Boosting Grease Efficiency & Life
The last 30 years have seen increased focus on improving the efficiency of automobiles and commercial vehicles, both from the perspective of resource conservation and greenhouse gas reduction. Government legislation has forced vehicle manufacturers to make engines and drivelines significantly more efficient than they were in the 1980s.
However, according to Gareth Fish, Lubrizol Corp.s grease technology manager, while the spotlight has been on engines and drivetrains, there was little or no focus on other components. In a presentation to ELGIs Annual General Meeting in Barcelona last April, he noted, A typical passenger car or light truck may use as many as 50 different greases to lubricate components. While the majority of the greases have no influence on efficiency, those used in wheel and accessory bearings, steering and transmission components can affect efficiency.
Another important factor is the significant downsizing of bearings in modern equipment. As explained by Piet Lugt at the International Colloquium Tribology at Technische Academie Esslingen in January, bearing size has dropped nearly 70 percent since the 1950s for the same load-carrying capacity. Thus, said Lugt, senior scientist at SKF Engineering & Research Center, the grease plays a far more important role in determining bearing life.
Lugt explained, The main sources of energy loss in grease-lubricated rolling-element bearings are seal friction, grease churning and viscous rolling resistance. Rolling motion predominates in the center of the raceway in deep-groove ball bearings, but is a mixture of sliding and rolling on the edges of the contact path. Fish added, The balls do slide against the cage pocket walls, but this has little measurable effect on efficiency.
Deep-groove ball bearings are used primarily in electric motors, driveshaft supports and accessory drive components. However, these bearings cannot be used as wheel bearings because they cannot support both radial and axial loads. Therefore, wheel bearings generally comprise opposed tapered roller bearings (in North America) or opposed angular contact bearings (in other markets, such as Europe).
Friction losses in both set ups arise from the rolling elements sliding against the cage and from the cage sliding against the raceways, Fish said. Sliding speeds, lubrication and contact conditions differ greatly from vehicle to vehicle, which leads to varying energy losses and efficiency.
He added that the coefficient of friction in sliding contact is an order of magnitude greater than for rolling contact. Therefore, if sliding can be reduced, frictional losses will be lower. One way to reduce sliding friction is to reduce the lubricants coefficient of friction. Thats why friction modifiers have been investigated since the 1920s as a way to lower friction under boundary lubrication in liquid lubricants, Fish said.
Many standard tests can be used to measure friction in greases. Historically, the sliding 4-ball wear tester was used; however, one important issue with this machine is that the bearing in the apparatus influences friction values, Fish reported. Some machines incorporate air bearings and others tapered roller bearings. The latter have higher resistance to motion, making comparative data difficult to interpret.
SKFs Lugt concurred. The R2F, FE8 and ball-on-disc testers provide far more reliable results.
In 2013, Lubrizol tested a series of friction modifiers in lithium complex and urea-thickened greases. The results showed that only organic molybdenum complexes – molybdenum dithiocarbamate and nonphosphorus molybdenum – produced low friction coefficients of less than 0.08, Said Fish. We also found that the friction modifiers do not work on their own, but need other additives such as zinc dithiophosphate or sulfurized extreme pressure additives such as found in fully formulated greases.
From this testing, Lubrizol determined that the two molybdenum complexes reacted differently with the two thickeners and that further optimization of the formulations was needed. Therefore, the company tested an additional 10 combinations of additives and molybdenum compounds in lithium, lithium complex and urea-thickened greases.
Most of the packages produced friction coefficients of around 0.06 for the simple lithium grease, Fish explained. Adding additional molybdenum dithiocarbamate improved running-in characteristics, but increased the treat level and net treat cost significantly.
While combinations of molybdenum complexes, organic friction modifiers, antiwear and other performance additives are important, Lubrizols testing showed that for complete energy efficiency, other factors need to be considered, including traction, film thickness, rheology, consistency and grease fill.
Traction: Traction is a measure of the internal friction of a lubricant. Fish said, Generally, the lower the traction coefficient, the less heat generated. However, determining traction coefficients of grease is difficult. The first issue is controlling temperature, he reported. Grease is an insulator with poor heat convection characteristics; therefore, controlling grease temperature during the test is difficult.
The second issue is starvation. In both traction and optical elastohydrodynamic testing, it is very difficult to get the grease to behave as it would in a bearing. In the test, the grease is pushed out of the contact area and cannot flow back into the contact. It has to be pushed back by a wiper, creating artificial conditions that need further development.
Film Thickness: Historically, the thickener was thought to play no role in lubrication. However, Fish said that research shows greases form slightly thicker films than base oil alone. Whats more, at low speed, grease forms much thicker films than expected. This has important ramifications when developing more energy efficient greases.
Rheology & Consistency: Grease stiffness, how easily it flows and how it behaves under shear, are important considerations in developing energy-efficient greases. Most greases soften when subjected to shear, said Fish. Therefore, comparing penetration grades will not indicate how greases will affect efficiency.
As grease runs-in, apparent viscosity typically drops as do churning losses. All greases behave differently and take different lengths of time and shear cycles to stabilize when subjected to shear.
