The problem has become even more pronounced in the last decade. While many years ago gears were oversized and capable of withstanding a degree of use and abuse, today's gear drives are precision components with higher power densities requiring a greater focus on lubrication.
In this multipart series, we'll examine the factors that impact precision lubrication in industrial, including lubricant selection, application and contamination control, as well as how to develop precision lubrication practices for enclosed gears.
Lubrication Fundamentals for Gear Drives
From a lubrication perspective, gears can be categorized based on their design (gear geometry), speed and load. For high speed gearing, surfaces are separated by a full oil film (hydrodynamic or elasto-hydrodynamic lubrication). Slow turning and/or heavily loaded gear drives tend toward boundary lubrication where point loading can result in surface separation between gear teeth that is equal to or less than the mean surface roughness of the mating gears (boundary lubrication). Table 1 gives a general overview of common gear types and the type of lubrication film expected under different loads and speed.
Table 1: Typical lubrication regimes found in gears
In most gears, the frictional force between the gear teeth is typically a combination of sliding and rolling friction. The degree of sliding versus rolling friction, in conjunction with the speed of rotation and applied load, all factor into how the meshing surfaces engage and ultimately how effective the lubricant is in reducing mechanical wear.
To illustrate this point, consider one of the simplest of all gear designs: a spur gear with involute gear tooth profile (Figure 1). For the sake of discussion, we can consider two points of interaction between the meshing gear teeth: the tip to root contact and the pitch line contact.
Figure 1: Meshing spur gears with involute gear tooth profile
In an involute spur gear design, such as that shown in Figure 1, the contact at the pitch line on each gear set is almost exclusively rolling friction. Rolling friction is defined by two surfaces that approach each other in a perpendicular direction (Figure 2a). Under rolling contact conditions, separation between the moving surfaces will depend on the applied load and speed. At higher speeds, the increased pressure on the lubricant under load causes a rapid increase in viscosity of the fluid. With sufficient pressure, the lubricant can undergo an instantaneous phase change from liquid to solid, which, in turn, can result in an elastic deformation of the mating machine surfaces. Elastic surface deformation results in the load being dissipated across a larger surface area, allowing the gears to transmit the applied load without mechanical wear as the surfaces are separated by a full oil film. This is an effect referred to as elasto-hydrodynamic lubrication (EHL). The key to effective elastohydrodynamic lubrication is having a lubricant with sufficient viscosity and a high viscosity pressure coefficient. Having too low a viscosity or poor viscosity pressure coefficient can result in metal-to-metal contact, dramatically reducing the life expectancy of the gears.
For lower speed gears under rolling friction, the rate at which the two surfaces approach is too slow to allow the EHL film to form. Under these conditions, boundary lubrication will occur, requiring the use of extreme pressure and anti-fatigue additives to prevent wear from occurring.
Figures 2a and 2b: Rolling and sliding contact between two friction surfaces
By contrast, at the tip to root of an involute gear tooth profile (Figure 1) and in many other gear geometries, sliding friction is the dominant frictional force. Sliding friction involves surface motion in a parallel direction (Figure 2b). In high speed gearing, the speed relative to the load is typically high enough that moving surfaces are separated by a full film of oil. However, unlike rolling friction where elastohydrodynamic lubrication is the norm, high speed sliding friction results in hydrodynamic separation between the moving surfaces, an effect akin to a water skier experiencing "lift" once the tow boat speed is high enough for the applied load (weight of the skier and water ski dimensions). Hydrodynamic lubrication requires the oil to have sufficient viscosity for the applied load and speed, both of which have an impact on oil film thickness.
For slower turning gears, such as the low speed gear in a gear reducer, the viscosity of oil necessary for hydrodynamic lift is too high compared to the ability of the oil to flow into the load zone. As a result, a hydrodynamic oil film cannot be maintained and, once again, boundary lubrication conditions dominate. Where boundary lubrication occurs in conjunction with sliding motion, severe sliding wear - sometimes referred to as adhesive wear, galling, or scuffing - can occur. In an involute gear geometry, this typically occurs just above and below the pitch line and frictional forces transition from pure rolling to sliding friction.
Sliding motion also results in higher localized temperatures, which causes a reduction in the oil's viscosity and can also cause the oil to be wiped away from the converging gear surfaces, further inhibiting the formation of an oil film. Under these circumstances, the meshing gears will be under boundary lubrication conditions. Where boundary lubrication is anticipated, special wear prevention additives must be used to protect gear teeth.
