcycle engines together for many years with common store-bought torque wrenches, with the threads on the fasteners always having either copper slip or Loctite of varying grades on them and haven't had a failure yet. I don't think it's as critical as all that. Main thing is that all the nuts are tightened down the same. If they are 20 per cent over due to lubrication, it does not seem to have any detrimental effect. Never stripped one yet, or had one fail.

As for calibration of torque wrenches, aircraft engineers do it. I have never bothered and never had a problem in the home workshop. The plain old beam type torque wrench with a direct reading pointer and scale is most reliable as there are no moving parts to jam or wear etc. But I have never had any problem with my "click" type torque wrenches either, which are used at home for hobby use. Might be a different story if you were using them all day every day at work etc.

The torque to be applied, should specify wherher lubricated or not..

Toyota specified "75 lb ft, DRY" for the zinc plated wheel bolts on the Yaris.

At Perkins when we went to yield tightening, we specified lubrication with soluble oil.

We found that anything between 5 and 20 % produced no different results, on 1/2 UNF W range steel bolts into cast iron.

In general, we found that lubrication reduced the spread of torque required to produce the same clamping force, (Which is why torques are specified, with the object of delivering a required level of clamping )

Howard

In aerospace it is normal to check torque wrenches against a calibrated gauge e.g. Acratork, before use.

I personally have a small acratork at home and use it to check my small (5/18" 8mm ~1N-m) preset torque wrenches used for precision coaxial connectors on RF test equipment. Using a torque wrench here produces repeatable measurments and prevents damage. Even if the connector is field replaceable they are expensive. The one on my 26.5 GHz spectrum analyser is about £500.

Torque is the least accurate measured method of tightening fasteners, but is unavoidable in most cases. One thing that must me adhered to is that the fastener must be torqued up the final 20% in one movement. If you stop even slightly short, possibly due to difficulties with swinging the torque wrench, the torque will not be correct. As soom as the tightening is interrupted, the subsequent torque needed to start things moving again will be much higher, this is called stiction. Backing off and re applying the torque without a pause right up to the full torque is the answer. Special requirements for stretch bolts must be adhered to exactly.

As has been stated several times, torque is pretty inaccurate, a seating torque followed by angular increases is far better and as others have said there are other more accurate ways of doing it.

In the general and regular automotive world, on a spread of fasteners that have the same tightening spec, the fact that they are all the same is more desirable. The accuracy of the torque wrench isn't that much of an issue as long as it's used and they are the all same and close to the spec.

And just to highlight what Old Mart said regarding the final tightening in 1 movement is a must. Getting close then having another bite at it so to speak will usually give a 'false' tight click or beep as the breakaway force required to make the fastener rotate again can be significantly higher than the required setting, especially on something like a flange nut that has a large surface area against the static face.

Personally, all the performance / race bike engines I've built / prep'd I'm with Hopper, generally loctite or a very small amount of light oil to prevent any galling or binding on pre cleaned fasteners and threads.

And yes, if it says replace the fastener then you really should as they will be 'stretch' bolts.

In the 1970s There were instances of Bedford CF ambulance wheels coming of whilst leaving the wheel nuts in place. Two explanations were offered one was metal fatigue due to over flexing when cornering, the other was put down to overtightening of the wheel nuts when changing tyres.

I guess both! Overtightening wheel nuts is a common pastime, and doing it to a vehicle that does high-speed cornering is begging to open up micro-cracks.

Anyone know about ambulance performance? Possibly an urban myth but I was told at school they were amongst the highest performing vehicles on UK roads. Were they souped up or just a big engine in a lightly loaded van? Judging by the way we were left at the lights by one, they certainly had much better acceleration than my mate's trusty Morris Marina!

Dave

Cracking of the metal behind the wheel nut is caused by too much axial load, whether by over-tightening or by reducing the nut to stud friction if the manufacturer has specified dry assembly.

Ford had serious problems with this in the 60's – it may well have since been addressed to reduce the severity of the problem. By all means feel free to experiment on your own car. I won't.

Air tools can overstress wheel studs or bolts very easily Once, another depot used 3/4 drive tools on 7/8" BSF wheelnuts.

My depot had the job, once we had got everything apart of fitting four new wheel hubs,all studs and nuts and all six wheels, not to mention brake shoes! The studs had been tightened beyond yield and the wheels had become loose.

Depending on the type of torque spanner used, the manner in which it is handled affects the torque finally applied..

In some cases "snatching" as the fastener is tightened resulted in the excess torque being applied.

The best results are achieved by slow steady application..

Because of stiction, or momentum, jerky rotation causes inaccuracy..

And torque wrenches vary in their accuracy and consistency. One very well respected tool supplier offered a dial type wrench. It was inaccurate, one of he worst that we had seen.

The Britool wrenches, correctly used, were accurate and consistent.

Conversely, one of the most accurate and consistent is the "Bendy Bar" type. I have two 1/2" drive ones which checked on different calibration rigs, consistently give accurate readings, The 3/8" drive one from Machine Mart needs a calibration chart made up to apply correct torques.

And the wrenches need to be calibration checked at regular, and frequent, intervals

Regularly may be once a shift, or if the fixings are safety critical, every time that the wrench is used.

One supplier told me that the wrenches were checked "Regularly", but could not even tell me how frequently, or show where the check meters were located. I found them for him, and insisted that "Regularly" meant "Once a shift". The problem had been drive hubs falling off. Now I knew why! After that we had no more problems. But I felt sorry for the Quality Manager for being put through the wringer, almost in public.

Tightening until the fastener just goes into yield gives the most consistent clamp ,load. But this requires the fastener material to be made accurately and from consistent material.

Since putting the fastener into yield produces a permanent extension. There is a very small number of times that the fastener can be reused, in some cases never again

It is most efficient use of the fastener and application of a consistent clamp load.

At the time, we had the largest yield tightening machine in the world. It was awesome. It applied a 9 ton axial load to 32 fasteners in less than 10 seconds..

Howard .

Edited By Howard Lewis on 24/11/2022 21:19:45

Edited By Howard Lewis on 24/11/2022 21:21:41