Well, they create torque by applying hammer blows to the drive spindle... hence the name impact wrench. Basically it's the same as if you were to tap on a wrench with a hammer.
The idea is that it's EASY for you to apply large amounts of torque to a bolt or nut, without breaking your arm off! The torque wrench can kick from you hand a bit, and you really should observe a few precautions, but overall it's easy to use and it does NOT put the 200,300, 400 ft-lbs of torque right through to you.
So note- the wrench can rotate suddenly as you start applying torque particularly to a very stuck nut. This can force your hand up against something, and pinch a finger or trap your hand. It could put you in a situation where you can't release the trigger- that can cause serious injury. Awareness of what direction the tool may move, and keeping a good grip on it is essential.
You can seriously hurt your fingers when changing sockets. Change sockets or extensions only when it's unplugged from the hose. It's way too easy to hit the trigger while pulling a socket off, and believe me, it hurts.
Grease, water, and rust can fly off the socket when it spins quickly. Eye protection is a must. I work on a lot of pipe flanges- 8 or more large bolts to tighten. When doing these, I shield the socket with my hand so stuff doesn't fly out and get me or co-workers in the eye. It happens, and it's unpleasant.
Tighten your wheel nuts by hand. All too often you will overtorque when using the torque wrench, and it might distort the brake rotor, or make it impossible to remove sometime on the road if you get a flat. I always use a breaker bar to tighten lug nuts to final torque.
DO not use regular (chrome) sockets on impact tools. They shatter. Results are disastrous. Enough said.
Oil your tools regularly. Squirt oil right into the inlet hose or fitting. Makes them last a long time. If you don't have air tool oil, use Marvel mystery oil.
Good luck with it!
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Rob Bareiss, New London CT ::: '87 244DL/M47- 225K, 88 744GLE- 209K, 91 244 183K. Also responsible for the care and feeding of: 88 745GLE, 229K, 88 244GL, 146K, 87 244DL, 235K, 88 245DL, 236K