Increasing equipment reliability with proper lubricant selection
By BY GENE M. FINNER
Modern lubrication technology can provide MRO specialists with proven, effective problem-solvers for today’s demanding applications. Different friction regimes require different lubricant forms, and proper lubricant selection must...
September 1, 2013
By BY GENE M. FINNER
Modern lubrication technology can provide MRO specialists with proven, effective problem-solvers for today’s demanding applications. Different friction regimes require different lubricant forms, and proper lubricant selection must consider the application load, environment, temperature and speed.
Inadequate lubrication or an incorrect lubricant choice can lead to equipment failure. When conventional or OEM-recommended lubricants fail, MRO specialists need to identify the proper lubricant to get the job done right. Knowing the friction regime involved, understanding the capabilities and advantages of different lubricant forms, and analyzing the specific application conditions are important considerations.
Lubrication friction regimes
Effective lubricating films must meet the requirements of one or more friction regimes or conditions. They are formed as a function of application speeds, loads, component geometries, substrate material properties and lubricant material properties.
Hydrodynamic regimes involve fluid lubricants forced between opposing surfaces by speed and load. The fluid keeps the surfaces apart with pressure. As pressure increases, the fluid may act like a solid, deform the surfaces, and create an elastohydrodynamic (EHD) regime.
Boundary regimes are not created by fluid under pressure, but rather by surface-active materials that form boundary films on and between the substrate surfaces. Anti-wear and extreme-pressure (EP) additives and solid lubricants can adhere to surfaces and provide boundary layers.
Mixed regimes combine hydrodynamic and boundary regimes; some applications have both. In startup and shutdown periods or with transient shock loads, conditions can occur in which the relative motion and/or the distance between surfaces converge toward zero.
Different lubricant forms
Any machine, component or surface requiring lubrication will have a primary lubrication regime based on its steady-state operation. Fluid-based lubricants are used for hydrodynamic and elastohydrodynamic lubrication. Solid lubricants are used for effective boundary-regime lubrication. A combination of lubricant forms often is needed for mixed regimes.
Greases are usually composed of 80% to 90% oil in a thickener and a few per cent of special additives, including solid lubricants. Greases work best when speeds are moderate to high, friction is rolling rather than sliding, and a regular lubrication service interval is possible.
Oils typically consist of 95% to 99% lubricating oil and a few per cent of additives to enhance pour point, viscosity index and resistance to oxidation or corrosion, and to aid wear prevention. Some oils are mixed with solid lubricants to provide an easily applied dispersion.
Anti-seize pastes consist of roughly equal amounts of oil and solid lubricants, along with small amounts of special additives. Pastes often are used where components are static for long periods of time and where loads are high, speeds are slower and friction is sliding rather than rolling.
Dry-film lubricants or anti-friction coatings (AFCs) consist of solvent, resin binder and solid lubricants plus other additives. Once dry, the lubricating film is tack-free and will not attract dust or dirt. Dry-film lubricants work best in applications where speeds are slow and loads are high.
Silicone compounds are sticky, non-hardening, thermally and chemically stable materials made with silicone fluid, inert fume silica and other additives. They are useful as light-load lubricants, non-curing sealants, electrical moisture barriers and electronic damping media.
Selecting an effective lubricant must consider critical application variables. These are summarized by the acronym LETS – Load, Environment, Temperature and Speed.
Load involves the amount of force or stress placed on the component. Heavy loads may require boundary lubrication with specialty greases, anti-seize pastes or anti-friction coatings with solid lubricants. Moderate loads may require greases or pastes with lower amounts of solids. Light loads can usually be handled by greases, oils or silicone compounds.
Environment affects the lubricant durability in the application. High humidity may require solid lubricants. Water-rich environments need lubricants with water-washout resistance. Exposure to chemicals and fuels may call for fluorosilicone grease. Dusty, dirty conditions may need a dry-film lubricant. Inaccessible components may require an oxidation-resistant grease, paste or AFCs.
Temperature includes the actual operating temperature, soak temperature at standstill and the ambient temperature. Synthetic lubricants generally have a wider service-temperature range than conventional greases and oils. Silicone-based lubricants are proven in cold temperatures. Some high-temperature greases and oils stiffen in extreme cold and oxidize or dry out in extreme heat.
Speed can run from high to low and involve rotational and reciprocating speeds as well as frequency of motion. Static conditions are harsh on liquid-based lubricants. Slower speeds can require use of higher-viscosity oils or, with heavier loads, anti-seize pastes or AFCs. Higher speeds can be best served with lower-viscosity lubricants to prevent problems with shear.
Effective MRO problem-solvers
MRO specialists in various industries can use specialty lubricants to help prevent or solve challenging lubrication problems. They know proper lubricant selection can help them optimize friction and wear control to save energy, maintain efficiency and lower maintenance costs. Here are some examples of the lubricant choices available.
• To gain extended lubrication reliability and avoid potential service disruptions, an electric utility lubricates circuit breaker triplatch-and-close bearings with fluorosilicone based Molykote 3451 Chemical-Resistant Bearing Grease.
• When a recommended grease for rolling element bearings in pillow blocks could not withstand frequent water washdown, a cheese plant switched to Molykote G-1502FM Grease and sharply reduced the number and costs of bearing failures.
• Replacing a lithium grease that required frequent service, a lime calcining plant chose Molykote Longterm 2 Plus Grease for
severe-duty bearings in its fans, crushers and conveyors.
• To eliminate harmful degradation by products of PAG-based compressor oils, a pet food manufacturer selected Molykote
L-4646 High-Temperature Compressor Oil for extended-life lubrication, cooling and corrosion protection.
• Oils failed under high contact pressures on a cement plant’s ball-mill support bearings; Molykote M-55 Dispersion provided
a solid lubricant film to prevent bearing wear, noise and failure.
• An aluminum-casting operation replaced an industrial grease with Molykote P-74 Paste for more effective, reliable lubrication of mixer-component roller bearings used in melting ovens.
• Applied to threaded pipe connections used in rock-drilling machines, Molykote G-N Metal Assembly Paste helps prevent microwelding, seizures and difficult assembly/disassembly.
• With corrosion protection, dry-film lubrication and high-pressure wear resistance, Molykote 3402-C Anti-Friction Coating is
helping extend the service life of yaw brakes for wind turbines.
• In reducing wear and preventing sticking of rods in metal-forging dies, Molykote 321 Dry Film Lubricant is helping to reduce
energy costs, extend die life, save replacement costs and increase productivity to meet ship schedules.
• To eliminate production stoppage, material leakage, costly repairs and frequent valve replacement, a mining operator is
using Molykote 111 Silicone Compound to lubricate and seal its knife valves, which were failing with a mineral-oil grease.
• For high-performance downhole electrical connectors and cables, an oil and gas supplier uses silicone-based Dow Corning
4 Electrical Insulating Compound to ease installation and fill gaps and voids for protection against moisture, chemicals and high pressures.
Gene M. Finner is product steward, Industrial Assembly and Maintenance, Dow Corning Corp. Dow’s application-driven and performance-matched Molykote brand Smart Lubrication solutions can help improve equipment reliability, extend maintenance intervals and reduce total cost of ownership. For more information, visit www.dowcorning.com.