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Are Synthetics Always a Wise Choice?
Mark Barnes, Noria Corporation
In the Jan-Feb issue of Machinery Lubrication magazine, we published an article entitled "Choosing Between Synthetic Lubricants and Conventional Oils." In this article, the author did an excellent job explaining some of the potential benefits of using synthetics lubricants. But are synthetic lubricants always a wise choice?
To answer this question, we must take a critical look not just at the benefits of synthetic lubricants, but also the potential pitfalls. For example, synthetic lubricants are typically made by taking small building-block molecules and joining them together, a process referred to as polymerization.
Additional Cost
Because of the amount of work required to convert these starting ingredients into finished base oils, synthetic oils are generally more expensive than conventional mineral oils, from three to five times more for common synthetics such as polyalphaolefins (PAO) to several hundred times for highly specialized fluids such as fluorinated polymers that are used in applications requiring inherent chemical inertness.
But can the additional cost for synthetic oils be justified? The answer to this question really needs to be "it depends." For example, imagine being able to extend the oil drain interval using a high-quality PAO synthetic-based oil by a factor of six or seven simply by purchasing a product that is five times the cost.
From this perspective, it would appear that the additional cost is money well spent, particularly when you consider the additional costs associated with an oil change. But if the oil change interval is driven not by base oil degradation, but rather due to additive depletion or the buildup of certain contaminants such as water, particles or soot, then perhaps the cost of a synthetic-based oil cannot be justified.
Solvency
Cost is not the only factor to consider. For example, there's the question of solvency. One of the main benefits of petroleum-based synthetic oils compared to their mineral oil cousins is the elimination of certain "undesirable" classes of molecules. Key among these is aromatics that contribute to a lower resistance to oxidation. But solvency can also be a good thing.
For example, there have been numerous studies showing that the poor solvency of certain types of petroleum-based synthetics (and for that matter, highly refined mineral oils) can result in a greater tendency to lay down varnish in high-temperature applications such as gas turbines because the oil does not have the solvency to keep the oil-wetted components clean.
Under these circumstances, the problem may be resolved by using oils with a higher natural solvency, perhaps a less highly refined mineral oil, or a nonhydrocarbon-based synthetic such as certain esters, or polyalkylene glycol (PAG) fluids which have excellent natural solvency.
Solvency also plays a role in lubricant formulation. With highly pure hydrocarbon synthetics or refined mineral oils, getting additives to dissolve in the lubricant can be a major challenge. Under these conditions, a less highly refined base stock, or a co-base stock that has greater additive solubility characteristics can be a distinct advantage.
Greater shear stability and reduced energy consumption are also often stated as advantages to synthetic lubricants. But while there is definitely truth to this statement, you must ask yourself what is causing excessive energy consumption. For example, in a worm drive application where significant energy losses can be attributed to sliding friction, the more uniform size of the molecules within synthetics lubricant can potentially allow for reduced energy consumption based on lower internal fluid friction.
But if the energy loss is not due to fluid friction but some other factor such as poor mechanical maintenance or the wrong oil level, even a pure, high-quality synthetic oil may not make a significant difference, particularly in applications such as spur gears, which are already 95 percent (or more) efficient at power transmission.
Relation to Water
Then there's the question of water. Some synthetics such as PAOs are distinctly hydrophobic - they resist water. Others, however, such as PAG and esters, not only attract water (hydrophilic), but in some instances react with water, which can lead to their downfall. Therefore, compatibility with other fluids and sealing materials should always be a concern.
So what's the answer, are synthetics really better? Instead of trying to answer this question directly, consider each lubrication point as requiring a lubricant that has a defined series of physical (viscosity, viscosity index, etc.) and chemical (oxidation resistance, active wear protection, etc.) performance properties.
These need to be decided based on machine type and application, as well as other factors such as ambient environment and accessibility for oil changes. Once this has been completed, the last step is to chose the product - whether that product be mineral or synthetic - that meets each performance criteria.
Once you take this pragmatic approach, the choice between mineral and synthetic becomes clear. In some cases, the only way to achieve the necessary performance properties will be to choose the appropriate synthetic oil. But in other cases, an acceptable level of performance can be achieved by selecting a good-quality, well-formulated mineral-based lubricant, avoiding some of the pitfalls outlined above.
Please reference this article as:
Mark Barnes, Noria Corporation, "Are Synthetics Always a Wise Choice?". Machinery Lubrication Magazine. March 2008
Mark Barnes, Noria Corporation
In the Jan-Feb issue of Machinery Lubrication magazine, we published an article entitled "Choosing Between Synthetic Lubricants and Conventional Oils." In this article, the author did an excellent job explaining some of the potential benefits of using synthetics lubricants. But are synthetic lubricants always a wise choice?
To answer this question, we must take a critical look not just at the benefits of synthetic lubricants, but also the potential pitfalls. For example, synthetic lubricants are typically made by taking small building-block molecules and joining them together, a process referred to as polymerization.
Additional Cost
Because of the amount of work required to convert these starting ingredients into finished base oils, synthetic oils are generally more expensive than conventional mineral oils, from three to five times more for common synthetics such as polyalphaolefins (PAO) to several hundred times for highly specialized fluids such as fluorinated polymers that are used in applications requiring inherent chemical inertness.
