Gears & Gear Drives

GEARS AND GEAR DRIVES GEAR TYPES .....................................................A145 GEAR TYPES 99%. Some sliding does oc- GEAR BASICS ...................................................A153 cur, however. And because ears are compact, SPEED REDUCERS ............................................A158 contact is simultaneous positive-engagement, across the entire width of G power transmission SELECTING GEAR DRIVES................................A162 the meshing teeth, a contin- elements that determine ANALYZING GEAR FAILURES...........................A168 uous series of shocks is pro- the speed, torque, and di- duced by the gear. These rection of rotation of driven rapid shocks result in some machine elements. Gear types may be objectionable operating noise and vi- grouped into five main categories: bration. Moreover, tooth wear results Spur, Helical, Bevel, Hypoid, and from shock loads at high speeds. Noise Worm. Typically, shaft orientation, ef- and wear can be minimized with ficiency, and speed determine which of proper lubrication, which reduces these types should be used for a partic- tooth surface contact and engagement ular application. Table 1 compares shock loads. these factors and relates them to the specific gear selections. This section on gearing and gear drives describes the major gear types; evaluates how the various gear types are combined into gear drives; and considers the principle factors that affect gear drive selection. Spur gears Spur gears have straight teeth cut Figure I — Involute generated by parallel to the rotational axis. The unwrapping a cord from a circle. tooth form is based on the involute curve, Figure 1. Practice has shown the tool traces a trochoidal path, Fig- Figure 2 — Root fillet trochoid generated that this design accommodates ure 2, providing a heavier, and by straight tooth cutting tool. mostly rolling, rather than sliding, stronger, root section. Because of this contact of the tooth surfaces. geometry, contact between the teeth The involute curve is generated occurs mostly as rolling rather than Spur gears are the least expensive to during gear machining processes us- sliding. Since less heat is produced by manufacture and the most commonly ing gear cutters with straight sides. this rolling action, mechanical effi- used, especially for drives with parallel Near the root of the tooth, however, ciency of spur gears is high, often up to shafts. The three main classes of spur gears are: external tooth, internal tooth, and rack-and-pinion. External-tooth gears — The most common type of spur gear, Figure 3, has teeth cut on the out- side perimeter of mating cylindri- cal wheels, with the larger wheel called the gear and the smaller wheel the pinion. The simplest arrangement of spur gears is a single pair of gears called a single reduction stage, where output rotation is in a direction opposite that of the input. In other words, one is clockwise while the other is counter-clockwise. Higher net reduction is produced with multiple stages in 2001 MSD ● Motion System Design ● A145 Chordal thickness Circular thickness Figure 4 — Internal (ring) gears produce a rangement, the plan- complex form of output with a planetary ets may be restrained configuration of sun, planets, and ring. from orbiting the sun and the ring left free closure in these applications, but to move. This causes some type of cover may be provided to the ring gear to rotate keep dirt and other contaminants in a direction opposite from accumulating on the working that of the sun. By al- surfaces. lowing both the planet carrier and the Helical gears ring gear to rotate, a differential gear drive Helical gearing differs from spur in Figure 3 — Spur gears have straight teeth is produced, the output speed of one that helical teeth are cut across the cut parallel to the rotational axis. shaft being dependent on the other. gear face at an angle rather than Rack-and-pinion which the driven gear is rigidly con- gears — A straight nected to a third gear. This third gear bar with teeth cut then drives a mating fourth gear that straight across it, Fig- serves as output for the second stage. ure 5, is called a rack. In this manner, several output speeds Basically, this rack is on different shafts can be produced considered to be a from a single input rotation. spur gear unrolled Internal (ring) gears — Ring and laid out fiat. gears produce an output rotation that Thus, the rack-and- is in the same direction as the input, pinion is a special Figure 4. As the name implies, teeth case of spur gearing. are cut on the inside surface of a cylin- The rack-and-pin- drical ring, inside of which are ion is useful in con- mounted a single external-tooth spur verting rotary motion gear or set of external-tooth spur to linear and vice gears, typically consisting of three or versa. Rotation of the four larger spur gears (planets) usu- pinion produces lin- ally