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August 1, 2011
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Certification Matters, Part III: Gearbox Principles And Lubrication

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By Ray Thibault, CLS, OMA I, OMA II, MLT, MLT II, MLA II, MLA III, Contributing Editor

This article is the third in this ongoing series on the important components of lubrication certification examinations administered by the Society of Tribologists and Lubrication Engineers (STLE) and the International Council for Machinery Lubrication (ICML).


Click here for more information on STLE and ICML certifications.

0708cover2Fig. 1. The simplest gear is the spur type. In this unit, the smaller gear is the “pinion.” The large one is the “bull.”The simplest gear is the spur type. In this unit, the smaller gear is the “pinion.” The large one is the “bull.” A key component of all types of applications, gears are used to transmit speed and power (torque) from one revolving shaft to another. They can change speed, torque and direction of rotation. Their major advantages as drives include the fact that they don’t slip, they’re able to carry high loads and they are compact in size. Figure 1 shows the simplest type, the spur gear.

Referring to Fig. 1, note that the smaller gear in the spur unit is the pinion; the larger one is a bull gear (also known as a “driven gear”). Typically, gears are speed reducers. The amount of speed reduction is based on the teeth ratio of the pinion and bull gears. For example, if the pinion has 15 teeth and the bull (driven) gear has 75, the reduction ratio is 5:1. A pinion rotating 3000 rpm would result in the shaft of the driven gear to be 600 rpm. The rotation of the bull gear is opposite the rotation of the pinion. To have the same rotation between the pinion and bull gear, an idler gear—which has no effect on speed and torque—is inserted between the pinion and bull gear. Several terms are used to describe the mating action of gear teeth:

  1. Pitch is the distance between a point on one tooth and the corresponding point on an adjacent tooth. It is the point on the tooth where rolling motion and the greatest force occur. Pitch circle is the circle formed by the point on each tooth at which meshing action is pure rolling.
  2. Sliding motion that occurs above and below the pitch point, called respectively the addendum and dedendum of the tooth, can result in high wear.
  3. The pure rolling at the pitch point results in an elastohydrodynamic lubrication regime characterized by a small solid-like lubricant film one micron or less in thickness. This small film does not prevent asperities (rough surface edges) from coming in contact with the pitch point, and causes initial pitting along the pitch line of the mating teeth. This is perfectly normal unless the pitting spreads destructively to the dedendum and eventually throughout the tooth.
  4. Clearance is the distance between the top of one tooth and the base of the tooth in the other gear.
  5. Backlash is the distance between the back of one tooth and the front of the next mating tooth.

Gear types and properties
Gears are classified by shaft orientation. The most common type makes up the parallel shaft group, shown in Fig. 2. 

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Fig. 2 The parallel shaft group is the most common gear type.

Some confusion can exist between double helical and herringbone gears: Most people consider them the same. In Fig. 2, though, notice that one of these gears has a strip in the middle and the other has continuous teeth. One definition holds that the double helical type has teeth slanting in opposite directions, while the teeth in the herringbone all slant in the same direction. Originally, that strip in the middle of the double helical was needed because of the manufacturing process. Eventually the process allowed all teeth to be continuous with no break in the middle. It should be noted that one of the major disadvantages of the helical gear is that it creates thrust along the shaft. This is eliminated by the use of herringbone (double helical) gears. Typical reduction ratios for parallel shaft gears do not exceed 10:1—and are more like 5:1. Table I summarizes the properties of parallel shaft gears.

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Table I. Properties of Parallel Shaft Gears (Click to enlarge)

As illustrated in Fig. 3, the next group of gears, which have shafts at right angles, are divided into intersecting and non-intersecting types.

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Fig. 3. The right-angle shaft gear group is made up of intersecting and non-intersecting types.

Both bevel and spiral bevel gears have shafts that intersect at the centerline, whereas worm and hypoid gears have non-intersecting shafts with one below the centerline. The properties of right-angle shafts are illustrated in Table II.

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Table II. Properties of Right-Angle Shaft Gears (click to enlarge)

It should be noted that hypoid gears are used primarily in automotive applications: They’ve replaced spiral bevel gears in differentials, which results in a much more compact arrangement since the shafts can pass each other. They also produce high torque.

Lubrication delivery systems
Most gears are lubricated by splashing oil from a sump onto the gear teeth and bearings. Achieving the right level/delivering the correct amount of lubricant is crucial. If the level is too low, you’ll find yourself dealing with lubricant starvation, increased wear, inadequate heat dissipation and foaming. Too much lubricant, on the other hand, may lead to churning, resulting in higher operating temperatures, a decrease in efficiency and greater foaming tendency. Typically, for parallel shaft and bevel gears at normal speed (1000 fpm- 4000 fpm), the oil level ranges from completely covering the bottom teeth up to three times the depth of the bottom teeth—the most common being twice the depth of the bottom teeth. At very low speeds (< 1000 fpm), the level of immersion can be 3-5 times the tooth depth. Lubrication of worm gears is different. Worm gears come in three designs, each with its own lubrication approach:

  • Worm on top… the oil level is typically one-third of the wheel diameter.
  • Worm on the bottom… the oil level is up to 50% of the worm, which is the center of the meshing zone.
  • Worm at the side… half the wheel is immersed to at least the worm height.

It’s best to adhere to what the OEM recommends for oil level during splash lubrication. The above levels are merely general guidelines.