But can the additional cost for synthetic oils be justified? The answer to this question really needs to be "it depends." For example, imagine being able to extend the oil drain interval using a high-quality PAO synthetic-based oil by a factor of six or seven simply by purchasing a product that is five times the cost.
From this perspective, it would appear that the additional cost is money well spent, particularly when you consider the additional costs associated with an oil change. But if the oil change interval is driven not by base oil degradation, but rather due to additive depletion or the buildup of certain contaminants such as water, particles or soot, then perhaps the cost of a synthetic-based oil cannot be justified.
Solvency
Cost is not the only factor to consider. For example, there's the question of solvency. One of the main benefits of petroleum-based synthetic oils compared to their mineral oil cousins is the elimination of certain "undesirable" classes of molecules. Key among these is aromatics that contribute to a lower resistance to oxidation. But solvency can also be a good thing.
For example, there have been numerous studies showing that the poor solvency of certain types of petroleum-based synthetics (and for that matter, highly refined mineral oils) can result in a greater tendency to lay down varnish in high-temperature applications such as gas turbines because the oil does not have the solvency to keep the oil-wetted components clean.
Under these circumstances, the problem may be resolved by using oils with a higher natural solvency, perhaps a less highly refined mineral oil, or a nonhydrocarbon-based synthetic such as certain esters, or polyalkylene glycol (PAG) fluids which have excellent natural solvency.
Solvency also plays a role in lubricant formulation. With highly pure hydrocarbon synthetics or refined mineral oils, getting additives to dissolve in the lubricant can be a major challenge. Under these conditions, a less highly refined base stock, or a co-base stock that has greater additive solubility characteristics can be a distinct advantage.
Greater shear stability and reduced energy consumption are also often stated as advantages to synthetic lubricants. But while there is definitely truth to this statement, you must ask yourself what is causing excessive energy consumption. For example, in a worm drive application where significant energy losses can be attributed to sliding friction, the more uniform size of the molecules within synthetics lubricant can potentially allow for reduced energy consumption based on lower internal fluid friction.
But if the energy loss is not due to fluid friction but some other factor such as poor mechanical maintenance or the wrong oil level, even a pure, high-quality synthetic oil may not make a significant difference, particularly in applications such as spur gears, which are already 95 percent (or more) efficient at power transmission.
Relation to Water
Then there's the question of water. Some synthetics such as PAOs are distinctly hydrophobic - they resist water. Others, however, such as PAG and esters, not only attract water (hydrophilic), but in some instances react with water, which can lead to their downfall. Therefore, compatibility with other fluids and sealing materials should always be a concern.
So what's the answer, are synthetics really better? Instead of trying to answer this question directly, consider each lubrication point as requiring a lubricant that has a defined series of physical (viscosity, viscosity index, etc.) and chemical (oxidation resistance, active wear protection, etc.) performance properties.
These need to be decided based on machine type and application, as well as other factors such as ambient environment and accessibility for oil changes. Once this has been completed, the last step is to chose the product - whether that product be mineral or synthetic - that meets each performance criteria.
Once you take this pragmatic approach, the choice between mineral and synthetic becomes clear. In some cases, the only way to achieve the necessary performance properties will be to choose the appropriate synthetic oil. But in other cases, an acceptable level of performance can be achieved by selecting a good-quality, well-formulated mineral-based lubricant, avoiding some of the pitfalls outlined above.
Please reference this article as:
Mark Barnes, Noria Corporation, "Are Synthetics Always a Wise Choice?". Machinery Lubrication Magazine. March 2008
Quote
Choosing Between Synthetic Lubricants and Conventional Oils
Synthetic lubricants continue to gain market share, thanks to higher performance properties that, for many uses, trump higher per-drum costs. Demand in the United States has grown to $2.2 billion per year, and is a resulting benefit of stricter environmental and worker safety requirements. Virtually all customers revisit the debate of "mineral oil vs. synthetic lubricants" on a regular basis. In certain situations, it is part of an overall demand planning exercise; other times, it's simply to assure the facility is receiving the best life cycle value. Acculube encourages this process, and assists customers in completing the math based on their specific situation, therefore finding the best program to fit their needs. Processes and products change, and the price volatility of crude at the producer level substantially impacts the equation; therefore a fresh look, at least annually, is worth the effort.
---------------------
Mineral Oils
Many factors differentiate mineral oils from synthetic lubricants including what they can accomplish, their requirements for efficient functionality, and composition.
Naturally occurring crude is a cocktail of hydrocarbons. Even after aggressive solvent-based refining, thousands of hydrocarbon compounds - as well as organic compounds of oxygen, sulfur and nitrogen - remain. These three compounds in particular are problematic because they enable oxidation and acid development, as well as facilitate the formation of sludge, particularly in high-temperature applications.
The varying molecules of refined lubricants also have differing shapes, resulting in irregular lubricant surfaces at the molecular level. These irregularities generate friction within the fluid itself which increases power requirements and reduces efficiency.
Synthetics
In contrast, synthetic lubricants are engineered products created by chemical reactions through the precise application of pressure and temperature to a specific recipe of components. All of the components are high in purity with strong molecular bonds. As a result, the end product is a pure compound, less vulnerable to oxidation, highly resistant to breakdown, and uniform in molecular size. This molecular size uniformity keeps synthetics from jellifying when it's cold (they do not contain waxes), and its specific molecular structure keeps it from thinning-out under heat; therefore, the lubricant's protective characteristics are more predictable. The saturated molecules created from the synthetic process are also nonhydrophilic and won't emulsify or produce undesirable by-products in high-humidity environments.
