Gear Drives Basics

Gear drives deliver power to industrial equipment such as bulk material conveyors, mixers, pumps and paper mills. The reliability that translates into greater uptime and profitability begins by specifying and selecting the proper drives for these critical applications. Many variables—such as service factor, gear drive rating, thermal capacity, speed variation and drive ratio—must be considered when sizing and selecting a gear drive. In addition, specific drive features may provide value-added benefits such as cooler operation or ease of serviceability that help reduce the total cost of ownership over the life of the drive—a win for any organization. Here are several major areas of importance to consider when selecting a gear drive.

Service Factor

The service factor (SF), a variable that combines external load dynamics, reliability and life, is used to calculate equivalent horsepower. Application and service duty play an intricate role in determining the proper service factor. Acceptable values of SF are determined by field experience. The American National Standards Institute (ANSI)/American Gear Manufacturers Association (AGMA) Standard 6013– A06 (Metric 6113–A06) for enclosed speed reducers publishes a listing of applications with their recommended service factors. Once an SF is chosen, the factor is multiplied by the motor nameplate power to establish the size of drive required by the equipment to be driven. A higher SF—or larger gear drive size— should be selected when peak running loads are substantially greater than normal operating loads. Gear drives that are supplied in combination with electric motors may be designated with a service class number such as I, II or III, rather than a numerical SF. Classes I, II or III are equivalent to SF values of 1.0, 1.41 or 2.0. Service class and service factor are used interchangeably; however, numerical designations are preferred because service class does not accommodate intermediate SF values. Note that published service factors are only the minimum recommended for a given application. Applications involving unusual or severe loading, or those requiring a higher degree of dependability, should be reviewed with the drive manufacturer. Typical values of SF will not accommodate systems that have serious, critical vibrations or repetitive shock loading. The system designer must identify vibratory or shock loading prior to gear drive selection. These conditions will require changes to be made in the inertia or spring constants of the drive system.

Gear Drive Rating

Published ratings of a gear drive are determined by the mechanical load-carrying capacity of gear tooth elements, rotating shafts and bearings. For example, the ANSI/AGMA Standard 6013–A06 establishes standards for industrial enclosed gear drives. The horsepower rating of a gear tooth is less than or equal to the durability (pitting resistance) of the surface, or strength (bending fatigue) rating, as determined by established AGMA criteria. As the SF is increased, the relationship between gear life (based on pitting resistance) and load is proportional to the increase in SF raised to the 8.78th power.

For example, if SF is increased by 30 percent, the gear tooth life will increase 10 times. Shafts support the gear tooth elements that transmit torque from the motor to the driven machine and also distribute the radial loads to the bearings. While shafts are designed for carrying torsional and bending stresses, they also minimize deflection by maintaining uniform contact across the gear face. Roller bearings are selected according to bearing manufacturers’ recommendations. Bearing life is defined as the number of hours of operation at a constant speed before the first evidence of fatigue develops on either.

The above is a brief introduction to Gear drive. In general, the gear drive is also a gear box, and the functions are the same. We will continue to update you with information about geared motors.

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