Home / Machinery / Fuels / Lubricants / Characteristics and grades of grease

Characteristics and grades of grease

Agriculture.com Staff 07/07/2010 @ 9:10am

Greases must be properly selected for specific applications. They must be compatible with common elastomers and seals, and tolerate some moisture contamination without degradation of their properties. Proper recommendations require a thorough knowledge of the various types of greases and an understanding of the machine to be lubricated.

This involves the selection of the correct grease for each application and instructions on methods of application, the amount of lubricant to be added, the frequency of inspections, and the relubrication intervals. Proper application is as important as choosing the correct grease. Over-lubrication with grease will cause trouble more quickly than with oil lubrication, and the results can be more disastrous. The type and amount of thickener are important, along with the base oil used and the presence of other materials or additives.

Greases are manufactured in open kettles or contactors in a grease plant. Manufacturers who make large volumes of one type of grease use a continuous process unit for making grease, because of its energy and labor savings. The grease thickener or soap is the component of the grease that immobilizes the lubricating oil, much like a sponge, and releases oil, as it is needed at a controlled rate. Greases are classified according to the type of thickener used and are described as follows:

Lithium Greases constitute the majority of greases sold in the marketplace. These are multipurpose extreme-pressure (EP) greases, which have moderately high dropping points, good water resistance, good load-carrying ability and good shear stability. They are formed by the reaction of single fatty acids and lithium hydroxide base.

Aluminum Complex Greases have short fibers, high dropping points, good water resistance, and good shear stability. Because of the short fibers of the thickener, this type of grease is suitable for centralized lubrication systems and for food-grade applications.

Calcium Complex Greases have high dropping points and inherent EP properties. However, they tend to have high soap contents and may harden excessively at bearing speeds above 1800 RPMs, particularly at elevated temperatures. Calcium sulfonate and calcium stearate greases have high dropping points and inherent corrosion inhibition as well as EP properties. They differ from the conventional calcium complex greases by the use of sulfonic or stearic acid instead of the usual fatty acid in the manufacture of the soap.

Lithium Complex Greases constitute the majority of high temperature greases sold in the marketplace. These are multipurpose EP greases, which have high dropping points, good water resistance, and good shear stability. They are formed by the reaction of di-carboxylic acids as complexing agents with lithium hydroxide base. They are multipurpose greases suitable for use over a wide temperature range and are very shear stable.

Polyurea Greases are multipurpose high temperature greases, which have high dropping points, good water resistance, and good shear stability. They are often used in sealed for-life applications. They may or may not contain EP additives. They are formed by the reaction of amines with isocyanate dissolved in oil creating a gel-like structure. They have unusual shearing characteristics - hardening under high shear and softening under low shear Ð which require careful consideration of the application.

Synthetic Greases. Most lubricating greases are made with mineral base oils. However, for some applications, such as at temperature extremes, synthetic base oils provide better performance. For example, synthetic greases withstand high temperatures such as experienced in auto racing or pulling heavy loads, where excessive heat is transferred to the bearings from the brake pads. During the cold winter months, synthetic greases provide superior low-temperature fluidity.

The relative hardness of the grease is measured by the Cone Penetration Test and classified according to the NLGI grades. It is dependent on the amount of thickener used in the grease. Consistency of the grease influences the ease of application and internal bearing friction as well as the degree of retention in the bearing.

/ag/files/sidebars/greasegradestable1.html

There are a wide variety of other characteristics that describe the performance of grease. These charactertistics include:

Oil Bleeding: A small amount of free oil is necessary to permit the lubricant in the bearing to creep by capillary action into narrow clearances. During prolonged storage, it is not unusual to have a moderate amount of oil separation. This is normal, especially when the grease is packaged hot and allowed to cool down in storage. This free oil may be remixed manually prior to use. Excessive bleeding is indicative of instability in the product.

Dropping Point: This is the temperature at which oil drops from a small amount of grease placed in a test cup, which is heated under the prescribed test conditions. At this temperature, the grease becomes sufficiently fluid to drip. This is important in selecting the correct grease for a given application.

Base Oil Viscosity: Base oil is the major component of grease. Load support at moderate to high shear rates is mainly attributed to the viscosity of the base oil, particularly in the absence of EP additives. Low temperature pumpability and ease of handling are also influenced by oil viscosity, but the amount of thickener plays a major role in grease pumpability.

Mechanical and Shear Stability: The ability to retain its consistency under prolonged working or shearing conditions is necessary for grease to prevent it from leaking out of bearings.

Additives in greases impart or enhance new properties to the grease such as oxidation or corrosion inhibition, antiwear, or EP properties. Solid additives like polyethylene, Teflon, molybdenum disulfide and graphite are helpful in boundary lubrication conditions such as with extremely heavy loads, shock loads, or intermittent starting and stopping.

These solid additives fill the valleys and form a solid film between the surfaces preventing metal-to-metal contact. Basic fillers like zinc oxide or magnesium oxide are desirable where exposure to salt water or corrosive acidic solutions is anticipated.

/ag/files/sidebars/greasegradestable2.html

Greases must be properly selected for specific applications. They must be compatible with common elastomers and seals, and tolerate some moisture contamination without degradation of their properties. Proper recommendations require a thorough knowledge of the various types of greases and an understanding of the machine to be lubricated.

CancelPost Comment
MORE FROM AGRICULTURE.COM STAFF more +

Farm and ranch risk management resources By: 07/07/2010 @ 9:10am Government resources USDA Risk Management Agency Download free insurance program and…

Major types of crop insurance policies By: 07/07/2010 @ 9:10am Crop insurance for major field crops comes in two types: yield-based coverage that pays an…

Marketing 101 - Are options the right tool… By: 07/07/2010 @ 9:10am "If you are looking for a low risk way to protect yourself against prices moving either higher or…

MEDIA CENTERmore +
This container should display a .swf file. If not, you may need to upgrade your Flash player.
Are We In a Climate Change?