How do you know which lubricant is the best fit for a given application? Typically, it is as simple as searching through a maintenance manual and selecting a product from the QPL (qualified product list). Unfortunately, this solution may not always provide optimum lubrication for a given gear set, or maximum efficiency in managing lubricant inventory.
While some original equipment manufacturers (OEMs) provide generic specifications that consider pertinent parameters, others give only a general specification that may not even consider operating temperatures. It is therefore important for the individuals responsible for selecting lubricants to posses a fundamental understanding of how to specify lubricants for gearing.
In addition to understanding and being able to interpret the specifications given by equipment manufacturers, it is important to understand why, and be able to make changes when necessary.
When selecting lubricants for industrial gearing, numerous factors must be considered beyond simply selecting a product from the maintenance manual’s QPL, including product availability, operating conditions, the preferred lubricant brand and product consolidation efforts. Proper lubricant selection is a cornerstone of any excellent lubrication program.
A good understanding of this allows the lubrication engineer to maximize machinery reliability under normal conditions, as well as use lubricant specification as a problem solver in abnormal conditions.
In order to choose the best lubricant for a gear set, the following criteria must be addressed:
Viscosity – Often referred to as the most important property of a lubricating oil.
Additives – The additive package used in the lubricant will determine the lubricant’s general category and affects various key performance properties under operating conditions.
Base Oil Type – The type of base oil used should be determined by the operating conditions, gear type and other factors.
Choosing an appropriate viscosity grade is usually as simple as finding the recommendation in a component’s maintenance manual. Unfortunately, the manual does not always exist or the machine operates outside the conditions for which the OEM’s recommendations were made. Therefore, it is important to understand the methods for viscosity selection and the factors that affect the requirement.
The viscosity for a gear lubricant is primarily chosen to provide a desired film thickness between interacting surfaces at a given speed and load. Because it is difficult to determine the load for most viscosity selection methods, the load is assumed and the determining factor becomes speed.
One of the most common methods for determining viscosity is the ANSI (American National Standards Institute) and AGMA (American Gear Manufacturers Association) standard ANSI/AGMA 9005-E02. In this method, assumptions are made concerning the load, viscosity index and the pressure-viscosity coefficient of the lubricant.
The chart in Figure 1 is applicable to spur, helical and beveled enclosed gear sets. Other charts exist for worm gears and open gearing. To use this method, the type of gear set, gear geometry, operating temperature and the speed of the slow speed gear must be determined.
After calculating the pitch-line velocity of the slowest gear in the unit, the required viscosity grade can be read from the chart using the highest likely operating temperature of the unit.
It is important to note that this method assumes the viscosity temperature relationship of the lubricant (viscosity index = 90). If the VI of the lubricant deviates from this value, additional tables for oils with VI = 120 and 160 are included, or a viscosity-temperature plot can be used to interpolate the appropriate ISO viscosity grade.
Although several common methods for gear lubricant viscosity grade selection are available, most should return similar values.
After selecting the viscosity grade, the basic type of lubricant must be chosen. While there are many variations, gear lubricants can generally be placed into three categories: R & O, antiscuff and compounded. The gear lubricant type that best fits a given application will be determined by the operating conditions.
Because there are no standard guidelines to help make this determination, the selection is somewhat subjective. Many equipment manufacturers will specify a viscosity requirement and leave this decision to the end user. Others will choose to be conservative and specify EP lubricants for the applications. It is therefore important to understand the general conditions that affect this requirement.
Rust and oxidation inhibited (R&O) gear lubricants do not contain antiscuff additives or lubricity agents. R&O gear oils generally perform well in the categories of chemical stability, demulsibility, corrosion prevention and foam suppression. These products were designed for use in gearing operating under relatively high speeds, low loads, and with uniform loading (no shock loading).
These lubricants are the best selection in applications where all surface contacts operate under hydrodynamic or elastohydrodynamic lubrication conditions. They do not perform well or prevent wear under boundary lubrication conditions.
Antiscuff gear lubricants, commonly referred to as extreme pressure (EP) lubricants, have some performance capabilities that exceed those for R&O oils. In addition to the properties listed for R&O lubricants, antiscuff lubricants contain special additives that enhance their film strength or load-carrying ability.
The most common EP additives are sulfur phosphorous, which are chemically active compounds that alter the chemistry of machine surfaces to prevent adhesive wear under boundary lubrication conditions.
In less severe applications, antiwear additives may also be used to provide wear protection under boundary lubrication conditions. Machine conditions that generally require antiscuff gear lubricants include heavy loads, slow speeds and shock loading.
In addition to sulfur phosphorous and zinc dialkyl dithiophosphate (ZDDP) antiwear additives, several common solid materials are considered antiscuff additives including molybdenum-disulfide (moly), graphite and borates.
One benefit of these additives is they do not depend on temperature to become active, unlike sulfur phosphorous compounds which do not become active until a high surface temperature is achieved. Another potentially negative aspect of sulfur phosphorous EP additives is they can be corrosive to machine surfaces, especially at high temperatures.
This type of additive may also be corrosive to yellow metals and should not be used in applications with components made of these materials, such as worm gears.
The compounded gear lubricant is the third type of common lubricant. In general, a compounded lubricant is mixed with a synthetic fatty acid (sometimes referred to as fat) to increase its lubricity and film strength. The most common application for these gear lubricants is worm gear applications.
Because of sliding contact and the negative effects of EP agents, compounded lubricants are generally the best choice for these applications. Compounded oils are also referred to as cylinder oils because these lubricants were originally formulated for steam cylinder applications.
High-quality mineral base oils perform well in most applications. In fact, mineral base oils typically have higher pressure-viscosity coefficients than common synthetics, allowing for greater film thickness at given operating viscosities. There are, however, situations where synthetic base oils are preferable.
Many synthetic base stocks have greater inherent resistance to oxidation and thermal degradation making them preferable for applications with high operating temperatures and, in some cases, allowing for extended service intervals. Additionally, synthetics perform better in machines subjected to low ambient temperatures due to their high viscosity index and low pour points.
The high viscosity index also makes synthetic products suitable for a wider range of ambient temperatures, eliminating the need for seasonal oil changes. Some synthetics may also offer greater lubricity which reduces friction in sliding contacts.
Selecting lubricants for industrial gearing is similar in most applications. There is no specific property or value to create a good specification. To identify the best choice for a given application, the right viscosity, base oil and type of lubricant must be selected and the appropriate performance properties evaluated. For more information on this topic, please see the references listed below.
Read more on gearbox lubrication best practices:
Robert Errichello. “Selecting Oils with High Pressure-Viscosity Coefficient - Increase Bearing Life by More Than Four Times.” Machinery Lubrication magazine. March 2004.
ANSI / AGMA 9005-E02 Industrial Gear Lubrication.