It seems counterintuitive that lubricants selected to optimize wear control may not be optimum when it comes to energy conservation. In fact, in view of today’s growing pressure to reduce demand on nonrenewable energy resources and increase operating profits, we are definitely facing a shift of emphases from past lubrication objectives. Energy-conserving lubrication offers motivation on several fronts. Consider the following:
1. When energy consumption is economized, equipment operating costs come down, translating to a boost in business profits, regardless of whether the energy source is renewable (hydro, solar, wind) or non-renewable (coal or petroleum). For many industries, the cost of energy far exceeds the cost of maintenance, machine repair and even downtime. A small percentage of reduction in energy consumption can translate into large returns.
2. Reduced demand on nonrenewable fossil fuels means cleaner air, reduced greenhouse gas emissions and a healthier environment (of growing political and social importance in view of the Kyoto Protocol on global warming, ISO 14001, Clear Air Act, etc.). When fuels don’t burn, there is no waste stream (smoke stack, tail pipe, etc.) and the risk of pollutants from emissions such as nitrogen oxides (the principle component of smog), sulfates, CO2 and unburned hydrocarbons is reduced proportionally.
3. With few exceptions, lubricants and lubrication methods that reduce energy consumption will also reduce heat and wear debris generation; however, the reverse may not hold true. When heat and wear debris are reduced, less stress is imposed on additives and the base oil. The result will be longer thermal and oxidative stability, and in turn, longer oil drains, lower oil consumption and the ancillary costs association with oil changes (as much as 40 times the cost of the lubricant itself!).
4. When lubricant consumption is reduced, so too is the disposal of environmentally polluting waste oil and certain suspended contaminants, some of which may be hazardous and toxic; ethylene glycol (antifreeze), for example.
5. When there is better economy in the consumption of both petroleum fuels and mineral-based lube oil, there is reduced dependence on foreign sources of crude oil, including those from politically unstable countries.
6. In certain countries, including European Union nations, reductions in the consumption of nonrenewable fuels can avert energy tax penalties such as the Climate Change Levy in the United Kingdom.
In recent years, I’ve seen a sharp increase in interest in energy-conserving lubricants and energy-conserving lubrication. Note, energy-conserving lubricants relate to formulation (basestocks and additives) and their selection for machine application. In contrast, energy-conserving lubrication includes the use and application of lubricants (change intervals, delivery methods, lube volume, etc.). Both can have a marked impact on energy conservation.
A closer look at these issues reveals that there is more to this than meets the eye. For one, energy economy and wear control do not necessarily go hand-in-hand. In many cases, they may be conflicting objectives. Why? For many organizations, environmental factors and energy costs fall low on the list of priorities compared to productivity and machine reliability. In such cases, the principle objective of the practice of lubrication is to reduce wear at the lowest possible cost. Add to this the misconception that lubricants capable of reducing wear must be equally capable of reducing friction and energy. Therefore, it seems timely to explore energy conservation as a principal lubrication objective more thoroughly.
Energy-Conserving Fluid Properties
When formulating or selecting lubricants, many have found a blurred line that distinguishes those properties that reduce wear from those properties that reduce energy consumption. The following properties are important in reducing friction and energy consumption:
When it comes to energy economy, viscosity can be both an inhibitor and an enabler. Recalling the well-known Stribeck curve, the oil film produced by hydrodynamic lubrication is directly influenced by viscosity. However, too much viscosity causes churning losses (internal oil friction) and heat production, especially in engines, gears, bearings and hydraulics. In addition to energy losses, this increased heat can more rapidly breakdown the oil and its additives. Viscosity Index Kinematic viscosity by itself defines an oil’s resistance only to flow and shear at a single temperature, typically 40ºC or 100ºC. However, in normal operation, lubricating oils transition through a wide range of temperatures. As such, it is the oil’s viscosity index (VI), combined with kinematic viscosity that defines what the viscosity will be at a specific operating temperature. Will it be too high when ambient start-up temperatures are low and too low when operating temperatures are high? Likewise, what will be the time-weighted average viscosity of the lubricating oil during the machine’s service life? It is this average viscosity that defines energy consumption, not the occasional temperature-based viscosity excursions that may have a greater impact on wear (cold starts for instance). In general, the significance of VI on energy conservation and wear is often sharply underestimated.
Fluids that exhibit shear-dependent viscosity changes (known as the non-Newtonian fluids) are known to reduce energy consumption in many machines. Good examples are VI-improved motor oils (multigrades) and many all-season hydraulic fluids. As fluid movement increases (shearing) during service, the oil’s effective viscosity self-regulates slightly downward, along with energy consumption. This, in part, explains why high-VI, multigrade motor oils are generally those that are designated energy-conserving by the American Petroleum Institute (API).
