Corrosion is an ongoing and costly issue for OTR fleets working in extreme mining environments. It causes immeasurable damage to mining equipment wheels and tires, negatively impacting equipment performance and making vehicles potentially unsafe for operators.
Corrosion occurs when a metal degrades to its natural state, through an electrochemical reaction with its environment. When iron, oxygen and water (air moisture) combine, the corrosion process is called oxidation. This reaction forms Fe2O3, known as iron oxide, ferric oxide, or rust. Rusting can be accelerated when iron is subjected to other minerals, chemicals and temperature fluctuations. The strength of wheel and rim components are compromised by rust and corrosion of contact surfaces. Oxygen, air and moisture not only corrode steel wheels, but they also react with rubber. With this type of corrosion, small particles of rust and dust can break off and clog valve cores, causing them to leak.
Let’s look at the stresses that affect a wheel and its studs. We want to know how wheels are attached and how to extend the life of those wheels and their studs.
It is relatively simple to bolt an object that is never going to move. You determine what pressure (tension) to apply to the object, bolt it to that tension, and you’re finished. The object remains in place. Bolting wheels, however, is more complicated. Wheels move - they start, turn and stop. They support different loads and travel at various speeds. As a truck runs down the road, and a wheel rotates, the load (stress) put on each stud changes dramatically. The studs experience different loads (cornering, tractive, normal), sometimes individually and sometimes collectively. If one of these loads applies pressure to a stud that is greater than the amount of tension torqued on the stud, there will be a momentary stretching of the bolt. Any stretch of the bolt, past where it has been tensioned to, will cause a brief relaxation of that bolt. And the result of any relaxation is a loss of pre-load. The majority of stud problems are caused by a reduction or loss of pre-load.
To examine stud and nut tension, we’ll conduct an experiment that tests the tension achieved in a fastener, given specific amounts of torque. This will demonstrate how friction affects the tension achieved in the fastener, and the dramatic consequences of increasing that friction.
With the help of a Skidmore-Wilhelm torque-tension tester, we will monitor the tension achieved in the fastener with varying amounts of torque. To ensure accurate torque is being applied, we will use the RAD-1800 pneumatic torque wrench.
Using a new stud and a new nut:
|ATTEMPT||TORQUE (FT/LBS)||TENSION ACHIEVED (FT/LBS)|
At this point, we want to see what the tension will be if we continue to use the same stud and nut, but decrease the amount of torque applied - back to the original 500 ft/lbs. If the result shows a lower tension than was originally achieved, we will then see how much more torque is required to achieve 28,000 ft/lbs (as shown above, this was originally arrived at with 600 ft/lbs of torque).
“If it ain’t broke don’t fix it!” We’ve all heard this before, and it is a suitable way to look at some things, but it doesn’t apply when dealing with rims and wheels. The challenge with wheel studs, and the hardware that keeps wheels attached, is that we don’t always know when something is broken. So, the best way to ensure that your wheels and hardware are in good working condition is through regular maintenance and a full understanding of potential problems.
Comprehensive information on maintaining studs and nuts is not widely available. In the trucking industry, highway trucks get the most analysis and we can use this information as long as we factor in that mining environments are more abusive and failures are more severe.