Traction Coefficient
Molecular size is also key to one of the synthetic lubricants' operational virtues - its traction coefficient or internal fluid friction (resistance). Traction coefficient is the shearing or tangential force required to move a load, divided by the load. The coefficient number expresses the ease with which the lubricant film is sheared.
Compared to mineral oil molecules, synthetic lubricants, for example, have up to a 30 percent advantage over mineral oils for traction coefficient. This means the force needed to move a load is less, which means less horsepower to do the work.
In a gear reducer, the lubricant in the tooth mesh is sheared, and the lower the traction coefficient, the lower the energy dissipated due to lubricant shearing. The difference is realized by low amperage draw on the motor and reduced lubricant /gear temperature.
Changing to a low-traction synthetic will reduce power consumption in a spur/helical gear by 0.5 percent for each reduction, and up to 8 percent for high-reduction worm gears.
Gear Wear
The issue of gear wear is also a consideration. A study cited in Machinery Lubrication magazine1 implied synthetic lubricants make gears more efficient than mineral oils. A polyglycol showed the highest efficiency (18 percent more than the high-performing mineral oil). Synthetic hydrocarbon (SHC) gear oil also increased the efficiency of the best gears by eight to nine percent. The performance of synthetic lubricants in food-grade applications in accordance with USDA-H1 food contact is also a benefit. Food-grade synthetics are sometimes believed to be inferior in performance to mineral oil lubes, a belief the study dispels.
Service Life
A popular topic concerning the difference between mineral oils and synthetic lubricants is service life. Synthetic lubricants as a class don't show their age, particularly at high temperatures, and have a longer service life. Often, the change interval is several times longer for synthetics at identical operating temperatures; however, the exact number depends on operating conditions, the additives and the specific synthetic used.
Synthetic lubricants have a lower friction coefficient in a gearbox, better film strength and a better relationship between viscosity and temperature (viscosity index, VI). This indicates synthetic lubricants can be used at lower viscosity grades and lower temperatures. When this is the case, the gap between the service lives of minerals and synthetics significantly increases.
Related to the oil change interval is the issue of product loss through evaporation and disposal. Both sludge and residue form more readily with mineral oil products. Evaporative losses are lower for synthetics due to the lack of lighter hydrocarbon structures. Disposal is more costly with some synthetics, but it is nowhere near enough to compensate for change-out intervals that are three to five times more frequent.
Safety
In regard to safety and insurance risks, the flash point for synthetics as a class is always higher, and reduced flammability is a key driver for synthetics' growing popularity in high-temperature applications.
Disadvantages
Synthetics, like most other lubricants, can have disadvantages. Material compatibility issues can occur with certain seals, metals, paints, coatings and plastics. Many ester-type synthetics do not perform well in the presence of water and can decompose or break down (hydrolysis). They also can cost more on a per-drum basis, though not necessarily on a life-cycle basis.
Summary
Synthetics are clearly superior in the extreme zone where temperatures, high loads or flammability are overriding factors. They also perform well in applications where needs are specific and complex. Synthetics are engineered to meet targeted performance benchmarks, and a synthetic formula can be (and probably has been) engineered for almost every combination of properties used in industry.
Reference
1. Dennis Lauer. "Synthetic Gear Oil Selection." Machinery Lubrication magazine, May-June 2001.
Please reference this article as:
, "Choosing Between Synthetic Lubricants and Conventional Oils". Machinery Lubrication Magazine. January 2008
Synthetic lubricants continue to gain market share, thanks to higher performance properties that, for many uses, trump higher per-drum costs. Demand in the United States has grown to $2.2 billion per year, and is a resulting benefit of stricter environmental and worker safety requirements. Virtually all customers revisit the debate of "mineral oil vs. synthetic lubricants" on a regular basis. In certain situations, it is part of an overall demand planning exercise; other times, it's simply to assure the facility is receiving the best life cycle value. Acculube encourages this process, and assists customers in completing the math based on their specific situation, therefore finding the best program to fit their needs. Processes and products change, and the price volatility of crude at the producer level substantially impacts the equation; therefore a fresh look, at least annually, is worth the effort.
---------------------
Mineral Oils
Many factors differentiate mineral oils from synthetic lubricants including what they can accomplish, their requirements for efficient functionality, and composition.
Naturally occurring crude is a cocktail of hydrocarbons. Even after aggressive solvent-based refining, thousands of hydrocarbon compounds - as well as organic compounds of oxygen, sulfur and nitrogen - remain. These three compounds in particular are problematic because they enable oxidation and acid development, as well as facilitate the formation of sludge, particularly in high-temperature applications.
The varying molecules of refined lubricants also have differing shapes, resulting in irregular lubricant surfaces at the molecular level. These irregularities generate friction within the fluid itself which increases power requirements and reduces efficiency.
Synthetics
In contrast, synthetic lubricants are engineered products created by chemical reactions through the precise application of pressure and temperature to a specific recipe of components. All of the components are high in purity with strong molecular bonds. As a result, the end product is a pure compound, less vulnerable to oxidation, highly resistant to breakdown, and uniform in molecular size. This molecular size uniformity keeps synthetics from jellifying when it's cold (they do not contain waxes), and its specific molecular structure keeps it from thinning-out under heat; therefore, the lubricant's protective characteristics are more predictable. The saturated molecules created from the synthetic process are also nonhydrophilic and won't emulsify or produce undesirable by-products in high-humidity environments.
