Most Americans have experienced inconvenience when a hurricane or blizzard brought down power lines, or a lightning strike disabled a transformer. Populations sometimes endure blackouts and brownouts when demand challenges the ability of power grids to provide electricity. Otherwise, most citizens confidently assume the availability of adequate and uninterrupted electric power.
Very different conditions prevail in other countries. The International Energy Agency estimated that over 300 million citizens in India lacked electric power in 2014, for example. And even where abundantly available, electricity can be costly, the agency states: In 2013, electricity costs for industrial and residential users, respectively, were $169.32 and $387.63 per MWh in Germany, versus $68.20 and $121.16 in the United States.
Electric power has been outpacing oil and coal as an energy source in recent decades. Global electricity consumption increased by a factor of 3.7 from 1973 to 2012 while total energy consumption almost doubled. During this period, reliance upon electricity rose from 9.4 percent to 18.1 percent of energy consumption worldwide. Future electricity demand will continue to rise, on trend with populations, industrialization, standards of living, use of plug-in hybrid vehicles and other drivers, the IEA indicates.
The workhorse machine for converting electrical energy into mechanical energy is the electric motor. So it makes sense that the single greatest demand for electricity consists of electric motors that power our home appliances, pumps, fans and industrial equipment. Citing IEA data, Shell Global Solutions US says as much as 46 percent of electrical energy is consumed by electric motors – most of which contain bearings lubricated with grease. We should consider the impact that we can make here, points out Greg Morris, the companys Americas grease product application specialist.
Motor efficiency is defined as the ratio of mechanical energy output to electric energy input. Inside a motor, electricity passes through a wire coil and magnetizes a metal rotor. The magnetic force between the rotor (an electromagnet) and the stator (a permanent magnet) turns a shaft that operates a mechanical device such as a fan. Bearings support the turning shaft, and grease lubricates these bearings. Friction between shaft and bearings (and other mechanisms) wastes energy and reduces energy efficiency.
Can better lubricants stem these energy losses? That was the issue raised in June at the National Lubricating Grease Institutes annual meeting in Coeur dAlene, Idaho. Several participants described how the right grease might make a difference in the consumption and conservation of electricity, one motor at a time. But to really help end users, Shells Morris added, industry needs to adopt a test that reliably measures the greases impact.
Key Grease Properties
While many original equipment manufacturers recommend specific commercial lubricating greases for their electric motor bearings, no single current specification guides grease selection. Most ball and roller bearings are lubricated with greases formulated with mineral base oils and lithium, lithium complex or polyurea thickeners. General purpose NLGI 2 greases are considered appropriate for moderate shaft speeds (up to 3,600 rpm) and ambient temperatures (up to 40 degrees C).
Typical EMB grease characteristics include good mechanical stability and oxidation and corrosion resistance. Certain premium greases aim to provide additional advantages such as extended use at high operating temperatures, enhanced resistance to wear, and low acoustic noise properties.
At the NLGI meeting, Solongo Wilson and Wayne Mackwood of Chemtura Canada in Toronto proposed that calcium sulfonate complex thickeners can be used to improve EMB greases. Their paper discussed formulation and testing of 10 calcium sulfonate complex greases and four control (lithium complex, polyurea) greases in various base oils, including paraffinic, naphthenic and synthetic types from API Groups I through V.
As Wilson explained, CSC greases are prepared by dispersing colloidal particles of amorphous calcium carbonate with sulfonate surfactants in base oil. Acid(s), base(s), alcohol(s) and water – or a commercial sulfonate pre-mix – are added, and the mixture is heated to form calcite, which gels and forms grease thickener. CSC thickeners impart high-quality performance properties without the use of additives, the authors pointed out: high dropping point, antiwear and extreme pressure protection, corrosion resistance, and mechanical stability. These greases are used in steel and paper mills, marine and similar demanding applications.
Wilson reported that the CSC greases dropping point (ASTM D2265) and oxidation stability (D5483) outperformed the controls. Appearance after testing on a hot plate (440 C for 60 seconds) and in an oven (170 C, until failure) confirmed the superior oxidation stability, she said. Consistency, four-ball wear and roll stability results were comparable for CSC and control greases.
Temperature effects are not limited to oxidation, Wilson pointed out. At high temperatures, greases can soften and leak out of bearings, and at low temperatures they can stiffen, interfering with bearing rotation and oil bleed. Wilson applied controlled-stress rheometry to assess these temperature effects across a range of temperatures, from -40 to greater than 195 C. Across this temperature range, the authors reported, shifts were smallest (that is, rheology was most stable) for CSC grease formulated in Group IV (polyalphaolefin) base oil.
Wilson and Mackwood concluded that CSC greases formulated in PAO show great promise for EMB applications. They plan to move forward with tests to measure bearing life and effects of sulfonate thickener content on film thickness. Some CSC greases are already in use in general electric motor bearing applications, Wilson added, and with further improvements in formulation, CSC could soon be in the list of three or four commonly recommended EMB greases.
Polymers: A Smoother Way?
Polymer thickeners for EMB greases were the subject of an NLGI paper presented by John J. Lorimor of Axel Americas, in N. Kansas City, Kans. With co-authors Mihir Patel, Brian Stunkel and Rob Heverly, all with Vanderbilt Chemicals in Norwalk, Conn., he investigated the impact of grease formulation on the three major contributors to energy inefficiency in mechanical systems: churn, traction and friction.
