Impact driver

A manual impact driver with screwdriver bits and adapters

Impact driver guide set with bits

A manual impact driver is a tool that delivers a strong, sudden rotational force and forward thrust when struck on the back with a hammer. It is often used by mechanics to loosen larger screws (bolts) and nuts that are corrosively "frozen" or over-torqued.^[1] The direction can also be reversed for situations where screws have to be tightened with torque greater than a screwdriver can reasonably provide.

Manual impact drivers consist of a heavy outer sleeve that surrounds an inner core that is splined to it. The spline is curved so that when the user strikes the outer sleeve with a hammer, its downward force works on the spline to produce turning force on the core and any socket or work bit attached to it. The tool translates the heavy rotational inertia of the sleeve to the lighter core to generate large amounts of torque. At the same time, the striking blow from the hammer forces the impact driver forward into the screw reducing or eliminating cam out. This attribute is beneficial for Phillips screws which are prone to cam out. It is also excellent for use with the Robertson square socket head screws^[2] that are in common use in Canada. It is less beneficial for slot head screws and is not beneficial at all for most other types.^{[citation needed]}

Compared to an impact wrench[]

An electric impact driver typically delivers less torque and accepts smaller tool bits than an impact wrench. This makes the impact driver (with its often lesser torque and smaller tool bit) more suited towards driving smaller screws in for example construction work, while the more powerful impact wrench is more suitable for driving larger bolts and nuts in heavy mechanical settings, such as lug nuts.

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