What causes galling?
Galling occurs when the protective oxide layer of a metal fastener wears during installation due to increased pressure and heat between the surfaces of the contact threads. In the absence of an oxide coating, the original metal threads are in direct contact with each other without obstructions, causing the nuts and bolts to "fuse" together. Before using, you have to check all the fasteners, but they still fail after installa
How to prevent thread galling?
1) Develop suitable tightening speed and correct assembly method
Lower installation speeds can often reduce or even completely prevent wear problems. As the installation speed increases, the frictional force also increases, resulting in heat generation, which leads to wear. In addition, excessive tightening can also increase friction and wear.
2) Use lubricating oil and coating as much as possible
Lubricating internal and/or external threads reduces friction, which is a major factor in wear. Always consider the end-use of the fastener before selecting a lubricant. If you are concerned about food or laboratory purity, some lubricants may be unacceptable. In addition, fastener installers must consider changes in the torque-tension relationship due to the use of lubricants, as excessive tightening can also cause wear.
Effective lubricants include:
Molybdenum disulfide is a high-temperature lubricant that can be used to prevent metal contact and minimize wear. Molybdenum disulfide is usually used when normal lubricants and oils do not meet the actual operating temperature of the fastener.
Film lubricants containing silver, aluminum or copper can be used to reduce the coefficient of friction. Most anti-seize agents used on assembly lines contain these metal particles.
Wear-resistant coatings such as Teflon, polyvinylidene fluoride, PFA and FEP are another option. These are fluoropolymer coatings that combine resin and a fluoropolymer lubricant to provide low friction, chemical and corrosion resistance, and no wetting and release or non-sticking below550 °F. performance.
Aqueous solid dry film lubricants such as German "gleitmo" fasteners, commonly known as friction coefficient stabilizers or German water waxes, may also be effective.
3) Fastener thread selection
A smooth surface texture will result in less frictional resistance. Rolling typically provides a smoother surface than the crepe, which reduces friction that can cause wear. In addition, the use of coarse threading can also help. The coarse teeth have a large thread allowance, while the fine teeth have more friction surfaces.
4) Selection of different metal materials (hardness)
Another technique to reduce wear is to use materials of different hardness for nuts and bolts. In doing so, the protective oxide layer of one material is more likely to wear, preventing the wear of the oxide layer of the other material. It must be noted, however, that galvanic corrosion may occur between different materials, especially when the fastener is exposed to a liquid medium.