Gearmotors such as this new S3 unit offer up to 145,000 lb-in. torque, 150 hp, and 1,250:1 ratio for food processing, water treatment, paper mills, chemical plants, and heavy duty pumps.

How you select geared speed reducers often means the difference between successful operation and failure. Common errors include undersized reducers, incorrect ratios, mismatched drive train components, and incorrect configurations. As the following example shows, ignoring one crucial factor leads to a reducer selection that is destined to fail.

Getting it almost right

Engineers at one plant found out how important it is to recheck all drive specifications when upgrading a conveyor line. For many years, the company ran a trouble- free packaging line using a 3-hp, 1,750-rpm motor coupled to a helical-gear speed reducer with a 9.3:1 speed reduction ratio. To increase production, they modified the conveyor to run 50% faster with the same load. This upgrade required more power.

The original application drew slightly less current than the full load ampere rating of the 3-hp motor, so the engineers increased the motor size approximately 50%, selecting a 5-hp, 1,750-rpm motor. Adding a 1:1.5-ratio V-belt drive between the motor and the speed reducer gave the necessary 50% increase in output speed. The engineers also reasoned that the rating of the present reducer (4.07 hp at 1,750 rpm) would increase somewhat proportionally to speed, giving about 6.1 hp, or enough to handle the increased power.

Within a few weeks, the once-reliable speed reducer was running hot and noisy. Workers quickly replaced the reducer with a new unit of the same size and type. While rebuilding the old unit, they found that the gears were pitted and worn, and the bearings were rough and discolored. The same thing happened to the replacement reducer after a few weeks.

The reducer manufacturer was called in, and found that the gears were the victims of a false assumption. Although the reducer’s mechanical rating did increase with speed, as expected, its thermal horsepower rating did not increase. Consequently, the transmitted horsepower exceeded the unit’s thermal horsepower rating, which caused the lubricating oil to overheat and break down. Furthermore, the reducer housing was not large enough to dissipate the excess heat generated by the increased power demand.

Had the engineers checked the thermal rating, they would have found that a larger reducer was needed.

Check important factors

To avoid committing such errors, remember that most speed reducers listed in catalogs are designed and rated only for standard operating conditions. However, reducers often encounter more severe conditions. Therefore, be sure to consider the following factors in making a selection:

Environment. Most standard reducers are intended for indoor or outdoor installation in a relatively clean and nonabrasive atmosphere with an ambient temperature range of 15 to 125 F. For temperatures above or below these limits, as well as excessively dusty and abrasive environments, consult the manufacturer. The same is true for corrosive or explosive atmospheres, as well as for high-altitude service (above 3,300 ft). Reducers suitable for washdown applications are available as an option.

Operating conditions. Unusual operating conditions include nonstandard mounting positions (inclined), high inertial loads, torsional vibrations, and a large number of starts or stops (over five per hour). Such conditions, as well as any application that involves the handling or safety of people, call for discussion with the manufacturer.

Motor type. Speed reducer catalogs usually contain information on motor compatibility, mounting arrangements, and dimensions. If you are using a hightorque motor (NEMA Design C), a slip motor, or any motor other than a NEMA Design B, consult the reducer manufacturer. The starting torque of a NEMA Design C (high-torque) motor, for example, may require a reducer with a higher torque capacity.

Get all the data

Make sure you have the following information before selecting a speed reducer:
• Type of reducer required — depends on such diverse factors as user preference for a particular configuration (inline, parallel shaft, or right angle), physical layout, size limitations, operator- friendliness, and cost-effectiveness.
• Duty cycle — including hours of operation per day, starts per hour, and reversals per hour.
• Motor horsepower (for motorized reducers).
• Demand (transmitted) horsepower or torque (for non-motorized reducers).
• Motor speed (rpm).
• Speed (rpm) of driven machine (output speed of reducer if not direct-connected).
• Details of sprocket or pulley, and its position on the reducer output shaft, if reducer is not directly connected to the driven machine.
• Unusual environmental and operating conditions, or special motor types (as described earlier).

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