torquing lug nuts, dry or greased?

In reply to dean1484 :

This is a back of the napkin approach and not going to be exact by any means but may give some insight into the safety factor.

Let’s start with a few assumptions:

Assume a grade 8 equivalent stud

Use a 2005 Mustang 1/2”-20 thread and recommended torque of 100 lb-ft.

Assume that the OEM does not want you to reach yield on this fastener since it is multiple use over its life

Based on the Fastenal information linked here: https://www.fastenal.com/content/feds/pdf/Torque-Tension%20Chart%20for%20A307%20Gr5%20Gr8%20Gr9.pdf

We see that for a 1/2”-20 fastener dry to achieve 75% of proof stress requires 120 lb-ft. For that same fastener lubed to achieve 75% of proof stress requires 90 lb-ft.

Now let’s assume we can take it right up to 100% of proof stress. To hit this with a dry bolt would require 160 lb-ft or about a 60% increase over the recommended torque level. With a lubed fastener, it would take about 120 lb-ft or 20% over the recommended torque level. In both cases there is still a safety factor if torqued to the recommended amount, but it isn’t inconsequential.

This doesn’t account for how the lug nut interfaces with the wheel and it may be an oversimplification but it does give some guidance on what the safety factor may look like.

What kind of torque limiting driver is used on the assembly line?  I assume not a torque wrench.

I work on Manufacturing all fasteners come in with a light oil on them, if they did not they would rust, I have to assume also that when initially assembled they are lightly oiled.

Who has access to a brand spanking new car, let’s check and see how dry the studs are.

 I’m not at all serious, just felt like asking.

In reply to fusion66 :

Thanks!!!!  Again I leaned something.  By your math the margin is allot less than I would have thought it would be.  I would have expected a factor of at least 100 percent.  

Edit:  to clarify this I was thinking it would be at least 2x the required clamping force or 100 percent greater if that makes sense.  

akylekoz said:

What kind of torque limiting driver is used on the assembly line?  I assume not a torque wrench.

I work on Manufacturing all fasteners come in with a light oil on them, if they did not they would rust, I have to assume also that when initially assembled they are lightly oiled.

Who has access to a brand spanking new car, let’s check and see how dry the studs are.

 I’m not at all serious, just felt like asking.

On the assembly line they use a fancy digital torque-sensing wrench that costs about $16,000.  (Avid "how it's made" watcher)

I also assume that a tiny bit of residual machining lube is a lot less of a variable than a gob of anti-seize or even 30wt

Hay GRM Staff.  Do you have any contacts over at a palace like say ARP that can chime in on this?  I am actually finding this very educational.  It would be great to get some in-site from an expert on both the dry versus lubed debate as well as just some general info / insight into the science/engineering behind Lug studs, nuts and bolts.  We have not even touch on the differences between how the clamping forces are actually applied to the rims.  

Ok to add a further bit of technical complication.  How does the lubing of the bearing surface on the rim change things? Dry threads plus lubed bearing surface versus all dry or all lubed.  I can see this as adding even more potential for over torquing.  Flat surfaced lugs with washers, the old lug nut with a tapered face that matches up to the steel rim.  The weird ball type that my 944 have. So many variables.

Wait, you aren't supposed to lube the threads with the axle grease that's leaking from the housing?!?

Throwing another discussion point in is that in the trucking world stud type mounting is ALWAYS done with a drop of oil on the stud and a drop of oil on the flange nut. Budd type is done dry but hub mount is done oiled. There are interesting videos put out but TIA about the how and why with them torquing a pb&j sandwich to show the difference between clamping force oiled, dry, rusty and oiled stud no oil on flange nut.

Javelin said:

Wait, you aren't supposed to lube the threads with the axle grease that's leaking from the housing?!?

We refer to those as self lubricating.  

In reply to dean1484 :

I contradict nothing.  Torque isn’t the issue.  Clamping force is the issue, but you can’t measure it.  This is also why I avoid discussions like this.  

Bolt stretch will give you clamping force. 

