What force is preventing car wheel bolts from being removed?

[Tom.G]

To clarify, applying 50 foot-pounds of torque to a WET connection does not stress the bolt the same as applying 50 foot-pounds to a DRY connection because of the difference in the coefficient of frictional between a WET and DRY connection.

If you are talking about twisting the bolt to failure, I agree. If you are talking about elongating the bolt to failure by applying too much tension, then the opposite is true; i.e. for the same torque, the lower friction of a wet thread will allow more nut travel along the threads than a dry thread will.

Two different things to worry about!

 

[AZFIREBALL]

for the same torque, the lower friction of a wet thread will allow more nut travel along the threads than a dry thread will.

...thereby tending to elongate the bolt.

 

[Dadface]

I think there must be a certain amount of bolt elongation (and compression of the system being bolted) in order to get a adequate clamping. The elastic /mechanical properties of the system parts should be such that they can withstand the optimum forces required.

 

[AZFIREBALL]

Dadface:

You are correct. Generally speaking, to get maximum benefit from a bolted joint the tightening torque should be that which places the tensional stress in the bolt just below the Yield Point of the bolt’s material. Any tighter than that causes the bolt’s thread root diameter tensional stresses to enter, what is known as the ‘plastic’ region, where the bolt becomes permanently elongated (distorted). Another consideration is the clamping system (all the bolts together) must be robust enough that no one element should never experience loads that causes it to exceed its Yield Point during the service life of the joint. Additionally, to reduce what is called ‘Fatigue Failures”, the stresses induced by the assembly process (preload on the bolt) should always be greater than the stresses it will see during normal operation.

 

[Herman Trivilino]

I'm not sure where this thread is going. Is there something that's not as obvious as it seems to me? An additional tangential force on the nut could have been added to the original diagram and that would have 'explained' everything, I think.

The original diagram is a free-body diagram of the wrench. But the thread title is a question about the bolt. Wondering about forces on one object while referring to a free-body diagram of a different object can be a source a confusion.

 

[lightarrow]

Just a somewhat humorous note. One time I had to remove the lug nuts to change a flat tire. I put the now standard right angled lug wrench on the lug and could not move the wrench. I ended up standing on it (my 170 lb) and with a flexing jump snapped the wrench into two pieces. Now those were tight lug nuts.

I would rather say that was a very economic wrench
Last time I had to use that ("cross like") wrench was 3 months ago on highway. To remove a bolt I had to jump on it and it suddenly rotated > 90º kicking painfully on my leg. .. Next time I'll use motocross shin guards

 

[sophiecentaur]

I would rather say that was a very economic wrench
Last time I had to use that ("cross like") wrench was 3 months ago on highway. To remove a bolt I had to jump on it and it suddenly rotated > 90º kicking painfully on my leg. .. Next time I'll use motocross shin guards

I think that's the problem when we aren't constantly playing with our vehicles - as when we were 'boy racers'. Joints dry out and a tiny amount of corrosion can change cause a lot of stiction.
I must say, tyre shops these days are well enough behaved when it comes to torquing up the nuts correctly. Way back they just used the pneumatic wrench and ZZZZZZZped up the nuts as hard as they'd go.
I was impressed to find that the L wrench, supplied with my Landrover actually has a telescopic extending section to increase the torque when needed. There's posh.

 

[Tom Rauji]

I'm a "gear head" and have been for over 50 years. Let me make a few points:

Wheels are either hub centric where the hub locates the wheel, or lug centric where the lug locates the wheel.

While the lug nut or bolt is held in place by friction, most of the locking is caused by distortion of the fastener. I actually lubricate fasteners on my wheels, like I do any fastener with a torque specification. My present drag race car puts at least 16,000 ft-lb total on the rear axles. None of my vehicles ever loosen a lug. They do not loosen connecting rod bolts either, even though the rod bolts are pre-lubed in assembly and bathed in oil. As long as the fastener is always under tension or "stretch", it won't back out.

So while friction holds it, the primary concern in a non-locked fastener is not allowing the fastener to reach zero tension.