Traction Coefficient
Molecular size is also key to one of the synthetic lubricants' operational virtues - its traction coefficient or internal fluid friction (resistance). Traction coefficient is the shearing or tangential force required to move a load, divided by the load. The coefficient number expresses the ease with which the lubricant film is sheared.
Compared to mineral oil molecules, synthetic lubricants, for example, have up to a 30 percent advantage over mineral oils for traction coefficient. This means the force needed to move a load is less, which means less horsepower to do the work.
In a gear reducer, the lubricant in the tooth mesh is sheared, and the lower the traction coefficient, the lower the energy dissipated due to lubricant shearing. The difference is realized by low amperage draw on the motor and reduced lubricant /gear temperature.
Changing to a low-traction synthetic will reduce power consumption in a spur/helical gear by 0.5 percent for each reduction, and up to 8 percent for high-reduction worm gears.
Gear Wear
The issue of gear wear is also a consideration. A study cited in Machinery Lubrication magazine1 implied synthetic lubricants make gears more efficient than mineral oils. A polyglycol showed the highest efficiency (18 percent more than the high-performing mineral oil). Synthetic hydrocarbon (SHC) gear oil also increased the efficiency of the best gears by eight to nine percent. The performance of synthetic lubricants in food-grade applications in accordance with USDA-H1 food contact is also a benefit. Food-grade synthetics are sometimes believed to be inferior in performance to mineral oil lubes, a belief the study dispels.
Service Life
A popular topic concerning the difference between mineral oils and synthetic lubricants is service life. Synthetic lubricants as a class don't show their age, particularly at high temperatures, and have a longer service life. Often, the change interval is several times longer for synthetics at identical operating temperatures; however, the exact number depends on operating conditions, the additives and the specific synthetic used.
Synthetic lubricants have a lower friction coefficient in a gearbox, better film strength and a better relationship between viscosity and temperature (viscosity index, VI). This indicates synthetic lubricants can be used at lower viscosity grades and lower temperatures. When this is the case, the gap between the service lives of minerals and synthetics significantly increases.
Related to the oil change interval is the issue of product loss through evaporation and disposal. Both sludge and residue form more readily with mineral oil products. Evaporative losses are lower for synthetics due to the lack of lighter hydrocarbon structures. Disposal is more costly with some synthetics, but it is nowhere near enough to compensate for change-out intervals that are three to five times more frequent.
Safety
In regard to safety and insurance risks, the flash point for synthetics as a class is always higher, and reduced flammability is a key driver for synthetics' growing popularity in high-temperature applications.
Disadvantages
Synthetics, like most other lubricants, can have disadvantages. Material compatibility issues can occur with certain seals, metals, paints, coatings and plastics. Many ester-type synthetics do not perform well in the presence of water and can decompose or break down (hydrolysis). They also can cost more on a per-drum basis, though not necessarily on a life-cycle basis.
Summary
Synthetics are clearly superior in the extreme zone where temperatures, high loads or flammability are overriding factors. They also perform well in applications where needs are specific and complex. Synthetics are engineered to meet targeted performance benchmarks, and a synthetic formula can be (and probably has been) engineered for almost every combination of properties used in industry.
Reference
1. Dennis Lauer. "Synthetic Gear Oil Selection." Machinery Lubrication magazine, May-June 2001.
Please reference this article as:
, "Choosing Between Synthetic Lubricants and Conventional Oils". Machinery Lubrication Magazine. January 2008
Quote
Synthetic Gear Oil Selection
Dennis Lauer
Synthetic gear oils are used whenever mineral gear oils have reached their performance limit and can no longer meet the application requirements; for example, at very low or high temperatures, extremely high loads, extraordinary ambient conditions, or if they fail to meet special requirements such as flammability. Even though additives can improve many properties of mineral oils, it is not possible to exert an unlimited influence on all their properties. This applies especially to physical properties like the following:
• thermal resistance
• low temperature properties (fluidity, pour point)
• flash point
• evaporation losses
Synthetic oils provide a number of advantages. However, they do not necessarily out-perform mineral oils in all respects and may even result in some drawbacks despite their advantages. The advantages of synthetic lubricating oils (depending on the base oil) include:
• improved thermal and oxidation resistance
• improved viscosity-temperature behavior, high viscosity index (in most cases)
• improved low temperature properties
• lower evaporation losses
• reduced flammability (in some cases)
• improved lubricity (in some cases)
• lower tendency to form residues
• improved resistance to ambient media
Possible disadvantages include:
• higher price
• reactions in the presence of water (hydrolysis, corrosion)
• material compatibility problems (paints, elastomers, certain metals)
• limited miscibility with mineral oils
Application-related advantages often prevail, increasing the use of synthetic lubricants as gear lubricants, especially under critical operating conditions. The most common synthetic types used include synthetic hydrocarbon oils (SHC), polyglycols (PAG) and ester oils (E).
Lubricating Oils Based On Synthetic Hydrocarbon Oils
Synthetic hydrocarbons are similar to mineral hydrocarbons in their chemical structure. They have nearly identical properties relating to their compatibility with sealing materials, disposal, reprocessing and miscibility with mineral oils. The main advantage is their excellent low temperature behavior. It is possible to manufacture food-grade lubricants for the food processing and pharmaceutical industries with SHC base oils using special additives.
