Greases are used to provide solutions to problems during machinery operation such as wear and corrosion, which, if not addressed, can result in equipment failure. A variety of grease technologies are available, designed to extend the operating life of different components and to decrease downtime, parts replacement and labor costs.
However, operating conditions for various types of equipment in many mining applications can create a host of lubrication problems, including extreme heat, excessive pressure, water washout and contamination. All of these problems can degrade lubricating grease and leave equipment components unprotected.
Most problems can be characterized by one of three main groups:
Excessive load and vibration. The vibration and force applied to a bearing or journal exceeds the load-carrying capability of the grease. The result is metal-to-metal contact, premature wear and increased heat. This is often seen in bucket pins and bushings on equipment such as excavators, as well as bearings in rock crushers.
Excessive or localized heat. Excessive heat can be generated during extended operation, or localized heat can result when metal asperities collide and friction produces temperatures in excess of 2,000 degrees Fahrenheit. Such temperatures break down the grease, leading to cold welding or galling, a form of wear caused by adhesion between sliding surfaces.
Contamination. Sand, mud, water and steam can wash away grease, leaving metal unprotected. Dust, fibers and abrasive particles can combine with grease and grind away metal. Acids, caustics and cleaning solutions can break down grease and corrode metal.
Often, operators use many different greases on the various types of machinery that they employ, which can lead to costly misapplication mistakes. There has been a concerted effort in the mining industry to minimize the number of greases used in a facility, a technique known as lubricant consolidation. As a result, many maintenance teams are in search of just a few greases to take the place of numerous types.
This means that the chosen greases must be carefully screened to meet multiple demands in multiple applications, while still avoiding the problems listed above. Understanding which grease is best for a given application can be a difficult task, but various controlled test methods can be used to make the selection process less painful.
In addition, several tests have been developed that can determine the health of an in-service grease and provide warning signs of impending bearing failure. These in-service tests can help identify misapplication, wear conditions, contamination and potential grease failure. For example, ASTM D7918 uses the die extrusion method to measure flow properties and evaluate wear debris, contaminants and oxidation of an in-service grease.
Mimicking Functional Problems
Many ASTM bench test methods are effective for gaining a fundamental understanding of the performance of grease. These tests are run in a controlled environment in order to reduce variables and provide reproducible results.
Certain tests stand out as indicators of grease performance under specific circumstances. The following list of tests was chosen by comparing the test results with actual application performance. Greases that demonstrate the highest performance in these tests have also been proven to provide the best corrosion protection, wear reduction and overall lubrication. While these tests should guide decision making, they should not be the single deciding factor for selection.
The Four-ball weld and wear test (ASTM D2596) produces measurements of a greases extreme pressure and wear protection ability in a dynamic environment.
Three steel balls are placed at the bottom of a test vessel, which is filled with the grease of interest. A fourth ball is held by a chuck and forced onto the three balls from above. Load is applied to the fourth ball as it is rotated at high speed. The load at which the balls begin to seize is considered the weld point. The wear dynamic is the size of the scar that develops prior to seizing.
The greater the weld numbers, the more stress the grease can withstand before the test specimens cold weld. The lower the wear scar value, the better protection the grease will provide while under dynamic stress. This is perhaps the best test method available for determining a lubricants load carrying and wear protection abilities. Superior grease will have weld numbers above 600 kilograms and scar values below 0.40 millimeters.
The water washout test method (ASTM D1264) measures a greases resistance to washing out of a bearing. In many applications, bearings are exposed to water or water based solutions from various processes, from washing down the equipment or from the environment.
First, grease is packed into the bearing and weighed. A cover plate is placed over the bearing housing, which is then subjected to a water spray. After exposure, the bearing housing with the grease is dried and weighed again. The difference in weight before and after the test is recorded. The lower the difference, the less grease has washed out, indicating greater resistance to water washout.
The dropping point test (ASTM D2265) determines the temperature at which the base oil begins to bleed out of the grease. When the temperature of a bearing exceeds the dropping point, the grease has the chance to run out of the bearing. A common rule is to select a grease that has a dropping point that is approximately 20 percent higher than the highest operating temperature a bearing will experience.
A sample of grease is placed in a small cup with a hole in the bottom and a thermometer, and the cup is placed in an oven. The temperature at which a drop emerges from the cup and falls to the bottom of an attached tube is the dropping point.
Its important to note that clay based (bentonite) grease, also known as non-melt grease, does not exhibit a dropping point. However, at temperatures above 550 to 600 F, the base oil may evaporate, leaving behind an abrasive clay.
The worked penetration test measures the change in a greases consistency after it has been physically stressed. When a bearings rolling element or journal turns, the grease experiences physical stress. The churning action can break down the structure of the thickener. When this occurs, the greases NLGI number, which measures its consistency, is reduced. This can lead to a thinner grease producing a compromised lubricant film, which can lead to increased bearing wear.
Worked penetration is measured using the cone penetration test (ASTM D217) after 60 to 100,000 strokes. The test vessel is filled with grease, and a stress plate is screwed onto the vessel. The vessel is then placed on a stressing unit that works the grease for a predetermined number of strokes. The consistency of the grease is then measured using a grease penetrometer, which drops a cone into the grease and measures the depth to which it penetrates the sample.
Selecting a grease for specific applications, or selecting one grease to replace many, is possible if a disciplined approach is taken. Consider the environment in which equipment is operating. Is the environment very hot (above 150 F) or very cold (below 32 F), and is the air humid or dry? Are there drastic temperature fluctuations? Is the equipment exposed to direct or indirect steam, or to sand, grit or loose dirt? What about acids, solvents or caustic materials? Will it be submerged in fresh or salt water?
Consider also the operating conditions, including heavy loads, high speeds, continuous shock and dissimilar metals in the bearings, journals or bushings. Then use the test methods listed above to begin evaluating the performance of different greases that may be suitable to meet the challenges a grease may face in your applications.
Michael D. Holloway has over 30 years’ experience in industry and is president of 5th Order Industry, which focuses on competency development and preparation classes for lubrication certification. He can be reached at Holloway@5thOrderIndustry.com.
Douglas Reid is the vice president of product development at CSW Industrials with responsibility for marketing, product development and lubricant research and development groups for the Whitmore and JetLube brands.