Scenario 2: The Click Wrench Said it Was Good

"The operator is very experienced and very diligent," as he describes the situation. "He knows the sound his tool makes when the nut is free-running and the noise it makes when the nut bottoms out on the wheel and starts to clamp the joint. He holds the tool on the nut, lets it bottom out, counts to five, and lets go of the trigger. He grabs his torque wrench and clicks the nut. All is good."

And to illustrate the point, MacMillan offers this cautionary tale...

"Imagine an operator grabs a handful of lug nuts and puts them in his apron. While in there one comes in contact with a little bit of grease and another comes

in contact with a little bit of dirt. When the technician installs those nuts, one will require a significantly different level of energy to make it turn. If the tech typically waits for his tool to impact the fastener for, say, five seconds at the bottom of the stud before he lets go of the trigger, the installation torque and the relative clamp load between those two fasteners will be dramatically different.

"Now the fastener is obviously stopped. This is when the effect of friction is highest [to manually push a car you must overcome inertia and friction to get it rolling, but once it gets moving, friction is minimized and gets a lot easier]. He takes his click wrench and successfully confirms the torque on both lug nuts. But the click wrench only measures resistance [friction]. It is incapable of knowing what is friction and what is clamp force. To get a real indication of how much torque is on the nut you must get it moving, ignore the high breakaway torque [mostly friction] and then grab an accurate torque reading before you increase the installation torque by turning the nut too far.

"The hub nut with grease on it will lose less energy to friction than the nut with dirt on it so there will be more energy left to clamp the wheel to the hub. The one with grease on it should make a better joint if it did not cause the stud to see too much torque and stretch beyond the elastic limit.

"All I know is that I am afraid of both of them," MacMillan concludes.

For More Information

There are literally countless sources of information on the topic of wheel fasteners, and wheel integrity in general, but the TMC's Recommended Practices Manual is the obvious place to start. Look for RP 222a and 222b (available separately as the 'Users' Guide to Wheels & Rims', RP 237, and RP 656.)

The following URLs may also be of special interest:

www.bridgestonetrucktires.com/us_eng/real/magazines/00v5iss1/ra5.asp

www.accuridewheels.com/steeldisc_hub.asp

www.youtube.com/watch?v=Oy0NVAQJ91Y (Alcoa video)

www.hayes-lemmerz.com

www.webbwheel.com/testyourself/

www.conmet.com/faqs.php

www.saaq.gouv.qc.ca/publications/lourds/wheel_loss.pdf

www.meritorhvs.com/MeritorHVS_Documents/TT3.pdf

www.gunite.com/literature/pdf/DiscWheelHubInstall.pdf

www.boltscience.com