November 2010, TruckingInfo.com - Feature
Attaching wheels to a truck hub is serious business. Yet it's not uncommon to see some big bruiser of a mechanic, one foot against the bottom of the tire, whaling away with a 1-inch-drive impact gun driving the wheel nuts home.
As with anything technical, there's a process for ensuring the job is done properly and safely. The above procedure is not recommended.
Wheel nuts must be torqued to specified values, and the stud, nuts, and wheel must be in good condition, otherwise torque values could be compromised and the equipment damaged.
When mounting wheels on a hub and securing them with a typical M22x1.5 stud and 33 mm two-piece flange nut, the clamped material will compress slightly and the stud will stretch slightly under the recommended 450-500 foot-pounds of torque applied to the nuts. The tension between the compressed and stretched material (called preload) is where we get the clamping force to secure the wheels - about 50,000 pounds per stud. When this preload is exceeded by external load created by the operation of the vehicle, or if the preload is relaxed, the joint will loosen.
This could happen in several ways:
* Over-torquing the nut can stretch the stud beyond its design yield, causing it to fracture or break.
* Torque readings could be lower than required due to contaminated or damaged threads, providing lower clamping force on the face of the flange nut.
* Contamination between the wheels or between the inside wheel and the hub face could compress or fall away, relieving the tension in the clamping system.
* The contact surfaces of the hub face and the wheels must be clean and free of contamination such as dirt, peeling paint, rust, grease, etc. to ensure proper clamping force. The studs should be clean and corrosion free, and the nuts should hand-turn onto the treads. Any resistance to hand-turning could indicate the stud has stretched and threads have deformed. The studs should be carefully examined and damaged studs replaced.
Two-piece 33 mm flange nuts used with hub-piloted wheels should be tightened to a torque of 450 to 500 foot-pounds. Other nut designs have different torque requirements. Check the manufacturer's recommendations for your equipment.
Alcoa recommends applying a coat of non-water-based lubricant to the wheel pilot or hub pads prior to mounting to minimize corrosion build-up between the wheel and hub pilot, which can make wheel removal difficult. Do not lubricate the face of the wheel, hub, or brake drum.
Lubricate the stud threads and the contact surfaces between the cap nut and the flange with a couple of drops of 30-weight oil. Lubrication isn't necessary with new hardware.
Position one of the hub's pilot pads at the twelve o'clock position, and lift the wheel onto the pilot. Hand tighten or use a low-torque nut runner to firm down the flange nuts. Lower the truck off the jack, and then tighten to the recommended torque following the proper sequence for your wheel type using a manual torque wrench. Never use a high-torque impact wrench to do the final torque. They aren't very accurate, and they could well over-torque the nut and damage the stud and/or the wheel.
Since the wheel and hub will further seat itself after a short distance, and the torque will drop, Alcoa strongly recommends retorquing the wheels after 5 to 50 miles of operation.
From the October 2010 issue of Heavy Duty Trucking magazine.