Regular care
will ensure longer wrench life and properly torqued
wheel assemblies.
Despite wheel and rim manufacturers’ admonitions
to use a torque wrench, most technicians use a
pneumatic impact wrench to install truck tire
and wheel assemblies -- because it’s fast,
not because it’s accurate.
Trouble is,
if lug nuts aren’t properly torqued the
wheels can fly off.
Most 1-inch
impact wrenches have working torque ranges of
around 100 to 1,000 lbs.-ft. and maximum ranges
of about 1,400 to 1,600 lbs.-ft. Manufacturers
recommend impact torquing of three to five seconds
(450 to 500 lbs.-ft.) for hub- and stud-piloted
wheel systems used on Class 7 and 8 vehicles.
However, if the wrench is left on the nut too
long, it can attain the top torque limit -- as
much as 1,500 lbs.-ft.
A lot of torque
is not better than a little for wheel studs. When
lug nuts are tightened, studs actually stretch.
The resulting tension holds on the wheels. When
the nuts are removed, the studs spring back to
their original length. Overtorquing stretches
the spring out, causing an undetected “yield”
that may result in broken studs.
If airflow
drops below 90 psi at the tool inlet with the
tool running, the impact wrench may not operate
within its working range. Compressor capacity
must be adequate to handle simultaneous tool use.
Check output regularly, fix leaks immediately
and ensure air lines are not restricted or collapsed.
If possible,
use an impact wrench to “snug up”
the wheels and then use a torque wrench to tighten.
This ensures wheels are torqued correctly every
time.
Proper wrench
maintenance includes in-line filters to remove
dust, corrosive fumes and excessive moisture;
drain them periodically. An in-line lubricator
will increase life, but wrenches should be cleaned
and lubricated when not in use and stored away
from dirt and moisture -- a good procedure is
to pour a liberal amount of air tool oil in the
inlet and run the tool before storing; also, check
clutch lubricant regularly and add only the amount
and type of lubricant specified by the manufacturer.
(A tool with a sealed oil lubricated clutch will
lose power if overfilled.)
Impact wrenches
should be checked and calibrated as a part of
regular tool maintenance -- not a repair. Installing
a nut on a vehicle with the impact wrench can
do this. The wrench should impact for 3-5 seconds.
Then measure the torque on the nut using a torque
wrench in the tightening direction. Note the torque
level when the nut just begins to move.
If working
range is not achieved, the wrench will need to
be inspected and perhaps rebuilt. If you don’t
want to mess with this, send the work to our service
center.
