Lubrication Basics
How to Select a Grease or Oil
Lubricants protect your components from wear and corrosion and should be treated as a critical design component. But different lubricant chemistries are better suited for different designs and the wrong lubricant can do your application more harm than good.
Similarly to how you validate other parts of your design, you want to make sure that your lubricant is compatible with your application's operating requirements. Here are some things you should consider when selecting a lubricant for your application.
The first question many engineers struggle with is: grease or oil? In some applications, oil may be the only viable alternative. Sintered or powdered-metal bearings are designed to be impregnated with oil. Small delicate mechanisms with extremely low starting torque, like those found in watches, micrometers, or other precision instruments, may also require oil if they lack the motive force to overcome even the lightest grease. With those few exceptions in mind, engineers should never quickly dismiss grease because it does offer cost and performance advantages over oil.
Grease generally stays where it’s put, so engineers can eliminate the cost of oil seals and seal design, which are essential to prevent leakage in an oil-lubricated component. Greases also prevent wear better than oils and are more forgiving, allowing engineers to be somewhat less exacting about perfectly mated parts. Importantly, greases can be formulated light enough to accommodate components with low start-up torque.
What is the operating temperature range of your application? A lubricant must be chemically stable at your high-temperature limit and have sufficient film strength to adequately prevent wear. At the lowest expected temperature, your lubricant must remain sufficiently fluid. Certain lubricant chemistries, like perfluoropolyethers, are better suited for applications exposed to extreme temperatures.
Some lubricants can “attack” certain plastics and elastomers. The base oil can infiltrate the solid material or cause the solid’s components to leach into the lubricant. The material compatibility of specific plastics and elastomers should always be tested by evaluating physical properties of your component such as tensile strength, dimensional stability, and gravimetric stability after immersion in the lubricant. Higher temperatures and lower base oil viscosities usually exacerbate chemical incompatibility.
Certain metals that come in contact with a lubricant may exhibit accelerated corrosion or lead to undesirable polymerization or “varnishing” and failure of the lubricant base oil. These problems can be avoided by identifying early in the design process the metal alloys used in the device and analyzing and testing their compatibility with candidate lubricants and additives.
The end-use environment for the device should be considered. Is it corrosive? Is dust an issue? Will the lubricant be exposed to water, steam, solvents, or solvent fumes? Will the part see temperature cycling? Is micro-vibration a possibility? Will your component be exposed to in-vacuum conditions that require virtually no outgassing or particle generation? Just as other parts of the component are designed to withstand operating conditions, the lubricant must also be designed with these constraints in mind.
Some applications require what we like to call “Lube for Life” lubrication. In applications where servicing is impossible or difficult, such as space or closed gear-box applications, a grease or oil must be able to lubricate the component for its entire life expectancy. Life testing data should be requested for such applications.
For most applications, the prevention of wear caused by friction is the primary reason for the use of a lubricant. In general, higher viscosity base oils support heavier loads. If the load in the contact zone is too great or the speed is too slow, asperities on the rubbing surfaces can collide, causing excessive wear. In this situation, which is referred to as boundary lubrication, extreme pressure (EP) additives may be necessary. Synthetic ester greases are particularly suited for preventing heavily loaded metal-on-metal wear. Under relatively light loading, the high surface energy of a silicone grease may help to re-wet the surface.
Lubricants do much more than just prevent wear. Lubricants can also be used to achieve specific design goals that would normally be fulfilled through mechanical parts. Grease can:
- Control precision motion
- Reduce noise, vibration, and harshness
- Eliminate Buzz, Squeaks, and Rattles
- Reduce torque
- Reduce temperatures
Perhaps most importantly, by spec-ing in a grease during the design phase, you protect your components before damage can be done to reduce risk and warranty claims. Our monthly newsletter has dozens of in-depth articles about how lubricants can enhance design performance to address common industry issues.
How the lubricant is applied to the device during manufacture is often critical to its success. The correct amount must be applied in the right location. In some applications, too much lubricant can be more detrimental than too little. Cleanliness of the lubricant is also an issue. Containers should be kept closed and exposure to contaminants minimized. Often the use of automated dispensing systems can preserve lubricant cleanliness and ensure that repeatable quantities are applied to the part. Some precision components, like those found in HDD bearings, may require that a lubricant be cleaned through microscopic ultra-filtration.
Different types of devices can have a wide variety of lubrication requirements. An electric switch that carries low current within a non-inductive circuit will require a different lubricant that one that has the potential to arc during the make–break of an inductive circuit load. A bearing that supports a rotating shaft will have a lubricant system different than a set of plastic gears. The ultimate application of the component and the likely mode of failure determined must be considered. Many specialized lubricants have been developed for specific parts, including:
Choosing a lubricant designed for the device at hand is the key. Some applications may also face industry constraints such as defense specifications, food-grade requirements, or biocompatibility concerns. Click here to explore our lubricants by industry.
