# Thread: Motor HP given Torque and RPM

1. ## Motor HP given Torque and RPM

Hi all,

Just curious, I saw a pneumatic impact wrench with the following specs: Torque = 350ftlbs, RPM =
3000, Using the formular T =Hpx5252/rpm
I'm getting 200Hp, does that make sense ? An average car engine makes around 100Hp

2. Hi Edward,

I didn't check your numbers, but at the point of impact that may well be the figure.

The torque quoted for an impact wrench is not a continuous torque. The energy is dissipated very quickly fractions of a second after the moment of impact. Then each successive impact applies brief moments of torque, but you are not really holding a 200hp motor in your hands.

3. Originally Posted by PinkertonD
Hi Edward,

I didn't check your numbers, but at the point of impact that may well be the figure.

The torque quoted for an impact wrench is not a continuous torque. The energy is dissipated very quickly fractions of a second after the moment of impact. Then each successive impact applies brief moments of torque, but you are not really holding a 200hp motor in your hands.
Thanks PinkertonD

I've been exploring the usefulness of pneumatic motors and I found out that the energy output of these motors is way less than the energy required to produce the compressed air which drives them, until I saw the specs on an impact wrench, which would make a big difference on the energy in and energy out ratio, so I'm still wondering if an impact wrench can be used as a motor with better energy efficiency than a standard air motor.

4. The output energy from the impact wrench is generated at the front end. Effectively, the motor winds up a spring (through a system of gears) which trips at a given number of turns or tension. It then release and unwinds with great speed, taking with it a lump of steel that rapidly builds energy. That slams into the ratchet that drives the socket to turn the nut/bolt as in that brief instant, energy is transferred.

The small sip of energy from the motor stared out as say 10000-rpm and the impact lump of steel maybe only traveling at say 40rpm. Given that someone once said, "matter/energy cannot be created or destroyed," and forgetting operating efficiencies of the system, then the impact steel will be around 10000 / 40 = 250 times greater than that of the motor.

It might be possible to use one as a motor, but the wear an tear on the resultant drive train would probably not be worth it. Also the speed would be jerky and not easily controllable as you are dependent on the spring tension and the load being applied forcing it's release.

All in all, not a sensible proposition. If it were there would be 200-hp air motors available for \$30.00 from Campbell Hausfield.

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