This month I want to slide into a column discussing lubrication (Sorry). We all intuitively know that we must lubricate any “joint” where there is any relative motion — relative motion from micro-motion between spline teeth to the “new normal” high rotational speeds typical of electric vehicles.
We are way beyond “oil” here …
Gear systems require lubrication to remain viable for the intended service life. Let’s define some terms. First, a lubricant is defined as a substance used to facilitate relative motion of solid bodies by minimizing friction and wear between interacting surfaces — generally accepted as the combination (generally a mixture) of a base oil and an additive package. The base oil can be segregated into three types of base oils: mineral, vegetable, and synthetic. Oil can be derived from naturally occurring resources such as crude oil that has been severely hydrocracked thus altering the carbon chain. In terms of chemical structure, oils are long-chain hydrocarbons, meaning, long chains of hydrogen and carbon molecules chemically bound together (as opposed to a mixture — think oil mixed with water here, they separate, which is a phenomenon we do not want). If you will remember back to your high-school days, your teacher drew this very pretty chain of carbon molecules, C, neatly surrounded by hydrogen molecules, H, nicely bound to the carbon molecules, etc. It’s not that way in the real world.
Lubricants can be grouped into one of three categories: mineral, synthetic, or vegetable. Vegetable oil has many uses, but none that we are interested in here. Mineral oil, which is derived from crude oil, can be produced to a range of qualities associated with the process used to refine the oil. Synthetics are “man-made” through a number of processes that synthesize the formulation with specific and unique properties designed for their intended use. The simplest organic compounds contain only the elements carbon and hydrogen and are called hydrocarbons. Even though they are composed of only two types of atoms, there is a wide variety of hydrocarbons due to the fact that they consist of varying lengths of chains, branched chains, and rings of carbon atoms, or combinations of these structures. In addition, hydrocarbons may differ in the types of carbon-carbon bonds present in their molecules. Much of the randomness of the branching is due to the methods and care used during refinement.
The American Petroleum Institute (API) had categorized all base oils into five groups, with the first three groups dedicated to mineral oils and the remaining two groups predominantly synthetic base oils. Groups I, II, and III are all mineral oils with each successive group number representing a designation for a refining process that produced a higher performing oil. Group I base oils are created using the solvent-extraction or solvent-refining technology. This technology extracts the undesirable components within the oil such as ring structures and aromatics. Group II base oils are produced using gaseous phase hydrogen in a process called hydrogenation or hydrotreating. This process is similar to solvent-refining, but it is more effective in converting undesirable components such as aromatics into desirable hydrocarbon structures. Finally, Group III base oils are made in much the same way as Group II mineral oils, except the hydrogenation process is conducted at high temperatures and high pressures. As a result, nearly all undesirable components within the oil are converted into desirable hydrocarbon structures. Generally speaking, the higher the Group, the greater the functional benefits, such as enhanced oxidation stability, thermal stability, viscosity index, pour point, and higher operating temperatures. More refinement unfortunately has a negative aspect as well. As the oil is refined more properties like additive solubility and biodegradability are reduced. It’s a balancing act to be sure.
Group IV is dedicated to a single type of synthetic called polyalphaolefin (PAO). They are the most widely used synthetic base oil. PAOs are synthetically generated hydrocarbons with an olefinic tail formed through a polymerization process involving ethylene gas. The result is a structure that looks very much like the purest form of the mineral oils described in Group III. The advantages of PAOs over mineral oil include a higher viscosity index, excellent low and high-temperature performance, superior oxidation stability, and lower volatility. However, these synthetic lubricants can also have deficiencies when it comes to additive solubility, lubricity, seal shrinkage, and film strength.
Group V is assigned to all other base oils, particularly synthetics. Some of the most common oils in this group include diesters, polyolesters, polyalkylene glycols, phosphate esters, and silicones. Diester (dibasic acid ester) is manufactured through a reaction of dibasic acid with alcohol. The resulting properties can be adjusted based on the types of dibasic acid and alcohol used. Polyolester is made through a reaction of monobasic acid with a polyhydric alcohol. Much like diesters, the resulting properties will depend on these two constituent types. Polyalkylene glycol (PAG) is produced through a reaction involving ethylene or propylene oxides and alcohol to form various polymers. Several PAG products are developed based on the oxide used, which will ultimately influence the water solubility of the base oil.
In general, synthetics can provide greater benefits when it comes to properties influenced by extreme temperatures, such as oxidative and thermal stability, which can contribute to an extended service life. In situations where the lubricant will encounter cold startups or high operating temperatures, synthetics like PAOs typically will perform better than mineral oils. PAOs also exhibit improved characteristics in relation to demulsibility and hydrolytic stability, which influence the lubricant’s ability to handle water contamination. While PAOs are ideal for applications such as engine oils, gear oils, bearing oils, and other applications, mineral oil remains the predominant oil of choice due to its lower cost and reasonable service capabilities.
For our purposes, the base oil is the carrier of the additive package. The primary function of the additive package is to improve the properties of the base oil under different operating conditions. Lubricant additives are chemical components that need to blend well with the base oil to function as a single fluid. The additive package is designed and developed specifically for an intended purpose. They function as dispersants, detergents, oxidation inhibitors, anti-wear agents, extreme-pressure additives, and viscosity index improvers. Additives are organic or inorganic compounds dissolved or suspended as solids in oil. They typically range between 0.1 percent to 30 percent by volume, depending on the application requirements. Speaking of requirements, I am going to focus on two major categories of use: gear lubricants and ATF (Automatic Transmission Fluid).
Unfortunately, I have run out of room for the column this month, so like any good series on TV, this one is to be continued …