Cordless impact wrenches are an incredible tool – and may people who work in auto repair shops would say they couldn’t live without one. But just how does a machine like this generate so much torque without taking your arm and your wrist with it? In this article, we’ll explore the history of the impact wrench and explain how they work.
Invented by Robert H Pott of Evansville Indiana, and first manufactured in 1939 by Chicago Pneumatic, the impact wrench was a groundbreaking tool from the outset. These early “power vane” wrenches were available in both electric and pneumatic versions, but were corded, unlike modern cordless impact wrenches.
All impact wrenches work by accelerating a rotating mass “the hammer” and hitting it against the anvil (where the socket attaches). After the impact, the hammer slides over the anvil and accelerates again. This mechanism of repeated striking is very similar to a regular hand hammer, only occurring in a rotational environment.
Most of the time, the hammer and the anvil are simply two cylinders with notches in the top and springs pushing them together. As the motor rotates the hammer, it slams into the anvil, then bounces over the notch before accelerating and striking it again.
But how does this mechanism allow an impact wrench to deliver so much torque as an output without twisting the operators arm? The answer lies in physics, and again a hammer is a helpful analogue. When you strike a hammer against something, you accelerate the mass (hammer head) and deliver a strong impulse force to whatever you hit. The reaction force you feel on the hammer however is nowhere near as strong. Now imagine that happening many times a second – it’s easy to see how so much force is generated.
Pneumatic impact wrenches are still used extensively in high performance environments – such as NASCAR pit teams. The downside is that you need an air compressor to power them. For most users however, a cordless battery powered tool delivers plenty of speed and power to get the job done.
How does an air impact wrench work?
Air impact wrenches are very similar in function to their electric counterparts – only they are driven by a compressor rather than an electric motor. The compressor passes high pressure air into the wrench, which is then connected to a vane motor.
A vane motor is a system to convert high pressure air into rotational movement. It does so by having a central shaft with 4-6 vanes attached to it. As air strikes the vanes, it rotates the shaft and generates rotational force.
Pneumatic impact wrenches typically generate both more RPMs of speed and more foot-pounds of torque too. Given the high performance requirement of motor racing teams, where seconds count, it makes sense that they would use the more high performance option.
Do impact wrenches need special sockets?
In short, yes they do.
Firstly, impact wrench sockets are carbonized, or drop forged in order to give them a harder surface finish. This harder finish enables them to absorb the impacting action of the wrench better without sustaining damage.
Secondly, impact wrench sockets feature much thicker walls and are generally heavier and built more substantially. This extra mass on the socket further adds to the rotational force applied by the anvil, helping the socket drive more force on to the fastening.
Finally impact wrench sockets grip the fastening slightly differently compared to a regular socket. Impact wrench sockets utilize what is called a “flank drive” design, which grips the nut or bolt along the flat sides. This is different to a regular socket which grips the corners of the nut or bolt. The flank drive design can more effectively transmit more torque to the fastening without rounding off the corners of the fastening.
You can, however, use impact wrench sockets with a regular ratchet wrench. They will be slightly more expensive and bulkier, but if you are looking to save some money then we recommend you take this approach rather than the opposite.
