Ask the Experts: Preventing Galling In Compression Fittings

Q. The compressed air system in our plant operates in a hostile, high-stress environment. How can I keep the system’s compression fittings from leaking or seizing up, while still allowing for remakes?

A. The compression fittings in a compressed air system are comprised of several stainless steel parts that slide against each other under high load. During initial make-up and subsequent remakes, compression fitting nuts experience high-contact stress in the threads and against the ferrule. The sliding and the stress will eventually cause the parts to gall or seize due to deformation at the contact points. The parts will actually “cold weld” themselves to each other at the sliding surfaces, which causes a very rapid increase in friction and makes further sliding impossible without damaging the parts or compromising sealing integrity.

In order to prevent this type of failure and allow for remakes, it is necessary to introduce an anti-galling or anti-seizing agent to the compression fitting. This material maintains separation of the surfaces during sliding, similar to using motor oil to prevent metal-on-metal contact in a car’s engine. At Fluid Flow, we recommend Molybdenum Disulfide (MoS2) as the ideal solid lubricant and anti-galling material for compression fittings.

Because of its unique crystal structure of very weakly bonded lamellae, MoS2 (or “Moly”) offers exceptional lubricating properties under demanding conditions including high pressure and temperature, corrosion exposure, and vibration. These lamellae shear under low force to provide lubrication with an extremely low coefficient of friction. The lamellae also strongly resist forces perpendicular to them and therefore don’t squeeze out from between mating surfaces like liquid lubricants do under extreme pressure. This combination of properties builds a boundary layer that effectively prevents pairs of lubricated surfaces from touching.

For over 30 years, our partners at Parker Hannifin have developed and used a black, bonded Moly coating that is baked on the nuts of their premium CPI™ compression fitting products. These single-ferrule compression fittings offer easy initial make-up with very low torque, plus more frequent and consistent remakes than silver-plating lubricants. Parker’s Moly-coated compression fittings also deliver exceptional, leak-tight performance in even the most hostile environments.

Click here to learn more about CPI-Series Single Ferrule Compression Fittings from Parker Hannifin. To learn more about the benefits of single ferrule compression fittings, please read this white paper.

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