The impact driver is a manual or power tool that delivers a strong, sudden rotational and downward force. In conjunction with toughened screwdriver bits and socket sets, they are 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.
A manual impact driver with screwdriver bits and adapters
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 it,