Myths about lubricants
Original article date: November 1997
Don’t believe everything you hear about selecting the proper lube for the job, says David M Richard, Maintenance and Repair Operation Industry Specialist from Dow Corning’s American base in Midland, Michigan.
Here is a sampling of some of the myths, misconceptions and misinformation surrounding lubricants, as well as the underlying realities.
Because lubricants can be exposed to harsh operating conditions in many applications, in most cases, the benefits of a superior lubricant outweigh the costs.
(1) Myth: Inexpensive greases are cost-effective
Fact: Lubricants that cost less are often made with lower performing raw materials and may be manufactured with lower quality control measures. Lower quality can lead to problems, malfunctions and – ultimately – costly downtime.
(2) Myth: Adding more lubricant can’t hurt
Fact: More bearings are ruined by over-greasing than by under-greasing. Switch lubricants instead. As grease is added, pressure builds within the bearing housing, especially in full or non-vented housings. Excessive pressure accelerates the process of oil “bleed” or separation from the thickener. If the separated oil drains away and leaves a bearing packed only with thickener, friction will cause the bearing to fail.
(3) Myth: Silicone lubricants are highly priced
Fact: Silicones are more stable, withstand a broader temperature range and offer better chemical resistance than their conventional counterparts, so often outlast them.
To guarantee a lubricant will correctly meet the needs of an application, it is important to match the product to the job. There are four major operational variables in an application:
- load
- environment
- temperature
- sp eed.
Determining whether a lubricant can withstand the load-carrying capacity of an application is critical to selecting the proper lubricant. Dispelling some of the myths about certain lubricants and their load-carrying characteristics may help make specifying the right product an easier task.
(4) Myth: Extreme pressure lubricants must be formulated with high viscosity fluids
Fact: Solids and surface-active additives can replace high viscosity fluids. Maintenance lubricants that contain solids or surface active additives are available for use in extreme pressure, moderate-to-high load and speed applications. Some high performance synthetic greases, for example, withstand high bearing loads. They lengthen lubrication intervals in high load, high speed applications. High viscosity fluid greases do not provide these same capabilities.
(5) Myth: PTFE solves all extreme pressure lubrication problems
Fact: While PTFE is a good wear inhibitor, it is not the primary choice for load-carrying capability. Try molybdenum disulfide or graphite instead.
(6) Myth: All silicone-based lubricants are poor extreme pressure lubricants
Fact: True of most, but fluorosilicone-based bearing greases may be used, for example, to protect heavily-loaded roller bearings from high pressure, metal-to-metal contact inside a die roll.
- Myth: A harsh operating environment leads to frequent lubricant and bearing replacement.
Fact: Proper lubrication under harsh operating conditions lengthens maintenance intervals and decreases equipment replacement.
- Myth: All effective lubricants must be wet like an oil, or semi-solid like a grease.
Fact: Dry, film bonded lubricants can be used as effective long term, extreme duty lubricants. Those containing molybdenum disulfide or graphite are very high performance lubricants.
- Molybdenum disulfide causes corrosion on metal
Fact: Well formulated, molybdenum disulfide products address corrosion problems effectively.
(10) Myth: The highest temperature rated grease is the best choice for all applications, because it covers the widest temperature range.
Fact: Lower-rated greases are more stable at lower temperatures.
- Myth: Mineral oil-based lubricants perform effectively and continuously above 140°C.
Fact: Mineral oil-based lubricants have increasing volatility over 125°C.
- Myth: The higher the dropping point, the higher the operating temperature of the grease.
Fact: The dropping point of a lubricant is not a true measure of performance at high temperature. Based on the separation of the fluid from the thickener at high temperatures, the figure has a very limited relevance to a grease’s melt point or functional range. For example, the realistic operating temperatures of petroleum-based greases are up to 55C° below their dropping points.
- Myth: Only fluid lubricants can be used on high speed bearings
Fact: Speeds approaching 1 000 000Dn have been achieved with some newer, high performing greases, which do perform well, even when high speed is combined with high load.
- Myth: The more molybdenum disulfide in a grease, the better
Fact: Excessive molybdenum disulfide in a grease can reduce its ability to resist separation.
Dow Corning
01734 507251
November 1997