Learn how K factor affects this choice.
by Phil Mahoney
September 4, 2018
Image 3Image 3. Consistency of applied load with a lubricated (A) versus nonlubricated (B) assembly (Image credit: Bickford. (1995), An Introduction to the Design and Behavior of Bolted Joints, New York, NY: Marcel Dekker)

Lubricants provide better accuracy and consistency in loading bolts in a flange assembly. Image 3 shows the difference in bolt stretch (which corresponds to the applied load) for a series of lubricated studs (A) versus clean, dry studs with no lubricant used (B). The bolt size used is a 2 1/4-inch bolt, 12 inches long, B6 Grade.

There is more consistency with the lubricated studs than the dry ones. In a flanged assembly with eight bolts, this improved accuracy helps ensure the applied stress on the gasket is equally distributed across its entire area. This accuracy may drop off at higher loads depending on the quality of the lube. Some lubes are designed to provide good lubrication under high loads (i.e., a high quality anti-seizes) while others are low friction at low loads but break down at the higher stresses resulting in greater variability in multiple assemblies. This can cause inconsistency in the bolt load, or in the worst case, galling of the threads resulting in none of the applied torque results in bolt load.

But even if the designer has selected a quality thread lubricant, there are a few other things to consider.

First, is the joint a one-time assembly, or is it assembled and disassembled repeatedly or adjusted during service? If the latter, the best option would be a quality anti-seize with good lubrication properties that prevents galling after the assembly has been in service for a period of time (often at elevated temperatures, which may break down the base lube). The highest quality products in this category can still provide a consistent—typically higher—K factor even after the base lube has broken down.

Second, the lubricant should be compatible with the media being sealed or the atmosphere in which the bolted joint resides.

Lastly, in systems monitored for emissions compliance, some lubes may emit volatile organic compounds (VOCs) at high temperatures that may result in a false positive. The monitoring equipment may indicate a VOC leak from a flange, when in fact it is the lube off-gassing.

Next Month: Packing Failure Modes

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