By the way, I'm very selective with rear wheels in my drag car. I won't use a wheel with a standard tapered lug centric nut (with are all standard thread direction and always have been on all of my vehicles since a 1950's Plymouth I owned). I use shank type nuts with thick wheels that close fit the nut shanks. This keeps most of the force as shear right at the hub surface. A regular taper nut (lug centric) spaced out away from the hub would greatly increase bending on the stud and I'd likely have failures.

I disagree with the notion friction is the critical parameter for locking the fastener. The critical parameter is the fastener stretch and elasticity, which can never be allowed to go to zero. While friction holds it, it is the stretch that prevents loosening. I'll continue to lube and properly torque my fasteners, because I want proper stretch.

 

[sophiecentaur]

I'll continue to lube and properly torque my fasteners

I guess that the instructions are not aimed at someone doing the job 'properly'. If there is a chance of a nut being under-tightened then a bit of sticktion under a dry nut may be 'statistically' safer than having a slippy nut. Your comment about con rod bolts applies all over a car but, in general, the more critical a nut is, the less likely that an amateur will be touching it. Almost anyone vaguely strong enough may be up for changing a wheel, even if they have no mechanical knowledge.

 

[Javier Lopez]

< I actually lubricate fasteners on my wheels>
The attaching force = area*coefficient of friction (steel to steel) /sin(angle) * force used to attach the bolt
The angle is the arctangent of screw thread mm/turn/2 divided by the diameter
The area =area of bolt inside the wheel holder

regards

 

[jack action]

If there is a chance of a nut being under-tightened then a bit of sticktion under a dry nut may be 'statistically' safer than having a slippy nut.

It might be actually the opposite.

The true aim when installing a nut & bolt fastener is to obtain a proper bolt elongation. The proper way of "torquing" a bolt is by measuring its length variation with a micrometer. Bolt elongation is proportional to the bolt axial stress and you want to have it as high as possible without exceeding the elastic/plastic deformation limit (i.e. yield strength).

Measuring a bolt length under stress can be difficult, sometimes impossible. But we can relate the torque applied on the bolt to its axial load (i.e. stress), IF we know the coefficient of friction. The friction coefficient is irrelevant; large or small it is still the axial bolt stress that matters. When a torque is specified, it must be specified in which conditions as well (dry or lubricated; often, even which lubricant is important).

That being said, if you applied a torque specified for a lubricated situation and the bolt is dry or rusty (i.e. higher sticktion), YOUR BOLT MAY BE UNDER-TIGHTENED! In the opposite situation, the bolt may be over-stressed and will failed.

The bolt is like a spring and you want to axially load it appropriately such that when the parts "compress" together under loads applied to them, then there is still an axial load (no matter how small it is) to prevent loosening. But it doesn't matter how high is the friction coefficient between the threads if the axial load becomes temporarily zero (which is basically the only situation where loosening is possible).

I personally prefer lubrication - especially on wheel studs that are prone to rust - just because I like to be able to remove them easily, like they were meant to be (Why use a nut & bolt if they are 'welded' together?). Lately, I also like to use stainless steel nuts & bolts (I hate rust) and lubrication is a must, otherwise they easily 'weld' together due to galling.

 

[Arthur Mitchell]

In my novel "A Grotto in the Sea," after my stout, protagonist stops to offer a mid-aged motorist and his wife help in removing their flat tire, and the older man said that two lug-nuts were not complying, and that he'd offered his "wife a go and she refused;" She replied, " I didn't want to embarrass Ken by breaking his nuts for him."

 

[Arthur Mitchell]

In my novel "A Grotto in the Sea," my protagonist stops to help a mid-aged couple remove their flat tire. the man said two lug-nuts were not complying and that he'd offered his wife "a go but she refused." she replied: " I didn't want to embarrass Ken by breaking his nuts for him."