Given these three forces, there are really only three ways in which a lubricant – in this case, grease – can influence energy efficiency, Lorimor said. One is to reduce the energy required to move the bearing through the grease itself. Second, between the sliding surfaces, energy is required to shear the lubricant film itself. And finally, under boundary conditions we get asperity contact between sliding surfaces.
Lorimor went on to compare three commercial NLGI grade 2 greases with different base oil viscosities: 1) a polyurea thickener in mineral oil (115 cSt at 40 C and 12 cSt at 100 C); 2) a lithium complex thickener in PAO (100 cSt at 40 C and 14 cSt at 100 C); and 3) a polymer in PAO (46 cSt at 40 C and 8 cSt at 100 C).
As he explained, churning losses depend on grease consistency and base oil viscosity. Some in-service greases will thicken due to oxidation, while certain polymer-thickened greases will soften with use.
Next, traction, or energy needed to shear a lube film between two moving surfaces, contributes to energy losses. Traction data were lower for PAO than Group I, II and III mineral oils of the same viscosity. Lorimor claimed that the molecular structure of PAO contributed to the superior energy efficiency of this base oil in his study. Likewise, the traction data were lower for the polymer-PAO grease relative to the polyurea-mineral oil and lithium complex-PAO greases.
Third, friction between surfaces in relative motion during boundary and mixed lubrication decreases energy efficiency. Experimental NLGI 2 greases were formulated from base oil, polypropylene, elastomer/rubber and additive(s). Stribeck curves (friction versus speed) were measured at specific loads and viscosities with tribometers. Friction data were significantly lower for one specific experimental grease than for the control grease, over the experimental ranges of speed, roll-to-slide ratio and temperature (from 40 to 120 C).
Lastly, the greases were packed in new 6203 deep-groove ball bearings. Each bearing was tested in a 1/3-hp motor (shaft speed 1,075 rpm) and electric energy consumption was monitored with a meter for 48 hours. The best experimental grease and the lithium complex-PAO greases reduced electric energy consumption by 3 percent and 2 percent, respectively, relative to the polyurea-mineral oil grease.
These outcomes led Lorimor to conclude that it is feasible to formulate greases with polymeric thickeners and low-viscosity base oils such as PAOs to reduce all three major sources of energy inefficiency in lubricated EMBs.
Proving Performance
Also speaking at NLGI, Shells Greg Morris returned to the issue of how to measure EMB grease efficiency, and highlighted a novel experimental method developed with Germanys Magdeburg University for this purpose. End users look for a number of properties when they choose a grease, Morris said – storage stability, final cost, temperature range, mechanical stability and preservation of component life.
But should we consider something else? he asked. He noted that energy consumption accounts for 95 percent of the cradle-to-grave expense for electric motors. Even though OEMs have increased motor efficiencies by as much as 35 percent in their premium models, versus standard equipment, he observed that friction and windage still account for 6 percent to 9 percent of energy losses in electric motors.
If about 46 percent of electrical energy is consumed by electric motors, we should consider the impact that we can make here, Morris urged.
Building on work led by Stefan Dagling, of Shell Global Solutions Deutschland, Morris described lab-scale test rigs used in independent projects with researchers at the University of Ghent (Belgium) and the University of Magdeburg (Germany). In one approach, unloaded greased bearings were tested in an electric motor. The motor was run at a defined initial speed, then the power was cut, and the run-down time was measured as the motor slowed to a target speed. Power dissipation was estimated by dividing the change in rotational kinetic energy by the run-down time.
In this experiment, significantly different results were observed for seven commercial greases (lithium complex and polyurea thickeners, PAO and mineral oils, 100-115 cSt at 40 C) during run-down tests, suggesting that the grease formulation has a measurable impact on power dissipation.
Next, a second lab-scale test rig was built to accurately and precisely measure frictional torque of bearings over a range of electric motor operating conditions. Results depended on grease formulation, rotational speed and temperature set-point. Frictional torque increased slowly with rotational speed up to 6,000 rpm in some cases, while there was a transition from slower to faster increase in torque with speed for other cases.
Significant differences in frictional torque and energy efficiency were reported for a range of lithium complex and polyurea thickened NGLI 2 greases, even though all were formulated with ISO VG 100 base oils and marketed as electric motor greases. As Morris pointed out, these results show that grease selection can influence friction and efficiency.
The test rigs and procedures provided useful comparisons of bearing greases, he added, and also showed that similar greases can deliver quite different effects. Bringing energy efficiency into the formulary decision-making process adds complexity to the decisions, commented Morris, but brings real value for the end user.
In conclusion, while frictional inefficiencies in individual electric motor bearings may seem small, these motors comprise the single largest demand on electricity worldwide. Improved greases and EMB energy efficiency provide opportunities for business development, and can have global implications.
Mary Moon, Ph.D., has broad R&D and management experience in lubricants, greases and specialty chemicals. She has commercialized products, led R&D innovation projects, developed test methods and solved problems in lubricants production, application and marketing. She can be reached by e-mail to marymoonphd@gmail.com or by phone at (267) 567-7234.