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August 21, 2017
An impact wrench is a socket wrench power tool designed to deliver a high-torque output with minimal exertion by the user. It stores energy in a rotating mass, accelerated by the motor, to then delivers it suddenly to the output shaft (the anvil), creating a high-torque impact. This system does not require a reaction arm and therefore doesn’t need a reaction surface to be operated.
The hammer mechanism is designed in such a way that after having delivered the impact, the hammer will go back to spin freely and will not stay locked. Thanks to this design, the only reaction force applied to the body of the tool is the one of the motor accelerating the hammer. As a result the operator feels very little torque, even though a very high peak torque is delivered to the socket.
The hammer design requires a certain minimum torque to allow the hammer to spin separately from the anvil. If only low torque is needed, then the tool will stop hammering and will instead smoothly drive the fastener, rapidly installing or removing it. It is the high number of repetitions of impacts that generates a high level of vibrations and noise.
Impact wrenches are mainly used for dismantling nuts as they do not deliver accuracy for bolting.
A pneumatic torque wrench, also referred as "nutrunner" is a planetary torque multiplier, or a gearbox coupled with a pneumatic air motor. Outside the gearbox is a reaction device which is used to absorb the torque and allow the tool operator to use it with very little effort. The torque output is adjusted by controlling the air pressure.
A nutrunner is used in situations where the tightness of screws and bolts is crucial. It allows the operator to measure the torque applied to the fastener so it can be matched to the specifications of a particular application. These planetary torque multiplier gearboxes have multiplication ratios and are primarily used anytime accurate torque is required on a nut and bolt or when a stubborn nut needs to be removed.
The pneumatic torque wrench is sometimes confused with a standard impact wrench, due to their similar appearance. The main difference is that a pneumatic torque wrench is driven by continuous gearing versus hammering for the impact wrench. All necessary torque is handled by the gearbox and the reaction arm. Having a reaction arm requires having a reaction contact like a nut or a strong metallic surface.
A nutrunner has a very low vibration level and excellent repeatability and accuracy, making it perfect for bolting.
It’s all about the job specification and accuracy needs. In demanding applications where safety is crucial, you must ensure you get the right torque, not too loose, not too tight. It is essential you get the appropriate tool for what you need to do.