You’ve designed the perfect gearbox for your product with bearings, gears, and materials all meant to last for the long haul. But don’t forget the role oil plays in gearbox performance.
Who, What, Where
“Synthetic Lubricants for Medical Devices,” May 18, 2000, covers lubrication concerns specific to the medical industry.
The right oil will bring lower wear rates, lower operating temperatures, and, best of all, greater energy efficiency. Whether the gearbox will be used in making food or in an industrial machine, the lubricant should be designed for the best performance under specific operating conditions.
Petroleum-based oils, also known as mineral oils, and synthetic oils are the two main classes of lubricants, although manufacturers sometimes combine the two into blends called semisynthetics. Each type has its own range of properties, costs, and applications.
Petroleum-based oils come from refined fossil fuels and are based on hydrocarbon chains of varying length. It’s easy to make these oils and to mix in performance-boosting additives, so they are the least expensive type of oil.
Petroleum-based oils also have limitations. Impurities such as sulfur sometimes linger in the oils’ base stocks. The variety of chain lengths can mean they break down more quickly when exposed to air, a drawback referred to as low oxidative stability. A typical mineral oil starts to break down after operating for about 5,000 hr, or less than a year of continuous operation, at or below 80°C.
As the hydrocarbon chains oxidize, Total Acid Number (TAN) increases. TAN measures the amount of alkaline potassium hydroxide (KOH) it would take to neutralize the acidic compounds that form as the oil degrades.
High TANs indicate the oil should be changed. Acidic oil can cause corrosion and other wear problems if it is not caught quickly. Although TAN doesn’t directly measure the condition of additives in the oil, there is a strong correlation between high TANs and the additives being used up.
Heat speeds any oil’s degradation, but some compounds have better baseline thermal stability than others. An accompanying graph shows the effect of lubricant temperature on performance life. For every 10°C the lubricant warms, performance life is cut in half. Petroleum-based oils survive longest when operating below 80°C. At 100°C continuous operating life is cut to about 50 days.
Synthetic oils are usually smaller molecular blocks built up to form short polymer chains. As a result, their composition is quite different from that of petroleum-based oils, and they are purer and more uniform. Some lubricant manufacturers react petroleum-based oils to control hydrocarbon chain length and stability. These products are sometimes called synthetics, too, but we’ll focus on the polymeric base stocks here.
Polyalphaolefin (PAO) is a widely used synthetic commonly formed from 1-decene, although shorter or longer monomers with the same CnH2n chemistry can also be used. The alpha designation means the chain has two double- bonded carbon atoms at its head. The rest of the chain can take on multiple configurations.
The chains’ flexibility and low-molecular weight keep the oil from crystallizing or getting too viscous at low temperatures. Standard PAOs have a pour point, the lowest temperature at which they flow, of –72°C compared to around –30°C for specially formulated petroleum-based oils.
PAOs also have a high-viscosity index, meaning the oils’ viscosities change little between the pour point and the top operating temperature of 140°C. Strong oxidative stability fosters up to three times the service life of petroleum-based oils. PAOs can run 15,000 hr at 80°C or withstand temperatures of about 95°C and still surpass the 5,000-hr mark where petroleum-based oils start to degrade.
PAOs have about 10% higher specific heat and 20% thermal conductivity than petroleum-based oils. This means equipment running on synthetics is better able to dissipate heat and run cooler for longer lubricant life.
PAOs are significantly more expensive than petroleumbased oils because of their engineered nature. PAOs are fully miscible in petroleum-based oils, so blends of the two can cut the up-front cost of the lubricant.
Another common category of synthetic oils is polyalkylene glycol (PAG). PAG is synthesized from alkylene oxide monomers with the help of a catalyst. The reaction leaves a polar molecule, so it cannot be mixed with PAO without risking phase separation or gellation. That polarity can be tuned combat corrosion or to make the grease soluble in water.
The molecule’s polar nature may also attract it more strongly to metal surfaces, cutting the coefficient of friction to nearly half that of petroleum-based oils.
Because the label PAG covers a variety of precursors, chain-end groups, and polarities, the range of properties available is wide, too. Like PAOs, PAGs can have a high viscosity index, over 200 for many formulations. Pour points can dip as low as –65°C or rise above the freezing point of water.
PAGs last 25,000 hr at 80°C before a change-out is needed, and can operate continuously for 5,000 hr at over 100°C. They excel in high-sliding applications like worm drives and at temperatures up to 160°C. The lower coefficient of friction that comes from using PAGs results in less power loss.
PAOs and some PAGs are also approved as H1 or H2 lubricants according to the USDA and NSF International. H1 lubricants can be used when they may incidentally touch food. They are also permitted in equipment that hangs above open food. They are inert in the human body at concentrations up to 10 ppm. H2 lubricants can be used in food manufacturing only if it is impossible for them to touch the food. For instance, they can be used on conveyers moving food that has already been packaged.
For OEMs, oil affects several design considerations, including the final product’s reliability. The effect of highquality oil on the gearbox’s energy efficiency depends on the gear type.
Designers can reap the biggest gains in gears that are typically low in efficiency such as worm drives. Worm gears run at approximately 60% efficiency when lubricated with a mineral oil. With a PAO, efficiency goes up to 70%, and with a PAG it rises to 78%. As efficiency rises, gearbox temperature drops 20°C or more, extending the life of the gears.
Many OEMs find that a 10 to 18% boost in efficiency, three to five times the interval between oil changes, and half the gear wear mean longer-lasting, more-reliable products that outweigh the extra cost of synthetic lubricants. According to some studies, in-plant applications can see a 2 to 8% drop in production costs by switching from a petroleum-based to a synthetic oil when energy use, lubrication labor and downtime, and component wear and maintenance are taken into account.