Powder Coat Affecting Lug Nut Seating?

[TireDesigner]

Hi everyone,

I have a problem that I inherited from a previous designer that I need help with. My company currently builds wheel assemblies for pivot irrigation systems (among other things), and our current design utilizes a steel T-wheel that is powder coated. The bolt holes in the wheel have a standard 45° bevel to match the lug nuts. Trouble is, farmers are reporting that the lug nuts keep coming loose in the field. They say they do not have this problem with pivot system wheels from other manufacturers. Part of the problem may be vibration - our wheels are completely solid and flat-free, as opposed to pneumatic wheels (which I would think would absorb more of the vibration). However, there are other solid wheels out there, and we haven't heard of this problem with those wheels.

My suspicion is that part of the problem may be the powder coating. When the steel wheel section is powder coated, they do the entire thing, including the lug nut bevels (not my decision - like I said, I inherited this design). I've read in some automotive forums that paint and/or powder coating on the lug nut/wheel mating surfaces may cause this type of problem (though I haven't seen any conclusive proof of this presented).

What do you think? My plan of attack is to try removing the powder coat from these surfaces on a couple of wheels and see what happens in the field. Unfortunately, our CEO (my direct supervisor) overruled me (wouldn't say why except that he thinks it's a "bad idea" - he's not an engineer). He said I should keep trying to find a locking wheel fastener(?? yeah, I know...).

Any input would be appreciated. Thanks.

Richard

 

[Compositepro]

Paint is like using a plastic washer under the nut. Plastics creep under load and the nut will loosen. I suppose you could put some serrations on the cone portion of the lug nut which will scrape the paint off as you tighten the nut.

 

[MikeHalloran]

Your CEO is the aperture of a rectum.

Powder coat is very difficult to remove, and very tough and strong. ... for a plastic. It will, as noted, creep under the load applied by the wheel nut, and will eventually relieve all of the preload applied, leaving the nut loose.

You could cross-drill the studs, castellate the nuts, and apply cotter pins or hitch pin clips. The plastic would still creeep, remove the preload, and the repeated shock would eventually fracture the studs, possibly after the rattling and looseness destroy your reputation.

The proper thing to do is mask the nut seat tapers before powder coating.
If cosmetic rust is of concern, You could electroplate or paint that area if you wish, or wet the tapers with paint or varnish at assembly.

I would paint the tapers and the studs with Never-Seez or Loctite PST at assembly, but that's just me.



Mike Halloran
Pembroke Pines, FL, USA

 

[CapriRacer]

I would also try more torque on the lug nuts.

My experience says that alloy wheels require more torque than steel wheels to stay tight - and the issue is very similar. Alloy compresses more and doesn't provide as much friction.

 

[CapriRacer]

And a couple of other thoughts:

Some serrations - either on the wheel or on the lug nuts. Those would act like lock washers. I'm thinking a simple sharp edged dent or 2 on the tapers of either.

 

[TireDesigner]

Thank you all for the responses. I really do appreciate it.

I'll try to use what you've said as ammunition to convince my boss that we need to remove the powder coat on the bevels. Just for clarification: these pivot wheels are going on someone else's pivot irrigation system, usually already in the field. I can only control the design of the wheel itself, not the wheel studs or the lug nuts. Nor can I paint or coat the lug nut/wheel interface after the lug nuts are applied, as the farmer will in most cases be doing the install.

 

[Tmoose]

I think to demonstrate the strength of his convictions the boss should be willing, nay, eager, to powder coat the wheel lug seat tapers of every vehicle owned or driven by a member of his family, since powder coating is a good idea. I'm surprised he has not already done so.

If there is nothing to be done to any part but the wheel, a weekly re-torque for several weeks is the only option I can think of.

 

[CoryPad]

Just to make it clear, it is standard practice for automotive wheels to be masked in the tapered contact region to prevent issues with torque-tension behavior like you described.

 

[CapriRacer]

So you only have the wheel that can be changed.

So my idea of serrating ((or notching) the lug nuts would not work - but you could do the wheel.

And more thoughts: How about roughening the taper a bit before powder coating? Or adding a bit of grit to the tapered area just before baking?

 

[Tmoose]

Boss says "keep trying to find a locking fastener" - sounds like optional nuts are not completely out of the question.