The role of pressure-viscosity (PV) coefficient on energy consumption is not well-defined in the literature. However, it is widely understood that many base oils exhibit a sharp increase in viscosity as pressure rises; a necessary quality of lubricants in achieving effective elastohydrodynamic lubrication (EHD). Some oils, such as mineral oils and PAOs, have higher PV coefficients than others, such as ester-based synthetics and water-based fluids. While high PV coefficients may be important at reducing contact fatigue wear, in some cases, this property may contribute to lower fuel economy. The high pressure-induced viscosity in sliding frictional zones and in hydraulic systems could result in exceedingly high viscous drag energy losses.
A fluid that is sponge-like and easily compressed has low bulk modulus. The more compressible a lubricant is, the more potential for lost energy and heat production. This is especially true in hydraulic and lube oil circulating systems.
Boundary Film-Strength Properties
Many lubricants and hydraulic fluids can gain considerable film strength under boundary and mixed-film lubrication from the base oil, without the need for additives. A phosphate ester synthetic is an example of a fluid with intrinsic lubricity. Most other lubricants rely on additives such as friction modifiers, antiwear agents, extreme pressure (antiscuff), solid lubricants and fatty acids. The effectiveness of these additives at reducing wear, friction and energy consumption can vary considerably between the different additive types employed. The performance of these additives also varies by machine and application (load, speed, metallurgy, temperature and contact geometry).
The consistency of grease can have an impact on energy consumption in ways similar to viscosity. The energy needed to move grease in frictional zones and in adjacent cavities by moving machine elements is affected by its consistency and shear rate (grease is non-Newtonian). So too, energy is required in some applications to pump grease to bearings and gears. Pumping energy losses is influenced, in part, by grease consistency and thickener type.
A grease that has good channeling characteristics helps keep the bulk lubricant away from moving elements, avoiding excessive churning and drag losses. Poor channeling characteristics may lead to increased energy consumption, heat production and base oil oxidation.
Role of Lubrication Practices
While lubricant formulation and selection are important, energy conservation is also influenced by machine design and lubricant application factors. A superior lubricant cannot offer redemptive relief for poor lubrication practices and/or machine design. Even the very best lubricants cannot protect against destruction caused by dirt and water contamination. One study found that particle contamination can increase fluid temperature by as much as eight degrees Celsius (due to increased friction). Increased cleanliness of crankcase oils has been found to reduce fuel consumption in diesel engines by one to four percent or more. Overgreasing bearings is known to increase frictional losses and raise bearing temperature. The same is true for bearings that are underlubricated. For bath lubricated bearings and splash lubricated gears, a change in oil level by as little as one-half inch (1.3 cm) can increase temperature by more than 10 degrees Celsius. This, of course, translates to greater energy consumption, shorter oil life and increased wear. Excessively aerated oils due to worn seals and wrong oil levels can have similar effects (loss of bulk modulus). There have also been studies showing the negative effects of overextended oil change interval on fuel economy in diesel engines. Additionally, overextended filter changes cause excessive flow resistance and fluid bypass. Both can often be corrected by the frequent and proper use of oil analysis in selecting the optimum oil and filter change interval, tailored to equipment type and its application.
Role of Machine Design
A machine’s design and the quality of its manufacture can also impact energy economy. Together with operating load and speed, machine design influences the type of lubricant that must be employed for wear protection and energy efficiency. I’ve mentioned the importance of viscosity films produced by hydrodynamic and elastohydrodynamic lubrication as well as boundary film strength from additives and polar base oil chemistry. These lubrication regimes relate to the contact dynamics associated with a machine’s design and operating conditions. Additionally, specific film thickness, also known as lambda, brings into the picture the influence of surface roughness and shaft alignment.
Many users and suppliers have reported energy savings from total-loss lubricant delivery technologies such as oil mist and centralized lubrication systems. The amount of fluid that a machine uses to lubricate frictional surfaces at any moment is extremely small compared to the amount of fluid some machines must keep in continuous motion. The advantage of some total loss lubrication systems is that there is minimal loss of energy from constant fluid churning and flow resistance of lubricants moving through lines. An example of internal fluid friction is observed when an oil is placed in a bottle and then shaken. The oil’s temperature will rise.
In addition, bath, splash and recirculating lubrication systems use the same oil over and over. As we all know, this reused oil over time can become impaired by loss of additives, base oil oxidation and rising concentrations of contaminants. In contrast, when well- engineered and in the right application, oil mist and other certain total loss systems can provide a continuous supply of fresh, clean and dry new oil. Energy consumption is also influenced by the size and type of fittings, oil lines and filters.
Wear Control vs. Energy Economy
Conflicting objectives may be created when lubricant selection and lubrication practices emphasize only wear control. Referring to the fluid properties listed above that influence energy consumption, many relate to drag losses from constant pushing and pulling of the lubricant around in the machine. This fluid friction occurs inside a machine’s wear zones but outside as well, such as fluid supply lines, grease cavities, oil ways and filters.
