What’s in a Lubricant: Characteristics of Grease
EP Editorial Staff | February 21, 2014
There’s more to selecting the right grease than you might have thought.
By Ken Bannister, Contributing Editor
Whenever I deliver a Lubrication Fundamentals workshop, I ask attendees to close their eyes and “picture” a lubricant. Most have reported the simultaneous appearance of two distinct images during this exercise: one is a container of liquid oil, the other a grease gun. Many individuals are shocked to learn that grease is made of 80% to 95% base oil.
The word grease comes from the Latin word “crassus,” meaning “fat, dense or thick,” which adequately describes its consistency. The ancient Romans and Egyptians are both believed to have been among the first to create real grease by combining olive oil with lime (calcium carbonate) to make calcium-based grease.
Making grease is similar to making soap: Both products rely on a chemical reaction to take place between oil, fat or fatty acids (often present in the oil or added) and an alkali base material (referred to as the “thickener”) to form soap-like material. This reaction is known as “saponification.” Grease uses a variety of metal hydroxide alkaline to make and define the grease type. For example, aluminum hydroxide makes aluminum grease, lithium hydroxide makes lithium grease, and calcium hydroxide makes calcium grease.
To give grease a wider temperature application range and enhanced properties, a second thickener known as a “complexing” agent is added to the mix. This agent is a salt, usually of the same metal hydroxide used to originally thicken the grease. If lithium is used as the alkaline agent, lithium salts are added to the mix to create lithium complex grease.
When grease is applied, it is always the base oil that lubricates the bearing surfaces. The chemical soap fraction, which acts like a sponge to hold the oil, is by default designed to “wick” or release the oil into the bearing area when the bearing temperature rises. When the bearing ceases operation and/or the temperature cools, the grease will reverse its action and “wick” back the oil into suspension, thus acting as a semi-solid living reservoir for the oil.
Considering grease as a lubricant of choice
Whenever rotating or interacting moving surfaces are encountered in a machine design, the designer must decide early on what lubricant is to be used. Grease’s unique capabilities give it advantages and disadvantages over just oil, as detailed in Table I, “Comparison of Grease to Oil.”
Because oil is always the lubricating medium, all decision factors involved in choosing the right viscosity and additives for the application will still apply. Before using a specific grease, check with the grease vendor or manufacturer to ensure that the product’s oil viscosity is suitable for your application.
Greases are classified and characterized in many ways. These characteristics are identifiers that allow us to evaluate the ability of a grease compared with others. They can be found on the lubricant specification sheet provide by the manufacturer. The most common characteristics are outlined here:
NLGI Consistency Rating…
Greases are manufactured in a variety of consistencies. These consistencies are distinguished into nine specific classifications using the National Lubricating Grease Institute (NLGI) number-rating system, from #000 (representing a grease with a very fluid appearance at room temperature), all the way up to a #6 (which appears block solid at room temperature). The most popular grease for use in grease guns is an NLGI #2 (which looks soft at room temperature). The recommended grease consistency for use in an automated centralized grease lubrication system is NLGI #1 or lower.
NLGI consistency is determined in the laboratory using an ASTM (American Society of Testing Materials) D-217 cone penetration test. In this procedure, grease is placed in a cup and a cone is dropped from a specified height at a room temperature of 77 F and allowed to penetrate the grease for five seconds. The depth of penetration is then measured carefully in tenths of a millimeter and rated according to an NLGI classification chart that assigns a rating number to penetration depth ranges. For example, if the cone penetration is between 265 (26.5mm) and 295 (29.5mm), the grease is classified as a NLGI #2.
(Note: If grease is classified with an EP letter rating prior to the NLGI number, e.g. EP2 grease, it signifies that the product is formulated with an Extreme Pressure additive such as Molybdenum Disulfide for use in low-speed, high-load applications.)
As seen in the NLGI rating system, grease is classified by its appearance at room temperature—which can range from very fluid to soft or smooth to block-like. Descriptions may vary slightly from manufacturer to manufacturer, such as “soft” versus “smooth.”
Manufacturers often color grease with dyes for identification purposes. Grease can be white, black, brown, green, blue or red. (White is typically reserved for food grade products.)
The thickener tells us what alkali base was used to manufacture the grease. Most popular thickeners are identified in Table II, “Grease Thickener Types.”
This denotes a grease’s ability to flow under pressure through a distribution system over a given temperature range.
Sometimes called “feedability,” the “slumpablity” designation refers to a grease’s ability to relax in its reservoir under gravity and be fed into the pump.
The temperature at which grease is liquid or soft enough to drip is referred to as its “dropping point.” (Note: This is not the maximum operating temperature.)
This term refers to the recommended optimum operating temperature range of a grease.
Water Resistance or Washout…
This characteristic reflects the ability of a grease to withstand the effects of water spray before its ability to lubricate and prevent rust formation is affected.
As grease is “worked” or sheared in operating conditions, its consistency may change. Those with better shear stability (i.e., retain their consistency) are preferred. Greases that harden are known as “rheopectic;” those that soften are “thixotropic.” LM&T
Ken Bannister is a certified Maintenance and Lubrication Management Consultant for ENGTECH Industries, Inc. He is the author of the Machinery’s Handbook Lubrication chapters, and the Lubrication for Industry textbook recognized as part of the ICML and ISO’s Domain of Knowledge. Bannister also conducts formal preparatory training for ICML MLT/MLA certifications and ISO LCAT certifications. For more training information, he can reached at (519) 469-9173; or by email at firstname.lastname@example.org.