Although different base oil types exist in the market and are widely in use, the majority of industrial lubricants are still based on mineral oils due primarily to low cost and easy miscibility with other oils. However, despite their widespread use, conventional lubricants reach functional limits in various scenarios. In addition to the limitations of natural resources, their safe and environmentally sound handling, use, and disposal often require considerable efforts.
At the same time, industrial operators’ expectations toward innovative specialty lubricants are increasing. They range from operational and food safety to biodegradability, longer life cycles, and reduced emissions and energy consumption.
While searching for a material that meets all of these general and branch-specific requirements, water is a visionary, yet obvious, raw material available worldwide that’s non-toxic and non-combustible. The benefits are clear, but some challenges include its low viscosity, evaporation, freezing point, corrosiveness, and sensitivity against microbiological growth. Overcoming these challenges would bring a greener solution to the gear industry with higher machine efficiency and energy savings than existing solutions. This paper presents one such lubricant concept based on water and highlights its benefits in comparison to a conventional mineral and polyglycol (PG)-based synthetic gear lubricant.
2 Definition and Properties of Hydro Lubricants
Hydro Lubricants unfold their innovative traits by using water either as a base oil or as an additive, hence the name “Hydro Lubes.” Initial results indicate it is a promising technology with a great potential to deliver high performance; some include high thermal and electric conductivity, super-low friction and good load-carrying capacity on the FZG four-square test machine. It is widely known that elastohydrodynamic (EHD) friction of mineral oil is higher than synthetic hydrocarbons (SHCs), followed by PG. This means PGs can offer better energy efficiency benefits compared to mineral oils. When looking for lubricant solutions that can offer friction benefits much better than PGs, water-based concepts have been an obvious choice. The Hydro Lubricant presented in this study is of ISO VG 460 according to  with a kinematic viscosity of approx. 460 mm2s-1 at 40°C suitable for gear applications. Water as base fluid is mixed with a synthetic fluid in order to obtain the viscosity grade. Table 1 lists some basic properties of the test Hydro Lubricant in comparison to a PG of similar ISO VG and a commonly used reference mineral oil in FZG testing protocol.
All lubricants contain typical gear oil additives such as extreme pressure (EP), anti-wear (AW), and corrosion inhibitors (CI). The viscosity indices (VI) of Hydro Lubricant measured according to DIN ISO 2909  are more than double compared to the reference mineral oil that causes a rather constant viscosity change with temperature as shown in Figure 1. The pour point measurements made according to DIN ISO 3016  demonstrate for the Hydro Lube that the freezing point of water can be overcome through proper additivation and a low temperature behavior comparable to a synthetic PG-based gear oil and better than the mineral oil. Furthermore, in Figure 1 the possible operating temperatures are shown where the low temperature operation is limited near the pour point of the fluids. The short-term maximum operating temperature for Hydro Lubes is about 90°C at ambient pressure, which is basically limited by the boiling point of water. Typical operating temperatures should be on a level about 60°C.
From the large study conducted, some example results related to corrosion protection, load-carrying capacity, and electrical properties are shown in the following sections.
2.1 Corrosion Protection
The ability of lubricants to prevent corrosion was evaluated by a copper corrosion test conducted according to DIN EN ISO 2160  but at a relatively low temperature of 80°C and a steel corrosion test according to DIN ISO 7120  method A. The results of the ISO VG 460 Hydro Lubricant are shown in Figure 2 where it can be seen that the Hydro Lubricant exhibits good anticorrosion properties. The Hydro Lubricant showed a copper corrosion value that equals the protection level commonly offered by conventional gear oils (see Figure 2a and 2b). The steel corrosion test also showed an excellent result with the Hydro Lubricant (see Figure 2c).
In addition to the static immersion tests, a dynamic test method to evaluate the corrosion-protection properties of the Hydro Lube was chosen. The test rig shown in Figure 3 is commonly used for testing the anticorrosive properties of roller bearing greases in a wet environment (EMCOR test). The Hydro Lubricant ISO VG 460 was filled instead of a grease, and the bearing was operated for 164 hours at 80 rpm intermittent operation in accordance to ASTM D6138-16 . After two test runs, no corrosion was observed on the outer bearing raceways, which equals a zero as a best possible test result.
Further tests have shown gearbox surfaces that are exposed to condensed water and not wetted by the Hydro Lubricant need to be protected by the application of a corrosion protection spray that is compatible with the gear fluid.
