SO the next time the guy at Discount tells you an impact gun is okay...

[altiain]

You can tell him this.

I’ve been attending a seminar this week on fastening technology and bolted joint design (hey, I’m an engineer. It’s what I do.) Anyway, one of yesterday’s subjects involved bolt tightening methods, and how to account for tool inaccuracy when designing bolted connections.

One of the numbers that stood out to me was this: with a well-maintained impact gun, the best accuracy you’ll ever get is +/- 60%. In other words, if your lug nuts have a nominal torque value of 80 ft-lbs, then even if that monkey down at Discount has a brand new gun and sets it properly (which they don’t), you could end up with nuts torqued anywhere from 128 ft-lbs to 32 ft-lbs. Using a “torque stick” doesn’t make a bit of difference in this scatter.

So, just a reminder: always make sure the monkey at your local tire shop reinstalls your wheels by hand and torques them with a torque wrench. This isn’t a perfect method, but it’s a lot more accurate than an impact gun, regardless of what the monkey tries to tell you!

[riverracer]

Ouch.... One of those Monkey's had his hands on my Honda nuts yesterday!

[Darron65]

+1 Impacts should only be used to loosen, not tighten.

[Radio-Active]

Even a torque wrench is only as good as the nut on the handle.

But I would rather have the nut use a torque wrench than an impact wrench.

[ray_sir_6]

They always use the impact to put them on, then a torque wrench to set. I see no problem doing it that way. I haven't seen many racecars losing wheels, and when have you seen someone using a torque wrench out there (pitstops)?? Did tons at the 25hr, and never had a wheel come off or snap a stud.

[POS Racing]

I’ve been attending a seminar this week on fastening technology and bolted joint design (hey, I’m an engineer. It’s what I do.) Anyway, one of yesterday’s subjects involved bolt tightening methods, and how to account for tool inaccuracy when designing bolted connections.

Let me guess, use bigger fasteners?? Did I pass??

[CosmosMpower]

Crazy, you'd think there would be some liability problems of people's wheels flying off on the highway more often from not enough torque or snapped wheel studs from over-tightening.

[altiain]

Let me guess, use bigger fasteners?? Did I pass??

Nope. Fail. In fact for fatigue lots of smaller fasteners are always better than fewer large fasteners.

Crazy, you'd think there would be some liability problems of people's wheels flying off on the highway more often from not enough torque or snapped wheel studs from over-tightening.

There's a reason wheels have multiple lug nuts. That's to prevent catastrophic failure if one bolt fails. Also, as noted above it improves fatigue resistance over using one large nut to capture a wheel.

[altiain]

They always use the impact to put them on, then a torque wrench to set. I see no problem doing it that way. I haven't seen many racecars losing wheels, and when have you seen someone using a torque wrench out there (pitstops)?? Did tons at the 25hr, and never had a wheel come off or snap a stud.

And usually when they do that, the wrench clicks without turning, which indicates that the nut is already overtightened.

I've seen plenty of wheels fall off of racecars, both professional and amateur. Impact guns are used in racing because they're faster, not because they're more accurate.

In addition, the wheel/hub interface on purpose-built race cars is designed with the use of impact guns in mind. Lugs are sized and torque values are picked so that if the gun hits the low end of the scatter the joint will still have the required clamping force, and if the gun hits the high end of the scatter it won't permanently deform the stud.

[Radio-Active]

I have had a stud broken off by a guy that used an impact wrench to install the lug nuts. In airplanes, they use nuts and bolts with holes in them to safety wire them together and resist loosening of the fasteners.

[Ataim]

http://www.youtube.com/watch?v=kyMDNzhfwn0

something is better than nothing.

[CosmosMpower]

There's a reason wheels have multiple lug nuts. That's to prevent catastrophic failure if one bolt fails. Also, as noted above it improves fatigue resistance over using one large nut to capture a wheel.

True, but if you're using the same pos airgun to tighten all 4 of 5 bolts they have an equal chance of all failing no?

[sammm]

True, but if you're using the same pos airgun to tighten all 4 of 5 bolts they have an equal chance of all failing no?

Would you rather have 1:1 odds or 5:1 odds?

[Titus]

Would you rather have 1:1 odds or 5:1 odds?

I think I'd rather have the comfort of knowing that I had re-torqued each and every lug nut myself...












With my $10 Chinese made HF special. I'm sure its acurate... it has to be!

[MX-5EE]

They always use the impact to put them on, then a torque wrench to set. I see no problem doing it that way. I haven't seen many racecars losing wheels, and when have you seen someone using a torque wrench out there (pitstops)?? Did tons at the 25hr, and never had a wheel come off or snap a stud.

They also probably use special $50k+ super accurate impact guns, especially in the high end, high dollar races.

But this is why my dad will ALWAYS just immediately go home and re torque all the lug nuts on his car after a tire change. He has given up on them no using an impact gun.

[wrxmr2eater]

They always use the impact to put them on, then a torque wrench to set. I see no problem doing it that way. I haven't seen many racecars losing wheels, and when have you seen someone using a torque wrench out there (pitstops)?? Did tons at the 25hr, and never had a wheel come off or snap a stud.

I have seen many endurance races where the lug nut has to be cut/torched/beaten off because it was incorrectly installed on the previous stop with the an air gun. I have also seen cars lose rims right after pit stops or worse, at speed in a few laps.

The chances of getting it wrong are high. Why take the risk on a car that can have your family in it?

[MX-5EE]

I have seen many endurance races where the lug nut has to be cut/torched/beaten off because it was incorrectly installed on the previous stop with the an air gun. I have also seen cars lose rims right after pit stops or worse, at speed in a few laps.

The chances of getting it wrong are high. Why take the risk on a car that can have your family in it?

That wasn't what I was trying to say. So I will break it down for you. I wasn't trying to make the point "because they use impact wrenches at races it's ok to use on your family car." I was trying to say "They may use impact wrenches on race crews because they have equipment not normally available for family cars." Which would in fact be an argument against using an impact wrench on a family car.

The 2nd part of my post was indirectly suggesting it may not be worth it arguing with the techs. Just drive it home and torque them down yourself.

[Rogue]

Thanks Iain, I was just fixing to go to one big lug in the centre!

No torque wrench needed, just beat the nut on with a big lead hammer.




I have a Snap-on torque wrench, but still had it calibrated just to make sure it was spot on.

Thanks Iain, for the PE insight.

We have a nationally known metallurgist here at 16X, I need it got his take on aluminum wheel fatigue. (from impacts)
I have twisted a 4-way off, on a rare set of aluminum wheels from a tire stores use of a big impact.
In fact I had to trailer the car back so the could use the same impact, because I had nothing that would budge the lugs.

[icepenguin66]

Yep, dumbasses always over tightening the nuts...and I work at a tire shop! We see quite a few stripped studs, etc from either us or usually from another shop when cars come in.