Thread galling is a common, yet seldom understood problem with threaded fasteners. Galling, often referred to as a cold-welding process, can occur when the surfaces of male and female threads are placed under heavy pressure. The frustrating aspect of fastener galling is that galled nuts and bolts may pass all required inspections (threads, material, mechanical, etc.), yet they still fail to function together.
Minor galling may cause only slight damage to the thread surface and the installer may still be able to remove the fastener. However, in severe cases galling can completely weld the nut and bolt together and prevent removal of the fastener.
If the tightening process is continued once galling begins, the fastener may be twisted off or have its threads stripped.
What is thread galling?
Thread galling, also known as seizing, is the regular abrasion of the threads of a fastener over time.
It most commonly occurs in fasteners that are likely to be subjected to a lot of motion, such as those that are meant to be fastened and undone regularly, or those that are subjected to regular vibration.
However, there are times when the galling can occur during the initial fastening as well. If you intend to use threaded fasteners, it’s important to not only identify galling when it occurs, but to also put in place measures to ensure that it does not occur in the first place.
Consequences of Thread Galling.
Potentially disastrous effects could result from galled fasteners seizing or experiencing fatigue breakage. It can increase maintenance and repair costs, as the galled or broken fasteners need to be removed and replaced, and the holes where the fasteners are applied may need repairs. And if features on fasteners that promote galling are not eliminated or mitigated, they may not only chronically affect the equipment, but the assembly line also, exposing it to recurring delays, downtime, and occupational hazards. With galling a serious threat to the quality and safety of expensive and crucial equipment, staff in executive management, operations and procurement would be wise to study this phenomenon also.
Galling’s consequences should not be underestimated. A galled fastener may fail to achieve the necessary pre-load – especially in dynamic loading. This jeopardizes the fastener’s task to securely fasten two surfaces together, which is complicated by the fact that an application may involve hundreds or even thousands of stainless-steel fasteners.
If critical fasteners in the rotating parts of a chemical pump, rotor blade, or propeller gall, the joint is likely to experience fatigue breakage, requiring costly maintenance and downtime for the customer. At worst, fatigue breakage of critical fasteners might result in accident or injury. Galled fasteners are also much more susceptible to corrosion, which can ultimately result in breakages.
Ways to minimize galling.
| Add fastener lubrication | The most efficient way to minimize risk of galling is to add an anti-seize solution. Some environments have special requirements like kitchens. Make sure the correct anti-seize is used based on the environment. |
| Reduce installation speed | As installation increases friction can cause a fastener to get hotter and hotter, reduce the speed will not stop the chance of galling completely but it will allow the heat to dissipate. |
| Take load off assembly before tightening | When tightening the nut onto the bolt, ensure there is no or minimal load on the assembly. Increased load will force the threading to push against itself while spinning, increasing the friction. |
| Use a torque wrench to avoid over-torque | Over-torqueing a nut can cause damage to the threading. |
| Use two different grades of the material | Some success has been found by using one softer grade and one harder grade material. Although in some situations this is not possible such as near the ocean where only grade 316 stainless steel should be used. |
| Ensure fastener threads are not damaged prior to installation | Damaged threading will increase the friction during installation. |
| Ensure there is no debris in fastener threading prior to installation | Debris sitting inside of threading will also greatly increase friction during installation. If bolts are covered in dirt use compressed air to remove it before use. |
| Use coarse threading where possible | Fine threaded fasteners are great tools but require many more turns per inch during that coarse threading. This, in turn, will generate more friction per inch leading to an increased chance of galling. |
| Keep fasteners in a controlled environment | Try to keep fasteners in a cool environment. If left in the sun, the fasteners will heat up resulting in an increased chance of galling. |
If seizing begins, immediately stop and remove the nut and bolt, use new fasteners and apply lubricant.