An impact driver is a manual tool that delivers a strong, sudden rotational and downward force, often used by mechanics to loosen larger screws (bolts) and nuts that are corrosively "frozen" or over-torqued. 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 down into the screw reducing or eliminating cam out. This attribute is beneficial for Phillips screws which are designed to cam out. It is also excellent for use with the Robertson square socket head screws 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.
Cordless impact drivers are not to be confused with the powered impact wrench.