Goodness of fit Parts should be dimensionally tight enough to prevent vibration and wear with consequent roughness which might facilitate galling. However, sufficient clearance is required to avoid fouling during assembly. Quality control Good housekeeping is needed to exclude dirt or abrasive materials from between mating surfaces. It is also vital that the thread profile match, i. Lubrication There are a wide range of lubricants containing nickel, molybdenum, copper, silver or graphite, or combinations of these.
They can be messy. Those primarily containing nickel are more suitable for high temperatures. Two suppliers of nickel and graphite containing anti-seize compounds are Devcon and Loctite. Their websites offer selection tables depending on service requirements. Molybdenum disulphide may also be suitable. The graph below shows that using a lubricant makes a substantial difference to the torque value for the bolt condition. Be aware of possible crevice corrosion between the grease and the steel if the grease dries out and cracks or, if the grease does not properly wet the surface so that a cavity forms allowing water to accumulate.
If graphite is used in large quantities then galvanic corrosion of the stainless steel is possible. Nickel plating has been used as a protection against such corrosion. Silver plating has also been used as a lubricant to prevent galling of nickel-copper Monel alloys in oxygen service. Material selection If the system is not lubricated or has insufficient lubrication, proper grade selection is vital to limit galling.
There can be an improvement obtained by using two different stainless steels across the interface. This is largely related to the magnitude of the hardness difference between the two steels and some experimental information is shown below. There also grades which are self-lubricating such as the high nickel grade Waukesha 88 which contains tin and bismuth or Waukesha 54C which contains tin and silver.
The sulphide particles in free machining also appears to provide lubrication. The corrosion resistance of these grades should be carfeully considered. It should be noted that while using bolts with nuts is often quoted as a cure for galling, the hardness difference is not necessarily sufficient to prevent galling even with a cold rolled, harder thread used against an annealed, softer nut.
It provides a layer of protection between the two materials, allowing for the materials to easily slide past each other without creating friction or heat. There are multiple types of anti-seize compounds, but is usually a grease containing copper, aluminum, or calcium oxide.
Sometimes the manufacturer will send the product with anti-seize already applied to the threads but is still a good habit to apply your own anti-seize before installation. Differing Stainless Steel Grades Sometimes using two different stainless steel grades will help prevent galling. By having dissimilar hardness ratings, the materials are much less likely to deform or fuse with one another.
In the worst case, the softer material will deform before damaging the other material, allowing you to remove the damaged part without affecting the other. This is an added protection, especially suited for stainless steel hardware. By using the bolt to pull the two pieces together, you are relying on the thread face to push the material together.
Standard threads are not designed for these dynamic loads and will only damage the threads unnecessarily. Instead, use a clamp or vise to pull the pieces together and then use the bolt to retain the position.
Clamps are designed for this type of force, not delicate threads of a bolt. This is a useful tip for any material, not just stainless steel. Installation speed Reducing the installation speed will help reduce friction between the materials. Unfortunately, some people experience issues with stainless Load More Related Articles.
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Chris November 6, at pm.
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