Enclosed gear drives are used in a diverse myriad of industries ranging from small to large manufacturing plants, steel mills, mines and quarries. In these industries, the number of enclosed gear drives can number from only a handful to thousands. Though some of these enclosed gear drives may not be used in critical operations, it is critical that they be properly lubricated.
Gear lubricants must work and perform in diverse conditions. These lubricants must often perform in the presence of large quantities of water, high operating and ambient temperatures or in highly contaminated environments, while still maintaining their ability to protect the enclosed gear drives from wear, especially during high load conditions. In addition to these factors, there are two major factors that affect how a gear lubricant must perform:
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The increased emphasis by the users of enclosed gear drives for longer lubricant life to reduce maintenance and disposal costs.
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Design changes by original equipment manufacturers of enclosed gear drives to improve gearbox efficiency. As a result of these design changes, enclosed gear drives have been downsized and built to operate at higher speeds and loads resulting in higher operating temperatures and increased gear and bearing distress. These smaller enclosed gear drives also have smaller oil capacities, so less gear lubricant is available to cool the equipment and suspend contaminants.
Types of Lubricants for Enclosed Gear Drives
The American Gear Manufacturers Association (AGMA) publishes a standard entitled “Industrial Gear Lubrication” (AGMA 9005-D94), which provides lubricant classifications and generalized application and servicing guidelines for industrial gearing that has been designed in accordance with applicable AGMA guidelines.1
The four types of gear lubricants described in this standard include: rust and oxidation-inhibited oils, compounded gear oils, extreme pressure (EP) gear oils and synthetic gear oils.
Rust and Oxidation-Inhibited Gear Lubricants
These lubricants are commonly referred to as R&O gear oils. They are generally petroleum base oils or synthetic blend base oils that are formulated with additive systems that protect against rust and oxidation.
In addition to rust and oxidation-inhibiting additive systems, some R&O gear oils contain minute amounts of antiwear additives. The viscosity grades for R&O are identified by a single-digit AGMA number 0 through 6, which corresponds to the ISO viscosity grades 32 to 320.
R&O gear oils perform well over a wide range of gear drive sizes and speeds in a temperature range of -5ºF to 250ºF (-15ºC to 121ºC).
Compounded Gear Lubricants
Compounded gear oils are a blend of petroleum base oils with rust and oxidation inhibitors, demulsibility additives and 3 percent to 10 percent fatty or synthetic fatty oils. These gear oils are frequently used in worm gear drives to provide excellent lubricity and prevent sliding wear.
Compounded gear oils are limited to an upper operating temperature limit of 180ºF (82ºC).They are identified by single-digit AGMA numbers with the suffix “Comp” from 7 to 8A, which corresponds to ISO viscosity grades 460 to 1,000.
Extreme Pressure Gear Lubricants
These lubricants are commonly referred to as EP gear oils. EP gear oils are petroleum base or synthetic blend base oils that contain multifunctional additive systems. The additive systems contain rust and oxidation inhibitors, EP additives, demulsifiers, antifoam agents, and in some cases solid lubricants that are collodially suspended, such as molybdenum disulfide, borates or graphite.
The EP additive system, which includes sulfur-phosphorous, borates and sulfur-phosphorous-boron chemistries, provides a chemically protective film that protects against welding, scuffing and scoring of the gears during boundary lubrication conditions, which can occur at start-up, stopping and high shock loads.
A single-digit AGMA number combined with the suffix “EP” from 2EP to 9EP corresponds to ISO viscosity grades 68 to 1,500. EP gear oils perform well over a wide range of gear drive sizes and speeds in a temperature range of -5ºF to 250ºF (-15ºC to 121ºC).
Synthetic Gear Lubricants
Synthetic gear lubricants differ from petroleum base gear lubricants in that they are formulated using synthetic base fluids. The most common types of synthetic base fluids used in the formulation of synthetic base gear oils include: polyalphaolefins (PAO), diesters, polyol esters and polyglycols.
Synthetic gear lubricants are used whenever petroleum base gear lubricants have reached their performance limit. In general, synthetic gear lubricants have the advantage of being stable over a wide range of operating temperatures, have a higher viscosity index (smaller viscosity changes with temperature variations), improved thermal and oxidation resistance and in some cases greater load-carrying capacities and better lubricity.
