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Application of Grease and Nonfluid Solid Lubrication

A. Bhatia, B.E.


Course Outline

The mineral and synthetic oil makes up some 95% of the total lubricants market since it is easier to handle, flows directly to the part needing lubrication and conducts heat away from the moving parts. However, the use of oil necessitates the use of an associated reservoir, sometimes together with a pump and piping, which is not always practicable.

Greases are most often used where a lubricant is required to maintain its original position in a mechanism, especially where opportunities for frequent relubrication may be limited or economically unjustifiable. This requirement may be due to the physical configuration of the mechanism, the type of motion, the type of sealing, or to the need for the lubricant to perform all or part of any sealing function in the prevention of lubricant loss or the entrance of contaminants.

This 2-hr course material is based entirely on the US Army Corps of Engineers document EM 1110-2-1424, "Lubricants and Hydraulic Fluids" and covers Chapter 5; "Grease" and Chapter 6, "Nonfluid Lubrication".
The course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the readers will:

Intended Audience

This course is aimed at students, mechanical and process engineers, O & M professionals, H &S professionals, machinery specialists, energy auditors, plant personnel's and general audience.

Course Introduction

The American Society for Testing and Materials (ASTM) D288 defines lubricating grease as: "A solid to semi fluid product of dispersion of a thickening agent in liquid lubricant. Other ingredients imparting special properties may be included." This definition indicates that grease is a liquid lubricant thickened to some extent in order to provide properties not available in the liquid lubricant alone. The liquid portion of grease may be a mineral oil or any fluid that has lubricating properties. The thickener may be any material that, in combination with the selected fluid, will produce the solid or semifluid structure. The other ingredients are additives or modifiers that are used to impart special properties or modify existing ones.

Greases are employed where heavy pressure exist, where oil drip is undesirable, and/or where the motions of the contacting surfaces are discontinuous so that it is difficult to maintain a separate lubricating film. Grease lubricated bearings have greater frictional characteristics at the beginning of operation and are designed to last the lifetime of the bearing without relubrication.

In this course, you will learn the basic types, properties and application of grease and non fluid solid lubrication.

Course Content

In this course, you are required to study US Army Corps of Engineers document EM 1110-2-1424, "Lubricants and Hydraulic Fluids":

EM 1110-2-1424, "Lubricants and Hydraulic Fluids" - Chapter 5 "Grease"

EM 1110-2-1424, "Lubricants and Hydraulic Fluids" - Chapter 6"Nonfluid Lubrication"


You need to open or download above documents to study this course.

Course Summary

Grease lubricants are semisolid type of shear thinning or pseudo-plastic fluid, which means that the viscosity of the fluid is reduced under shear and as the rate of shear increases, the viscosity drops to approximately the same as the base mineral oil.

These have several important advantages: They resist being squeezed out; they are useful under heavy load conditions and in inaccessible parts where the supply of lubricant cannot easily be renewed, and they tend to form a crust that prevents the entry of dirt or grit between contact surfaces. Some greases are labeled "EP", which indicates "extreme pressure". Under high pressure or shock loading, normal grease can be compressed to the extent that the greased parts come into physical contact, causing friction and wear. EP grease contains solid lubricants, usually graphite and/or molybdenum, to provide protection under heavy loadings. The solid lubricants bond to the surface of the metal, and prevent metal-to-metal contact and the resulting friction and wear when the lubricant film gets too thin. Solid lubricants are especially useful at high and low temperatures, in high vacuums, and in other applications where oil is not suitable.

Composition of greases

Grease is a mixture of a lubricant and a thickener; often it is made from a mineral oil and soap.

1) Fluid Components - Most of the greases produced today have mineral oils as their fluid components. These oils may range in viscosity from as light as kerosene up to the heaviest cylinder stocks. In very low or high temperature applications or in applications where temperature may vary over a wide range, synthetic greases or grease with solid additive such as graphite, Molybdenum disulphide or PTFE are used.

2) Thickeners - The principal thickeners used in greases are metallic soaps such as calcium, aluminum, lithium and barium. Calcium- and sodium-based greases are the most commonly used; sodium-based greases have higher melting point than calcium-based greases but are not resistant to the action of water. Lithium-based grease has a drip temperature at 350° to 400°F and it resists moisture, hence it is commonly used as lubricant in household products such as garage door openers. Some grease which are made with mixtures of soaps, such as sodium and calcium, are usually referred to as mixed base greases. Modifications of metallic soap greases, called "complex" greases, are becoming increasingly popular. These complex greases are made by using a combination of a conventional metallic soap forming material with either organic or inorganic complexing agent, and may or may not involve another metallic constituent. Greases of this type are characterized by very high dropping points, usually above 500ºF (260ºC), and may also have excellent load carrying properties.

3) Additives and Modifiers - Additives and modifiers commonly used in lubricating greases are oxidation or rust inhibitors, pour point depressants, extreme pressure antiwear agents, lubricity or friction reducing agents, and dyes or pigments. Most of these materials have much the same function as similar materials added to lubricating oils.

Grease characteristics

The general description of grease is in terms of the materials used in its formulation and physical properties, some of which are visual observations. The type and amount of thickener and the viscosity of the fluid lubricant are formulation properties. Color and texture, or structure, are observed visually. (Light colored or white greases may be desirable in certain applications.)

1) Consistency - Consistency is defined as the degree to which a plastic material resists deformation under the application of a force. In the case of lubricating greases, it is a measure of the relative hardness or softness, and may indicate something of flow and dispensing properties. Consistency, similar to viscosity, varies with temperature and, therefore, must be reported at a specific temperature. Consistency is reported in terms of ASTM D217, "Cone Penetration of Lubricating Grease" or National Lubricating Grease Institute (NLGI) Grade.

2) NLGI Grease Grade Numbers - The NLGI has standardized a numerical scale for classifying the consistency of greases. The NLGI grade and corresponding penetration ranges, in order of increasing hardness. This system has been well accepted by both manufacturers and consumers. It has proved adequate for specifying the preferred consistency of greases for most applications

3) Dropping Point - The dropping point of grease is the temperature at which a drop of material falls from the orifice of a test cup under prescribed test conditions. The dropping point of grease is not considered to have any bearing on service performance other than that, at temperatures above the dropping point, the grease becomes more fluid and may approach the operating limits of satisfactory performance.

4) Oil Separation - A certain amount of bleeding (oil separation) of the oil is necessary in order to perform the lubrication function. Some oil release resulting in free-oil on the surface of the grease in containers in storage is normal. However, excessive separation, while not usually harmful, may result in loss of the user's confidence in the product.

Because of their essentially solid nature, greases do not perform the cooling and cleaning functions to the same extent associated with the use of a fluid lubricant. With these exceptions, greases are expected to accomplish all other functions of fluid lubricants.

Selection of the most appropriate grease is not always straightforward, and the equipment manufacturer or grease supplier should be consulted if in any doubt. Selection must be based upon a number of criteria, including thickener characteristics, base fluid characteristics, and additive content, compatibility with elastomers used in the equipment, compatibility with grease already present and cost.
Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

Take a Quiz

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.