11. GEAR TRANSMISSIONS

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Gregory Dorsey
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1 11. GEAR TRANSMISSIONS GENERAL CONSIDERATIONS Gears are one of the most important elements used in machinery. There are few mechanical devices that do not have the need to transmit power and motion between rotating shafts. Gears not only do this most satisfactorily, but can do so with uniform motion and reliability. In addition, they span the entire range of applications from large to small. To summarize: 1. Gears offer positive transmission of power. 2. Gears range in size from small miniature instrument installations, that measure in only several millimeters in diameter, to huge powerful gears in turbine drives that are several meters in diameter. 3. Gears provide high load capacity within small overall dimensions. 4. Gears can provide position transmission with very high angular or linear accuracy; such as used in servomechanisms and military equipment (due to rigorously constant transmission ratio). 5. Gears can couple power and motion between shafts whose axes are parallel, intersecting or skew. 6. Gear designs are standardized in accordance with size and shape which provides for widespread interchangeability. 7. Gears are reliable, long life and easily maintained. On the other hand, some of their shortcomings are: 1. Gears require high precision for both manufacturing and mounting. 2. Gear manufacturing is not so simple so that it is considered a costly process. 3. Noise and vibrations are important concerns for many applications. 4. Gears transmit rigidly the torque (they have practically no damping characteristics) GEAR CLASSIFICATION There are several criteria of classification. A summary is given in the following: i) Relative position of the axes (a) parallel (spur and helical gears, rack and pinion); (b) intersecting (bevel gear); (c) skew (worm gear, hypoid gear, zerol gear, crossed helical). ii) The shape of the wheels (a) cylindrical (spur and helical gears, rack and pinion); (b) conical (bevel gear); (c) hyperboloidal (worm gear, hypoid gear, zerol gear). iii) Tooth line (direction) (a) straight (parallel with the axis of rotation, inclined single and double- in respect with the axis of rotation); (b) spiral (worm gear, hypoid gear, zerol gear). iv) Type of the profile (a) involute (spur and helical gears, rack and pinion); (b) cycloid; (c) arc of circle (Novikov). v) Type of axis (a) fixed; (b) mobile (planetary/epyciclic/ gears). Cap. 11 Gears

2 Parallel Gearing Spur gears Spur gears are the simplest and the most common type of gears used to transmit motion between parallel shafts, having the teeth parallel to the shaft axis. It has the largest applications and, also, it is the easiest to manufacture. They could be with external or internal gearing. Internal gears have their teeth pointing inward and are commonly used in the planetary gear set used in automatic transmissions and transfer cases. In planetary gear sets an outer ring gear has internal teeth that mate with teeth on smaller planetary gears. These gears, in turn, mesh with a center or sun gear. Many changes in speed and torque are possible, depending on which parts are held stationary and which are driven. In a planetary gear set, one gear is normally the input, another is prevented from moving, or held, and the third gear is the output gear Helical gears A helical gear can be imagined as an ordinary spur gear machined from a stack of thin shim stock, each limitation of which is rotated slightly with respect to its neighbors. Consequently, the teeth longitudinal axis is inclined in respect with the shaft axis. When power is transmitted by a pair of helical gears, both shafts are subjected to a thrust load. To eliminate this, double helical or herringbone gears can be used, with the price of increased manufacturing and mounting cost. As helical gears rotate, each tooth comes first into engagement on one side, with contact spreading along the tooth as rotation continues. Thus teeth come in contact gradually, which makes for smoother and quieter operation than with spur gears. A common application is in automotive transmissions, for which quietness is a priority Herringbone gears As shown before, Herringbone gears are actually double helical gears with teeth angles reversed on opposite sides. This causes the thrust produced by one side to be counterbalanced by the thrust produced by the other side. The two sets of teeth are often separated at the center by a narrow gap for better alignment and to prevent oil from being trapped at the apex. This median gap is also necessary as tool run-out if the gear is cut with both sides together (the alternative is to cut separately each side of the double gear and then, to mount them together). Herringbone gears are best suited for quiet, high-speed, low-thrust applications where heavy loads are applied. Large turbines and generators frequently use herringbone gears because of their durability rack and pinion gears Rack and pinion gears convert straight-line motion into rotary motion, and vice versa. Rack and pinion gears also change the angle of power flow with some degree of speed change. The teeth on the rack are cut straight across the shaft, while those on the pinion are cut like a spur gear. These gear sets can provide control of arbor presses and other devices where slow speed is involved. A typical application is in automotive steering boxes Intersecting Axes Gears Straight Bevel Gear This is a gear in which the teeth have tapered conical elements that have the same direction as the pitch cone base line (generatrix). These gears permit the power flow to turn a corner. The straight bevel gear is both the simplest to produce and the most widely applied in the bevel gear family. The most commonly used spiral beveled gear set is the ring and pinion gears used in heavy truck differentials. Bevel-type gears are also used for slow-speed applications that are not subject to high impact forces. Handwheel controls that must operate some remote device at an angle use straight bevel gears Spiral Bevel Gear This is a bevel gear with a helical angle of spiral teeth. It is much more complex to manufacture, but offers a higher strength and lower noise. Cap. 11 Gears