2.2 Load-carrying Capacity
The load-carrying capacity was evaluated by a modified scuffing test using an FZG four-square test machine according to ISO 14635-1 . The Hydro Lubricant was tested at a reduced starting temperature of 60°C (FZG A/8.3/60) instead of 90°C, which was kept constant by cooling. For a constant lubricant composition, the breather plug was closed.
In this test, elevated surface temperatures due to high surface pressures and sliding velocities cause a local welding of the tooth flanks of pinion and wheel. A higher failure load stage from this test is an indicative measure of high relative scuffing load-carrying capacity of gear lubricants. The tested Hydro Lubricant reached a failure load stage greater than 12. In comparison, some industrial PG-based gear oils achieve an API GL-5  classification as a measure of highest scuffing protection.
The result clearly indicates the water-containing Hydro Lubricant provides a good tooth flank surface protection even at high temperatures in the gear mesh.
2.3 Electrical Resistivity
Another important property of lubricants that contain a significant amount of water is the low resistivity. As shown in Figure 4, the specific electrical resistance of the ISO VG 460 Hydro Lubricant is comparable to tap water and therefore significantly lower than conventional PG gear oil. As the resistivity is depending on the polarity of the base oil, these values are higher for mineral or SHC oils .
This behavior is particularly beneficial in systems where electric discharge (ED) is a problem, such as those found in electric motors. It is well known that bearings used in variable-speed electric motors experience fluting — a damage commonly found on the surface of bearing raceways caused by electric arcs that surpass the lubricant. The resulting electric discharge could potentially degrade the lubricants by high local temperature rise. One way to reduce the risk and severity of damages caused by ED is to use lubricants with high electrical conductivity . The Hydro Lubricant presented in this study possesses an excellent electrical conductivity property compared to conventional oils, indicating that it could potentially reduce the damage caused by ED.
3 Evaluation of Gearbox Tightness
If a gearbox is lubricated by a Hydro Lube, the compatibility of lubricant and seal is of utmost importance. A gearbox must be tight over a long seal lifetime to achieve long service intervals and avoid breakdowns. The compatibility of sealing material and Hydro Lubricant ISO VG 460 was tested first with static immersion tests and afterwards in dynamic radial shaft seal (RSS) tests in order to provide a long-lasting solution.
3.1 Static Compatibility
The static elastomer compatibility was evaluated according to DIN 1817  with the seal elastomer 75 FKM 585 from Freudenberg Sealing Technologies. The test was performed at a reduced temperature of 80°C instead of 130°C frequently used for FKM rubbers, and the lubricant composition was kept constant with a reflow device. The test results show a good static compatibility between the lubricant and elastomer. According to the limits of Freudenberg Sealing Technologies FB 7311008-eng , two points of a possible three were achieved.
3.2 Test Rig and Method Description
The dynamic elastomer lubricant compatibility that correlates more with practice  than static testing was tested on a test rig shown in Figure 5 according to . The engine drives a shaft, and the RSS is mounted in a cover. The lubricant is filled to the middle of the shaft, and the test rig housing can be pressurized to increase the stress for the RSS.
The test rig was operated for 42 cycles of 20 hours of running time at 3,000 rpm and 4 hours of stand still. A lubricant sump temperature of 45°C was chosen, and furthermore, an overpressure of 0.25 bar was applied to simulate a gearbox with a closed breather plug that causes a higher radial force on the seal edge. During the test period, the tightness of the seal was controlled visually once a day.
3.3 Seal Description
A special seal with two seal lips shown in Figure 6 was used. The rubber parts highlighted in blue were made of the elastomer 75 FKM 585 that was used for the static compatibility testing under 3.1. The left seal lip a) was in direct contact with the Hydro Lubricant whereas the seal lip b) was a second seal lip in contact with the ambient air. The space between the two seal lips was lubricated with a grease based on lithium soap, synthetic oil of the viscosity 110 mm2s-1 at 40°C, and consistency of NLGI grade 1.
The seal edge a) uses the geometry of a sinus curve instead of the conventional seal edge b). The sinus provides a wider contact area and less friction and heat generation. The additional conventional seal edge b) with a reduced spring strength is used for higher operational safety.
