Measuring impact gun torque?

[m_c]

I've got an old electric impact gun I'd like to measure the torque of, however I'm struggling to come up with any kind of semi-accurate method to do so, without spending lots of time and/or money.

My ideas so far involve threaded rod and a spring stack (suitable spring is the current stumbling block), or a weighted bar (tonight's latest idea, but would need to some work to construct something suitable).

I do have a couple good torque wrenches for comparison.

Anybody got any other suggestions?

[magicniner]

You need to keep the system as simple and compact as possible so that the power of the impulse isn't lost to forms of work other than rotary, I'd try

A big bolt, say M10 with a significant unthreaded length.
A block of nylon the length of the unthreaded portion of the bolt with a 10mm hole through.
Slit one side to allow the hole to open up and close down.
Stick it in a vice and with very short blasts on the impact gun (to avoid generating too much heat) tighten up until you can't move the bolt head then test with a torque wrench.
A better test might be to start from locked and back off until the first signs of movement and then test with a torque wrench.

- Nick

[m_c]

I have tried similar with a M12 bolt and stack of washers, however I suspect there wasn't enough stretch in the bolt/washers to make it accurate enough, as readings seemed to vary quite widely.

My thought this morning was to get a bit bar, drill a row of holes to accept a row of nuts/bolts, torque them down at increasing torques, then see what the gun can slacken. What I really need to know is how tight a bolt the gun can slacken, as I'm wanting to test fi I can replace it with some form of high torque motor/gearbox.

[cropwell]

I have tried similar with a M12 bolt and stack of washers, however I suspect there wasn't enough stretch in the bolt/washers to make it accurate enough, as readings seemed to vary quite widely.

My thought this morning was to get a bit bar, drill a row of holes to accept a row of nuts/bolts, torque them down at increasing torques, then see what the gun can slacken. What I really need to know is how tight a bolt the gun can slacken, as I'm wanting to test fi I can replace it with some form of high torque motor/gearbox.

..
You could use a Belville washer as the spring

[m_c]

I've been on the lookout for something suitably springy, such as some Belleville washers.

We used to have truck body front mount springs lying around at work, which would of been ideal, but sods law dictates when you need something, you can't find it.

If I can't find something soon, I'll just have to buy a suitable spring. This is one of those projects that's been continually shoved down the todo list, but is now rapidly approaching the top of the list :-/

[m_c]

Just found a reasonably priced supplier of Belleville washers, so have ordered some M10 and M12 ones to do some testing.

I can now rest easy in the fact I'll walk into work tonight and find the other springs!

[magicniner]

I have tried similar with a M12 bolt and stack of washers.

You haven't, what I suggested is a tubular clutch that you tighten onto a shaft in a vice, very simple and close to a dynamometer, no nut or thread involved ;-)

[m_c]

You haven't, what I suggested is a tubular clutch that you tighten onto a shaft in a vice, very simple and close to a dynamometer, no nut or thread involved ;-)

I think I just heard a penny drop!
I'll give that a try when I get time, as well as playing with the springy washers.

[magicniner]

as well as playing with the springy washers.

For this you should use the best available friction modifier on the thread to minimise the difference between short sharp impulses and manual torque wrench, Tungsten Disulphide paste is best (but dificult to get, I had to import my stock) as it's coefficient of friction decreases at higher pressures, where Molybdenum Disulphide's coefficient of friction drops,

- Nick

[m_c]

I had a quick play tonight.

Tightening the nut/bolt/washers seemed to max out around 75Nm. Setting the torque wrench to 70Nm there was no noticeable movement, 80Nm things definetly moved, and 75Nm there was slight movement.

Slackening on the other hand, tightening the nut to 120Nm (starting to push the limit of the M12 8.8 nut/bolt), the gun managed to slacken it after a couple seconds.

The values are around what I would of guessed the impact gun would be capable of, and correspond to the calculated figures for tightening at partial power.
These measurements were to work out the feasibility of replacing this electric impact gun with a stepper/servo motor and gearbox to get a more consistent torque, however the cheapest planetary gearbox capable of the required torque I've found so far works out at over £300 delivered :-/

So it's back to either a new electric impact gun, or an air impact gun, but I only have about 8 1/2" between the top of the socket, and an immovable object.