surrounding a smaller central ear travel of the rack. pinion (sun). Conversely, move- Normally, the ring gear is station- ment of the rack causes the pinion to Figure 5 — Rack-and-pinion gearing ary, causing the planets to orbit the rotate. produces linear travel from rotational sun in the same rotational direction The rack-and-pinion is used exten- input. Shown here is spur gearing. Helical gearing is also available, but is not as as that of the sun. For this reason, sively in machine tools, lift trucks, common because the helical teeth create this class of gear is often referred to as power shovels, and other heavy ma- thrust, which produces a force acting a planetary system. The orbiting mo- chinery where rotary motion of the across the face of the rack. Worm rack is tion of the planets is transmitted to pinion drives the straight-line action also available, the axis of the worm the output shaft by a planet carrier. of a reciprocating part. Generally, the (pinion) being parallel to, rather than In an alternative planetary ar- rack is operated without a sealed en- perpendicular to, the rack. A146 ● Motion System Design ● MSD 2001 compact than double-heli- shafts. This overhung load (OHL) cals. However, the gear cen- may deflect the shaft, misaligning ters must be precisely gears, which causes poor tooth con- aligned to avoid interfer- tact and accelerates wear. Shaft de- ence between the mating flection may be overcome with strad- helixes. dle mounting in which a bearing is Cross-helical gears — placed on each side of the gear where This type of gear is recom- space permits. mended only for a narrow There are two basic classes of range of applications where bevels: straight-tooth and spirals. loads are relatively light. Straight-tooth bevels — These Because contact between gears, also known as plain bevels, teeth is a point instead of a have teeth cut straight across the face line, the resulting high slid- of the gear, Figure 9. They are subject ing loads between the teeth to much of the same operating condi- requires extensive lubrica- tions as spur gears in that straight- Figure 6 — Helical gears have teeth cut tooth bevels are effi- across the face at an angle for gradual cient but somewhat loading. noisy. They produce thrust loads in a di- straight, Figure 6. Thus, the contact rection that tends to line of the meshing teeth progresses separate the gears. across the face from the tip at one end Spiral-bevels — to the root of the other, reducing the Curved teeth provide noise and vibration characteristic of an action somewhat spur gears. Also, several teeth are in like that of a helical contact at any one time, producing a gear, Figure 10. This more gradual loading of the teeth that produces smoother, reduces wear substantially. quieter operation The increased amount of sliding ac- than straight-tooth tion between helical gear teeth, how- bevels. Thrust load- ever, places greater demands on the ing depends on the di- lubricant to prevent metal-to-metal rection of rotation and contact and resulting premature gear Figure 7 — Double helical Figure 8 — Herringbone whether the spiral failure. Also, since the teeth mesh at gearing uses two pairs of gears have opposed teeth angle at which the an angle, a side thrust load is pro- opposed gears to eliminate joined in the middle. teeth are cut is posi- duced along each gear shaft. Thus, thrust. tive or negative. thrust bearings must be used to ab- sorb this load so that the gears are tion. Thus, very little held in proper alignment. power can be trans- The three other principle classes of mitted with cross- helical gears are: double-helical, her- helical gears. ringbone, and cross-helical. Double-helical gears — Thrust Bevel gears loading is eliminated by using two pairs of gears with tooth angles op- Unlike spur and posed to each other, Figure 7. In this helical gears with way, the side thrust from one gear teeth cut from a cylin- cancels the thrust from the other drical blank, bevel gear. These opposed gears are usually gears have teeth cut Figure 9 — Straight-tooth bevel gears are manufactured with a space between on an angular or conical surface. efficient but somewhat noisy. the opposing sets of teeth. Bevel gears are used when input and Herringbone gears — Teeth in output shaft centerlines intersect. these gears resemble the geometry of Teeth are usually cut at an angle so Hypoid gears a herring spine, with ribs extending that the shaft axes intersect at 90 deg, from opposite sides in rows of paral- but any other angle may be used. A Hypoid gears resemble spiral- lel, slanting lines, Figure 8. Herring- special class of bevels called miter bevels, but the shaft axes of the pinion bone gears have opposed teeth to gears have gears of the same size with and driven gear do not intersect, Fig- eliminate side thrust loads the same their shafts at right angles.

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