Applying torque

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.

As I recall matters the emergence of these wheel crack problems more or less coincided with the steady introduction of air tools into the wider garage community.

Back in the late 1960's when i had a Saturday job on the pumps at the village "one mechanic and tyre sales" off-shoot of a larger concern 10 miles away in town it was still the norm to tighten wheel nuts manually with a spider. Teenage Clive was trusted to loosen wheel nuts with a longer bar, lift the car on the big trolly jack and finally spin the nuts off to take the wheel in for a new tyre to be fitted. After the wheel had been re-shod on I'd put it back on, spin the nuts up with the spider and give an extra heave when the wheel was back on the ground. Before the car departed the mechanic would check all the nuts giving them an extra heave with the spider.

I was well pleased when he got to the stage of saying "why am I doing this, you get them plenty tight enough" and trusted me to tighten them. I suspect the absentee bosses wouldn't have approved.

I'd be very surprised if we regularly got above, or even to, 50 ft lb or so with a spider. Any over tightening due to lubrication, a teeny smidge of grease went on things, would seem moot.

Clive

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

The common 1/2" drive pneumatic impact wrench gets a bad press, as most of them can't tighten(or loosen!) nuts as much as a normal size person with a 600mm breaker bar. That's because most of them are run off knackered compressors, joined with small bore hoses and connectors.

I would suggest that pretty much anyone who doesn't use a torque wrench is overtightening common fasteners by a significant amount - the M6 bolts for K-series or Vauxhall V6 cam bearing caps only need 10Nm, which is easily applied with a couple of fingers on a 10mm spanner.

When I worked in a steam turbine drawing office there was a rule not to use anything less than 5/8 bsw. They reckoned that even the most horny handed fitter couldn't overtightening them unless he went and got a scaffold pole to put over the spanner. A lot of the sets went to developing countries so perhaps skilled men were in short supply. Getting that way in the UK.

When Rolls-Royce Oil Engine Division supplied engines for British Rail, three car DMUs, troubles with broken cylinder head bolts eventually arose.

They should have been tightened to 175 lb ft. One of our service engineers visited a depot which was suffering badly with the problem of broken bolts found the fitters tightening the bolts by walking around the engine pushing a pole over the breaker bar on the socket!.

The habits of tightening large BSW fasteners died hard! 5/16 UNF was watchmaking to them.

Howard

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My introduction to such torque levels on cars came in 1972 with the lovely Hillman Imp [effectively the Coventry Climax fire-pump] engine in my secondhand Singer Chamois.

… which justified my purchase of a low-range torque wrench.

MichaelG.

Edited By Michael Gilligan on 25/11/2022 08:35:22

Those two are why I have a 1/4" torque wrench. Using it for the first time was an eye-opener at just how little effort is required to torque such small fasteners correctly.

I did refit an XK cylinder head using a 3/8" TW, which was much harder work than if I'd used the 1/2" one I had at home.

Anyone with experience of LT/Sprinter vans from twenty years ago will know the consequences of not following the exact procedure for torquing the wheel bolts: the rear wheels fall off! It was a pity there wasn't a similar procedure to keep the side door attached to the van.

Further to Howards last comment I put 5/16 UNF into the bolt stress calculator spreadsheet from Security Locknut at the link given in my earlier post.

For a 2" long grade 8 bolt it reckons 27 flt lb on a dry joint, 20 ft lb lubricated, is enough to reach maximum safe stress on the bolt of 120,000 psi. Takes 52 degrees rotation angle from full contact, finger tight I guess, to full torque.

Bolt stretches by 6 thou.

Gives a clamp load of 5,200 lb per square inch. Equivalent to about 2 tons on my thumb nail. Ouch.

Bit more playing around suggests that tightening to nominal torque / maximum safe stretch is 25° per inch from finger tight.

Frankly its all a bit scary how easy it is to generate, ahem, very large stresses in a bolt with correspondingly high clamp loads.

Clive

Perhaps they put the good engines in the Chamois. When I met her, SWMBO had an Imp. I spent a lot of time underneath it. Great when it was going, but suffered from lack of development, particularly overheating.

Perhaps they put the good engines in the Chamois. When I met her, SWMBO had an Imp. I spent a lot of time underneath it. Great when it was going, but suffered from lack of development, particularly overheating.

Interesting discussion. In the early 70s, I cured a rattle from the o/s front wheel of my brother-in-law's Morris Traveller by taking the hub cap off and recovering the snapped of stud that had fallen victim to a pneumatic wrench. Changed the studs on all four wheels.

