As an assembly technique, threaded connections offer a number of advantages. They're a simple, effective method when a connection requires a predetermined tension or compression. Their torque can be measured in most cases, even in service. They offer unequalled convenience for component disassembly, permitting repeated use of a fastener and helping ensure accurate reassembly.
However, threaded connections can present problems. Selecting the right lubricant can mean the difference between a failure in service and a trouble-free connection. As exotic alloys and extreme temperatures become more common, choosing a proper lubricant for threaded connections becomes more significant. Sufficient lubrication takes on increased importance in light of the potential costs of an assembly that comes loose in service, or a seized bolt that prevents disassembly, especially in the field.
Most manufacturers rely on torque wrenches to determine bolt tension. Yet the torque wrench fails to compensate for the fact that much of the energy used to install an unlubricated bolt is spent overcoming friction, not tightening the connection.
Without a lube, lost energy should be considered in calculating the bolt tension, a practice that's often impractical. The most accurate methods for determining bolt tension depend on measuring how much the bolt has been stretched during installation, but these approaches are too complicated and expensive to use under normal circumstances. Unless the threads and mating surfaces are lubricated properly, it's uncertain how much of the torque applied to the bolt head is converted into bolt tension.
Correct bolt tension is essential to the life of a threaded connection, especially when it's subjected to shock loading or thermal cycling, either of which can pull the connection apart. Much like a wire that breaks from repeated bending, continuous expansion and contraction can fatigue the bolt. For maximum service life, the recommended preload force holding the threaded connection together should be 80 percent of the bolt's yield strength.
The 80 percent value is how the fastener or machine manufacturer calculated torque values for the field use. The millwright simply uses a torque value to tension critical fasteners. The idea is to apply enough torque to deform the bolt elastically while avoiding the plastic region of its stress-strain curve. Too much torque permanently deforms the bolt and destroys its strength.
Conventional oils and greases are almost always derived from a petroleum base, and lubricate most effectively by separating metal surfaces with a fluid film. Film formation is a result of lubricant viscosity, surface tension, applied load and the relative speed between moving metal parts.
Based on the characteristics of conventional oil-based lubricants however, low sliding velocities, minimum clearances and high loads may cause the surface peaks of the threaded connections to penetrate the lubricating film. This penetration results in metal-to-metal contact and can lead to galling or seizing.
Conventional lubricants are not designed to operate under the conditions encountered in threaded connections. For instance, when fasteners are at rest, conventional lubricants tend to drain or squeeze out. When exposed to low temperatures, petroleum-based lubricants thicken, and when exposed to high temperatures, they thin out or volatize off. Petroleum-based lubricants also may present a fire or explosion hazard when exposed to oxygen or oxidizing chemicals. Further, these "wet" lubricants attract dirt, dust and other contaminants, which produce abrasion that can damage a threaded connection.
An alternative to petroleum-based lubes can be found in a group of products known collectively as specialty lubricants. They prevent metal-to-metal contact when it's not possible to establish a fluid film with a conventional lubricant. In a threaded connection, a specialty lubricant acts as a protective layer that allows metal surfaces to slide over each other without breaking through the lubricant film. Of the many types of specialty products available, solid lubricants provide the best results when used on threaded fasteners.
Solid lubricants prevent metal-to-metal contact when speed and load characteristics prevent the formation of a hydrodynamic fluid film. Solid lubricants include graphite, zinc, copper, nickel and various metal salts, which cling to metal surfaces. Another popular solid lubricant, molybdenum disulfide, attaches itself to the surface and can be "burnished" into the metal. Under load conditions, the powder's molecular structure prevents contact between metal surfaces as they slide upon one another.
Coefficients of friction and thermal stability make solids effective thread lubricants. The lube's coefficient of friction can be controlled by varying the type and amount of solids, useful for varying the amount of torque required to tighten a bolt to the necessary tension. In addition, some solid lubricants, such as nickel, are effective at temperatures above 2,000 F. Powdered lubricants don't attract abrasive dust and dirt.