Lubricating Oils Based On Polyglycols
These lubricants ensure especially low friction coefficients, which makes them suitable for gears with a high sliding percentage (worm and hypoid gears). With the appropriate additives, they provide excellent antiwear protection in steel/bronze worm gears, and have a good extreme pressure performance. In gear systems, higher polarity polyglycols allow greater interaction on the metal gear surface. This gives polyglycols mild extreme pressure performance even without additives.
Polyglycol oils may have a negative impact on sealing materials and may dissolve some paints. At operating temperatures above 212°F (100°C), only seals made of fluorinated rubber or PTFE are resistant. Before using PAG oils in production applications, it is advisable to test compatibility with paints, seals and sight glass materials.
Miscibility with mineral oils is limited; mixtures should therefore be avoided. Polyglycols are neutral toward ferrous metals and almost all nonferrous metals. If the application has a loaded contact with one component consisting of aluminum or aluminum alloys (rolling bearing cages containing aluminum), there may be increased wear under dynamic load (sliding movement and high load). In such cases, compatibility tests should be conducted. If a worm gear is made of an aluminum bronze alloy, polyglycols should not be used because the reaction in the load zone could result in increased wear.
Lubricating Oils Based On Ester Oils
Ester oils are the result of a reaction of acids and alcohols with water splitting off. There are many types of esters, all of them having an impact on the chemical and physical properties of lubricants. In the past, these lubricating oils were mainly used in aviation technology for the lubrication of aircraft engines and gas turbines as well as gear systems in pumps, starters, etc.
Ester oils have a high thermal resistance and excellent low temperature behavior. In industrial applications, rapidly biodegradable ester oils will gain importance because it seems possible to achieve the same efficiency as with polyglycol oils by selecting an appropriate ester base oil.
Certain ester oils may exhibit low hydrolytic stability. Hydrolysis is the cleavage of the ester into an alcohol and an acid in the presence of water. Ester lubricants need to be hydrolytically stable because they are often exposed to humidity in use. In practice, hydrolysis may be a less serious problem than commonly reported. The hydrolytic stability of an ester depends on:
• the type of ester used
• the type of additives used
• how the ester was processed
• the application
Application-Related Advantages of Synthetic Lubricating Oils
The following application-related advantages result from the improved properties of synthetic lubricating oils as compared to mineral oils:
• improved efficiency due to reduced tooth-related friction losses
• lower gearing losses due to reduced friction, requiring less energy
• oil change intervals three to five times longer than mineral oils operating at the same temperature
• reduced operating temperatures under full load, increasing component life; cooling systems may not be required
Reduction of Gearing Losses and Efficiency Improvement
Because of their special molecular structure, synthetic lubricating oils based on poly-alphaolefins (a type of SHC) and polyglycols ensure that tooth-related friction is considerably lower than with mineral oils. It may be up to 30 percent lower than if a regular mineral gear oil with EP additives was used. Because the friction coefficient of synthetic oils is lower, tooth-related friction is reduced, thus increasing the gear’s efficiency.
The efficiency of gears with a high sliding percentage, worm and hypoid gears, for instance, may increase up to 15 percent if a synthetic oil is used instead of a mineral oil. Even in the case of spur, helical and bevel gears (which have a naturally high gear efficiency), it is possible to increase gear efficiency of up to one percent by using a synthetic gear oil. This may not seem like much at first, but it may result in considerable cost savings depending on the nominal output of the gear unit, especially in the case where several gears are deployed.
Table 1. Potential Reduction of Gearing Losses and Improvement of
Efficiency if Using a Synthetic Gear Oil Instead of a Mineral Oil.
Type of Gear
Effect Worm Gears, Hypoid Gears Spur and Helical Gears, Bevel Gears with Axis Not Offset
Reduction of Total Losses 30% and more 20% and more
Improved Efficiency 15% and more up to 1%
Reduction of Operating (Steady-State) Temperature 68°F (20°C) and more up to 54°F (12°C)
Table 1 shows the extent synthetic oils can reduce gear losses, especially in gear systems with a high degree of load-dependent losses.
Advantages of Synthetic Gear Oils Based on Reduced Friction
Increased Gear Efficiency
• Smaller gears with smaller motors can provide the same power output
• Higher power output can be achieved with the same power input
Reduced Oil Temperatures
• Extension of the oxidative life (five times longer than mineral oils in some cases)
• Extended component life (where reduced wear and friction is achieved)
• Cooling systems may no longer be required
Reduced Energy Consumption
• Reduced costs for electric current or fuel consumption resulting from lower total energy losses in the gearbox; 30 percent and more for worm gears
• Costs for electric power have been reported as high as 10 percent
Improved Efficiency and Reduced Wear When Using Synthetic Oils
Tests show synthetic oils make gears more efficient than mineral oils. A polyglycol oil in the study resulted in the highest degree of efficiency: 18 percent more than the high performance mineral gear oil. SHC gear oil also made the test gears eight to nine percent more efficient. Its performance as a food-grade lubricant in accordance with USDA-H1 is also an excellent added advantage. Food-grade lubricants are often thought to be inferior to normal lubricants, an opinion which this study seems to disprove.
Synthetic base oils have excellent wear protection behavior, which is enhanced by appropriate antiwear additives. Wear is particularly low when the PAG gear oil is used.
