This week Tech Know is looking the topic of fastener thread damage and galling, which is a problem that can cause permanent damage to bolt and nut threads. The coating on critical OEM shear-head bolts, such as Hendrickson QUIK-ALIGN pivot bolts, helps prevent galling. But it can still occur if there is any damage to the thread before assembly. Therefore, to minimise any likelihood of damage, it is important to visually check the threads for damage before assembly.
Galling is where materials adhere to one another. In fasteners, this means that metal from one surface, usually the bolt, transfers over to the moving nut. As the nut moves further down the thread the galled-up metal grows in size as it collects more metal from the adjoining surfaces.
Heat from friction increases the likelihood of the metal thread galling. Prevailing torque locknuts have added friction, which is why they are particularly susceptible to galling.
Not all metals have the same susceptibility to galling. Aluminium, stainless steel and titanium are particularly prone to galling. On the other hand, brass is used in bushings because of its resistance to galling.
New bolt threads can easily be damaged in transit. Even a small amount of damage can initiate galling as the fastener is tightened. Once the metal begins to gall, it will simply get worse as the bolt is tightened until the thread is beyond repair. Therefore, it is essential to visually inspect every QUIK-ALIGN and shear-head bolt before assembly to ensure there is no damage that could cause galling.
Threaded bolt holes can also cause galling due to weld splatter or hole misalignment. So, they should be checked as well. It only takes a few seconds to run a bolt through with your fingers to check the thread. Any problems can be quickly repaired with a tap before serious damage occurs.
Galling Prevention Tips
There are several steps you can take to reduce the likelihood of galling:
- Inspect the threads for damage before assembly.
- Tighten the fastener slowly to reduce heat from friction.
- Stop tightening if it does begin to gall. If the galling is near the start of the thread, you may be able to save the bolt with a thread die. At the very least, it will be easier to remove the bolt before the metals have completely fused together.
- Never reuse critical nut or bolts.
- Some fastener use allows for the application of a thread lubricant, such as anti-seize. However, never apply lubricants to coated shear-head bolts, for example Hendrickson shock absorber and QUIK-ALIGN pivot bolts.
- Run the bolt through by hand for a few threads to confirm thread consistency.