This is typically the power of the prime mover and its speed. If your motor regularly runs at a lower load than the nameplate, the demand or service horsepower may be used. Gear drives designed and selected in accordance with American Gear Manufacturers Association (AGMA) standards will permit starting and momentary overloads of 200 percent of the unit rating. The unit rating is defined as the maximum power that can be transmitted without exceeding the lowest individual rating of the gearing, housing, shafting, keys, bearings, fasteners and other components of the basic gear drive and auxiliary systems.
Ratio
To arrive at the specific gear ratio required, divide the motor full-load speed by the revolutions per minute (rpm) of the driven equipment. Exact ratios are determined by dividing the actual number of gear teeth by the mating pinion teeth—both of which are whole numbers.
AGMA offers standard ratios. Typical manufacturer’s deviations between AGMA nominal and exact ratios are ±3 percent for a single reduction gear drive, and ±4 percent for a double reduction.
For applications with variable frequency drives, exact gear ratios become less important. In that case, it is best to select a manufacturer’s standard ratio. These will provide lower costs and shorter delivery, with ready availability of off-the-shelf stock spare parts.
Configuration
Gear drives are available in a variety of sizes with various shaft configurations to meet your space requirements. The most popular are parallel shaft, concentric and right angle, with the low-speed shaft either horizontal or vertical. Space should be allowed to access areas such as the lubrication points, the dipstick and the inspection cover.
Service Factor (SF)
Most applications have startup loads, overloads and expectations for life and reliability that cannot be completely captured in the motor loads. The minimum service factor (SF) is a variable that includes the combined effects of stress cycle, reliability and overload factors. It is used to calculate an equivalent horsepower. Application and service duty play an intricate role in determining the proper SF. Appropriate values of SF are determined by field experience. AGMA Standard 6013-B16 (metric 6113-B16) for enclosed speed reducers also contains a listing of applications with their recommended service factors.
A higher SF or larger gear drive size should be selected when peak running loads are substantially greater than normal operating loads. For example, an application that places a torque load on a drive in excess of its rated capacity will inevitably result in distress and, in severe cases, breakage.
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. Class I, II or III are equivalent to SF values of 1.0, 1.41 or 2.0, respectively. Service class and service factor can be used interchangeably. However, numerical designations are preferred because service class does not accommodate intermediate values of SF.
Special consideration for the type of prime mover shall be used as well for determining service factors.
Published service factors are only the minimum recommended for a given application. Some applications require special procedures and may need to be referred to 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 the gear drive selection. These conditions will require changes to be made in the inertia or spring constants of the drive system. Other applications that fall under non-standard selection procedures include a high frequency of starts, reversing service, brake-equipped applications, oversize prime movers, and speed variation or multi-speed applications.