Extended Oil Change Intervals Using Synthetic Oils
Synthetic oils have better resistance to aging and high temperatures and a longer service life than mineral oils. Depending on the base oil (SHC or PAG), the oil change intervals may be three to five times longer at the same operating temperature.
Approximate oil change intervals of gear oils at an operating temperature of 176°F (80°C) are:
• Mineral oil: 5,000 operating hours
• SHC oil: 15,000 operating hours (extension factor 3)
• PAG oil: 25,000 operating hours (extension factor 5)
Synthetic oils have a lower friction coefficient than mineral oils in a gearbox and a more favorable viscosity-temperature relationship. This generally permits the use of synthetics at lower viscosity grades and also offers the possibility of reduced oil temperature during operation. In such cases, the life extension factors for oil change intervals of synthetic oils are longer than the values stated above, which refer to identical oil temperature. The following comparison of test results illustrates this advantage. Three lubricants were tested in a splash lubricated worm gear test rig.
The test records show the following oil sump temperatures after 300 operating hours:
Mineral oil: 230°F (110°C)
SHC: 194°F (90°C)
PAG: 167°F (75°C)
The life extension factors of synthetic oils as compared to mineral oil are as follows:
Mineral oil = 1
SHC = 9.5 times longer
PAG = 31 times longer
Synthetic Oils Help Save Maintenance and Disposal Costs
As compared to mineral oils, the oil change intervals of synthetic oils may be five times longer under the same thermal conditions. Despite the fact that the purchase price and the costs of disposal of synthetics are higher than that of a mineral oil, the extended oil change intervals can offset these costs when taking into account the gear unit’s extended service life. A comparison of the costs for mineral and synthetic offers real opportunity to reduce maintenance cost and improve machine reliability. There is also the advantage of reduced environmental impacts with lower lubricant disposal rates.
Selecting the Gear Oil Type
In order to select which type of gear oil to use in a gearbox, you must understand the gearbox’s mode of operation. The application factor (KA) identifies the type and magnitude of load the gears will experience.
Viscosity
Viscosity is the most important physical property of a lubricating oil. Because the viscosity changes with temperature, the rate of change is an important property identified by the Viscosity Index (VI). Most mineral-based gear oils will have a VI of 95. A lower VI indicates that the oil’s viscosity changes to a greater extent with change in temperature. Conversely, a higher viscosity index indicates a much lower rate of change in viscosity with respect to change in temperatures. The advantage of a high VI is that in lower temperatures, the oil will tend not to increase viscosity as much as a lower VI product. The ability of an oil to maintain a small viscosity differential over the operating range of the gearbox provides a more consistent lubricating film to the gears and more predictable wear performance.
Viscosity Selection
As stated earlier, the correct viscosity is an important parameter in proper selection of a gear oil. The manufacturer of the gearbox or gear system generally offers a viscosity recommendation and these recommendations should be followed in most cases. If the OEM of the gear unit has not provided a recommendation and the viscosity has not been calculated based on lubrication theory, it can be selected in accordance with various worksheets and graphs. The differing viscosity-temperature and viscosity-pressure behavior of synthetics as compared to mineral oils should also be taken into account.
The correct viscosity must be selected independently of any specific gear stage, realizing that a compromise is required for multistage gears. The selection of the correct viscosity is based on the oil’s expected operating temperature, such as sump temperature or the temperature of the injected oil. This temperature is calculated by determining the gear’s thermal economy, taking into account the frictional losses; or in the case of gears already installed, by measuring the temperature of the sump. It might be required to select a lower viscosity to ensure lubricant is supplied during a cold start or at lower ambient temperatures. In each individual case, it is necessary to check the viscosity at the existing starting temperature, especially in the case of oil circulation systems.
A typical worksheet method for determining the viscosity required for a spur gear drive and a worm gear drive starts with the calculation of the force-speed factor. Because of different viscosity-temperature (VI) behavior of different oils, different ISO viscosity grades are selected for the same Kluber Viscosity Number.
Conclusion
This article presents only a few of the important factors in gear lubricant selection. Technical and performance specifics about lubricants lead to better, more precise decisions in making lubricant selection. It is also helpful to use a reputable lubricant supplier who is knowledgeable in selection options that affect energy consumption, machine life, lubricant consumption and waste oil generation. Included in these options should be the consideration of synthetic lubricants.
Please reference this article as:
Dennis Lauer, "Synthetic Gear Oil Selection". Machinery Lubrication Magazine. May 2001
Dennis Lauer
Synthetic gear oils are used whenever mineral gear oils have reached their performance limit and can no longer meet the application requirements; for example, at very low or high temperatures, extremely high loads, extraordinary ambient conditions, or if they fail to meet special requirements such as flammability. Even though additives can improve many properties of mineral oils, it is not possible to exert an unlimited influence on all their properties. This applies especially to physical properties like the following:
• thermal resistance
• low temperature properties (fluidity, pour point)
• flash point
• evaporation losses
Synthetic oils provide a number of advantages. However, they do not necessarily out-perform mineral oils in all respects and may even result in some drawbacks despite their advantages. The advantages of synthetic lubricating oils (depending on the base oil) include:
• improved thermal and oxidation resistance
• improved viscosity-temperature behavior, high viscosity index (in most cases)
• improved low temperature properties
• lower evaporation losses
• reduced flammability (in some cases)
• improved lubricity (in some cases)
• lower tendency to form residues
• improved resistance to ambient media
Possible disadvantages include:
• higher price
• reactions in the presence of water (hydrolysis, corrosion)
• material compatibility problems (paints, elastomers, certain metals)
• limited miscibility with mineral oils
Application-related advantages often prevail, increasing the use of synthetic lubricants as gear lubricants, especially under critical operating conditions. The most common synthetic types used include synthetic hydrocarbon oils (SHC), polyglycols (PAG) and ester oils (E).
