Wow Matt,
Don't you love it when a hot topic is started for a simple inquiry? Open a Big can did-ya?
Do a google search and find many discussions on forums on wet/dry torque.
Why do we torque wheel nuts, what are we shooting for here?
To get maximum clamping force to keep the wheels on for safety/longetivity.
What if we under-torque nuts?
They work loose, wreck studs/wheels and wheels fall off on the road (people die = $M liability) (38 yrs ago, I had a wheel pass me once, it was mine!)
What if we over-torque nuts?
The studs stretch, later (or sooner) break, wheels-off on the road, same problem.
No one wants either to happen, so the answer for billions of wheel nuts in the world is somewhere in the middle.
"Ya pays your money, Ya takes your shot"
So there are definitions of torque, friction co-efficients, lubrication, torque angle, bolt stretch, elastic limit, etc.
Plus all the variables with wheel alloys, nut designs, hub/stud piloting, rolled/machined threads, bolt/nut grades, applications, load ratings, service conditions, maintenance frequencies, technician skill/competence, ad infinitum!
What matters here is an strength of materials engineering term called elastic limit.
It is a point on a curve you would not want to exceed, lest you break what you are loading (tightening). Simplified: if you stretch a stud that is elastic, it will recover when unloaded.
If you over stretch a stud, it will not recover, EVER!!!
IT WILL BREAK!!!!
It is a liability waiting to happen at some unopportune time. OOPS.
Wet torque (never-seize, oil, grease, loctite, etc.) changes the the friction when running nuts on. Each is a variable in itself.
Note: oil is [generally] the most predictable for truck technicians, so wheel manufacturers spec. "drops of oil".
Reduced friction means stud stretches more for any given torque (example quoted above of mechanic breaking studs).
But what is the answer? wet/dry torque? Well gentle readers that is a function of all the variables quoted above and more.
Torque, per se is at best, an indicator of stretch. Rusty/dirty studs or bad threads use up the torque, just overcoming friction.
The nut may not be "tight" to the wheel at all.
When building race engines, con-rods/bolts are the Achillies Heel when high RPM loaded. We measure bolt stretch with a dial indicator gage. Thus staying within the elastic limit.
Can't do that on a wheel stud. So the next best thing, Torque Angle. Fasteners are torqued to a base point, then a torque angle is applied. The angle is directly related to stretch.
Each thread has a "thread pitch" (amount hut advances in one turn). If you had the engineering specs for allowable stretch on each fastener and included many of the above variables noted, you can calculate the angle (360 per turn) an thence get it in the right ballpark.
Maintenance shops often use plastic pointer indicators on each nut to "indicate" if a nut is working loose. Look on your average city bus or well maintained fleet trucks. Most of us in our hobby would consider it overkill for our old Bolts.
What really matters to us? Tight nuts, yes, but not too tight! I have old vehicles that get very low miles per year, as do most of us. I may not take the wheels off for a hand full of years, it is more likely to rust in place (thus the never-seize)
Yesterday I picked up a '45 2Ton farm truck, by p.o. estimated wheels had not been off since late '50's. YAH! I know what break-away torque means. Black (impact) socket, 1" drive, 5 ft. bar and pipe. I am (290lbs) standing on the pipe at the 4 ft mark and bouncing. Nothin! You do the math! Time for the "hot wrench".
One of my customers this summer brought me a wheel nut with stud broken off inside. His torque wrench was bad and he had no idea, just kept puttin the torque to her. ALL studs now need to be replaced, ALL have now exceeded their elastic limit and would fail soon after. ("OR, does ya want to take yer chances???") He was lucky it happened right in front of him, rather than on the road. When I (digital) calibrated the torque wrench, it was 50% over! SHUZBOT!
Another friend was wanting to get his 13yr old son to help. Good for the kids to get involved. Assume nothing about their understanding of the task. He was to tighten nuts with the preset (click) torque wrench. We could hear him working on the far side of the vehicle, Click, Click, good. Then when he came to the near side, we saw him torque the nuts, Click, then add 1/4 to 1/2 turn "for good measure". "GODFREY DANIEL!!!!!, ==== STOP THAT!"
As ever, opinions vary! Others may differ or add or cite other resources. Do what is best for you and your old Bolt.
Thanks kind Bolt Friends for suffering thru a long post. Keep your nuts tight, oil in the sump and ROll-ON, rubber side down.
Rick Metcalfe
Professional Engineer, Inventor, part time College Professor, Racecar Owner/Builder/Racer, & owner of too many Bolts per SWMBO (she who must be obeyed!)
Last edited by Rick Metcalfe; 10/24/2009 3:53 AM.
