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.
Bolt tension
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 lubricants
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
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.
A careful choice of lubricant will improve the accuracy of bolt tension measurements. Solid lubricants minimize frictional losses and permit close adherence to specified tightness. In some cases, it's impossible to achieve the desired tightness without the proper solid lubricant.
For example, a leading manufacturer of power generating equipment produces water-level gauges for high-pressure steam turbine boilers. Each gauge requires eight gaskets of various materials, including copper and mica. Following a specified sequence for bolting up the gauges is as critical as it is for assembling an engine head or turbine casing. Proper torque is so essential to leak-tight, safe operation that the company furnishes a special torque wrench with each gauge it sells and specifies that cap screws be coated with solid lubricants and tightened to 60 inch-pounds of torque.
Solid lubricants can be used in various forms for different applications. High concentrations of lubricating solids (greater than 50 percent) are usually found in paste form, which increases the effectiveness for extreme pressure applications. Greases for antifriction bearings generally contain small amounts (about two percent) of solid lubricant particles (typically molybdenum sulfide or graphite), which allow the grease to function more effectively at higher loads and boundary conditions.
In addition, dispersions of solid lubricants in various fluids can be used alone or added to conventional oils. Adding various binders to solid lubricants allows them to be applied like a paint to leave a dry, corrosion-resistant film for surface lubrication, known as anti-friction, or A-F coatings.
Metal deformation
Even the most carefully machined metal surfaces appear as jagged peaks and valleys when examined under a microscope. Around the circumference of a thread, the high points of contact are distributed asymmetrically, resulting in off-center loading and increasing the severity of contact pressures and frictional conditions. With conventional lubrication, galling and seizing could take place because the surface peaks of the threads collide with each other and may interact. Wear particles are formed when the peaks break off. Increased wear or galling occurs, and a seizure takes place when metal fragments prevent further movement.
A specialty lubricant on a threaded connection acts as a protective film that prevents the contact points from colliding and breaking through the lubricant layer. For example, airline mechanics apply solid lubricants to the threaded connections on jet engine mountings to permit tensioning bolts properly, while eliminating the danger of galling and seizing, thereby allowing nondestructive disassembly.
Reduced maintenance
Though threaded connections are convenient for disassembly, even routine maintenance and repair operations involving these fasteners can pose problems. Threaded connections that are susceptible to corrosion, oxidation or carbonization can represent significant maintenance expenses if disassembly is required.
For instance, a leading vinyl acetate manufacturer discovered a problem on the threads of head bolts and grate support pins used in its chemical reactors. The first time the grates had to be removed for cleaning, corrosion seized the bolts and pins completely.
Four maintenance men worked 18 hours to burn out the threaded connections, which presented an explosion hazard and held up other operating crews during downtime. The company spent thousands of dollars on labor, new parts and lost production. After the firm switched to a solid lubricant, subsequent grate removal required two maintenance technicians and two hours work. Maintenance costs were reduced significantly, as the annual cost of solids-based thread lubricants was comparatively negligible.
Specify "smart" lubes
Proper lubricants for threaded connections are typically designed to meet three fundamental needs. The first is true torque values that standard torque control techniques can replicate. Further, correct lubrication and tightening of critical connections ensures proper assembly seating.
Second, because they resist oxidation and many chemicals, the lubricants protect against corrosion. They also reduce the destructive contact between dissimilar metals and withstand greater temperature stresses than conventional petroleum-based products. And third, they allow for nondestructive disassembly, which saves labor and equipment costs.
For maximum benefit, the wise engineer will specify the proper lubricant before evaluating a threaded fastener for its torque/tension conversion accuracy and dismantling performance. Given the proper lubricant, the bolt controls the torque/tension relationship.
The diverse group of solid lubricant materials offers improved performance in applications that exceed the environmental or operational limits of conventional products, especially under extremes of temperature, loads and environment. Solids-based lubricants offer performance capabilities that exceed those of conventional products.
David Como is Senior Lubricants Specialist/Product Steward with the US Lubricants Expertise Group at Dow Corning Corp., Midland, Mich. He can be reached at d.como@dowcorning.com and 989-496-8306.