Having started driving ambulances in 1966 until I retired in 2008 I drove several eras of ambulances. During my service I also trained ambulance drivers and instructors, . The only modifications I was aware of, apart from the electrics, were higher ratio back axles. The J1 Bedfords in service from the 1950s up to the introduction of Ford Transits and Bedford CFs had a 3.5 litre engine and were certainly fast once wound up and could approach 80mph on a straight road, faster than most family cars of the time. The early transits were quick with the initial acceleration, but lacked top speed and it was not until they put the V6 petrol engine in that the top speed improved. The last Transits in service when I retired were capable of 90 mph with good acceleration. The later vehicles with tail lifts were the first in general use to weigh over 3 tons, approaching 4 tons in weight with the equipment. I hope this answers some myths of the times, some one once told me that they had a reinforced front end in case they hit anything this was certainly a myth as you can see of any accidents involving ambulances.

From memory, the torque to tighten 5/16 UNF bolts and setscrews (Without putting them into yield ) was 12-15 lb ft

Stretching memory even further, 3/8 UNF was something like 21 -25 lb ft

Until yield tightening was implemented, the W range 9/16 bolts were tightened to about 120 lb ft, but did not produce a reliably consistent clamp load. hence th change to yield tightening, and the end of our troubles.

Howard

Howard

That torque calculator program is interesting to play with. Gotta say I'm going to pay a bit more attention to stretch in future.

Looks like your W range could have just about gotten away with grade 5 bolts, yield at 121 ft lb, rather than grade 8 or high tensile. Which seems very dangerous as you can bet someone will stuff a wrong un in. Always felt that half the art of design is outsmarting the things stupid people do or the mistakes normal folk under stress can make.

I do wonder how the relative inaccuracy of simple torquing as opposed to stretch measurements varies with absolute clamp load and bolt length. Instinctively you'd think the resistance to turning would go up disproportionately more with increasing clamp load so the higher the clamp load the less accurate torquing becomes. Bolts are effectively torsion springs so longer bots are inevitably more springy which probably makes torquing less accurate.

Presumably the inherent spring an increasing resistance at higher clamp loads is something that needs to be taken seriously when specifying the angle of turn when using the "turn X° from torque Y" method. I've never really understood how the "turn X° from torque Y" is any great advantage over a good torque wrench when tightening to clamp load. All the inherent variability seems to be the same except for swopping torque wrench accuracy for bolt torsional stiffness variation. Different with torque to yield where, effectively, the spring effect is overstressed and you need to be sure the bolt is firmly in the yield region.

Wonder if its different for nuts and studs. Objectively turning a bolt or tightening a nut down on a stud should be the same except for the thread length in a nut being constant as opposed to increasing as you screw a bolt in. Which difference should be more theoretical than real given (typically) half a turn or less from touch to tight and the end loading stress distribution that means only the outside few threads in a hole carry the load

But we all know that sometimes spinning the job works better than spinning a drill.

Clive

The Rolls-Royce C range oil engines used aircraft practice (as you would imagine ) for the Big End bolts.

They were fully machined waisted bolts, and were tightened to produce a defined stretch.

Needless to say, we never had a rod come adrift!

Strangely, on other places on the engine, non critical fasteners, no torque or tightening instructions applied. It was left to the individual holding the spanner, or socket wrench!

NEVER let a tyre change outfit near a locking wheelnut , EVER.

I have a little useful book.

It has torque settings of many older vehicles cast and vans as the book is a little out of date but some very useful information.

David

Edited By David George 1 on 26/11/2022 15:29:51

I have been reading this thread with interested- seeing as I was the one that started it.

A couple of things have confused me & I may have missed them in the text.

I can grasp the point about striction when applying torque. I can understand the principle of rotating a bar a certain number of degrees rotation to acheive a certain torque- although the starting point may be a bit variable surely?

But I do not understand stretch- well I understand that a stud or bolt will extend in length as torque is applied. But how on earth can one measure that when it is buried in the clamped work piece?

The next point, & once again I apologise if I have missed it - What is yield tightening? I know that mild steel reinforcing rod can be placed under tension. It will give then stop giving, then actually become "stronger". That is part of the process of making high tensile reinforcing rod.

But how is this applied to a bolt or stud? Or is yield tightening something else?

Apologies for the ignorance but I am genuinely interested. I will ask about wheel nuts later.

Edited By Sam Longley 1 on 26/11/2022 15:43:31

Better advice is to never refit locking wheelnuts. They are all terrible and likely to cause you some major inconvenience at some point.