Long Live Your Engine
In any given lubricant (such as engine oil, transmission fluid or gear oil), three main factors influence the useful life of the lubricant: viscosity stability, oxidation and contamination. While it’s important to understand how these factors affect oil life, it’s equally important to realize that none of these factors can be measured or monitored except through a thorough and ongoing oil-analysis program.
First, let’s examine the oil property known as viscosity. Viscosity is defined as resistance of an oil to flow at a given temperature. Viscosity is typically measured and reported at two temperature set points: 40C (104F) and 100C (212F).
To maintain sufficient viscosity to support heavy loads in gears and bearings, the thickness of the oil film must be greater than the combined surface finish on the bearing balls (or rollers) and the bearing race. Likewise, the oil-film thickness in a gear mesh must be greater than the combined surface finish of the gears in mesh. Under the right conditions (speed, load and temperature), these surfaces never come into contact, due to the separating oil film.
The ratio between the oil-film thickness and combined surface finishes of the parts is known as the lambda factor. The lambda factor should always be greater than 1.0 to minimize wear and maximize part life. Oil-film thickness is determined by oil viscosity, oil temperature, applied load and surface speeds. Where speed is insufficient to build an adequate oil film (the lambda factor is less than 1.0), the contacting parts are said to be operating under boundary lubrication.
Antiwear agents and/or extreme pressure (EP) additives are included in oil formulations to protect against wear caused by boundary lubrication. These antiwear agents and EP additives are complex polymers designed to decompose at a predetermined temperature and form a surface film on the highly stressed parts. These protective films are “sacrificial,” as they are consumed over time. This protective film then carries the load without harming the metal parts.
Selecting the correct viscosity for the operating conditions (speed, load and temperature) ensures that gears and bearings remain durable, with very little pitting or other damage, over long periods of time.
In most multigrade lubricants (engine oils, transmission fluids and gear oils), the base oils are selected based on their cold-temperature properties where equipment is operated at extreme starting conditions. These lighter base oils allow reduced cranking torque because the oil can more easily flow if it exhibits lower viscosity at low temperature. To have sufficient viscosity for gears and bearings at operating temperature, formulators add Viscosity Index Improver (VII) additives to the base formulation.
All lubricants exhibit a measurable Viscosity Index. Viscosity Index indicates the amount the viscosity alters with changes in temperature. Viscosity Index is based on two temperature points: 40C and 100C, as shown in Figure 1. The base oil, or base oils, used to blend a lubricant will exhibit some measurable Viscosity Index. The higher the Viscosity Index, the less the viscosity changes with temperature.
VII additives are used in multigrade lubricants, such as in SAE1 5W-30 and 15W-40 engine oils or in most automatic-transmission fluids and gear oils. These additives are made up of very long chained polymers and are designed to give additional viscosity to the base oil at operating temperature.
Viscosity Index Improvers are very long chained molecules (polymers) designed to expand with increased temperature, resulting in higher viscosity than would be available with only the base oil.
With time, these VII polymers are cut up (sheared) as they pass through highly loaded gears and bearings. The process is known as shear-down (see Figure 2). Shear-down is permanent, and the lost viscosity is never gained back. Topping off with new oil will temporarily increase the viscosity, but the affect is not lasting, and soon shearing will again decrease the viscosity.
Shear-down can progress to the point where there’s no longer sufficient viscosity to lubricate gears, bearings and other heavily loaded moving parts, and part wear follows. Figure 3 shows a typical curve demonstrating viscosity change (shear-down) with extended use.
All lubricants oxidize over time. The oxidation rate depends on the following:
• Initial oil quality
• Total amount of heat the oil absorbs during the change interval.
A common rule of thumb states that the oxidation rate doubles for every 10C (18F) rise in oil temperature.
During the oxidation process, some of the hydrogen bonds in the base oil degrade, allowing oil molecules to combine with oxygen from the surrounding air. This leads to the formation of acids, which causes the oil to have increased acidity over time. If the oil isn’t changed and oxidation is allowed to continue, the oil molecules may degrade to a point where they cross-link or bond together to form viscosity growth, the end stage of oxidation.
Left unchecked, the oil will eventually become very thick and viscous and reach a mayonnaise consistency. This is known as runaway oxidation (see Figure 4).
We’ve seen that lubricants can suffer viscosity loss through shear-down and they can oxidize at raised temperatures. These factors shorten oil life and can often occur simultaneously, depending on initial oil quality. In addition to viscosity change and oxidation, lubricants also tend to collect debris.
This debris can be ingested through the breather or introduced when new oil is put into the system or when the oil is topped off. Debris can also accumulate from wear metals and water from condensation. Most oils contain dispersants to handle some of this debris and keep it in suspension, but as time goes by, the debris tends to build up and begin to block filters. If the debris is from wear metals, this may result in secondary pitting wear in bearings and gear meshes, depending on the size and hardness of the debris.
In summary, oil life is a function of the amount of permanent viscosity loss (shear-down) suffered by the lubricant, the oxidation state of the lubricant and the amount of debris present. If the oil is run too long, at some point it will no longer be useful and will lose its ability to provide sufficient lubricating film or protect effectively against runaway oxidation. This can be assessed only by measuring and monitoring the oil properties and corresponding wear of parts through oil analysis.
The main factors that affect oil life can be measured and monitored using oil analysis. Measured parameters include viscosity at 40C and 100C, TAN (Total Acid Number), TBN (Total Base Number), water content, soot content, wear and additive metal contents, and contamination debris through particle count.
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