Types Of Gears

The gears generally used for pump applications are parallel-shaft helical or herringbone gears and right-angle spiral-bevel gears. Spur gears are used on occasion, particularly in

FIGURE 1 Spiral-bevel gear, right-angle vertical FIGURE 2 Four-speed change gear, variable-speed pump drive drive

FIGURE 1 Spiral-bevel gear, right-angle vertical FIGURE 2 Four-speed change gear, variable-speed pump drive drive low-power, low-speed pump drives. Straight bevel and hypoid gears are also occasionally used in right-angle drives. As in the case of spur gears, straight-bevel gears are limited in power and speed. Hypoid gears are employed only infrequently for pumps because they are generally more costly than the other right-angle types. Worm gears, too, are employed only on occasion, in cases where an overall package requires a compact gear arrangement or when a high ratio of speeds is called for. Worm gears are limited in power capacity, and the efficiency of this type of drive is lower than that of other types. Figure 3 shows the various types of gears.

Parallel-Shaft Gearing A high-speed parallel-shaft gear drive is shown in Figure 5.

HELICAL VERSUS SPUR GEARS Spur gears transmit power between parallel shafts without end thrust. They are simple and economical to manufacture and do not require thrust bearings, but they are generally used only on moderate-speed drives.

One of the first decisions that must be made when considering a parallel-shaft gear reducer or power transmission drive is whether the gears should be spur or helical and, if helical, whether they should be single or double. It is generally acknowledged that helical gears offer better performance characteristics than do spur gears, but because helical gears, size for size, often are somewhat more expensive, some users have shied away from them to keep the cost of the gear drive to a minimum. However, cost studies that compared similar size spur and helical gears found that helical gears are actually a better buy.

The geometries of spur and helical gears are both the involute tooth form. Slice a helical gear at right angles to its shaft axis and you have the typical spur gear profile. Typical of a spur gear, however, is that, when driving another spur, its teeth make contact with the teeth of the mating gear along the full length of the face. The load is transferred in sequence from tooth to tooth.

A more gradual contact between mating gears is obtained by slanting the teeth in a way to form helices that make a constant angle—a helix angle with the shaft axis. Tooth contact between the teeth of mating helical gears is gradual, starting at one end and moving along the teeth so, at any instant, the line of contact runs diagonally across the teeth.

Types Gears
FIGURE 3 Types of gears

The effect of the tooth helix is to give multiple tooth contact at any time. Gear geometry can be arranged to give from two to six or more teeth at any time.

Because of the greater number of teeth in contact, a helical gear has a greater effective face width (up to 75% more) than an equivalent spur gear. Also, the effect of the tooth helix on the profile geometry in the plane of rotation is to make the pinion equivalent to a pinion with a greater number of teeth, thereby increasing its power capacity. A helical gear is capable of transmitting up to 100% more power than an equivalent spur gear. Furthermore, a helical gear set will give smoother, quieter operation.

The recommended upper limit of pitch line velocity for commercial spur gears is around 1000 ft/min (300 m/min). The upper limit for equivalent helical gears is about five times that, or 5000 ft/min (1500 m/min). Of course, as precision goes up, so do the permissible operating speeds for both spur and helical gears. Velocities in the 30,000-ft/min (9100-m/min) range are not uncommon for helical gears.

In addition to the normal radial loads produced by spur gears, helical gearing also produces an end thrust along the axis of rotation. The end thrust is a function of the helix angle: the larger the helix angle, the greater the thrust produced. Mounting assemblies and bearings for helical gearing must be designed to receive this thrust load.

Types Gear Drive

FIGURE 4 Cross section of spiral-bevel vertical FIGURE 5 High-speed parallel-shaft gear drive pump drive

FIGURE 4 Cross section of spiral-bevel vertical FIGURE 5 High-speed parallel-shaft gear drive pump drive double helical gears Gears of this type have two sets of opposed helical teeth. Each set of teeth has the same helix angle and pitch, but the helices have opposing hands of cut. Thus, the thrust loads in two sets of teeth counterbalance each other and no thrust is transmitted to shaft and bearings. Also, because end thrust is eliminated, it is possible to cut the teeth with greater helix angles than is generally used in helical gears. Tooth overlap is greater, producing a stronger and smoother tooth action.

The advantages attributed to helical gears are also applicable to double helical gears. Double helical gearing finds application in high-speed pump applications where a large helix angle must be combined with tooth sharing and elimination of end thrust for extremely smooth gear action.

Single helical gears, however, have some attractive advantages over double helical gears, the most significant being that, in the former, the external thrust loads do not affect gear tooth action. With a double helical gearing, a thrust load on the member with a thrust bearing tends to unload one of the helices and overload the opposite one. See transmission of external thrust forces in Section 6.3.1, "Pump Couplings and Intermediate Shafting."

Furthermore, the gear face for a single helical gear can be made narrower than for a double helical gear because the need for a groove between the two helices is eliminated. This leads to the use of a narrower, stiffer pinion with less tooth deflection and torsional windup and, generally, to a more favorable critical speed condition.

An axial vibration of the pinion, without a thrust bearing on a double helical gear set, is sometimes referred to as apex runout. This vibration can be caused by pitch circle runout where one helix is out of phase with the other. This tends to unload one helix cyclically and induce the vibration. The vibration will generally be the pinion because it has no thrust bearing, but it will be at the frequency of the member with the thrust runout problem.

When pitch circle runout, tooth spacing errors or lead errors are present in an element of a single helical gear set, the vibration will, when loaded, be radial because each member has a thrust bearing to restrain its axial movement.

continuous-tooth herringbone gears Gears of this type are double helical gears cut without a groove separating the two rows of teeth. Because of the arched construction of these gears, they are often known as "the gears with a backbone." Continuous-tooth herringbone gears are used for the transmission of heavy loads at moderate speeds where continuous service is required, where shock and vibration are present, or where a high reduction ratio is necessary in a single train. Because of the absence of a groove between opposing teeth, a herringbone gear has greater active face width than the hobbed double helical gear and therefore is stronger. There is also no end thrust, as the opposing helices counterbalance one another. The bearing arrangement of herringbone gears is usually the same as double helix in that the pinion does not usually have a thrust bearing. See transmission of external forces under Section 6.3.1.

Much of the success of the continuous-tooth herringbone gear is due to the greater number of teeth in contact and to the continuity of tooth action, which is an outgrowth of the larger helix angle. These larger helix angles can be fully utilized without creating bearing thrust loads. Continuous-tooth herringbone gears normally are furnished with a helix angle of 30°. Herringbone gears are generally of a lower American Gear Manufacturers Association (AGMA) quality level than hobbed or ground gears.

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