Gear Oil Lubricant Selection
Lubricants used in gearing come in three basic classes:
Rust and oxidation (R&O) inhibited oils
Extreme pressure (EP) oils
Compounded (COMP) oils, sometimes referred to as cylinder oils
The type of lubricant used will depend on the type of lubrication regime (hydrodynamic, elastohydrodynamic, boundary, etc.) and the type of gear set. For higher speed application where full film conditions exist, simple rust and oxidation inhibited oils are used. Aside from their lubricating properties, these oils need to exhibit good oxidation resistance to counter the effects of the heat generated and good corrosion resistance to counteract the effects of any ambient moisture ingress and protect any yellow metals that may be present.
For slower speed or higher loaded gears where full film separation is simply not possible, extreme pressure additized gear oils should be used. There are several different types of EP additives, from chemical films that react with and coat the gear surfaces to solid suspensions that improve lubricity under sliding contacts. However, all have the same basic function: to reduce the coefficient of friction under boundary lubricating conditions.
For worm gears in particular, it is often recommended to use a compounded oil rather than an EP additized oil. The reasons for this are twofold. First, some chemically active EP additives can be corrosive to yellow metals (brass, bronze, etc.), which are commonly used for the ring gear in worm drives or in bearing cages. Second, compounded oils contain lubricity agents based on fatty materials that do a better job of reducing the coefficient of sliding friction, the dominant frictional force in most worm drives. For gears containing yellow metals, chemically active EP additives are not recommended at elevated temperatures (140 F-150 F and higher).
Like any other lubricant application, viscosity is the single most important decision when selecting a gear oil. Viscosity is selected based on the speed and size of the gears by calculating the pitch line velocity and the ambient operating temperatures. While this is a good starting point, other variables, such as shock loading or cold ambient start-up conditions, also must be factored in for an optimized lubricant selection. Viscosity mismatch is one of the most common errors with gearbox lubrication. Table 2 provides some good general guidelines on viscosity selection.
Oftentimes, OEM recommendations are adopted in selecting the correct gear lubricant. While this is an excellent practice in many applications, where extreme conditions, such as high or low operating temperatures or shock loading, are present, OEM guidelines should be considered as just a starting point and should be adjusted up or down depending on the application.
Table 2: Recommended viscosity grade for enclosed gears based on the pitch line velocity of the slower speed gear and operating temperature
In some instances, such as high or low operating temperatures or the desire to extend oil drain intervals, the use of high performance gear oils, including synthetics, is recommended. Where their usage can be financially justified, these lubricants offer significant benefits in low temperature start-up, high temperature thermal and oxidative stability, and improvements in film strength. Although there are several different types of synthetic gear oil, the two most common are poly-alpha-olefin (PAO) and poly-alkylene glycol (PAG). Both have their relative merits, but their use should be considered judicially since the cost of switching to a synthetic gear oil can often far outweigh the benefits. When selecting synthetic gear oil, it's not uncommon to drop down one ISO viscosity grade from the OEM recommendation since the effective viscosity of a synthetic gear oil at elevated operating temperatures often matches that of the OEM recommended mineral oil grade due to the higher viscosity index of synthetic fluids.
Increasingly, PAG fluids are being used in gearboxes, including initial factory fill. While these types of lubricants offer some distinct advantages, including lower coefficients of sliding friction and better deposit control, care must be exercised when using PAG fluids since they are chemically incompatible with hydrocarbon fluids, including conventional mineral-based gear oils and PAO synthetics.
While lubrication selection is an important first step in ensuring gearbox reliability, equally important is how the gearbox is maintained throughout its life. The degree to which water, moisture and other contaminants are controlled, as well as the introduction of precision maintenance practices, such as basic inspections and oil analysis, impact the longevity and performance of in-services gearboxes. In Parts 2 and 3 of this series, we'll examine how to ensure gear drives in your facility continue to operate reliably and efficiently.
Mark Barnes, CMRP, is Vice President of the Equipment Reliability Services team at Des-Case Corporation. Mark has been an active consultant and educator in the maintenance and reliability field for over 17 years. Mark holds a PhD in Analytical Chemistry. www.descase.com