Are the wheels centered by a pilot diameter feature on the hub, or by the fasteners?
Do the steel wheels have some stamped in curvature to provide a belleville washer effect?

http://taxilimoautoparts.com/images/rr1 024.JPG

Preventing the fastener from un-screwing is desireable, but only to prevent a mid term problem of fastener loss.
Lug nut$ with nylon patche$ might serve that purpose, but I would not expect them to prevent loss of preload from powder coat embedment, and the resulting beating up of the studs, nuts, wheel, and hub.

As mentioned by others The loss of fastener preload can result in fastener fatigue due to 10X higher alternating stress. Then that busted hunk of fastener with the nut still secured by a cotter pin or whatever will fall into the furrow, and the surviving (loose) fasteners, possibly laden with hungrily progressing fatigue cracks will be left to handle even greater loads.

 

[TireDesigner]

The steel wheels don't have any belleville washer profile. It's just a T-wheel with chamfered holes. See attached/linked picture.

 

[MikeHalloran]

Ah, a really primitive wheel.

You can probably find a 'stock' locking lug nut, retaining clip, and tab washer from Stage 8.

They will prevent the nuts from loosening enough to fall off.

They will NOT compensate for loss of preload from yield of the powdercoat, so the nuts will still get loose, and if nobody does anything about it, the loose nuts will eventually cause the studs to fracture.

On second thought, maybe Stage 8 could provide locking lug nuts with Belleville washers roll staked on the cones. I would still use the clips and tabs, but inserting an actual Belleville washer in the stack might be enough to offset the yield of the powdercoat. Don't expect to get specials like that for free.





Mike Halloran
Pembroke Pines, FL, USA

 

[NormPeterson]

Boss says "keep trying to find a locking fastener" - sounds like optional nuts are not completely out of the question.

Sounds even more like the (inherited) wheel design is the boss's own baby and he needs it to work no matter what.


Norm

 

[Tmoose]

Hi Tire designer,

Are the wheels centered by the studs, or a pilot diameter?

Aren't automotive lug nuts commonly 60 degrees included angle?

 

[Xwheelguy]

60° is probably the most common but there are others, particularly wheels with bushings (inserts) in the conical nut seat and through-hole. Some are spherical as well. And from my experience, coatings of any type on the nut seats, pilot hole (hub or center hole) and even mounting surface are verboten not only to avoid fitment, but in the case of the nut seats to prevent loss of torque. The depth that the holes are drilled can also impact torque - too deep and I've seen torque occur before.

Tim Flater
NX Designer
NX 8.0.3.4
Win7 Pro x64 SP1
Intel Xeon 2.53 GHz 6GB RAM
NVIDIA Quadro 4000 2GB

 

[TheBlacksmith]

Sounds like you're trying to fix the wrong lug nut(s). Overwhelming replies here indictae that best practices in the auto industry do not allow the powder coating to be applied in the nut seating area. Since you are supplying the wheels to the end user for installation, this is the only item (whell coating) you have 100% control of. The end user will install the wheels with whatever tapered nuts they have on hand, torque them or not and recheck them or not.

 

[TireDesigner]

Thanks for the tip on Stage 8. I'll check them out.

I have confirmed that the chamfer used on the irrigation system lug nuts (at least all the ones I've seen so far) is 45°.

The center hub fits through the center pilot hole with a slip fit, so there is some centering action there, at least during installation (see picture). And yes, this is definitely an ancient wheel design - very heavy.

 

[Tmoose]

I wish my Studebaker Hawk had hub centric wheels. Seems like lug centered wheels were common back then.
The newest car I know of with lug centered from the factory was a 1978 Honda Accord.

 

[Xwheelguy]

Sorry for the poor typing - posted in spurts....should have read:

"...verboten not only to avoid fitment ISSUES, but in the......." and "...too deep and I've seen torque LOSS occur before."

Tim Flater
NX Designer
NX 8.0.3.4
Win7 Pro x64 SP1
Intel Xeon 2.53 GHz 6GB RAM
NVIDIA Quadro 4000 2GB

 

[TireDesigner]

Thank you again for all the responses. I'll try to remember to post an update in the future when we get this issue sorted out.