Gearbox essentials

Use these guidelines for selecting, commissioning and maintaining gear reducers.

By Donald L. Borden, P.E.

PlantServices.com

Gears have been with us for at least 3,000 years and they’ve been improved over the years. About 200 years ago, a typical large gear would be a wooden rim with wooden teeth inserted around the circumference (Figure 1). The teeth were pinned in place, simplifying maintenance because a broken tooth could be changed out easily.

Figure 1. About 200 years ago, a typical large gear would be a wooden rim with wooden teeth inserted around the circumference.

A modern gear drive can be considered as a box of machine components: bearings, shafts, shaft seals and gears. The gear teeth cut into the blank have a shape known as an involute. The involute shape, when manufactured to specific tolerances, then transmits motion at a constant speed. A major feature of a gear drive is that for a constant horsepower, torque is an inverse function of speed. With a speed reduction, the turning torque increases in proportion to the reduction ratio. It’s possible to use this characteristic to increase the prime mover’s turning torque by a factor of as much as 1,700. Achieving this kind of performance requires paying attention to the best practices for selecting, installing and operating geared drives.

Pick a winner

Manufacturers of gear drives are offering many different types of gears. If you can’t find a supplier in your area, contact the American Gear Manufacturers Association for suggested manufacturers. The selection of an appropriate manufacturer depends on the product’s features, as well as your own economic considerations.

Define your horsepower and speed-reduction requirement. Describe your application, the service time in hours per day, the environment for the operation, the mounting position for the drive and any size limitations. The AGMA establishes suggested service factors for different applications, based on the experiences of its members. Multiplying these service factors by the motor horsepower determines the required horsepower used for selecting the size of the gear drive. These factors take into consideration twin realities: an applied load is never uniform and the degree of load variation is a function of the specific application.

A gear’s dimensional accuracy is important and the AGMA developed accuracy specifications for the elements of gear geometry. The levels of accuracy are known as quality numbers. AGMA quality level 9 is adequate for most commercial gear drives. Drives that need greater reliability must be made more accurately, and an AGMA quality of 12 to 15 might be required. Remember, the higher the level of accuracy, the higher the price of the gear.

Use a solid foundation

Once you determine the size of the drive, your next consideration is the mounting surface. Provide a solid foundation that won’t sag or crack. Once the drive is properly aligned to the prime mover and driven equipment, it must stay aligned.

If you use a concrete foundation, allow the concrete to cure completely before mounting the drive on it. Don’t mount the drive directly on the concrete, though. The best mounting uses structural steel mounting pads that are grouted into the concrete. The steel mounting pads provide the drive with a flat, level surface, making the installation of the drive an easier task. After the drive is in operation, check the foundation periodically for cracks and check the shaft alignment. Mount the drive above the floor level to facilitate oil changes. The drain plug should be at the lowest point on the housing.

Connect and align

A drive usually has two shaft connections: an input shaft for the prime mover and an output shaft for the driven equipment. Possible connecting devices include couplings, chains, pulleys or sheaves with V-belts. Whatever device you use, mount it as close as possible to the drive housing to minimize the overhung load caused by the pull of the chain or belt. The rotating shafts, of course, require guards that meet OSHA requirements. The guards might be available from the drive manufacturer.

Align the drive with flat, broad shims inserted under the drive’s mounting pads. Starting at one end of the drive, level along the length and then across the width. Shaft extensions can be used for leveling, as well as pads machined into the housing that facilitates lateral and longitudinal alignment.

Check each mounting foot with a feeler gauge to ensure that each pad provides support, thus minimizing housing distortion when the drive is bolted down. After the unit is aligned with the driven machine and bolted down, align the prime mover to the unit input shaft. If the prime mover was factory-aligned to the drive, ensure that the alignment didn’t shift during transit.

It’s important to maintain shaft alignment because shaft misalignment might have an adverse effect either on the device connecting the shafts or on the bearings and the drive’s internal shafting. A gear drive is designed to accept a certain amount of overhung load, and bearings are selected and shafts are designed to accommodate those anticipated loads. With the exception of an elastomer coupling, a misaligned coupling produces an appreciable radial overhung load (at right angles to the shaft axis) on the shaft that can overload the bearings and can stress the shaft beyond its fatigue strength.