Lubricating Oils Based On Synthetic Hydrocarbon Oils
Synthetic hydrocarbons are similar to mineral hydrocarbons in their chemical structure. They have nearly identical properties relating to their compatibility with sealing materials, disposal, reprocessing and miscibility with mineral oils. The main advantage is their excellent low temperature behavior. It is possible to manufacture food-grade lubricants for the food processing and pharmaceutical industries with SHC base oils using special additives.
Lubricating Oils Based On Polyglycols
These lubricants ensure especially low friction coefficients, which makes them suitable for gears with a high sliding percentage (worm and hypoid gears). With the appropriate additives, they provide excellent antiwear protection in steel/bronze worm gears, and have a good extreme pressure performance. In gear systems, higher polarity polyglycols allow greater interaction on the metal gear surface. This gives polyglycols mild extreme pressure performance even without additives.
Polyglycol oils may have a negative impact on sealing materials and may dissolve some paints. At operating temperatures above 212°F (100°C), only seals made of fluorinated rubber or PTFE are resistant. Before using PAG oils in production applications, it is advisable to test compatibility with paints, seals and sight glass materials.
Miscibility with mineral oils is limited; mixtures should therefore be avoided. Polyglycols are neutral toward ferrous metals and almost all nonferrous metals. If the application has a loaded contact with one component consisting of aluminum or aluminum alloys (rolling bearing cages containing aluminum), there may be increased wear under dynamic load (sliding movement and high load). In such cases, compatibility tests should be conducted. If a worm gear is made of an aluminum bronze alloy, polyglycols should not be used because the reaction in the load zone could result in increased wear.
Lubricating Oils Based On Ester Oils
Ester oils are the result of a reaction of acids and alcohols with water splitting off. There are many types of esters, all of them having an impact on the chemical and physical properties of lubricants. In the past, these lubricating oils were mainly used in aviation technology for the lubrication of aircraft engines and gas turbines as well as gear systems in pumps, starters, etc.
Ester oils have a high thermal resistance and excellent low temperature behavior. In industrial applications, rapidly biodegradable ester oils will gain importance because it seems possible to achieve the same efficiency as with polyglycol oils by selecting an appropriate ester base oil.
Certain ester oils may exhibit low hydrolytic stability. Hydrolysis is the cleavage of the ester into an alcohol and an acid in the presence of water. Ester lubricants need to be hydrolytically stable because they are often exposed to humidity in use. In practice, hydrolysis may be a less serious problem than commonly reported. The hydrolytic stability of an ester depends on:
• the type of ester used
• the type of additives used
• how the ester was processed
• the application
Application-Related Advantages of Synthetic Lubricating Oils
The following application-related advantages result from the improved properties of synthetic lubricating oils as compared to mineral oils:
• improved efficiency due to reduced tooth-related friction losses
• lower gearing losses due to reduced friction, requiring less energy
• oil change intervals three to five times longer than mineral oils operating at the same temperature
• reduced operating temperatures under full load, increasing component life; cooling systems may not be required
Reduction of Gearing Losses and Efficiency Improvement
Because of their special molecular structure, synthetic lubricating oils based on poly-alphaolefins (a type of SHC) and polyglycols ensure that tooth-related friction is considerably lower than with mineral oils. It may be up to 30 percent lower than if a regular mineral gear oil with EP additives was used. Because the friction coefficient of synthetic oils is lower, tooth-related friction is reduced, thus increasing the gear’s efficiency.
The efficiency of gears with a high sliding percentage, worm and hypoid gears, for instance, may increase up to 15 percent if a synthetic oil is used instead of a mineral oil. Even in the case of spur, helical and bevel gears (which have a naturally high gear efficiency), it is possible to increase gear efficiency of up to one percent by using a synthetic gear oil. This may not seem like much at first, but it may result in considerable cost savings depending on the nominal output of the gear unit, especially in the case where several gears are deployed.
Table 1. Potential Reduction of Gearing Losses and Improvement of
Efficiency if Using a Synthetic Gear Oil Instead of a Mineral Oil.
Type of Gear
Effect Worm Gears, Hypoid Gears Spur and Helical Gears, Bevel Gears with Axis Not Offset
Reduction of Total Losses 30% and more 20% and more
Improved Efficiency 15% and more up to 1%
Reduction of Operating (Steady-State) Temperature 68°F (20°C) and more up to 54°F (12°C)
Table 1 shows the extent synthetic oils can reduce gear losses, especially in gear systems with a high degree of load-dependent losses.
