Synthetic lubricants save resources and increase efficiency

Design engineers know that manufacturers are continually challenged to minimize operating costs by reducing maintenance expenses and service time, as well as improving efficiency of equipment. One immediate and cost-effective way to do so is by using high-quality lubricants in machinery and their components, such as gearboxes.

Recent studies suggest that an appropriate industrial lubricant can increase energy efficiency by as much as 30%. In other words, if a machine is operating at 60% efficiency, the efficiency level can be raised to 80% by switching to a high-quality lubricant. Furthermore, the use of a synthetic oil could provide a longer bearing and gear set life, thereby reducing downtime, as well as saving general resources due to a reduced demand for manufactured components.

Lubricant properties

Figure 1. Lubricating oils consist of a combination of synthetic and/or mineral oil base with additives to enhance specific properties.

The primary lubricant types are petroleum-based oils (also known as mineral oils), semi-synthetic oils and synthetic oils (Figure 1).

Petroleum-based oils come from crude oil refining. These consist of hydrocarbon chains and contain a variety of impurities. They’re the least expensive oil, but they exhibit low oxidation stability. This means they must be changed out more often than other oil types, typically after 5,000 hours of operation at 175°F (80°C). In addition, their thermal stability permits operating at temperatures only as high as 200°F to 215°F (90°C to 100°C).

The two main synthetic oils for gear and bearing lubrication are polyalphaolefin (PAO) and polyalkylenglycol (PAG), or simply polyglycols. Each type possesses certain advantages that are a function of physical properties, application, environment and other variables. Most of the synthetic oils are produced from other crude oil compounds by synthesizing the oil in a chemical process. As a result, their composition is quite different from that of petroleum-based oils.

Figure 2. The properties of lubricant types varies.

Synthetics have greater purity and uniformity that offer several advantages, such as a higher viscosity index, oxidation stability, better low-temperature behavior and lower friction coefficients (Figure 2).

While PAOs provide the best low-temperature behavior of the synthetic gear oils, polyglycols offer the best thermal stability, best viscosity temperature behavior and lowest friction coefficient.

Figure 3. The prolonged service time of synthetic lubricants can reduce equipment downtime and resources.

Compared to mineral oil at an operating temperature of 175°F (80°C), oil change intervals with PAO could be extended up to three times, and with PAG it is possible to achieve five times the service life. As you can see, how often a manufacturer is required to change gear oil depends on the chemistry of the lubricant being used. The slope of the lines in Figure 3 represents the 10K rule: For every 18°F (10°K or C) you increase the temperature of the lubricant above about 180°F (80°C), you halve its performance life.

Gear oil and gearboxes

Choosing the right gearbox lubricant provides the benefits of lower wear rates, lower operating temperatures and greater energy efficiency. Years ago, engineers would design a gearbox before deciding how to lubricate it properly. These days, design engineers factor in the oil viscosity because it directly affects load-carrying capacity. Viscosity index is important because it indicates how the oil’s viscosity varies at elevated or high temperatures. The higher the viscosity index, the less the oil viscosity change has to be considered.

The pressure viscosity coefficient is defined by how the oil changes viscosity under load. As pressure increases, the viscosity also increases so it can carry the load and, ultimately, lubricate the metal components of a gearbox.

Plant professionals need to understand that gear oil affects several design considerations, including reliability. How large a gearbox energy efficiency increase can be achieved by using high-quality oil depends on the gear type. You should specify expectations from every lubricant: reducing friction and wear, protecting against corrosion, dissipating heat and providing an overall sealing effect, for example. Depending on specific plant operating conditions and manufacturing processes, lubricants might perform a variety of wide-ranging functions.