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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).
Gear types and properties
Gears are classified by shaft orientation. The most common type makes up the parallel shaft group, shown in Fig. 2.
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.
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.
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.
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:
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.
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:
Table III notes the most common viscosities and gear oil types for enclosed gearboxes.
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.
Table IV reflects general guidelines only: There are many exceptions. The following are additional comments on oil selection for reducer gearboxes:
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.
Fig. 4. Destructive pitting is caused by surface overload conditions.
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:
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.)
The next installment in this series discusses the Basic Principles of Fluid Power. Look for it in the September/October issue of LMT.