Each type of synthetic base fluid has different characteristics and some of them may have limitations or disadvantages such as compatibility with elastomers, paints, backstops, clutches, reactions in the presence of moisture and higher price.
Synthetic gear lubricants can also contain rust and corrosion inhibitors, EP additives, demulsifiers, antifoam agents and in some cases solid lubricants. They are identified by single-digit AGMA numbers with the suffix “S” from 0S to 9S, which corresponds to ISO viscosity grades 32 to 1,500.
A Gear Lubricant’s Key Performance Properties
To meet the lubrication needs of modern enclosed industrial gear drives, a gear lubricant must possess the following key performance properties:
- thermal and oxidative stability
- thermal durability
- compatibility with seal materials
- protection against excessive gear and bearing wear
- high-temperature extreme pressure protection (EP gear oils)
- gear and bearing cleanliness
- demulsibility characteristics
- rust and corrosion protection, especially to yellow metal components
- antifoaming characteristics
Many of these key properties can be identified by examining the lubricant supplier’s technical data or specification sheets and comparing them against the minimum performance requirements for industrial gear lubricants as set by the following widely recognized specifications:
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U.S. Steel 224 Specification for nonleaded EP industrial gear lubricants - This specification identifies high load capacity and thermal stability.
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AGMA 9005-D4 - This specification closely mirrors U.S. Steel 224 and also includes minimum physical and performance specifications for R&O, compounded oils and synthetics.
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Cincinnati Milicron P-34/P-35/P-59/P-63/P-74/P-76/P-77/ P-78 - These lubricant specifications also include minimum performance for thermal stability and antirust protection.
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DIN 51 517 Part 3 CLP - Developed by the Deutsche Institut fur Normung of Germany, this specification addresses petroleum-based gear lubricants containing additives to improve rust protection, aging characteristics and EP protection.
In addition to these platform specifications, a number of supplemental tests can be conducted to assess the performance of a gear lubricant in applications where optimum performance is required. Some of these tests will be further discussed.
The major industrial gear lubricant performance requirements and tests used to evaluate performance are summarized in Tables 1 and 2.
Table 2: U.S. Steel 224, DIN 51 517 Part 3, Cincinnati Milicron
Thermal and Oxidative Stability
Design changes to improve gearbox efficiency have produced smaller gear drives that operate at higher speeds, loads and temperatures, producing increased gear and bearing stress. These enclosed gear drives also have smaller capacities resulting in less lubricant being available to cool the gears and bearings and suspend contaminants.
In addition to design changes, enclosed industrial gear drives can be subjected to overheating due to inadequate load-carrying capacity, misalignment and inadequate clearances.
As a consequence, industrial enclosed gear lubricants can be subjected to extreme thermal stress. This thermal stress can cause oxidation and thermal breakdown of the gear lubricant. Oxidation and thermal breakdown of the gear lubricant can result in an increase in the gear lubricant’s viscosity, the buildup of acidic components and the formation of sludge, varnish, lacquer and carbon deposits on the gears, bearings and seals.
The buildup of these deposits can increase frictional drag, operating temperature and energy consumption; reduce gearbox efficiency; and cause premature and catastrophic wear to the gears and bearings. The accumulation can also cause abrasion to the seals, premature seal hardening and brittleness, thus resulting in decreased seal life and increased lubricant leakage.
Because of these factors, it is important that an enclosed gear lubricant exhibit thermal and oxidative stability.
A thermally stable gear lubricant is defined as a gear lubricant that keeps critical parts clean, with respect to deposits and sludge, when subjected to sustained high-temperature service.
Several tests measure an industrial gear lubricant’s thermal and oxidative stability properties that enable it to prevent and reduce oxidative thickening and deposit formation. The accepted test methods used to evaluate thermal and oxidative stability include:
- ASTM D943 Thermal and Oxidation Stability Test (R&O Gear Oils only)
- ASTM D2893 and U.S. Steel S-200 Oxidation Stability Characteristics of Extreme Pressure Gear Lubricants test methods
- ASTM D5704 (L-60-1) Thermal Oxidation Stability Test
- FTM 3462 Panel Coker Test
ASTM D943 Thermal and Oxidation Stability Test
The ASTM D943 Thermal and Oxidation Stability Test is used to evaluate the oxidation stability of rust and oxidation-inhibited gear lubricants in the presence of oxygen, water, copper and iron metals. The test method is used for specification purposes and is valuable in estimating the oxidation stability of rust and oxidation-inhibited industrial gear lube, especially those that are prone to water contamination.