3 Zerol Gear Zerol gear is a special case of spiral bevel gear with zero degree of spiral angle tooth advance. Teeth are crowned, so that tooth contact takes place first at the tooth center. It has the characteristics of both the straight and spiral bevel gears. The forces acting upon the tooth are the same as for a straight bevel gear Face Gear (Crown Gear) This is a pseudobevel gear that is limited to 90 (pitch angle) intersecting axes. The face gear is a circular disc with a ring of teeth cut in its side face; hence the name face gear. Tooth elements are tapered towards its center. The mate is an ordinary spur gear. It offers no advantages over the standard bevel gear, except that it can be fabricated on an ordinary shaper gear generating machine Nonparallel and Nonintersecting Axes Gears Worm and Worm Gear Worm set is the name for a meshed worm and worm gear. The worm resembles a screw thread; and the mating worm gear a helical gear, except that it is made to envelope the worm as seen along the worm's axis. The outstanding feature is that the worm offers a very large gear ratio in a single mesh. However, transmission efficiency is very poor due to a great amount of sliding as the worm tooth engages with its mating worm gear tooth and forces rotation by pushing and sliding. With proper choices of materials and lubrication, wear is contained and noise is low. Like the screw, the worm can have one or more threads. Worm gears usually provide right-angle power flows. It is a one way gear (non back-driveable) the worm can turn the gear, but the gear cannot turn the worm. This is because the angle on the worm is so small that the gear cannot overcome the friction of the worm. The most common use for the worm gear is in applications where the power source operates at high speed and the output is at slow speed with high torque. Many steering mechanisms use a worm gear connected to the steering shaft and wheel and a partial (sector) gear connected to the steering linkage. Small power hand tools frequently use a high-speed motor with a worm gear drive. Also, many elevators are driven by a worm gearing as its one-way feature provides safety operation Double Enveloping Worm Gear This worm set uses a special worm shape in that it partially envelops the worm gear as viewed in the direction of the worm gear axis. Its big advantage over the standard worm is much higher load capacity. However, the worm gear is very complicated to design and produce, and sources for manufacture are few Hypoid Gear This is similar to spiral bevel gear but differ in that the axis of the pinion is offset from the center axis of the ring gear, so that the two axes do not intersect each other. It is widely used as the first stage in automobile gear boxes. This permitted the drive to the rear axle to be nonintersecting, and thus allowed the auto body to be lowered. However, it is complicated to design and is the most difficult to produce on a bevel gear generator Crossed Helical /Spiral/Skew/Screw/ Gear A classical pair of helical gears can be made to operate on nonparallel, nonintersecting shafts. In this case, the theoretical contact is a point, so that they can carry only light loads. A common application is driving the distributor and oil pump from the camshaft in automotive engines. Cap. 11 Gears

4 Fig Parallel Gearing Fig Straight Bevel Gear Fig Spiral Bevel Gear Fig Zerol Gear Zerol gear Spiral bevel gear Fig Fig Spiral Bevel Gear Fig Face Gear Cap. 11 Gears

Fig. 11-8 Worm Gear Fig. 11-9 Double Enveloping Worm Gear Fig. 11-10 Hypoid gear Fig.

Types of Gears and their efficiency Categories of Gears Types of Gears Efficiency(%)

5 Helical Rack Double Helical Gear Straight Bevel Gear Intersecting Axes Gears Spiral

5 Fig Worm Gear Fig Double Enveloping Worm Gear Fig Hypoid gear Fig Crossed helical gear Table Types of Gears and their efficiency Categories of Gears Types of Gears Efficiency(%) Parallel Axes Gears Spur Gear Spur Rack Internal Gear Helical Gear Helical Rack Double Helical Gear Straight Bevel Gear Intersecting Axes Gears Spiral Bevel Gear Zerol Gear Nonparallel and Worm Gear Nonintersecting Screw Gear Axes Gears Hypoid Gear Cap. 11 Gears

6 11.2. TOOTH GENERATION Figure Shaving with a rack cutter Figure Gear cutting by hobbing with a hob Figure Gear cutting by milling Cap. 11 Gears

Catalog Q Conversion For those wishing to ease themselves into working with metric gears

Catalog Q Conversion For those wishing to ease themselves into working with metric gears 1.3.4 Conversion For those wishing to ease themselves into working with metric gears by looking at them in terms of familiar inch gearing relationships and mathematics, Table 1-5 is offered as a means