Maximize your thixotropy
Understanding, care and diligence will ensure you get maximum benefits from your thixotropic lubricants.
Thixotropy is the ability of a grease to move from a semisolid state when at rest to a relatively fluid state when it is mechanically manipulated or placed under pressure, as it would in a gearbox or bearing application.
This is characteristic of greases, defined as solid or semifluid lubricants consisting of a thickening agent in a liquid lubricant. Additives imparting special properties may be included. A grease typically consists of 75% to 95% base fluid, 2% to 20% thickener and 3% to 25% additives. Base fluid, thickener and additives are interrelated and must be balanced for top performance.
Base fluids can be asphaltic, naphthenic, paraffinic, hydrocracked, synthetic or vegetable. The costs range from low for naphthenic to high for silicone base fluids, and performance characteristics vary.
Grease thickeners may be calcium, lithium, barium or aluminum complexes, benton, clay or silica, and polyurea. Performance additives include antioxidants, antiwear, extreme pressure or metal deactivators, rust and corrosion inhibitors and tackiness additives. Not all of those are compatible, so pay attention to matching.
Greases provide surface coating for moving parts to prevent metal-to-metal contact so they must be water and corrosion resistant, provide shear and oxidation stability, and exhibit high-temperature stability and lowtemperature mobility.
Handling with care
Be sure to store, handle and dispense grease properly. Mike Deckert, vice-president of FLO Components Ltd., an ISO 9001 certified automatic greasing systems specialist firm and leading supplier of lube solutions in Mississauga, Ont., has this advice:
• Never assume that product in a drum is clean. Drums can become contaminated, depending on where and how they are stored. Always sample what’s in a new drum.
• Separation will occur when grease sits too long in a drum. Rotate your inventory.
• Storage temperature is critical. Cool is fine, too warm or very cold is usually detrimental. If stored outside, place a cover over the drum. When in use, replace bungs and covers. Tilt drums where possible and position cover ports horizontal to the tilt. If drums are stored on their sides, position cover ports horizontally.
• Movement of drums from point to point can be dangerous. Use a hand dolly or forklift with drum clamps.
• No smoking signs and fire extinguishers are a must in storage areas. Solvents should be in a separate room.
• If in cold weather the product requires heating, never use a torch. Use a band heater properly located around the drum and never heat above 20 degrees C.
• The most common and recommended types of grease fittings and adapters for dispensing are NPT and “Zerk” units. There are also specialty fittings and vents and adapters.
• Grease guns can be manual, air-operated or electric. The critical issue is how to use them properly. How much is a shot? It depends on the type, the loading and the supplier. Going by “eight shots for this coupling and 10 shots for that bearing” is misleading and dangerous. Much depends on the grease gun. General heavy duty guns have 33 strokes per one ounce of output at a maximum pressure of 7,000 psi. Heavy duty dual pressure guns deliver 18 strokes at low pressure (6,000 psi) and 33 strokes at high pressure (7,000 psi). High pressure models deliver 25 strokes, and volume models deliver nine strokes per one ounce output at 7,000 psi and 3,000 psi, respectively.
• Standardize grease guns and mark them properly. Most grease guns have a three-way load: bulk filler, bulk suction and cartridge. For all of them, cleanliness is godliness.
• Grease gun lubrication should be performed while the machine is running. That assures better replacement throughout a bearing or gearbox and a lower temperature increase normally caused by relubrication.
• The use of air-operated pumps warrants some special considerations. Because of their high output, there is a risk of seal damage. Since these pumps can develop high pressures, only high-pressure hose and accessories should be used. A follower plate is recommended to prevent cavitation.
• Air-operated pump performance greatly depends on selected pump features, air supply, the type of grease used, operating temperature, the length and size of hose, and the number and size of fittings and swivels in the system.
• Drum cover and pump stem should always be properly sealed to prevent injuries, and drum change is a two-man job, unless a pump hoist is used.
• The amount of grease that will completely replenish the lubricant capacity of each point once every eight hours of operation is the volume that provides 0.002 inches of film. However, dirty, hot or corrosive environments require higher volumes.
• The relubrication interval depends on the application and operating environment, as well as on the need to purge to keep out dirt. Other factors that determine lubrication intervals are machine speed, operating temperature and the type of grease used. Harder grease holds better.
Grease quality, proper selection and performance characteristics are important. Understanding, care and diligence will ensure you get maximum benefits from your thixotropic lubricants.
Information for this article was provided at an educational workshop on grease organized by the Hamilton Section of the Society of Tribologists and Lubrication Engineers (STLE). Steve Gahbauer is an engineer and the former engineering editor of PLANT. E-mail firstname.lastname@example.org.