According to Fish, Apparent viscosity is important, but shear history also plays a role. What is clear is that testing greases when they are new, before they have a chance to shear soften, will give a different indication of energy efficiency.
Researchers have observed that urea greases, with grains of rice or rice pudding structures, can lose consistency temporarily under shear. They can soften by as much as 120 penetration points when worked for 100,000 double strokes, said Fish. Letting grease sit without shear allows it to recover up to 100 penetration points.
Urea greases can also have fiber-like structures similar to those of lithium soap greases. These greases shear soften up to 120 penetration points, but do not recover after standing, he noted. Fibrous urea greases typically have lower thickener content (~10 percent wt) than rice pudding urea greases (~12 percent wt).
Fibrous urea-thickened greases used in constant velocity joints made a definite contribution to energy efficiency. They start with NLGI 2 consistency but soften to NLGI 1 when pumped into the joints, Fish observed. When worked in the joints, they soften to between 0 and 00 grades. He added that softening contributes to lower running temperature, reduced plunging resistance and lower vibration transmission compared to grease that does not soften, such as anhydrous calcium or lithium soap greases.
Some calcium complex greases soften 100 penetration points and mimic the behavior of fibrous urea greases. However, Fish noted that one challenge with greases that soften readily is keeping them sealed in the bearing. Improved sealing systems have been developed that also allow the use of shear-stable NLGI 0 grade lithium soap greases, which contribute to energy efficiency and also reduce noise and vibration.
The properties of lithium soap greases differ depending on how they are made. If quenched rapidly, they predominately consist of small fibers that do not thicken the grease very well. They also have poorer yields and higher soap content, but they have lower bleed rates and better shear softening resistance, Fish revealed. Also, the higher soap content of quenched soap greases causes higher churning losses.
Slow cooling of lithium soap forms larger fibers that thicken better but do not have the shear stability or lower bleed of small fibers. Greases that consistently bleed small amounts of oil have been shown to provide better lubrication than those that do not readily bleed oil.
Steady oil bleed helps form thicker elastohydrodynamic films. In mixed film lubrication, this reduces metal-to-metal contact and, in turn, reduces friction losses and improves efficiency.
Manufacturing simple lithium soap greases with a partial quench helps balance yield, bleed and rheological properties, resulting in good lubrication and better efficiency, Fish said. Lithium complex greases manufactured with a well-controlled complexing reaction typically shear soften slightly by up to 30 penetration points. Higher levels of complexing produces higher dropping points, but also higher thickener content for the same NLGI grade, which can negatively affect energy efficiency.
Grease Fill: The final design parameter Fish covered was the amount of grease packed into a bearing or component. It is customary to pack only 20 to 50 percent of the available free volume of a bearing with grease. And around 30 percent of the volume is filled in CV joints, he said.
If the grease is significantly over-packed, thermal runaway will result, and the component will fail prematurely. If the grease is slightly over-packed or has too high apparent or base oil viscosity, the component will run hot.
When a component is packed properly, it will heat up as it runs in, the thickener will shear down and additives will react to form a protective antiwear layer, Fish explained. The surfaces are protected, and the component will settle to a stable steady-state running temperature.
Energy efficient greases require minimal running in and experience only nominal temperature rise at the start of operation. Toward the end of their life, greases can no longer lubricate effectively, and temperature rises as the grease breaks down.
Enhancing Grease Life
Lugt explained that one drawback of grease lubrication is that grease is usually the weakest component in a bearing. It has a shorter lifetime than the bearing parts. Therefore, bearing system reliability is usually determined by grease performance.
Lugt pointed out that the key factors in ensuring long grease life are creating a sustainable elastohydrodynamic film, controlling chemical and mechanical degradation and matching the grease to the bearing design. With regard to this last item, he emphasized the importance of carefully observing the greases low and high-temperature limits, and he presented guidance on the temperature limits of various grease types. He emphasized that the high-temperature performance limit should not be excessed during grease performance testing.
The only true way to evaluate the efficiency of a system is to measure energy input and the useful energy coming out, said Lubrizols Fish. For bearings or automotive components, the measure is torque in vs. torque out.
He related that measuring the small torque losses in a single highly efficient small deep-groove ball bearing is very challenging. Research shows that after running in, there is no measurable difference in the friction torque of two greases with different fiber structures. However, testing has confirmed that the amount of thickener influences churning losses, showing that the higher the thickener content, the less efficient the grease.
Many applications use tapered roller bearings with significantly more sliding contact than deep-groove ball bearings. Measurements in the FE8 test rig showed that it takes more than 150 hours to reach steady state. To better
discriminate between greases, its necessary to couple more bearings together to magnify the individual losses, Fish noted.
Lubrizols work also showed that packing the bearings consistently is difficult, and measuring the efficiency of greases is particularly challenging. The energy losses in small deep-groove ball bearings are very small and difficult to quantify, he said. In addition, to test bearings with more sliding, Lubrizol developed a modified test machine to measure energy efficiency of tapered roller bearings.