5 Lubrication Myths That Are Costing You Money

Stephen Sumerlin

Gearbox Efficiency And Lubrication

Can your choice of lubricant improve gearbox efficiency?

According to the U.S. Energy Information Administration (EIA), the United States generated 1006 billion kWh of electricity in 2007.[1] It is generally accepted that electrical motors account for about 70% of industrial electrical power consumption. Assuming that electric motors are all driving gearboxes, then every 1% increase in gearbox efficiency saves the equivalent yearly output of an 800 MW power plant. In other words, small changes in efficiency can have a large aggregate impact. That’s why lubrication decisions can be so important to a plant’s energy management efforts. Unlike other efficiency-improving ideas, lubrication changes require no changes to existing equipment.

Oil churning, seal drag and friction account for most of the losses in gearboxes. To some extent, these three sources are all affected by lubrication. Seals ride on a thin oil-lubricant film. Churning losses are due to the gearbox components moving through the oil sump.

Fig. 1. The Stribeck Curve relates friction between load-bearing surfaces as a function of relative oil-film thickness and lubrication regime.

Fluid friction
The Stribeck Curve, shown in Fig. 1 relates friction between load-bearing surfaces as a function of relative oil-film thickness and lubrication regime.[2] Relative oil-film thickness is the ratio of film thickness to surface roughness. The thicker the film relative to surface roughness indicates a reduced likelihood of contact by surface asperities. Figures 2 through 4 illustrate the relationship between film thickness and surface roughness.

The traction coefficient is up to 30% lower for synthetic oils than for mineral based—possibly due to the synthetic’s uniform molecular structure

Removing Water Contamination In Lubricants

Water contamination is often called the scourge of the machine. An ongoing battle ensues between lubrication technicians in the wet process industries like pulp and paper, in outdoor machinery applications like construction and mining, and where frequent machine wash downs occur such as food processing. Its effects are not limited to these industries, however. Water contamination rears its ugly head in almost every plant and industry.

Water affects both the oil and the machine. It promotes oxidation of the lubricant’s base oil and washes out some additives which are attracted to water. Later, water will typically separate to the bottom of the sump. It hydrolyzes (chemically attacks) additives, which compromise their performance, and in some cases, produces highly corrosive by-products. A water-degraded lubricant cannot fully lubricate and protect the machine, which leads to excessive wear and failure. Water also attacks the machine directly. The following is a summary of common water-induced wear mechanisms:

Rust and Corrosion - Water directly attacks iron and steel surfaces to produce iron oxides. Water teams up with acid in the oil to increase the corrosive potential in the attack of ferrous and nonferrous metals. Rust and corrosion lead to rapid surface deterioration when abrasive particles are present. Rust particles are also abrasive. Abrasion also exposes fresh nascent base metal which is more easily corroded in the presence of water and acid.

Vaporous Cavitation - If the vapor pressure of water is reached in the low-pressure regions of a machine, such as the suction line of a pump, the pre-load region of a journal bearing, etc., the vapor bubbles expand. Should the vapor bubble be subsequently exposed to sudden high pressure, such as in a pump or the load zone of a journal bearing, the water vapor bubbles quickly contract (implode) and simultaneously condense back to the liquid phase. The water droplet impacts a small area of the machine’s surface with great force in the form of a needle-like micro-jet, which causes localized surface fatigue and erosion. Water contamination also increases the oil’s ability to entrain air, thus increasing gaseous cavitation.

Film Strength Loss - With elastohydrodynamic (EHD) contacts such as rolling element bearings and the pitch line of a gear tooth, the great strength of the lubricating film occurs because the oil’s viscosity increases as pressure increases. Water does not possess this property. Its viscosity remains constant (or drops slightly) as pressure increases. As a result, water contamination increases the likelihood of contact fatigue (spalling failure) caused by surface-to-surface rolling contact. In these high-pressure zones, hydrogen-induced wear, a complex tribo-chemical reaction, also occurs, causing embrittlement and blistering.

Managing Water Contamination
The best way to manage water contamination is to keep water out of the oil in the first place. Water enters the sump or reservoir at those points where the machine interfaces with its environment. Following are tips for water exclusion:

• Manage new oil properly.
• Use desiccant breathers or other tank headspace protection.
• Use and maintain high quality shaft and wiper seals.
• Avoid shafts, fill ports and breathers when washing down machines. Avoid high-pressure sprays in the areas of seals if possible.
• Maintain steam and heating/cooling water system seals.

Step-by-Step Grease Selection

Jarrod Potteiger, Noria Corporation

How do you know if you’re using the right grease? You might be using a high-quality grease. You may have put a lot of effort and money into selecting the best quality grease in the pursuit of lubrication excellence. But don’t confuse the quality of the lubricant with the quality of the specification. Considering this lubricating oil analogy, the best quality turbine oil would most likely not make a good engine oil.

Most users are aware of the importance of selecting the right lubricant for a given application. When it comes to selecting lubricating oils for manufactured equipment, it’s easy to determine which products meet the original equipment manufacturer (OEM) requirements. OEM specifications for a lubricating oil normally include viscosity at operating or ambient temperature, additive requirements, base oil type and even special considerations for different environmental conditions. Grease specifications, on the other hand, often lack the detail necessary to make a proper selection, leaving it up to the lubrication engineer to create the specification.

A common OEM grease specification might be to use an NLGI (National Lubrication Grease Institute) No. 2 lithium grease of good quality. Using this information alone, one could select the right consistency and thickener type. A similar specification for an oil-lubricated application would be to use a “good quality lubricating oil.” What?!