There are many lubricant selection scenarios in which wear is reduced at the expense of greater energy consumption. This might occur when viscosity selected is too high. To the other extreme, exceedingly low viscosity can periodically bring surfaces into boundary conditions (mechanical rubbing) and sharply increased sensitivity to particle contamination.
The thinner the oil film, the more risk of abrasive wear from silt-size particles that grow in population in many poorly filtered lubricants and hydraulic fluids. There is always a disproportionately higher population of small particles in lubricants than large particles.
How to Resolve the Dilemma
Lubricant suppliers are increasingly pitching the importance of energy conservation when selecting lubricants. Lubricant users are also seeing increased corporate pressures to keep costs down and profits high. For many, this temptation has led them to take the plunge into energy-conserving lubrication. However, with change comes risk.
Fortunately, there may be ways to reduce this risk. Begin with machines where the opportunity for reduced energy consumption is the greatest. Consider incremental changes in viscosity and VI for those lubricants that don’t have film-strength enhancing AW and EP additives. Typically, viscosity is reduced and VI is increased in this strategy; however in some cases, improvements are in the direction of viscosity increase. Take small steps, for example, in half-grade increments in ISO viscosity grades at a time (this is achieved by onsite blending). Synthetic base oils, or paraffinic mineral oils with VI improvers, can provide the enhanced VI. For gear oils, compressor lubes, hydraulic fluids and other lubricants with AW or EP additives, a combination of viscosity, VI and additive technology changes might be prescribed.
Reducing viscosity may necessitate improving filtration; for example, from 12 microns to 6 or 3 microns. This will counteract the increased sensitivity of the machine to smaller particles as oil films contract slightly in response to lower viscosity. With reduced viscosity, finer filtration is often easier to achieve with less impact on pressure drop. Also, design a monitoring plan to assess positive and negative consequences of changes in lubricant selection and lubrication practices. Once these operating conditions have been baselined, representing the conditions before the change, monitor these same conditions after the lubricant/lubrication change.
For stationary equipment, consider monitoring bearing metal temperature, thermal emissions (thermography), oil temperature, acoustic emissions, vibration, motor current and wear metal production rate. If electric motor amperage decreases along with bearing metal and lubricant temperature, an improvement in energy consumption has likely been achieved without a negative tradeoff. This should be confirmed by looking at any changes in wear metal production rate.
The value gained by optimizing lubrication to achieve wear control and energy economy can translate to huge savings for many organizations, which can become a substantial prize for the whole team. However, stepping back and looking at the big picture - our planet earth and the future generations that will inhabit it - is there anything more important than protecting our environment? Let’s be environmental stewards and go for the grand prize!
Lubrication and Energy Consumption: Is Independent Testing Needed? Energy conservation is a relatively new concept in American culture and industry. Not until the oil crunch of the 1970s did the American public and government and business institutions seriously recognize the limited availability of nonrenewable energy resources. Most industrial facilities in the United States were designed and built long before that realization, and are therefore not necessarily designed with energy efficiency in mind. Initiatives to increase operating profits in the face of diminishing margins by reducing operating costs, such as maintenance and energy consumption, have attempted to mitigate original design deficiencies through capital-intensive engineered solutions.
Why is this important? Energy prices continue to rise. Even a relatively small oil refinery, for example, may spend $30 million or more annually on energy, the vast majority of which is needed to power machinery. In many cases, the amount spent on energy exceeds the cost for labor! For example, a large centrifugal compressor installation that is driven by a 6,000-hp motor costs as much as $200,000 annually to power. All of this machinery is lubricated to reduce wear and friction. Yet while wear is monitored and minimized in the name of reliability and productivity, friction management (surface and fluid internal) for the purpose of energy conservation has not received as much attention - until recently.
Marketing materials from lubricant suppliers often directly or indirectly refer to energy consumption reductions achievable through lubricant application. How much of this information is founded in real science and how much is based upon weakly supported testimonials and an unproven hypothesis? How transferable are results from one test situation to another? By what standards are such tests performed? Are the methodologies appropriate and the results statistically significant? What are the relevant variables? How will wear be impacted? What efficiency gains are achievable in real installations? Which products best deliver these benefits? These are all valid questions that have not been answered to my own satisfaction. It is difficult to know what to believe, especially when the consumer is inundated with conflicting claims from a variety of sources, all seemingly credible and plausible on the surface.
Despite the absence of resolute empirical verification, there are substantial returns on minimal investment available to the practitioner who effectively balances efficiency and reliability priorities. A very modest one-percent reduction in a $30 million energy budget translates to $300,000 of cost savings that contribute directly to the bottom line. This exceeds the total maintenance budgets of many facilities! If achieved through optimized lubricant specification, this benefit could be realized without any appreciable investment of capital - a priority for most managers.