 

[sophiecentaur]

I personally prefer lubrication

That certainly would seem logical to me but spend a half hour googling the topic and you will keep reading that manufacturers say no lubrication. This must be for a good but probably a very subtle reason.
I think that not enough is made of the importance of threads being clean. A rag with some trace of oil in it will certainly do a good job of clearing out the thread of grot that gets onto the outer exposed few turns of the wheel studs.

 

[OCR]

Yes, sir... . ✔

The true aim when installing a nut & bolt fastener is to obtain a proper bolt elongation.

Yes, sir... . ✔

The proper way of "torquing" a bolt is by measuring its length variation with a micrometer.

I obtained an A&P license in 1973, and that is the technique we were taught, and HAD to use, at least on aircraft engine connecting rod bolts...

That technique is a mandate, is contained within the FARs, including, but not limited to Part 21, Part 39, Part 43, and Part 145, since my school is a certificated repair station.

And believe me... we did EVERY THING by the book... .
Good post, Jack... .

.

 

[jack action]

but spend a half hour googling the topic and you will keep reading that manufacturers say no lubrication.

On the first result page on my favorite search engine, I get these links from fastener manufacturers/sellers who all specify "lubricated" torque values:

• http://www.alliedsystems.com/pdf/Wagner/Forms/80/80-1057.pdf
• https://www.fastenal.com/content/feds/pdf/Torque-Tension Chart for A307 Gr5 Gr8 Gr9.pdf
• https://www.boltdepot.com/fastener-information/Bolts/US-Recommended-Torque.aspx
• http://www.zerofast.com/proper-bolt-torque

The reason why some manufacturers put more emphasis on the dry torque, I think it is because of the human factor, as explained here:

Obviously lubrication of threaded bolts has lots of benefits, but with it comes the overriding concern that the lubricant will change the torque required to develop proper tension on the bolt—and how that could affect the integrity of the joint. Some estimate that adding a lubricant could reduce the required torque reading by up to 40 percent!

So for example, let’s say a company has always used a torque of 100 ft-lbs to install a bolt in a certain application, and let’s say that that 100 ft-lbs was spec’d for use with dry threads with no lubricant. Then one day, an operator says, “Hey I read this article that said thread lubricant is good for the bolted joint to reduce wear, so I’m going to try it.”

But when they apply the lubricant and then use that same torque spec to tighten it down, they wonder why the threads got damaged or the bolt head broke off.

That’s where some of the caution about using thread lubricant comes from, and why some companies might decide that, even though it has beneficial properties, they won’t use it because they’re not sure what the effect on bolt tightness will be.

 

[sophiecentaur]

I get these links from fastener manufacturers/sellers

Which all seems fine to me. However, in car owners' handbooks etc. etc. the instructions say "dry".
I am not arguing in favour of that but I would like to know the reasoning behind it. It has to be because wheel nuts are the only ones which the ham fisted 7 stone weakling or King Kong gets their hands on. Can I assume that dry protects the thread from over tightening and that, on balance is statistically lower risk?
PS A few years ago I collected my car from 'the menders' and found one wheel nut missing and the others finger tight. I was staggered by the casual response of the firm when I drove back in.

 

[jack action]

Can I assume that dry protects the thread from over tightening and that, on balance is statistically lower risk?

That would be my guess.

If you are asked to assume "dry" all the time and you encounter a "lubricated" torque value without knowing it and you don't lubricate, what happens? Your nut is not preloaded to its maximum value, but it is still preloaded. You have a lower safety factor, but it will probably do its job except for some extreme cases. Also, the nuts will slowly unscrew (probably not even all of them), giving early signs that something is wrong. Everyone will assume the mechanic did not do his job well by undertightening the nuts.

On the opposite, if you are asked to assume "lubricated" all the time and you encounter a "dry" torque value without knowing it and you do lubricate, your nut is overtighten and will most likely fail. It will also fail instantly without warnings. And once one stud failed, all others will probably fail soon after, just as fast. Whether it fails in use or not, humans being humans, the company will probably be blamed for cutting corners on safety by using cheap material. In addition, if you ask for lubrication, a majority of people won't do it, either because they don't know about it (backyard mechanics), they can't (no lubricants around) or they're too cheap (garages who cut corners for profit).

