Lubricant Friction and Wear Testing Michael Anderson^ and Frederick E. Schmidt^
THIS CHAPTER PROVIDES VARIOUS METHODS TO EVALUATE THE FRIC-TION AND WEAR PROPERTIES of lubricants and materials. If fric-tion and wear can be controlled then the engineer can select materials and lubricants with a high degree of confidence. Many laboratory tests are used to evaluate the interaction of materials under a broad range of test conditions and con-trolled environments.
In this chapter, the following topics will be discussed: History of tribology testing Basic types of tribology test systems and reasons for their
Fundamentals in designing tribology tests How to select a test device to simulate a field condition Contact geometry used in bench tests Standard and commonly used test devices Designing special application bench tests Common terminology relating to friction and wear testing
HISTORY OF TRIBOLOGY  From the beginning of time, man has tried to overcome fric-tion and wear. The earliest application of friction is its use for building fires. To early man, fire offered many advantages in-cluding safety, light, warmth, and cooked food. Man also needed weapons to kill animals for food. Primitive tech-niques were used to sharpen sticks and stones. As simple as these would appear, this use of friction greatly enhanced man's quality of life during this primitive period.
Later, as man began to cultivate the land to provide food to supplement his diet of animal meat and fish, agricultural tools became a necessity. Not only must they be durable, but they also had to be shaped. Simple manufacturing tech-niques were employed such as grinding. More durable mate-rials were more difficult to make. As time went on, man used new techniques to help in this manufacturing stage. Simple engineering methods were employed such as pottery wheels.
At the time the great pyramids and monuments in ancient Egypt were being built, man was beginning to use not only engineering techniques, such as rolling elements (logs) to re-duce friction, but he was also introducing liquid media be-tween the surfaces. Sometimes, these liquids were simply hy-drated earth (clays, soaps, or other materials). Nevertheless, lubrication was becoming a part of life.
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Even though man was employing simple engineering prin-ciples and lubrication for manufacturing, it wasn't until the late 15th century, when Leonardo DiVinci first deduced laws governing the motion of a block over a flat surface, that the science of friction and lubrication was developed. During this time, primitive testing devices were developed to measure the force of one object moving against another. Scientist during this time also realized that measured forces were less when a material such as pig fat was introduced between sliding or moving surfaces; hence, the study of lubrication had begun.
During the years that followed, friction, wear, and lubrica-tion studies increased. As the industrial revolution brought more advanced machines for transportation and power gen-eration, engineering became part of the curriculum at uni-versities. These studies included the fundamentals of friction, lubrication, and wear. With new extraction techniques for obtaining crude oil and the ability to refine this oil, lubricants became more commonplace. As lubricants became more widely used, technology was needed to eveduate the differ-ences in properties and in various applications. In 1927, the first commercial tribomoter was introduced to blenders and manufacturers of finished lubricants. This tester "Pin and Vee Block test machine" provided suppliers with a method of measuring anti-wear and extreme pressure properties of the lubricants they were selling. Subsequently, tribometers such as the Timken^ tester. Four Ball Wear and Four Ball EP, Block-on-Ring, and others were introduced to evaluate lubri-cants and materials under a variety of test conditions. These machines are described in this chapter.
Further developments in transportat ion, medicine, and space exploration have provided impetus for the develop-ment of new lubricants and materials. With these technolo-gies has come the development of test machine designs and test methods to meet the challenges of these new applica-tions. Today, over 225 commercial and independent testing devices  have been developed.
BASIC TYPES OF TRIBOLOGY TEST SYSTEMS
Laboratory testing of lubricants used in a tribology system involves different levels of sophistication. A tribological sys-tem consists of all relevant test parameters, materials in con-tact including the lubricant, if present, and any external, en-vironmental conditions . Each level of test sophistication
^ Timken Corporation, Canton, OH.
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1018 MANUAL 37: FUELS AND LUBRICANTS HANDBOOK
has benefits and drawbacks. The three levels of test sophisti-cation Eire as follows : Laboratory bench test devices (simple geometric contacts) Component bench test devices (use of actual parts and as-
semblies in a laboratory test rig) Field tests (materials and lubricants tested in actual
systems) The most representative test program is one that uses the
proposed material combination in the actual field situation [4,5]. Generally, this approach is not practical for several rea-sons. Costs can be prohibitive; the time to develop a mean-ingful program may be too long; and the environmental or ambient conditions are difficult if not impossible to control. These limitations leave many uncontrollable variables and possibly, a wide scatter of test data. Even component testing, which is a laboratory test rig that uses the particular section of the machine (or field application) that is of interest and in which the parts are made of the materials under evaluation, is more cost effective but rarely used as a first approach [3,6]. These instrumented laboratory test devices possess the same limitations as field tests, except that the ambient conditions are more controllable. However, laboratory bench tests are designed to move test pieces with simplified geometry under a variety of test loads, speeds, and environmental conditions. Although these simplified devices cannot produce exact op-erating conditions, they have the potential to produce results that provide meaningful data for a range of similar applica-tions . The wide use of these test devices, such as the Four Ball and Timken machines, for determination of extreme pressure properties reflects the low cost and ease of such measurements and the belief that test results correlate to some extent with performance . The use of simple bench testing reduces the test evaluation to a single, specific tribo-logical condition simulating, as close as possible, the operat-ing conditions for the material and lubricants. Data gener-ated from these tests are compared and those materials and lubricants are selected that jaeld the best wear life or perfor-mance for further testing under more specific test designs. Repeatability of the obtained test results can be better when the test is kept as simple as possible. Figure 1 gives a relative economic comparison versus repeatability for bench tests, component tests, and field tests.
The ability to use a bench test offers many benefits including: Simplicity of operation Lowest testing cost Accelerated test results Real time presentation of data to facilitate recognition of
BENCH TESTS Test tvDe
* * *
FIG. 1Economic comparison of test types.
Accurate and precise indication of wear rates and perfor-mance properties given the test parameters under which the test is conducted
Inexpensive and uniform consumable test pieces Test pieces from a wide range of materials and conditions Small volumes of test fluid Controlled test environments and ambient conditions Convenience of operation.
Commercial test devices offer significant benefits over test equipment made in-house. Because commercial test machines are made in quantity to the same manufacturing specifications, they can offer better test result comparisons between the laboratories using them. Commercial testers are often used when developing standardized test methods because of the availability of users willing to cooperate in the development of precision statements. In most cases, the test parameters are listed in standardized test methods. However, they may not provide the user all the necessciry in-formation for evaluating his materials. Therefore, standard-ized test methods can be suitable starting points, but the user may need to modify the test parameters to achieve meaningful test results [3,8]. Usually more data can be ob-tained throughout the test rather than just the final speci-fied endpoint or reported test result. Many data occur dur-ing the course of a test, including but not limited to, changes in lubricating mechanisms, changes in surface ar-eas giving different contact pressures, development of lubri-cating films and surfaces, and so on. Therefore, the opera-tor must identify these changes and develop test methods that facilitate obtaining as much pertinent information as is possible or required.
Commercial test devices provide the following benefits: E