Truck Sales & Leasing
January/February 1999

Impact Wrench Use And Maintenance

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 a service center.

A consultant to fleets and manufacturers, Peggy Fisher was Roadway Tire Co. president for nine years and Roadway Express Tire, Trailer & Dolly maintenance manager for 10. She is among the leaders of The Maintenance Council of ATA, and chairs three Tire & Wheel task forces. Send your tire and wheel questions to: Ask Peggy, P.O. Box W, Newport Beach, CA 92658; or e-mail pfisher@heavytruck.com.