So, from the companies POV, I guess standardizing to "dry" torques is statistically the least damaging choice.

 

[Tom Rauji]

The specified lug torque on my F250 4X4 pickup truck is 190-210 ft-lbs. On my car which has a 1/2 inch stud, torque is 90-120 for tapered (lug centric) nuts on steel wheels. My uniform shank aluminum wheel lugs are spec'ed 140-160 ft-lbs.

I don't think a driver over tightening the lugs is likely with tools supplied in vehicles.

I have not found anything in any of my manuals about lube or no lube, although I have seen anti-seize mentioned in the past.

 

[cjl]

Yes, sir... . ✔

Yes, sir... . ✔

I obtained an A&P license in 1973, and that is the technique we were taught, and HAD to use, at least on aircraft engine connecting rod bolts...

That technique is a mandate, is contained within the FARs, including, but not limited to Part 21, Part 39, Part 43, and Part 145, since my school is a certificated repair station.

And believe me... we did EVERY THING by the book... .
Good post, Jack... .

.

Interesting. In high performance car engines, the technique used is not to measure bolt elongation, but rather to intentionally slightly yield the bolt. Because yield stress is very consistent, this allows for a very consistent clamp force, at the expense of having to replace the bolts every time you disassemble it. Usually, on a torque to yield bolt, the assembly instructions will be in the form of a torque plus a rotation, and in some cases, it can be quite elaborate - for example, from my Subaru's service manual, for installing the cylinder head:

(1) Clean the bolt threads and the bolt holes in the cylinder block.
CAUTION: To avoid erroneous tightening of the bolts, clean out the bolt holes sufficiently by blowing with compressed air to eliminate engine coolant etc.
(2) Apply a sufficient coat of engine oil to the washer and bolt thread.
(3) Tighten all bolts to 40 N·m (4.1 kgf-m, 29.5ft-lb) in alphabetical order.
(4) Retighten all bolts to 69 N·m (7.0 kgf-m, 50.9 ft-lb) in alphabetical order.
CAUTION: If the bolt makes stick-slip sound during tightening, repeat the procedure from step (1). In this case, the cylinder head gasket can be reused.
(5) Loosen all the bolts by 180° in the reverse order of installing, and loosen them further by 180°.
(6) Tighten all bolts to 40 N·m (4.1 kgf-m, 29.5ft-lb) in alphabetical order.
(7) Retighten all bolts to 69 N·m (7.0 kgf-m, 50.9 ft-lb) in alphabetical order.
(8) Loosen the bolts E, D, and A by 360° in this order.
(9) Tighten the bolts A, D, and E to 10 N·m (1.0kgf-m, 7.4 ft-lb) in this order.
(10) Tighten the bolts A, D, and E to 30 N·m (3.1kgf-m, 22.1 ft-lb) in this order.
(11) Further tighten the bolts A, D, and E by 98 — 102° in this order.
(12) Further tighten the bolt A by 123 — 127°.
(13) Further tighten the bolts D and E by 78 — 82° in this order.
(14) Loosen the bolts F, C, and B by 360° in this order.
(15) Tighten the bolts B, C, and F to 10 N·m (1.0 kgf-m, 7.4 ft-lb) in this order.
(16) Tighten the bolts B, C, and F to 30 N·m (3.1 kgf-m, 22.1 ft-lb) in this order.
(17) Further tighten the bolts B, C, and F by 98 — 102° in this order.
(18) Further tighten the bolt B by 123 — 127°.
(19) Further tighten the bolts C and F by 78 — 82° in this order.

In general, if you just see a torque value specified, it's probably a bolt with a fairly high safety factor, so it's not as critical.

 

[OCR]

In high performance car engines, the technique used is not to measure bolt elongation, but rather to intentionally slightly yield the bolt.

✔... Torque-to-Yield (TTY) .

...it can be quite elaborate - for example, from my Subaru's service manual, for installing the cylinder head...

Lol... my God, that's a nightmare... .

.