ROCHESTER HILLS, Mich. (April 28, 2003)—A positive trend has emerged in the commercial truck tire industry in the last couple of years thanks to efforts of industry associations such as the Technology and Maintenance Council (TMC) and the Tire Industry Association (TIA).
We are now seeing more and more fleets and commercial tire dealers purchase and actually use torque wrenches to ensure that the wheels and rims they install on equipment are torqued to the proper level. Sales of torque wrenches have increased dramatically, according to tire and automotive equipment suppliers.
Kevin Rohlwing, senior vice president of education and technical services with TIA, has been instrumental in this movement. He has worked with torque wrench manufacturers and suppliers and written several articles on this subject that have appeared in the trade press as well as TIA's Commercial Tire Service Today publication. Mr. Rohlwing also has emphasized the importance of using a calibrated torque wrench in TIA's Training and Certification program for commercial tire service personnel.
At TIA's World Tire Expo in Louisville, Ky., last month, torque was once again the focus of a seminar. Only this time, a different aspect of proper wheel system installation was addressed. Ken Kelley, vice president of engineering at Walther Engineering & Manufacturing Co. and a member of the TMC task force that developed The User's Guide to Wheels and Rims, focused on fastener maintenance and its importance to ensuring that proper torque is achieved.
I moderated this session and thought it provided some valuable information worth sharing. The following are some highlights of Mr. Kelley's presentation.
Components of torque
There are plenty of places that you can go to find the correct procedures for installing wheels and rims on vehicles. The User's Guide to Wheels and Rims, wheel and rim manufacturer service and maintenance manuals and TIA's training products all provide this information.
These materials also contain the standard torque requirements for wheel fasteners by thread size and wheel system. However, these standards are based on mating fasteners that have threads that are in good condition. When the condition of fasteners and their threads are poor, the torque-tension relationship changes.
There are two components of applied torque. The first is tension, or the amount of stretch the stud experiences. (Yes, steel studs stretch to some degree when a fastener is tightened on it.) Tension is measured in pounds and also is referred to as clamp load. Using fasteners in good condition, 475 ft.-lbs. of torque should produce about 47,000 pounds of tension, which is the amount required to keep standard stud and hub piloted wheels on. (See Chart 1)
The second component of applied torque is friction—the resistance to motion when a nut is installed on a stud. This resistance increases significantly if the fasteners are rusty, dirty, damaged or not installed properly.
Chart 2 illustrates the tension that is obtained when rusty studs and lug nuts are used. That same 475 ft.-lbs. that was applied with a torque wrench now produces only about 25,000 pounds of tension, which is not enough to keep the wheels on. See why it's so important to use a wire brush to clean the threads of the fasteners?
Besides corrosion, there are a few other factors that affect fastener performance. These are:
c Excessive lubrication;
c Improper installation and removal techniques; and
c Improper replacement parts.
Excessive lubrication severely reduces the amount of friction in a fastening system. You may be thinking, “Well, how bad is that? Without friction we can really get the nuts on tight!”
And that's exactly right. Excessive lubrication can reduce the friction so much that you end up over-tightening and perhaps yielding the studs. When studs yield, they lose their ability to spring back and therefore their ability to hold tension. When that happens, no tension is achieved and the studs are broken, although they may still be in one piece. This condition is insidious in that is very hard to detect with the naked eye. If you're lucky, the stud will break and you'll know to replace it.
Chart 3 illustrates what happens to the torque-tension relationship when excessive lubricant is used. In this graph, 475 ft.-lbs. of torque now results in more than 70,000 pounds of tension, which exceeds the yield strength of the stud.
The hub-piloted system is designed to have two drops of SAE 30W oil applied to a point between the nuts and flanges and two drops to the last two or three threads at the end of each stud. Thirty-weight oil is specified since that oil's characteristics are standardized by the Society of Automotive Engineers (SAE). Every oil can made by every manufacturer is the same.
The stud-piloted wheel system is a dry system and no lubricant should be used on studs or nuts. Despite this, many people apply anti-seize compounds or other corrosion resistant materials to prevent inner and outer cap nuts from freezing together.
However, unlike SAE 30W oil, these compounds can be different from manufacturer to manufacturer and from can to can. As a result you can never be sure how much tension you are applying to a stud when you use this material.
In addition, anti-seize compounds or other products such as WD-40, silicone sprays or rust inhibitor materials that spray lubricant or smear it all over the place are not good, since you run the risk of getting lubricant in places that should not have it at all. This can result in loose wheels, since places that should have a lot of friction, such as the disc face, are now slippery.
Never apply lubricant with a brush or spray. Use a small oil can that will allow you to apply one drop of oil at a time to the appropriate location on the stud or nut.
Fasteners require care
Fasteners are commonly damaged when they are improperly installed, removed or are subject to over- or undertorque. For example, studs in demountable wheels can be bent as a result of overtorquing. The threads in studs and nuts can be stripped and deformed as a result of excessive torque, mismatched fastener hardware and corrosion and dirt on the threads.
One very common cause of damage to studs is the use of a hammer to install them. The problem with using a hammer is that you can miss hitting the stud and damage the stud head, which will result in loose wheels.
Or you might hit the hub pilot—which will prevent good seating of the wheels—or the hub flange, which will result in oil leaks. If you are really a bad shot, you could hit the ABS exciter ring on the hub, which will turn up as an ABS problem that has to be diagnosed and may require replacing the hub.
Great care has to be taken when installing studs with a hammer. Chart 4 illustrates what happens when stud heads do not have full contact with the hub. This “flatline” indicates that tension is not being achieved and that the stud is not working. The stud may either be yielded or it has not been properly installed in the hub and has minimal contact with the hub face.
If you use a hammer, make sure you support the hub flange. Use a block of some type on top of the stud when applying hammer blows. This will prevent stud damage. Avoid direct contact of the hammer to the stud head. Bent heads will hold the bolt off the back of the hub.
Also use a feeler gauge to check for any space between the hub and stud head. If there is space, the stud is not properly installed. And avoid hitting other hub components and adjacent fasteners.
When installing replacement fasteners, make sure you select nuts and studs with the same thread size, thread direction, overall length and bolt type. There are many different types of studs on the market, so choose the proper one for replacement. Some studs have clipped heads, which are flat on one side so that they fit in the hub boss. Replacing one with a round head will not work in that type of hub, since the boss will then hold the stud away from the hub.
Stud length also is important. Although there is no standard established for the number of threads that should protrude past a nut, it is imperative that you have sufficient threads for adequate thread engagement. Failure to use the correct length may lead to insufficient clamp load on the disc wheels.
Fasteners have a finite life. There is no existing standard in the industry that states how many times fasteners can be used, but you should inspect them routinely during wheel installations for excessive rust and damage and replace them if their condition is suspect.
To promote good fastener life, protect fasteners from corrosion by employing the liberal use of a wire brush on both the nuts and the studs. Use only approved lubricants and apply them sparingly. And remember to use proper installation and removal procedures and make sure only the correct replacement fasteners are used.