Due to the lack of specificity in most grease recommendations, it is important to learn how to properly select greases for each application in the plant. Proper grease specification requires all of the components of oil selection and more. Other special considerations for grease selection include thickener type and concentration, consistency, dropping point and operating temperature range, worked stability, oxidation stability, wear resistance, etc. Understanding the need and the methods for appropriate grease selection will go a long way toward improving lubrication programs and the reliability of lubricated machinery. Let’s walk through the grease selection process step by step, starting with the most important property.

Base Oil Viscosity
The most important property of any lubricant is viscosity. A common mistake when selecting a grease is to confuse the grease consistency with the base oil viscosity. Because the majority of grease-lubricated applications are element bearings, one should consider viscosity selection for those applications. While most would not use an EP 220 gear oil for an oil-lubricated electric motor bearing, many people will use a grease containing that same oil for an identical grease-lubricated bearing. There are several common methods for determining minimum and optimum viscosity requirements for element bearings, most of which use speed factors, commonly denoted as DN or NDm. Speed factors account for the surface speed of the bearing elements and are determined by the following formulas:

DN = (rpm) * (bearing bore) and
NDm = rpm * (( bearing bore + outside diameter) / 2)

The NDm value uses pitch diameter rather than bore diameter because not all bearings of a given bore have the same element diameter, and thus have different surface speeds. Knowing the speed factor value and likely operating temperature, the minimum viscosity requirement can be read directly from charts like Figure 1.

Figure 1. (Courtesy of ExxonMobil)

Figure 1 assumes the base oils’ viscosity index. To be more precise, one would need to use a chart that identifies the viscosity at operating temperature, then determine the viscosity grade from a viscosity/temperature chart for a given lubricant.

Additives and Base Oil Type
Once the appropriate viscosity has been determined, it’s time to consider additives. The additive and base oil types are other components of grease that should be selected in a fashion similar to that used for oil-lubricated applications. For instance, a lightly loaded high-speed element bearing does not require extreme pressure (EP) additives or tackifying agents, while a heavily loaded open gear set does.

Most performance-enhancing additives found in lubricating oils are also used in grease formulation and should be chosen according to the demands of the application. Figure 2 shows some common additive requirements by application. Most greases are formulated using API Group I and II mineral oil base stocks, which are appropriate for most applications. However, there are applications that might benefit from the use of a synthetic base oil. Such applications include high or low operating temperatures, a wide ambient temperature range, or any application where extended relubrication intervals are desired.

Detecting and Controlling Water in Oil

Moisture is considered a chemical contaminant when suspended or mixed with lubricating oils. It presents a combination of chemical and physical problems for the lubricant and machinery, respectively. The potential problems, states of existence and methods for measuring moisture are discussed here.

Effects of Water on Equipment and Lubricants
The effects of water are insidious. Failure due to water contamination may be catastrophic, but it may not be immediate. Many failures blamed on lubricants are truly caused by excess water. The following are some of the effects of water on equipment:

·         Shorter component life due to rust and corrosion

·         Water etching/erosion and vaporous cavitation

·         Hydrogen embrittlement

·         Oxidation of bearing babbitt

·         Wear caused by loss of oil film or hard water deposits

Rust and Corrosion

Water attacks iron and steel surfaces to produce iron oxides. Water teams up with acid in the oil and corrodes ferrous and nonferrous metals. Rust particles are abrasive. Abrasion exposes fresh metal which corrodes more easily in the presence of water and acid.

Water Etching
Water etching can be found on bearing surfaces and raceways. It is primarily caused by generation of hydrogen sulfide and sulfuric acid from water-induced lubricant degradation.

Erosion
Erosion occurs when free water flashes onto hot metal surfaces and causes pitting.

Vaporous Cavitation
If the vapor pressure of water is reached in the low-pressure regions of a machine, such as the suction line of a pump or the pre-load region of a journal bearing, the vapor bubbles expand. Should the vapor bubble be subsequently exposed to sudden high pressure, such as in a pump or the load zone of a journal bearing, the water vapor bubbles quickly contract (implode) and simultaneously condense back to the liquid phase. The water droplet impacts a small area of the machine’s surface with great force in the form of a needle-like micro-jet, which causes localized surface fatigue and erosion. Water contamination also increases the oil’s ability to entrain air, thus increasing gaseous cavitation.

Hydrogen Embrittlement
Hydrogen embrittlement occurs when water invades microscopic cracks in metal surfaces. Under extreme pressure, water decomposes into its components and releases hydrogen. This explosive force forces the cracks to become wider and deeper, leading to spalling.

Film Strength Loss
Rolling element bearings and the pitch line of a gear tooth are protected because oil viscosity increases as pressure increases. Water does not possess this property. Its viscosity remains constant (or drops slightly) as pressure increases. As a result, water contamination increases the likelihood of contact fatigue (spalling failure).

The effects on lubricating oil can be equally harmful:

·         Water accelerates oxidation of the oil

·         Depletes oxidation inhibitors and demulsifiers

·         May cause some additives to precipitate

·         Causes ZDDP antiwear additive to destabilize over 180°F

·         Competes with polar additives for metal surfaces

Maximum Recommended Water Concentrations
Oil, unless it is dried, contains some dissolved water. Figure 1 shows the amount of dissolved water that can be found in ISO 220 paper machine oil and ISO 32 turbine lubricant before it turns cloudy.

Figure 1. Dissolved Water as a Function of Temperature
in Paper Machine Oil and Turbine Oil

Table 1 helps determine the relative life of mechanical equipment versus the amount of water in the lubricant. To use the chart, estimate the current moisture level in the system along the y-axis, move toward the right to the target moisture level. The top of the chart gives the estimate of how much the life of the oil is extended. For example, by reducing moisture from 2,500 ppm to 156 ppm, machine life is extended by a factor of 5.