PICTURE -- Big vs. small impact wrench

I'm no pipsqueak, but this guy looks like he could frighten a 5" diameter bolt loose!!!

An impact wrench uses the motor to spin up a big chunk of metal that functions as a hammer. The hammer is then allowed to strike an anvil to which the socket is attached, conveying torque to the fastener. It's the same idea as wanging on a slugging wrench

for example) with a hammer only the tool does it all for you. If it's a big heavy hammer then the tool is going to convey some torque back to the operator as the hammer spins up, but since the tool can take its time about spinning up the hammer it it conveys much less force to the operator than the tool applies to the fastener.

Did you happen to see the "Dirty Jobs" episode in which one of the jobs was changing a tire on a heavy recovery vehicle? The motor pool sergeant weighed about half what the tire did (and she was kinda cute too) and had no trouble handling one. On the other hand, Mike Rowe, trying to do it by brute force and awfulness instead of by using the tools designed for the job, managed to drop one on a cameraman, who fortunately managed to avoid injury.

Good for Joe. Citing some over sterioded wrestler/boxer/football player as an example of using power tools is a weak argument at best. I know there are all types of tools. I've worked with a lot of them. I know some are sweet to use, and have very little negative effects on the operator. And then I know some that will kick the living shit out of you.

BTW, I used to make a living using these things.

Steve

I think anyone who REALLY knows and understands tools will say, beef don't matter. And little guys can get just as much done or more by using their heads, leverage, and by using the tool properly.

It is common for a power tool to eat the lunch of even the beefiest operator when that operator doesn't use the tool properly, or tries to use muscle over technique. Many amputees will verify this fact of life.

There is no glory in someone being so burly as to use power tools in an unsafe and unintentioned way over the skinniest guy using it right and getting the job done and going home with all his fingers. If I had to work with either, give me the skinny safe operator rather than the beefcake showoff.

Steve

Yet a real pipsqueak could get the big bolt off there using the proper technique and tool. Sometimes big guys are plain vanilla dangerous.

Steve

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And perhaps spinning it up *without* a load would quickly accelerate the square drive and some internal parts to full speed without using the impact, thus transferring more to the user's hands -- which could explain Iggy's experience. He did not say that he had it on an appropriate nut load -- but that it did try to twist out of his hands. (The 1" or 1-1/2" one, not the 2-1/2" one IIRC.)

Enjoy, DoN.

I was a Teamster for 37 years. One of the favorite sayings was, "Give a lazy man a job, and they will find the quickest easiest way every time."

Steve

No load, no nut.

Here's the old video of that impact spinning up.

I held it as hard as I could.

Tim.

wrong application - the energy transfer is to accelerate a rotary hammer that then smacks against an anvil delivering an impluse - when the hammer hits the anvil, neither is connected in any significant way to the handle - so you have to hold the torque of accelerating the hammer, not the impact torque (beause the acceleation is caused by the air motor, and the reaction torque of the air motor is against the body of the tool)

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I suspect that it would have been easier to control with a well torqued in the socket. There would have been less spinning up.

Not going to bother visiting the web site. Video is a pain to view on this system -- especially with my preferred browser.

Enjoy, DoN.

You are not thinking hard enough about the problem that is solved by impact wrenches.

The only net torque that must get transfered to the operator is the torque required to make the nut and socket and rotate in free space - aka when not attached to anything. This is because as the nut comes off, the only thing that actually moves, is the nut and the internal parts of the wrench, and therefor the only torque that must be transfered to the operator is the torque required to turn the nut a few rotations if it were floating in weightless in space - which is basically something even a 50 lb kid could support.

The very large amounts of torque applied to break the nut free is applied, and then removed - so the net change is zero. In a perfect impact driver, if the nut didn't move, the operator would feel no torque at all.

It works because it applies a large amount of torque in a short time - which causes some part of the wrench to start moving, but then a smaller torque is applied in the opposite direction for a longer period of time, causing the moving parts to come to a stop - with no need to transfer any net motion or torque to the operator if the nut didn't move. And when the nut does move, the only torque transfered to the operator is the torque to overcome the angular momentum of the nut.

Still, life is not perfect, and I'll bet that big ass wrench could knock a very big guy on his butt at times.

Without naming names,I think that many of us can benefit from reading the wikipedia article on impacts.

The torque applied to the bolt is the same, averaged over time, as the torque transmitted to the operator. The key is AVERAGED. The torque applied to bolts is HUGE but for short periods of time, and then zero for much longer periods etween blows. Because the impact is heavy, and is held in soft hands, the torque on operator hands is much more continuous and, on average, not that huge.

Example: forget impacts at all. Suppose that I pound a steel plate with a sledgehammer. The force on the plate may be equivalent to several tons, but it is very intermittent. Force that is acting on my body is much more continuous and is never more than a few dozen pounds of force.

i

I've never looked at the design but it's got to be a bit more complex than that.

The rotary hammer can't just be accelerated in a single direction. It's got to be accelerated in both directions. So after it "hits" the anvil, it's got to be brought back for the next strike. This requires that the hammer transfer torque in both directions. As a result, the net torque sent to the operator can in theory be zero. To make that actually work, the wrench would need have at least two moving parts - like two hammers which were taking turns making the strikes and using the momentum of each other to move - with a net transfer of zero torque to the operator. I don't know if real torque wrenches work that way, but you would think those really large ones would almost have to work that way - or else just require they have long handles with two operators to absorb the torque.

I think my logic on this is correct - but maybe I'm just crazy....

Curt - take an impact wrench apart and see how it works - it's pretty much what I said - I've taken (smaller ones) apart and fixed them, they are quite simple.

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No, it hits again on the next rotation.

The wikipedia article has a video of the operation of the mechanism.

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Do you have a better link? For me, that only played the sound.

The principal of the impact action is the same regardless of the size.

Precicely, but again that assumes tha tthe impact is operating properly.

I don't remember my mechanics very well, but I do recall that the mathematics of "impulse" is different from simple statics. Many shots from a BB gun can move a floating aircraft carrier but the shooter never feels more than a minor recoil VERY many times. The energy delivered is large but in tiny, rapid increments.

That breaks it down pretty well.

No, they are pretty simple tools, they make an electric drill look complicated.

Logic is correct but not for this particular tool. Take a look at a schematic of an impact.