Advantages of Synthetic Gear Oils Based on Reduced Friction
Increased Gear Efficiency
• Smaller gears with smaller motors can provide the same power output
• Higher power output can be achieved with the same power input
Reduced Oil Temperatures
• Extension of the oxidative life (five times longer than mineral oils in some cases)
• Extended component life (where reduced wear and friction is achieved)
• Cooling systems may no longer be required
Reduced Energy Consumption
• Reduced costs for electric current or fuel consumption resulting from lower total energy losses in the gearbox; 30 percent and more for worm gears
• Costs for electric power have been reported as high as 10 percent
Improved Efficiency and Reduced Wear When Using Synthetic Oils
Tests show synthetic oils make gears more efficient than mineral oils. A polyglycol oil in the study resulted in the highest degree of efficiency: 18 percent more than the high performance mineral gear oil. SHC gear oil also made the test gears eight to nine percent more efficient. Its performance as a food-grade lubricant in accordance with USDA-H1 is also an excellent added advantage. Food-grade lubricants are often thought to be inferior to normal lubricants, an opinion which this study seems to disprove.
Synthetic base oils have excellent wear protection behavior, which is enhanced by appropriate antiwear additives. Wear is particularly low when the PAG gear oil is used.
Extended Oil Change Intervals Using Synthetic Oils
Synthetic oils have better resistance to aging and high temperatures and a longer service life than mineral oils. Depending on the base oil (SHC or PAG), the oil change intervals may be three to five times longer at the same operating temperature.
Approximate oil change intervals of gear oils at an operating temperature of 176°F (80°C) are:
• Mineral oil: 5,000 operating hours
• SHC oil: 15,000 operating hours (extension factor 3)
• PAG oil: 25,000 operating hours (extension factor 5)
Synthetic oils have a lower friction coefficient than mineral oils in a gearbox and a more favorable viscosity-temperature relationship. This generally permits the use of synthetics at lower viscosity grades and also offers the possibility of reduced oil temperature during operation. In such cases, the life extension factors for oil change intervals of synthetic oils are longer than the values stated above, which refer to identical oil temperature. The following comparison of test results illustrates this advantage. Three lubricants were tested in a splash lubricated worm gear test rig.
The test records show the following oil sump temperatures after 300 operating hours:
Mineral oil: 230°F (110°C)
SHC: 194°F (90°C)
PAG: 167°F (75°C)
The life extension factors of synthetic oils as compared to mineral oil are as follows:
Mineral oil = 1
SHC = 9.5 times longer
PAG = 31 times longer
Synthetic Oils Help Save Maintenance and Disposal Costs
As compared to mineral oils, the oil change intervals of synthetic oils may be five times longer under the same thermal conditions. Despite the fact that the purchase price and the costs of disposal of synthetics are higher than that of a mineral oil, the extended oil change intervals can offset these costs when taking into account the gear unit’s extended service life. A comparison of the costs for mineral and synthetic offers real opportunity to reduce maintenance cost and improve machine reliability. There is also the advantage of reduced environmental impacts with lower lubricant disposal rates.
Selecting the Gear Oil Type
In order to select which type of gear oil to use in a gearbox, you must understand the gearbox’s mode of operation. The application factor (KA) identifies the type and magnitude of load the gears will experience.
Viscosity
Viscosity is the most important physical property of a lubricating oil. Because the viscosity changes with temperature, the rate of change is an important property identified by the Viscosity Index (VI). Most mineral-based gear oils will have a VI of 95. A lower VI indicates that the oil’s viscosity changes to a greater extent with change in temperature. Conversely, a higher viscosity index indicates a much lower rate of change in viscosity with respect to change in temperatures. The advantage of a high VI is that in lower temperatures, the oil will tend not to increase viscosity as much as a lower VI product. The ability of an oil to maintain a small viscosity differential over the operating range of the gearbox provides a more consistent lubricating film to the gears and more predictable wear performance.
Viscosity Selection
As stated earlier, the correct viscosity is an important parameter in proper selection of a gear oil. The manufacturer of the gearbox or gear system generally offers a viscosity recommendation and these recommendations should be followed in most cases. If the OEM of the gear unit has not provided a recommendation and the viscosity has not been calculated based on lubrication theory, it can be selected in accordance with various worksheets and graphs. The differing viscosity-temperature and viscosity-pressure behavior of synthetics as compared to mineral oils should also be taken into account.
The correct viscosity must be selected independently of any specific gear stage, realizing that a compromise is required for multistage gears. The selection of the correct viscosity is based on the oil’s expected operating temperature, such as sump temperature or the temperature of the injected oil. This temperature is calculated by determining the gear’s thermal economy, taking into account the frictional losses; or in the case of gears already installed, by measuring the temperature of the sump. It might be required to select a lower viscosity to ensure lubricant is supplied during a cold start or at lower ambient temperatures. In each individual case, it is necessary to check the viscosity at the existing starting temperature, especially in the case of oil circulation systems.
A typical worksheet method for determining the viscosity required for a spur gear drive and a worm gear drive starts with the calculation of the force-speed factor. Because of different viscosity-temperature (VI) behavior of different oils, different ISO viscosity grades are selected for the same Kluber Viscosity Number.
Conclusion
This article presents only a few of the important factors in gear lubricant selection. Technical and performance specifics about lubricants lead to better, more precise decisions in making lubricant selection. It is also helpful to use a reputable lubricant supplier who is knowledgeable in selection options that affect energy consumption, machine life, lubricant consumption and waste oil generation. Included in these options should be the consideration of synthetic lubricants.
Please reference this article as:
Dennis Lauer, "Synthetic Gear Oil Selection". Machinery Lubrication Magazine. May 2001
