วันพุธที่ 27 ตุลาคม พ.ศ. 2553

A Basic Analysis of Grease Thickeners

A Basic Analysis of Grease Thickeners

John Schlobohm
Castrol Specialized Industrial Business Unit

1001 West 31st Street
Downers Grove, Illinois 60515
USA
(630) 743-3571
john.Schlobohmsr@castrol.com

Key words: Thickener, Grease, Soap, Base Oil

ABSTRACT
Greases are complex compounds constituted of base oils, thickeners and additives designed to effectively meet specific lubrication requirements. There are several ways greases can be classified: synthetic or mineral based, biodegradable or non-biodegradable, high temperature or low temperature, or according to the product’s thickener system. The most widely used classification is the one based on the thickening system. This paper will discuss the various thickeners used in making greases today, along with their advantages and disadvantages.

Grease development

In simplest terms, lubricants are divided into oils and greases. In general, oil tends to flow and grease tends to stay put. However, advances in the formulation of lubricants have made those simple definitions incomplete. The first greases were simple animal fats and lards smeared on wooden axels and shafts to reduce friction. Greases were also used to maneuver stone blocks used in construction. The early settlers used hand soap and animal fat to lubricate machines that developed friction. As mechanical systems became more complex, base oils were refined and thickener systems developed to meet changing needs. Today, greases are expected to work without failure under adverse conditions, including extreme heat, massive water wash, tremendous shock loads and frigid cold.

The carefully engineered properties of premium and high-performance greases enable them to change viscosity to meet changing operating conditions – blurring the once clear distinction between greases and oils. Modern greases have three elements: base oils, additive packages and thickeners.

Base oils are of three general types: mineral, animal/vegetable and synthetic. Most industrial oils are mineral based. However, synthetic oils have moved rapidly into the marketplace. Although called synthetic, these fluids are often manufactured from petroleum feedstock.

Prior to around the 1950s, base oils were the sole lubrication property within greases. Today, however, advanced thickener and additive packages are available that assist in the lubricating operation. Additive packages aid in failure prevention. They are used to reduce wear, inhibit corrosion and rust, and reduce the impact of shock loading on bearing elements. Other additives contain anti-oxidants, anti-wear (AW), extreme pressure (EP) and solid packages.

Greases can be classified by a variety of terms: synthetic or mineral based, biodegradable or non-biodegradable, high temperature or low temperature, or according to the product’s thickener system. Thickener classification is the most frequently used and most helpful method of determining which grease is appropriate for a given situation. Therefore, this paper will discuss greases based on their thickening systems. Advantages and disadvantages of various types of thickeners will be discussed as well as some applications for the different grease types.

Thickening systems

The thickener provides the gel-like consistency to hold the liquid lubricant (base oils and additives) in place. The gel-like characteristic make greases preferred over oil lubrications in applications where leakage can occur, or where the sealing action of the extra film thickness is needed.

Soap thickened greases



Many thickeners are the result of a chemical reaction between a long-chain fatty material (animal or vegetable origin) and an alkali in the base oil. Thickeners fall into several categories as shown in the following table.



NLGI Grade

In general, lubricating greases are composed of 85 to 95% base fluid and 5 to 15% thickener and other additives. The National Lubricating Grease Institute (NLGI) Grade is a standard used to classify the consistency (thickness) of grease. The NLGI grade is determined using the American Society for Testing Materials ASTM D-217 cone penetration test. In the test, a standard cone-shaped weight is placed on the surface of a cup of grease. The weight is allowed to penetrate the sample for five seconds. The NLGI grade assigned to the grease is based on a micrometer measurement of the depth of penetration. NLGI grades range from 000 (softest or thinnest and pourable at room temperature) to 6 (hardest or thickest and resembling a solid block of grease at room temperature).

Final statement


Other advanced thickeners are currently undergoing testing. If history is an indicator, tomorrow’s greases will some day relegate today’s products to the same antique status as unrefined animal fats and hand soap. Possibly the first “perfect grease” will emerge -- one that will eliminate friction and never break down under high temperatures or become unpumpable in the cold. But until that product is developed, an expanded knowledge base regarding today’s products will make selecting the proper grease for specific applications easier.

No matter what the incentive is when purchasing greases -- cost, performance or special needs -- the oil industry has a product that will work in most any application. As noted at the outset, thickeners are only one-third of the overall lubrication puzzle. Additive packages and base oils make up the other two-thirds. Greases are engineered so that these three parts work in harmony to produce the best possible results at a particular cost level. Four elements demand consideration when making a grease choice: operating environment, equipment cost, lead-time on replacement parts and delay cost if the equipment is down. If any one of these items is a factor, making an informed choice of the best product available will deliver overall cost savings.


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