The lubricant is subjected to test conditions that include water, catalytic metal (copper) and oxygen for as long as is necessary to force the oil to oxidize. The acid level is routinely checked, with a growth in acid content indicating that the test is rapidly deteriorating.
ASTM D2893 and U.S. Steel S-200 Oxidation Stability Characteristics of EP Gear Lubricants Test Methods
Both of these tests are intended to simulate service conditions on an accelerated basis. The test method is used to measure the ability of EP gear lubricants to resist oxidation and the formation of deposits when subjected to high operating temperatures. At the end of the test period, the viscosity is checked for change (maximum of five percent increase), the test utensils are inspected for sludge or residue and the lubricant color is checked for evidence of change (darkening).
ASTM D5704 (L-60-1) Thermal Oxidation Stability Test
The L-60-1 test is used to measure a gear lubricant’s ability to resist thickening and deposit formation characteristics under sustained high-temperature conditions. This test method provides an accurate indication of how a gear lubricant performs in the field when subjected to high-temperature conditions.
The test method utilizes a specially built gear case in which two spur gears immersed in the gear lubricant being evaluated are run for 50 hours at 325ºF (163ºC) in the presence of a copper catalyst and 1.1 liters/hour of air flow to promote oxidation at an input shaft speed of 1,750 rpm.
At the end of 50 test hours, the gear lubricant is evaluated for viscosity increase and the test gears are visually inspected and rated for carbon, sludge and varnish deposits. A thermally and oxidatively stable gear lubricant exhibits no more than a 50 percent increase in viscosity, and the gears and bearings exhibit little or no signs of deposit formation.
FTM 3462 Testing for Carbon Coking Tendency
The Panel Coker Federal Test Method (FTM) 3462 is used to determine the deposit formation tendencies of a gear lubricant and its effectiveness in keeping the system clean and free from deposits in extreme high-temperature environments. The Panel Coker apparatus is designed to continually splash the lubricant onto a hot plate (panel) that is sitting 25 degrees off of horizontal.
The lubricant evaporates as it drips from the plate. At the end of the test, the heater is turned off and the panel is allowed to cool. Once cooled, the test panel is removed, washed with solvent to remove any excess lubricant, and weighed to determine the amount of deposits adhering to the surface of the panel. The darker the color of the panel, the more deposits that have been left. A cleaner panel indicates fewer deposits.
Thermal Durability and EP High-Temperature Protection
In addition to being thermally and oxidatively stable, a gear lubricant must exhibit thermal durability. A thermally durable industrial gear lubricant is defined as a gear lubricant that has the ability to resist radical changes in its EP additive chemistry.
When an industrial gear lubricant is subjected to high operating temperatures, it can become thermally stressed to a point where its EP additive chemistry undergoes radical changes in its chemical structure. These changes can result in a loss of the lubricant’s EP ability to provide protection against excessive gear tooth and bearing wear, spalling and overall distress.
Compatibility with Seal Materials
Most seals commonly fail due to hardening and deposit formation, especially when high operating temperatures are encountered. Thermal stress of the industrial gear lubricant during use can cause the industrial gear lubricant to breakdown and form carbon and varnish deposits in and around the seal lip. Once formed, these deposits can abrade the seal material, causing cracking and tearing of the seal.
Seal leakage can be further aggravated by loss of seal material elasticity caused by chemical interactions of the seal material with the industrial gear lubricant’s basestocks and additive chemistry. These chemical interactions can cause the seals to either excessively swell or shrink.
Therefore, it is important that the industrial gear lubricant incorporate a careful balance of additive chemistry and base oils in order to prevent seal failures. Generally, PTFE (Teflon®) and Viton® seal materials provide the most universal compatibility.
Reference
1. ANSI/AGMA 9005-D94. Industrial Gear Lubrication. p. 1.
