Impact
wrenches differ basically from screwdrivers
and nutrnunners in the method by which torque
is applied to the fastener. Screwdrivers
and nutrunners have high torque motors which
deliver torque directly to the fastener
until air is shut off or a limiting clutch
in the tool functions. The operator must
resist the tendency of the tool to rotate
in the opposite direction.
An
impact wrench has a relatively low torque
motor which imparts rotary inertia to a
special type of clutch. The clutch converts
this inertia into a series of rotary blows
which gradually build up torque in the fastener.
The effect is similar to that obtained when
a hand wrench is placed on a nut and the
opposite end is struck with a hammer. Torque
reaction is not transmitted back through
the tool to the operator, therefore fasteners
can be safely tightened to high torques
with hand-held tools