Edit:  actually measuring bolt stretch is very difficult what I meant was stud stretch. 

fasted58 said:

Interesting to note that those studs are highly-proofed extreme quality studs, and the lug nuts are 15/16" hex, and are 45 degree taper instead of the 60 degree taper used on street applications, and the lug nuts are heavily "bullet nosed" - the first three or four threads in the nut are NOT threads, they are open, so that the nut is self starting.  This has the benefit, combined with the shallow taper, of the nuts not having threads where the nut actually seats to the wheel, so that any overtorque will definitely, 100% not result in pinched threads.

Just an addedum.

Let's say you are Joe Average.  Let's say you own, for instance, a Kia minivan.

Let's say some berkhead overtorqued all of your lug nuts because they oiled the threads, and now they are all pinched at the tip, and they will not torque properly, or even thread onto the studs without forcing the issue.

Do you:

A) Replace all 24 lug nuts at $9 per lug nut (yes, $9 for EACH NUT because they are a special design that holds the hubcaps on)

B)  Replace all 24 lug nuts for a total expense of nearly $300 and then forward the bill to the previous hack who worked on the minivan, who will no doubt tell you to pound sand, or

C) have someone spend over a half an hour at $125/hour to chase the threads in the lug nuts and clean the residual gopp from the threads in the nuts and on the studs, while silently ranting at the hacks at NTB who berked over this guy's vehicle, and adss to the grudge this person has against anyone who adds random goops and lubes to taper seat lug nuts?

It would be interesting to put strain gauges  between a hub and a rim. Put one between each stud/lug and see how torque translates to clamping force. It would be interesting to see how evenly the force is applied and if the tightening pattern of cross tightening the bolts actually matters. 

Obviously the the dry versus lubed could also be looked at. 

Now it is off to eBay to see if there are cheep strain gauges out there that would work. I actually have the hardware for the computer interface if the gauges use a 0-5 volt output. 

Hay don’t bring “goop” in to the discussion. Goop is a whole different level of lubrication. Goop is very sensitive. Goop has feelings. Goop is you friend. 

dean1484 said:

It would be interesting to put strain gauges  between a hub and a rim. Put one between each stud/lug and see how torque translates to clamping force. It would be interesting to see how evenly the force is applied and if the tightening pattern of cross tightening the bolts actually matters. 

Obviously the the dry versus lubed could also be looked at. 

Now it is off to eBay to see if there are cheep strain gauges out there that would work. I actually have the hardware for the computer interface if the gauges use a 0-5 volt output. 

The studs are not the problem.

The lug nuts getting pinched at the tip, are the problem.

Overtorque a taper seat lug nit,and it will never torque the same.  It will probably gall, and may actually require breaking the stud to remove.

With steel wheels, the lug nut seat will also open up with successive overtorque.  Eventually, commonly seen on certain Chryslers, the lug nut will bottom out before properly tensioning the stud.

Knurled. said:

Just an addedum.

 

Let's say you are Joe Average.  Let's say you own, for instance, a Kia minivan.

 

Let's say some berkhead overtorqued all of your lug nuts because they oiled the threads, and now they are all pinched at the tip, and they will not torque properly, or even thread onto the studs without forcing the issue.

 

Do you:

A) Replace all 24 lug nuts at $9 per lug nut (yes, $9 for EACH NUT because they are a special design that holds the hubcaps on)

B)  Replace all 24 lug nuts for a total expense of nearly $300 and then forward the bill to the previous hack who worked on the minivan, who will no doubt tell you to pound sand, or

C) have someone spend over a half an hour at $125/hour to chase the threads in the lug nuts and clean the residual gopp from the threads in the nuts and on the studs, while silently ranting at the hacks at NTB who berked over this guy's vehicle, and adss to the grudge this person has against anyone who adds random goops and lubes to taper seat lug nuts?

The answer is A, but you're shopping at the wrong place.  Lug nuts are lug nuts.  Auto Zone, O'Reilly, Advance, all have lug nuts   Should be a standard 12mm-1.50 lug nut.  Sold in sets of 20 for $40.  Dorman 12mm lug nuts are $1.99 each at the Zone.  And yes, those are with the shoulder.

Even if they don't have the shoulder, they sell plastic washers that hold the hubcap on.

In reply to Curtis :

Difficulty:  They need to have a certain register, and you need them NOW, not tomorrow or next week.

