Everything You Need To Know To Find The Best Industrial Gear Oil

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.

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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.

Gear Oil Selection Criteria

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.

Viscosity

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 -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.

Figure 1

Although several common methods for gear lubricant viscosity grade selection are available, most should return similar values.

Gear Lubricant Type and Additive Selection

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.

R&O Gear Lubricants

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 (Extreme Pressure) Gear Lubricants

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.

Compounded Gear Lubricants

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.

Base Oil Selection

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:

Aleman Moil contains other products and information you need, so please check it out.

How to Inspect a Gearbox

Best Ways to Reduce Gearbox Oil Leakage

How to Flush Gearboxes and Bearing Housings

References

It would be great if industrial gears ran in cool, clean and dry environments. However, conditions in gear-driven operations such as steel mills, manufacturing plants and other strenuous industrial applications are anything but cool, clean and dry. That’s why lubricant selection can be so challenging.

Changes Impacting Gear Oil Lubricants

Harsher Environments

Even with regular lubricant maintenance, heat, higher loads and pressures, and contaminants such as water can compromise a gear system. Today’s gear-driven equipment, and the lubricants that protect and allow them to perform well over the long haul, must withstand increasingly harsh environments that also cause quick consumption of essential gear oil additives.

This is partly due to the trend toward smaller machines and exposure to diverse applications and punishing operating conditions. In addition, maintenance and plant managers expect higher performance, less downtime and more productivity to decrease costs and improve profits.

Gearbox Size

Today’s gearboxes typically are smaller and made from newer, lighter-weight materials than before. But, these smaller, lighter pieces of equipment are pushed to produce more power and, at the same time, be more durable and reliable than before.

Downsizing gearboxes means less oil and additive to lubricate and protect gears. However, at the same time, equipment loads are increasing. That translates into higher temperatures and more rapid oxidation.

Oxidation harms industrial gear oils because it can form sludge that can shorten both oil and gear life. The results are expensive downtime, repair or replacement costs.

Selecting the Right Oil

To handle increased demands, today’s industrial gear oils must contain high-performance additive chemistry. The goal is to keep the lubricant thermally stable and robust enough to ensure that it lasts longer, protects better and performs more efficiently, while at the same time keeping the system clean and carrying away heat and contaminants.

This is no easy task. Consider industrial gear oils that at one time were widely acceptable for a given application. Even if these oils meet minimum industry specifications, which can remain unchanged for up to 10 years, they may not be durable enough to protect your equipment.

There are five factors to keep in mind when selecting industrial gear oil that will provide you optimum performance and profitability. Each is discussed in this article.

Fluid Cleanliness

Smaller gearboxes must do the same amount of work as, or even more than, their larger predecessors. But spaces are smaller and tolerances are tighter. That translates to higher speeds and loads. The trend toward smaller reservoirs means the system must cycle the fluid more often with less time to dissipate heat, release foam, settle out contaminants and demulsify water.

Constant gear rolling and sliding produces friction and heat. The heavier operating loads common in today’s industrial settings increase metal-to-metal contact or boundary lubrication, producing even more heat and pressure. To meet longer drain intervals for environmental and cost reasons, the fluid stays in the system longer. Therefore, fluid cleanliness and performance retention becomes critical.

Highly viscous lubricants generate heat from internal fluid friction and also may consume more power to turn the gears. The rate of oxidation in the fluid can increase, which decreases the fluid’s effectiveness and life. In addition, higher operating temperatures increase sludge and varnish formation, which can damage equipment by forming deposits that can block filters, oil passageways and valves.

On the other hand, less viscous lubricants generate less heat, minimizing the chance of exceeding recommended operating temperatures or damaging equipment.

Lubricants play a critical role in removing contaminants such as dirt, water, wear particles and other foreign matter that can damage gears and bearings and impact efficient, smooth running of the gears.

As the lubricant travels through the filter system, contaminants, which can originate outside the system or from wear inside, should be removed. Even other lubricating fluids that find their way into the system can cause contamination if they are incompatible, thereby reducing performance.

Because they don’t move easily through the filtration system, highly viscous lubricants can be difficult to filter. Pressure at the filter can increase and, if sufficiently high, will trigger a system bypass, allowing contaminant-laden lubricant to circumvent the filters. Equipment damage can follow. Worn gears and higher levels of iron in the lubricant are signs of an ineffective filtration system.

Less viscous lubricants can flow more easily through the filtration system. Contaminants are effectively removed, reducing the likelihood of gear and bearing damage, and increasing equipment life. Another benefit is that the lubricant may need to be changed less frequently, resulting in less downtime and cost.

Fluid Durability

Industrial gear oils must be durable enough to withstand in-service conditions and to retain that performance over time. Although many fluids may meet the industry specification when new, they rapidly lose performance while in service. Industrial gear oils formulated for extended durability will keep gears operating properly and protect equipment investment by extending life, reducing downtime, maximizing productivity and lowering maintenance costs.

Industrial gears often operate under heavy loads and require extreme-pressure protection for gear components. Typical industrial gear oils do not always provide high extreme-pressure performance at low-viscosity greases. This challenges the notion that industrial gears performing in harsh environments must have highly viscous lubricants to be adequately protected.

Figure 1. Industrial Gear Oil Trends

Fluid Demulsibility

It would seem easy enough to keep a gearbox dry, but water can creep into the system, particularly the reservoir, in a variety of ways. Mist from water used in routine plant maintenance can enter the reservoir breather, forming condensation in the reservoir after hot-running equipment cools after shutdown. Or, water may enter in some other way. In any case, it can lead to corrosion and decrease performance.

It is vital for the gear oil to be formulated to quickly separate water at both the high and the low temperatures found in industrial gearboxes. The ability to rapidly drain water from the system helps extend the life of both the component and the oil.

Universal vs. Dedicated Fluids

There are two types of industrial gear lubricants. The first, so-called universal gear oils, are formulated so they may also be used in automotive gear applications. Universal fluids may contain components that are both unnecessary for and harmful to industrial gear performance. Or, they may not contain components that are necessary in industrial applications.

For example, water separation is not necessary in automotive gear oil applications. However, water separation is critical in industrial gear oil applications; therefore, demulsibility additives must be incorporated.

The second type of gear oil lubricant is called a dedicated fluid. These fluids are tailored for industrial applications by carefully formulating the lubricant with additive components specifically designed for such applications.

The Right Additives

Additives used to enhance extreme-pressure properties in gear oil can be prone to thermal instability, resulting in sludge formation. However, technology is available that provides the optimum balance of thermal stability for sludge-free gearboxes and also extreme-pressure protection for heavy-duty durability.

The combination prolongs gearbox life, maximizes efficiency and eliminates downtime. But most important, high extreme-pressure performance and cleanliness are maintained across a full spectrum of viscosity grades, down to ISO VG 68. Using a lower-viscosity grade can improve efficiency while maintaining durability for optimum performance.

In industrial settings, equipment downtime significantly impacts the bottom line. A lower-viscosity lubricant with optimized additive technology effectively protects gear-driven equipment and ensures its operation at maximum performance.

About the Author:

Tim Cooper is The Lubrizol Corporation’s industrial additives product manager for Europe, Africa and the Middle East. He is responsible for the industrial additives product line, which encompasses hydraulic, turbine, industrial gear and grease additives. Tim has worked at Lubrizol for 23 years in a variety of technical and commercial positions in both the United Kingdom and the United States. These roles have covered a broad spectrum of activities including additives for industrial lubricants, paints and coatings, specialty monomers and surfactants. He earned an honor’s degree in applied chemistry from Trent Polytechnic in . For more information, visit www.lubrizol.com.