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It seems to be that the consensus behind impact drivers is that when the tool stalls, the hammer camms off the anvil, picks up speed turning 180 degrees, and then the moving hammers strike the anvil, losing all their energy, and using inertia of the hammers stopping to generate a pulse, a spike of torque.

However looking at some people operating example models or slowly operating an open driver module, it looks like as soon as the hammers cam over the anvil, and the spring picks up tension, in this moment, before the hammers do any lateral traveling, it looks like with no further rotational input, the spring releases, and the hammers surge in speed, with no assistance from the motor.

You can observe this when someone is operating a driver without a motor, just with manual operation. As soon as the spring is released, and they arent moving their hands, the hammer jolt forward.

So it looks like the spring actually stores energy for impact purposes, and it transfers it's compression into rotational force.

My question is how? It's not very clear and no one shines any light on it or brings it to attention or shows the inside of the impact driver's walls.

Everyone just explains that the motor drivers the hammers and ignores the hammers launching themselves via the spring.

Does the inside of the impact module have groves, is there another piece of metal the spring rests on that is inside those groves?

So is it the case that when the motor spins, it drives the hammers, when the driver stalls, the motor drivers the spring instead, the spring becoming a capacitor for kinetic energy whenever the output shaft isn't moving? So half the time the hammers get power, half the time the spring gets power? Then the output shaft benefits from the motor and the springs combining momentum?

How would you double the torque? put in a stiffer spring? Couldn't you put in a 2000 pound spring and use 20 times torque reduction on the motor to make a metal shattering tool? that would probably destroy itself.

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enter image description here https://www.wonkeedonkeetools.co.uk/cordless-impact-drivers/how-does-the-impact-function-work

Coil springs can be both compressed and twisted.

The compression of the spring is what allows the hammer to cam out. The twisting or winding up of the spring is what stores the energy for the rotational impact.

That's why the ramp that the hammer cams out on is so steep. If you were just wanting the hammer to cam out so that the motor spin unloaded to pick up speed to slam into the anvil, you could use a gentler ramp. But if you're trying to wind up the spring you need a much steeper ramp so the motor can continue to spin while the hammer stays still as much as possible without actually remaining still so that it can eventually unwind.

How would you double the torque? put in a stiffer spring? Couldn't you put in a 2000 pound spring and use 20 times torque reduction on the motor to make a metal shattering tool? that would probably destroy itself.

No. For all intents and purposes, it would just behave like a a regular drill which has no camming out all because your wrists wouldn't be able to stop the drill from rotating when it is supposed to be camming out. It would just torque and break your wrists like a high torque drill would. It wouldn't even shatter your wrists because no camming would occur and thus no shattering impact.

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  • $\begingroup$ ah i see. So the spring tries to stay straight, and it gets twisted into oblivion after which it slips past the anvil the first time and smacks into the anvil the second time. Very smart. So basically just a bigger motor would be one way to improve it. I suppose it might be possible to put a stiff spring on a cam of its own, and wind it a bunch until it slips off the track and comes back violently. Anyway thanks for the enlightenment. $\endgroup$
    – sergbot
    Mar 14 at 20:38

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