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FIGURE 7 Shaft arrangements for gear drives noise and vibration, the allowable maximum speed of the bearings, the pumping and churning of the lubricating oil, the friction of the oil seals, and the heat generated in the unit.

At high speeds, it is possible for inaccuracies in the gear teeth to produce failure even though no power is being transmitted. Gear reducers built in accordance with AGMA specifications are recommended to operate at speeds given in Table 1.

power range OF gear reducers The power-transmitting capacity of a reduction gear unit is a function of the output torque and the speed of the reducers. Some types of reducers, such as worm gear reducers, are more satisfactory for high torques and low speeds, whereas others, such as helical herringbone (or double helical), are suitable for high torques and also high speeds. Therefore the range of powers suitable for various units is considerable. A listing of this range obtainable in standard types of reducers is given in Table 1, with a brief explanation of why the range indicated is maintained. These powers are not fixed at the values given because they are continually changing. Although the values given are general, there are many special reducers available outside this range.

ratios and efficiencies The ratio of a gear reducer is defined as the ratio of the input shaft speed to the output shaft speed. Different types of gearing allow different ratios per gear stage. Spur gears usually are used with a ratio range of 1:1 to 6:1; helical, double helical, and herringbone with ratios of 1:1 to 10:1; straight-bevel with ratios of 1:1 to 4:1; spiral-bevel (also zerols and hypoids) with ratios of 1:1 to 9:1; and worm gears with ratios of 3j:1 to 90:1. Planetary gear arrangements allow ratios of 4:1 to 10:1 per gear stage.

Factors influencing the efficiency of a gear reducer are

1. Frictional loss in bearings

2. Losses due to pumping lubricating oil

3. Windage losses due to rotation of reducer parts

4. Frictional losses in gear tooth action

It is not uncommon in many types of reducers to have the combined losses due to items 1, 2, and 3 greater than the loss due to item 4. For this reason, in some cases the power lost in the reducer remains practically constant regardless of the power transmitted. Therefore, it must be realized that the efficiency specified for a reducer applies only when the

TABLE 1 Guide for gear selection and design (reflecting generally accepted design criteria)
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