Impact driver

An impact driver is a 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 what 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 the screw reducing or eliminating cam out. This attribute is most beneficial for philips screws which normally cam out as part of their design. It is less beneficial for flat head, aka. common, screws and not beneficial at all for most other types.

Another type of impact driver uses a motor to automatically deliver the downward and rotational forces. These have the advantage of greatly increased speed. They are most often used in construction and manufacturing to replace screwdrivers where speed and operator fatigue are an issue. In some situations however, this type falls short since current designs cannot deliver the heavy downward blow of a manual unit. This can be especially true on very stubborn fasteners.

These are not to be confused with a motorized tool (usually powered by compressed air), with a similar name and function, known as an impact wrench. These also use a hammering action to apply torque to fasteners. It is notable however that these do not provide the downward blow that impact drivers offer for firmly seating the bit into some types of fasteners. This is particularly desirable on hex head fasteners and others where the downward seating action is unnecessary and potentially damaging. Buyers must be aware that some tools improperly advertised as impact drivers are actually just impact wrenches.