3.4 Dynamic Compatibility Test Result
Test results have shown an operation with a single radial shaft seal like seal lip b) causes high wear of shaft and seal, avoiding long seal lifetimes. However, a radial shaft seal with a single seal lip of type a) operated tight for 1,008 hours at low wear rates of 0.15 to 0.19 mm of the seal edge. The deviation of two tested radial shaft seals of type a) was high, which is why the seal concept in Figure 6 was evaluated.
The tested modular sealing solution with two seal lips shown in Figure 6. was tight over 1,008 hours of operation and is a good candidate for long time tests or serial applications.
4 Friction Measurements
Efficiency testing has been performed on a ball-on-disc tribometer as well as on an FZG four-square test machine. Exemplary test results are shown in this chapter.
4.1 Tribometer Tests
Friction measurements were made on steel/steel contact using a ball-on-disc tribometer (EHD2, PCS Instruments) at a realistic test temperature of 60°C and a mean speed ensuring EHD full film condition (2.5 m/s). It is evident from Figure 7 that the Hydro Lubricant exhibits a super-low friction compared to the conventional PG gear oil.
It should be noted that among the different base oil types available, PGs are known for their low friction properties due to their molecular structure. The Hydro Lubricant presented in this study shows a much lower friction than the current low friction PG, indicating that the Hydro Lubricant can potentially offer a significant energy saving benefit.
4.2 Gear Efficiency Testing
The gear efficiency testing was performed on an FZG four-square test machine modified to evaluate the efficiency performance of Hydro Lubricants with a higher correlation to practice than the ball-on-disc tribometer tests.
4.2.1 Test Rig and Method Description
The FZG four-square test machine shown in Figure 8 used for this test is equipped with a torque measuring clutch and a loss torque meter in order to evaluate the efficiency with the smallest possible error.
The test method according to FVA 345  is shown in Figure 9 where a mapping of the gearbox efficiency at different speed and load conditions is performed. A temperature variation causes different lubrication regimes at the mentioned operating conditions.
In the test procedure, a candidate oil compared with a reference oil as an absolute evaluation of one lubricant is not possible. In addition to the efficiency mapping, the steady state temperature is measured after five hours of constant operation at load stage KS7 and 8.3 m s-1 peripheral speed. For the tests with the Hydro Lubricant, the test stage with sump temperature of 120°C was removed, and the breather plug was closed .
4.2.2 Gear Efficiency Test Results
The test of the Hydro Lubricant and the PG gear oil on the FZG test rig showed excellent repeatability . Loss torque measurements at load stage KS7 are shown in Figure 10 over a speed variation from 0.5 to 20 m s-1. The lubricant sump temperature was kept at 60°C by cooling both test and slave gearboxes.
Both lubricants show an increase of losses with higher peripheral speeds, which is representative for a Stribeck curve in the regime of fluid friction. Furthermore, the Hydro Lubricant exhibits lower friction properties over the whole speed range. Depending on the operating point, this causes energy savings in the order of 10 to 48 percent.
In the next step, the load stages were varied to lower (KS5) and higher (KS9) flank pressures while the speed and temperature were kept constant. It can be seen from Figure 11 that the level of losses increases with the load, and the energy saving benefits are highest at load stage KS7 (1343 N mm-2) at 8.3 m s-1. Nevertheless, the Hydro Lubricant shows lower losses than the PG gear oil over all loads.
In addition, different fluid sump temperatures were investigated at a moderate speed and load shown in Figure 12. The loss torques of both lubricants decrease with increasing sump temperatures and what correlates with the viscosity-temperature behavior shown in Figure 1. The energy savings of the Hydro Lube versus the PG gear oil are 32 percent at 40 and 60°C. A higher viscosity compared to the PG gear oil due to evaporated water might explain the lower savings of 27 percent at 90°C.
The test rig was operated safely at a sump temperature of 90°C as mass temperatures of the gears were even higher. Hence, a short-term operation with sump temperatures of 90°C is possible with a Hydro Lube whereas a constant operation at sump temperatures of about 60°C is recommended.
The steady state temperatures of both test lubricants and the mineral oil reference were evaluated at load stage KS7 at peripheral speed 8.3 m s-1 over a duration of five hours. The results are shown in Figure 13, combined with the loss torque representing the efficiency of a gearbox. This test simulates the operation of most gearboxes in practice as heating or cooling devices are seldom due to cost reasons.
