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 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, also known as common, screws and is not beneficial at all for most other types.
Cordless motorized impact driver, battery, charger, and some bits
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 the impact wrench
, which is a motorized tool (usually powered by compressed air
), with a similar name and function. These also use a hammering action to apply torque
to fasteners. The difference is that impact wrenches do not provide the positive engagement that impact drivers offer as mentioned above. This is desirable though on hex head fasteners and others where the downward seating action is unnecessary and potentially damaging. To add to the confusion, they look identical to motorized impact drivers.
Buyers must also be aware that some tools improperly advertised as impact drivers are actually just impact wrenches. The only way to verify that a motorized impact driver is truly what it claims to be (without taking it apart) is to try it before purchasing.