Knurled. said:

fasted58 said:

Interesting to note that those studs are highly-proofed extreme quality studs, and the lug nuts are 15/16" hex, and are 45 degree taper instead of the 60 degree taper used on street applications, and the lug nuts are heavily "bullet nosed" - the first three or four threads in the nut are NOT threads, they are open, so that the nut is self starting.  This has the benefit, combined with the shallow taper, of the nuts not having threads where the nut actually seats to the wheel, so that any overtorque will definitely, 100% not result in pinched threads.

 

 

It's also worth noting that it's not at all uncommon for lugs to be both over and under tightened during pit stops resulting in a loose wheel.

dean1484 said:

It would be interesting to put strain gauges  between a hub and a rim. Put one between each stud/lug and see how torque translates to clamping force. It would be interesting to see how evenly the force is applied and if the tightening pattern of cross tightening the bolts actually matters. 

Obviously the the dry versus lubed could also be looked at. 

Now it is off to eBay to see if there are cheep strain gauges out there that would work. I actually have the hardware for the computer interface if the gauges use a 0-5 volt output. 

I used to deal with these types of applications for strain gages for a living. Stain gage would be the best way to measure the difference, and many of the OEMs would have us instrument bolts and studs for them to do these types of tests.

You need to instrument the stud/bolt with the desired strain gage configuration, then you use the strain gage to measure the bolt stretch to calculate clamping force. Three axial gages equally spaced will allow you to get clamping along with bending moment and direction. Two axial and two circumferential gages can be used for a full bridge to measure clamping force and cancel out bending. 

If you turn the base of the stud down to a few thousandth below the minor diameter of the thread you should have clearance for the strain gage and enamel insulated wire.

To get the signal from the strain gage you’ll need to be able to make a full Wheatstone bridge, then measure a millivolt signal. 

If you are good with electronics you might be able to do it a little differently, and use the strain gage as the resistor of an RLC circuit. If you go this route you’ll be measuring the time to charge and discharge the capacitor and still working with small changes. 

In the interest of furthering discussion, understanding, and expanding the overall confusion, here are some of my experinces and learnings:

1. Dry and clean seems to get me the most reliable removal torque readings in my own backyard testing. This was with an old bendy bar, and a newer digital style. Not super scientific, but tried dry, wd40, and bearing grease. Wd40 made little difference, but helped on gnarlier studs. Grease was really variable for some reason, maybe thread/face deformation i did not consider then. 

2. Overtorqued nuts almost always stretch or otherwise distort the threads (possibly bolt?) on my lugs before it damages the hardened nuts. None of my applications use lug bolts. While this is apt to be highly variable, its what I have seen.

3. Many mechanic friends like to use light penetrating or wd40 type spray oil, as it seems to fit the 'lightly lubricated' spec, and claim it makes them go on and stay on better. This is 100% hearsay. The one guy I questioned about torque felt the oil was too thin and solvent to change the clamping force. 

4. I can't put my finger on why, but I was always told to torque things outside a case dry. Perhaps as it will be its natural state after a few years outdoors. 100% hearsay.

5. Most torque specs on nuts I have seen is dry. A cursury google finds ford requiring 'dry, rust and debris free' as well as the trucker style drop of oil on inner and outer on 1 ton and larger truck models. 

6. Nascar guys have/had a perfect ratio and thickness of rubber cement they would use to keep the lugs on the wheel when throwing the new one on, that would stretch just enough to stay on when pushed against the lugs, and not decrease clamping force unduly. According to rumor, many races were won and lost on getting it right.  Imagine being a tire guy when one went rolling off under the car...!

Of all these things, I think repeatability is the most critical here, as I think there is going to be a range of torque/clamping force that works in most situations. 

Any extra considerations for dissimilar metals, such as Ti or Al lug nuts? Is lube then needed to prevent galling? 

does anyone know what the total clamping force is of a wheel?  I am actually looking at things both load cells and strain gauges and i need to know the range of force it will have to work in..

In reply to dean1484 :

Here’s some reading that might get you the info you’re looking for:

Wheel stud failures

That is an interesting article.  And that does give me an idea as to the clamping pressures I need to deal with.  

Thanks!!!!