Preventative maintenance should be the key focus. Here are some basic problems and solutions:
- Minor corrosion on threads (stud or nut): Brush thoroughly with a wire brush.
- Excessive corrosion on threads: DISCARD.
- Mutilated threads (stripped or otherwise): DISCARD.
- Broken studs, one only: Replace it, as well as the studs on either side of it.
- Fastener wear: Replace fasteners after a predetermined number of uses.
It is important to replace any parts that are overly corroded, because the corrosion will increase the amount of friction in a joint. The same reasoning applies to studs with mutilated threads: replace them, because of the increased friction they introduce.
If you replace a broken stud, you should also replace the studs surrounding it. When a stud breaks, it causes an uneven load distribution, which causes increased stresses on the surrounding studs. In a highway tractor application, for example, if you have two broken studs, you replace all eight. Mining equipment is not quite so vulnerable, because wheel attachment is over-engineered. There are numerous studs, so you don’t need to replace every stud every time; but if broken studs are close together, you might also want to replace those on either side of the broken ones.
There are a number of factors that affect the torque applied to a fastener.
- Torque tool accuracy
- Operator skill
- Bolting procedure
- Condition of the threads
Our goal is to gain a better understanding of these factors so that we can reduce the margin for error. We need to understand the following:
Most bolted joints provide less clamping force than is desirable.
- Nuts can vibrate loose if there is not enough pre-load.
- A fastener will resist vibrating loose if it is loaded higher than the external loads placed on it.
- A few major vibrations will contribute more to loosening than will many minor vibrations.
You may need to change fasteners more regularly in some applications than in others. And there are factors other than friction involved. When a nut is tightened, it compresses at the point where the nut meets the wheel. This compression of the bottom of the nut is called “nut dilation” – it is a slight, but permanent change to the nut, causing increased friction. As you reuse the nut, it continues to dilate, increasing the friction. The result is a lower pre-load. After between four to six uses, the preload may be reduced by as much as 50%. This means that you need to double the amount of torque placed on the stud. Also, the more challenging and extreme the environment, the more often fasteners need to be replaced!
"Our goal is to gain a better understanding of torqueing factors so that we can reduce the margin for error with fasteners."
RIMEX recommends torque values for a number of fasteners. All of our recommendations are “dry” values, meaning that no lubrication is used. Because lubrication comes in many forms, and from different manufacturers, it introduces a variable that is difficult to accurately predict. For example, the friction factor could vary greatly between different types of lubrication. Reduced friction can cause problems such as:
- Stripping of threads when a stud/nut is being fastened
- Dilation of nuts
- Vibration loosening of nuts
Therefore, we recommend that all lubrication should be stopped completely when fastening wheels and rims.
Problems and Causes
Here are some common problems with studs and nuts, along with their causes:
|Damaged threads near the hub||Loose wheel|
|Mutilated threads along the stud||Improperly installed wheels|
|Broken studs (during installation)||Excessive torque|
|Broken studs (while in use)||Loss of preload. Either over-torqued or under-torqued|