Be aware that there is a speed limitation on the use of splash lubrication. Speed is measured in meter/second or feet/minute (fpm) and calculated by multiplying the circumference of the gear (π x diameter). For example, a 12” diameter gear, running at 1000 rpm, will have a speed of 3140 fpm (3.14 x 1 ft x 1000 rpm). With no design changes, a splash-lubrication system can usually operate up to 4000 fpm. By installing baffle plates and oil pockets, the speed can reach 11,000 fpm. At higher speeds, a pressure-circulation system is used. The two major types are dry sump (where the oil is stored outside the gearbox), or wet sump (where the oil is in the gearbox). In a pressure-circulation system, oil is sprayed directly at the teeth contact points.

Lubricant selection
The most important property for an oil used to lubricate enclosed gears is correct viscosity. The major variable in viscosity selection is the speed of the gears expressed in pitch line velocity, which is defined as speed of the gear in rpm times the circular pitch diameter in inches. The American Gear Manufacturers Association (AGMA) publishes viscosity recommendations based on pitch line velocity. Robert Errichello, a world-renowned expert on gear failure analysis, has developed the following simplified formula for determining the correct viscosity to use on enclosed gears:

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Table III notes the most common viscosities and gear oil types for enclosed gearboxes.

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Keep in mind that Table III is only a summary of the most common viscosity grades for enclosed gears: It incorporates the old AGMA system, which has been changed. New tables no longer include the AGMA number. Many gearboxes still reflect the old system where viscosity grades were also expressed as a single digit number. Referring to Table III, we see the outdated AGMA number for ISO 220 gear oil is 5.

To purchase the most up-to-date AGMA Classification System chart, go to www.AGMA.org. Once the correct vis-cosity has been determined, the oil type must be selected. Options include rust and oxidation (R&O) inhibited lubricants, synthetics, extreme pressure (EP) products and compounded oils. Table IV lists the types of oils used on enclosed reducer gearboxes.

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Table IV reflects general guidelines only: There are many exceptions. The following are additional comments on oil selection for reducer gearboxes:

  • The most common viscosity grade for both parallel shaft and right-angle intersecting gears is ISO 220 EP. When in doubt about using EP oil, go with it.
  • Worm gears experience high sliding, and the ring gear is typically bronze. EP additives are not recommended because of the additive attack on the yellow metal at high temperatures.
    • Although new EP additives are less aggressive, in some cases such additives are not activated by the yellow metal and steel contact and serve no purpose in the formulation.
    • Traditionally, worm gears were lubricated with oils compounded with synthetic animal fat to provide protection during the severe sliding that occurs.
    • PAOs have been successfully used in worm gears by providing protection during severe sliding without the use of EP additives. They also lower the temperature of the box and are completely compatible with mineral oils.
    • PAGs are the lubricants of choice on new gearboxes as they provide the best efficiency. They’re also used in small, sealed-for-life worm gearboxes.
  • If changing from another type of oil to a PAG, follow proper flushing procedures. PAGs are incompatible with mineral oils and PAOs.
  • Although hypoid gears are used mainly in automotive applications, they do have a few industrial uses. The severe sliding that occurs in these types of gears calls for very aggressive EP additives in high concentrations. Typical EP industrial oils will not provide the necessary protection for hypoid gears.

This information is just a start: More details on gear oils can be obtained from product data sheets.

NOTE: While a product data sheet provides useful information, the true test of a gear oil is how it works in the system. Adhere to OEM guidelines and consult with your lube supplier for further information.

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Fig. 4. Destructive pitting is caused by surface overload conditions.

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Fig. 5. Ridging on the side of a deformed gear tooth indicates a condition known as plastic flow, which is caused by severe overload. (This is NOT a lubrication-related condition.)

Gear failure modes
The major factors affecting gear life are load, environment, temperature and speed. Wear modes are summarized as follows:

  1. Adhesive wear is caused by an inadequate lubricant film under severe boundary-lubrication conditions. This situation may require the use of higher-viscosity oil with an EP additive.
  2. Abrasive wear is caused by hard particles in the oil gouging the gear teeth. The degree of wear is related to hardness and amount of particles present. Some have the misconception that gear oils don’t need a high cleanliness standard. That is incorrect: Gear oils should be kept as clean as possible. Very clean oil in a gearbox would be 17/15/13.
  3. Surface fatigue is material failure caused by repeated surface and sub-surface stresses beyond the endurance limit of the metal that result in surface pitting.
    1. Initial pitting occurs at the pitchline where the lubricant film is very thin, resulting in asperity removal creating a smoother surface. This is perfectly normal and should be no cause for concern.
    2. Normal pitting occurs in the root part of the tooth (dedendum) and usually stabilizes. It occurs primarily when gear loads are close to the maximum and should be watched closely.
    3. Destructive pitting starts at the pitchline and progresses until the tooth is destroyed and is caused by surface overload conditions. This can be seen in Fig. 4.
  4. Plastic flow is the deformation of gear teeth due to severe overload—this condition is NOT lubrication-related. It can be caused by unhardened teeth subjected to heavy loads (particularly shock-loading) causing surface material to be squeezed out at the tips of the teeth, shown in Fig. 5. (The ridging on the side of the tooth indicates plastic flow.)
  5. Tooth breakage is primarily caused by severe surface overload, but also could result from severe surface fatigue that significantly weakens the tooth.

Conclusion
Gears are an integral part of many manufacturing processes: A failure of a gear can have an enormous impact on production. These components must be lubricated properly and maintained to achieve long life. Oil analysis is an important predictive tool in monitoring gear wear. (This topic will be discussed in a later installment of our series.)

Coming up
The next installment in this series discusses the Basic Principles of Fluid Power. Look for it in the September/October issue of LMT.

Ray Thibault is based in Cypress (Houston), TX. An STLE-Certified Lubrication Specialist and Oil Monitoring Analyst, he conducts extensive training for operations around the world. Telephone: (281) 257-1526; email: rlthibault@msn.com.

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