The loss torque of the Hydro Lubricant is 17 percent lower compared to the PG gear oil and on the same level as a mineral oil of ISO VG 100. In combination with the better thermal properties, this causes a temperature reduction of 13°C compared to the PG and 9°C to the mineral oil of lower viscosity. One potential reason for the lower level of loss torque reduction against the PG in the steady state temperature test compared to the 32 percent at the same speed in the previous measurements (see Figure 10) is the temperature difference of 13°C, which in turn causes the Hydro Lubricant to have a higher viscosity.
Some key challenges of Hydro Lubricants like pour point and corrosion protection have been solved by proper and advanced formulation. Various radial shaft seals for industrial gearboxes have been evaluated and suitable solutions of Hydro Lubricants and radial shaft seals are available. The low electrical resistivity offers benefits in systems with high risk of damages caused by electrical discharge. The demonstrated lower friction, higher efficiency and lower operating temperatures of the Hydro Lubricants in comparison to a PG-gear oil of similar ISO VG 460 unfold new possibilities in the design of gearboxes. With the wide range of demonstrated benefits, Hydro Lubricants offer a more sustainable solution for gearboxes with high demands in low friction and temperature reduction.
- DIN ISO 3448, 2010, Flüssige Industrie-Schmierstoffe – ISO-Viskositätsklassifikation (ISO 3448:1992).
- DIN ISO 2909, 2004, Mineralölerzeugnisse – Berechnung des Viskositätsindexes aus der kinematischen Viskosität (ISO 2909:2002).
- DIN EN ISO 2160, 1999, Mineralölerzeugnisse – Korrosionswirkung auf Kupfer – Kupferstreifentest (ISO 2160:1998).
- DIN ISO 7120, 2000, Mineralölerzeugnisse und andere Flüssigkeiten – Bestimmung der Korrosionsschutzeigenschaften in Gegenwart von Wasser (ISO 7120:1987).
- ASTM D6138-16, Standard Test Method for Determination of Corrosion-Prevention Properties of Lubricating Greases Under Dynamic Wet Conditions (Emcor Test).
- DIN ISO 14635-1, Gears – FZG test procedures – Part 1: FZG test method A/8,3/90 for relative scuffing load-carrying capacity of oils (ISO 14635-1:2000).
- API Publication 1560, Seventh Edition, 1995, “Lubricant Service Designations for Automotive Manual Transmissions, Manual Transaxles, and Axles” F15607, American Petroleum Institute, Washington, D.C.
- Gemeinder, Y., 2016, “Lagerimpedanz und Lagerschädigung bei Stromdurchgang in umrichtergespeisten elektrischen Maschinen” Ph.D., IWV, Bonn.
- DIN ISO 1817, Elastomere – Bestimmung des Verhaltens gegenüber Flüssigkeiten (ISO 1817:1999).
- Freudenberg Sealing Technologies, FS PLM 111 0119, 2018,“Statische Ölverträglichkeitstests mit Freudenberg Simmering – Werkstoffen zur Freigabe für den Einsatz in Flender Getrieben (Tabellen T 7300),” Flender GmbH, Freudenberg Sealing Technologies.
- Dr. Adler, M., 2017, “Understanding the Dynamic Influences of Gear Oils and Radial Shaft Seals” American Gear Manufacturer Association Fall Technical Meeting, AGMA.
- Freudenberg Sealing Technologies, FB 7311008-eng, 2018, “Dynamic oil compatibility tests for Freudenberg radial shaft seals to release the usage in FLENDER-gear units applications (Table T 7300),” Flender GmbH, Freudenberg Sealing Technologies.
- Forschungsvereinigung für Antriebstechnik E.V., FVA 345, 2003, “Method to Determine the Frictional Behaviour of Lubricants Using a FZG Gear Test Rig,” FZG Gear Research Centre, Technische Universität München.
- FZG, 2018, “Expertise Investigations of the Efficiency of four Lubricants,” FZG Gear Research Centre, Technische Universität München.
Printed with permission of the copyright holder, the American Gear Manufacturers Association, 1001 N. Fairfax Street, Suite 500, Alexandria, Virginia 22314. Statements presented in this paper are those of the authors and may not represent the position or opinion of the American Gear Manufacturers Association. (AGMA) This paper was presented October 2019 at the AGMA Fall Technical Meeting in Detroit, Michigan. 19FTM11