Shaman said:
I don't need multiple identities on the net, Shaman is the one and only nick
name I use.
Now, you can believe me or not.
I think that Oz is a troll more than anything. Re-post and answering old
thread...
But He is right on this one, at least for my Costco shop. They use a torque
impact tool, then after they torque the nuts at 80 Lbs by hand with a torque
wrench. Seen it with my eyes. Open.
I would want to be sure that they are actually stopping below the proper
torque with the impact wrench. The very nature of the way an impact
wrench works makes them prone to overtightening. Torque on fasteners is
based on both the stretch of the bolt/stud and the friction between the
stud and the nut. An impact operated tool effectively removes the
friction force and thus it is very easy to get a torque that is too
high. I don't have exact figures or know where to find them right
off-hand, but I do know that an impact wrench set to 50 lb-ft will
tighten a nut much more than will a manual torque wrench also set to 50
lb-ft.
Each impact cycle of the impact wrench is exerting much more force than
is a smoothly applied force. If you don't believe me, just look at any
structural dynamics text book. Typically, an impact from a force of a
certain magnitude is considered to provide approximately twice the
affect initially as provided by that same force applied slowly and
smoothly. Most building codes also build in this 2X factor so that is
another source.
So, I would not at all be surprised that an impact wrench set at 50
lb-ft would torque a lug nut as tightly as would a manual torque wrench
set at 100 lb-ft.
The bottom line is that an impact wrench should be used only to remove
fasteners, never to install them.
Matt
