Test specifications for industrial gear oils have not changed much in the past several decades, even as gearbox design has undergone significant revisions. That situation changed recently with upgrades by the German Institute for Standardization (DIN) and original equipment manufacturers like Siemens.
According to David Oesterle, industrial prod-uct manager at Lubrizol, the primary focus of the upgrades is bearing protection, specifically micropitting on the rolling elements. In a presentation at the STLE Annual Meeting in Detroit, Mich., United States, last May, he explained the implications of the upgrades and their effect on formulating industrial gear oils.
Industrial Gear Oil Specifications
Many types of fluids are used as industrial gear oils, and global consumption of the fluids was over 925,000 metric tons in 2012. Oesterle, who is based in Wickliffe, Oh., United States, said, For many years, industry relied on a set of specifications to govern the performance of industrial gear oils, including AGMA 9005-E02, DIN 51517, U.S. Steel 224 and ISO 12925-1. Table 1 compares the requirements of the various specifications.
The baseline for most testing is the DIN specification, which specifies basic properties, performance and, especially, antiwear performance, he added. (See Table 2.)
These specifications changed little for many years; however, OEMs are now beginning to update their requirements, Oesterle said. For example, the Siemens MD Revision 13 specification issued in 2011 now requires the FVA 54 test for micropitting, paint compatibility, Loctite sealant compatibility, static and dynamic seal testing and the Flender dynamic foam test. These new requirements not only add more severe test conditions, they also add significantly to the time required to qualify a lubricant.
Another upgraded specification is Hansen BUI-TEC-2009-4-001 Revision C. Historically, Hansens specification covered all transmission types, including general industrial and wind turbine gearboxes, Oesterle said. As a result, it was very complex. Hansen recently sold its wind turbine transmission business to ZF; therefore, its general industrial specification will evolve and become more realistic for industrial applications.
As written today, BUI-TEC-2009-4-001 Revision C for general industrial transmissions, which was introduced in 2012, now includes DIN 51517-3 testing, the FVA54 micro-pitting test at 60 and 90 degrees C, eight different material compatibility tests and a heavy focus on bearing tests, including DIN 51819-3 FE8 and Schaeffler FAG Steps 1 through 4. Bearing wear and fatigue are the main criteria, Oesterle said.
FE8 is the standard short duration bearing test found in DIN 51517-3. The test rig consists of cylindrical roller thrust bearings with brass cages. It runs for 80 hours at 7.5 rpm under an axial load of 80 kiloNewton. Oil temperature is 80 degrees C. These are often referred to as D7.5/80-80 conditions. The specification calls for less than 30-milligram weight loss on the rolling elements.
Schaeffler FAG Step 1 is a short duration test to determine the effectiveness of additives on rolling bearings operating under boundary lubrication conditions. Test conditions are similar to those of DIN 51819-3 but under a 100-kN load and an oil flow rate 100 cubic centimeters per minute. Test requirements are less than 30-mg weight loss on the rolling elements and no rippling or micropitting.
Schaeffler FAG Step 2 is a longer duration fatigue life test that determines the effectiveness of additives under moderate mixed-friction conditions. It uses the FE8 test rig under DIN 51819 conditions but with cylindrical roller thrust bearings with polyamide (nylon/plastic) cages. Axial Load 90 kN, speed is 75 rpm and test duration is 800 hours. Passing requirements are less than 30-mg weight loss on the rolling elements and no visible surface damage. Damage to the cage material is not taken into account as a reportable parameter.
Schaeffler FAG Step 3 is a fatigue life test under elastohydrodynamic lubrication conditions that measures the aggressiveness of additives toward bearing surfaces. It uses the L11 test rig with a deep groove ball bearing and an auxiliary cylindrical roller bearing. Radial load is 8.5 kN, meaning that the bearing is essentially unloaded. Speed is 9,000 rpm and test duration is 700 hours. Requirements are no visible fatigue damage.
Schaeffler FAG Step 4 is a long duration test to evaluate an oils tendency to form residues at high temperatures (100 to 140 degrees C). It uses the FE8 test rig with cylindrical roller thrust bearings with a brass cage and an oil aging chamber. Water additions are an option. Axial load is 60 kN, speed is 750 rpm and test duration is 600 hours. Requirements are less than 30-mg weight loss on the rolling elements, no fatigue damage and no heavy residue on the bearings or in the preheater chamber.
Pressure on Formulators
As can be seen from the brief description of the tests above, the new requirements add significantly to the time required to qualify a formulation. The FAG tests alone account for 2,260 hours of testing, over three months for one sample. Exacerbating the problem is that only a few labs are authorized to run certain tests:
Walter Mader – Internal paint compatibility;
Loctite – Liquid sealing compound compatibility;
Flender – Oil foam testing and gray staining;
Freudenberg – Elastomer shaft seals (static and dynamic).
This situation has caused a significant backlog in the industry, Oesterle said.
Adding to the challenge is that the additional test requirements increase the cost of qualifying a formulation. For example, he said, a first-time pass can cost between $18,000 and $20,000 per viscosity grade. If a formulator wants approval on viscosity grades ISO 150 through 680, the baseline costs jumps to $100,000 assuming a first time pass. And the costs go up from there if a formulation doesnt pass the first time. Only the best industrial gear oil formulations will pass, said Oesterle, and, to date, there have been very few approvals globally.
As a result, there is significant pressure on industrial gear oil formulators to get it right the first time. Testing shows that all premium industrial gear oils are not equal even though they may claim the same performance, said Oesterle, and top-tier formulations from 15 to 20 years ago are not passing the more severe tests. They produce significantly different bearing weight loss results, which translates into potential failures in the field.
To evaluate the capabilities of industrial gear oils in the new tests, Lubrizol conducted a benchmarking exercise in the DIN 51819-3 FE8 test on two Siemens MD approved additives. Results in an API Group I, ISO VG 150 base oil are shown in Table 3.
Results for the new additive in an ISO VG 68 base oil were a weight loss of 1.7 mg on the rolling elements and 101.7 on the cage.
Future industrial gear oil formulations will have to run a more severe gauntlet of tests, said Oesterle. They will not only have to provide enhanced gear and bearing life but also resist oil degradation at high temperatures for extended drain intervals. In addition, they must resist corrosion and demulsibility to provide trouble-free operation in gearboxes prone to water contamination.
Finally, they must be compatible with ferrous and non-ferrous metallurgy, internal paints and both liquid and elastomeric seals used in todays high-performance gearboxes. The challenge is signifi-cant, Oesterle concluded, and only the best formulations will pass the more severe requirements.