It is estimated that somewhere between 25 and 30 percent of all energy produced is consumed by friction*, which means that we are wasting a substantial portion of our energy resources.
One way to reduce energy loss from friction is through efficient lubrication of industrial equipment. In many cases, this can be achieved by switching lubricants from conventional to synthetic fluids.
In an ideal world, all of the energy put into a machine would be available at the output, i.e. it would be 100 percent efficient. But, even the most advanced systems suffer some form of efficiency losses.
To illustrate this point, consider this example: a new, highly efficient motor driving a gearbox, driving a mechanical fan. The motor will experience frictional losses in its bearings, as well as electrical losses in its windings; the gearbox itself will experience frictional and churning losses as the gears turn; and the fan will experience further frictional losses.
In this scenario, when we multiply the efficiency of each component together, we can expect to see an overall system efficiency near 88 percent, without taking into account electrical supply and switching losses, or losses in any drive couplings, chains, and other such parts. That means that, even at peak condition, a motor-gearbox-fan system can anticipate more than 10 percent energy loss as a result of mechanical friction, fluid friction and electrical losses.
To improve mechanical efficiency, there are three major areas of opportunity where a high performance lubricant can help operators regain efficiency:
- Reduced friction at moving surfaces such as gears, bearings and seals
- Reduced fluid losses from oil churn and pumping
- Reduced internal fluid friction under elastohydrodynamic lubrication (EHL) conditions in gears and bearings
Reducing Traditional Frictional Losses
Frictional losses such as these have sparked industry interest in lower viscosity lubricant solutions across-the-board. This trend is most clearly evident in the automotive industry where the conventional 15W-40 engine oil grade has been largely replaced by 5W-30 synthetic lubricant grades with even lower viscosity solutions¯including 0W-20, 0W-16, and 0W-8 grades¯being introduced.
In the automotive example, where journeys can be short and lubricants do not have much time in service to reach operating temperature, one can clearly see the benefit of a low-viscosity lubricant. While thicker oil can provide good protection of parts, the energy required to move mechanical parts through the cold oil results in increased energy losses from viscous churning. These lower viscosity oils provide effective equipment protection through rapid circulation, while reducing friction within the engine at low-temperature start-up, thereby delivering higher fuel economy.
Similarly, in industrial applications synthetics can improve operations in a gearbox operating at low temperatures. Consider the below trial:
After an overnight soak at different low temperatures, we compared the performance of mineral and synthetic gearbox oils at start up. As is evident in the graph, below, in this test the mineral oil required much more energy at start up than did the synthetic PAO gear oil, 35 percent more at -10°C (14°F). Beyond that temperature, the viscosity was ultimately too high to allow the gearbox to begin operations.
In general, synthetic lubricants typically have high viscosity indices (VI) which means the rate of change of viscosity with temperature is lower thus:
- Providing higher viscosities at high temperatures, which improves protection and reduces friction at boundary conditions
- Delivering low viscosities at lower temperatures, which carries all of the benefits previously mentioned.
Addressing Elastohydrodynamic Losses
Finally, synthetic lubricants offer benefits under elastohydrodynamic conditions, which occur when non-conforming surfaces are in contact, such as gears, cams and rolling element bearings. In such circumstances, small contact areas create very high pressure, which deforms the surfaces elastically to increase the contact area. Under such high pressures, the viscosity of the lubricant increases dramatically to almost a solid oil film, allowing the load to be supported by a thin film of oil.
As the surfaces move relative to each other, the oil film, which is in contact with both surfaces, must shear. The force required to shear the oil is indicated by the traction coefficient. As a general rule of thumb, the traction coefficient is lower for lubricants having more uniformly structured base oils. And, a low traction coefficient generally indicates that less energy will be used when shearing oil in the contact zone.
So, what does this mean for industrial applications?
Energy costs typically far outweigh the maintenance costs in most industrial rotating equipment, so any small improvement in energy efficiency can translate into major savings.
For example, a power station in South Africa ran a trial on 13 gearboxes where the company switched from a mineral to a PAO synthetic gear oil and saw energy consumption drop by 2.2 percent on average. With 288 gearboxes on site, the incentive to convert to a synthetic product is clear¯it will result in major cost savings**. Additionally, synthetics offer indirect cost savings benefits as a result of extended oil drain intervals, diminished wear on parts, fewer deposits and reduced required maintenance.
"Why Synthetics Work: Background on Energy Efficiency Benefits" by, Sandy Reid-Peters used with permission of the Mobil SHCTM Club (all rights reserved.)
*Holmberg, Kenneth, “Global Energy Consumption Due to Friction in Passenger Cars, Transportation and Industry”**Du Bois, Charles, “Reduce Friction to Save Energy,” The South African Mechanical Engineer, Vol. 61, September 2011