Select speed reducers to perform reliably

Oct. 1, 2000
Specifying a geared speed reducer or gearmotor can be tricky. But you can avoid common mistakes and ensure the reducer’s reliability by considering all of the important application factors.

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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Selection methods

You can use one of two methods to select various types of geared speed reducers, including inline, parallel-shaft, and right-angle (worm gear) configurations.

The first method, used mostly for gearmotors, is the Service Class system as described in ANSI/AGMA Standard 6019. The second method is the Service Factor system described in ANSI/AGMA Standard 6010.

Service Class system. Use this method where the motor horsepower already has been determined. In this case, the speed reducer may be a motorized or motor-adaptable unit with a NEMA C-face adapter, a dedicated-flange motor, or a scoop-type bracket for foot-mounted motors.

Here’s the procedure:

1. Determine whether your application is Class I, II, or III from the AGMA (or manufacturer’s) Service Class Table based on the type of application and duty cycle that most closely matches yours, Table 1.

2. Refer to the manufacturer’s selection table for the type of reducer and service class required. Find the column for the motor horsepower required, and the row showing the reducer ratio (input speed/output speed) or the desired output speed. At this intersection, you’ll find a reducer model that is suitable for the application.

3. Calculate output shaft overhung load (if applicable) according to the box “How to check shaft overhung load,” and make sure that the selected reducer can handle this load.

4. You have now selected a basic speed reducer. To be sure you cover all the additional details that must be specified when ordering the unit, see the box “Ordering information checklist.”

Service Factor system. Use this method where the driven or transmitted power and speed are known and the reducer is supplied either with or without a motor. This method also works where the motor is not directly connected to the reducer, such as with a V-belt drive between motor and reducer, or a power takeoff from a line shaft.

For this method, use the following steps.

1. Using the AGMA (or manufacturer’s) Service Factor Table for the type of reducer you need, Table 2, determine the service factor for the type of application and duty cycle that most closely matches yours.

2. Determine the equivalent (or selection) horsepower by multiplying the transmitted horsepower by the service factor obtained in Step 1. Or use the same procedure to determine required output torque.

3. Find the manufacturer’s rating table for the type of unit required (inline, parallel- shaft, or right-angle) and the input speed (rpm). Locate the speed reduction ratio or output speed required, then the reducer size that gives a rating equal to or more than the horsepower or torque calculated in Step 2.

Make sure the unit’s maximum thermal horsepower is equal to or more than the demand horsepower. Otherwise, use the next larger size that meets these requirements.

4. As before, calculate the output shaft overhung load (if applicable) according to the box “How to check shaft overhung load,” and make sure the selected reducer can handle this load.

5. You have selected a basic speed reducer. To be sure you cover all the additional details that must be specified when ordering the unit, again see the box “Ordering information checklist.”


To illustrate the selection process, let’s use the Service Factor System to choose a speed reducer for a uniformly fed belt conveyor operating 24 hr/day. Requirements include demand power of 3.3 hp, plus 1,750-rpm motor (input) speed and 55-rpm reducer output speed. The reducer will be directly connected on both input and output shafts.

1. In Table 2, find the Service Factor of 1.25 based on 24 hr/day operation.

2. Multiply the demand power (3.3 hp) by the service factor (1.25) to obtain equivalent horsepower (4.13 hp).

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3. Select a unit from the manufacturer’s rating table for inline reducers, Table 3. In the output speed column, find 56 rpm, which is closest to the 55 rpm speed desired. Then locate a reducer with a rating of at least 4.13 hp mechanical. In this case, select model 35B because its rating is 5.8 hp. This unit also has a maximum thermal rating of 11 hp at 75 F ambient temperature. Therefore, it is satisfactory in both mechanical and thermal ratings.

4. The unit is directly connected, so no overhung load check is required.

5. As stated earlier, see the box “Ordering information checklist,” to be sure you cover all the details that must be specified when ordering.

These procedures will help you select the right speed reducer for your application. After the unit arrives, carefully follow the installation and alignment instructions. These factors play a major role in ensuring long, trouble- free service.

Ordering information checklist

Be sure to specify the following information when ordering a speed reducer:
1. Type of reducer (inline, parallel-shaft, or right-angle worm gear).
2. Size (from manufacturer’s selection or rating tables).
3. Nominal speed reduction ratio (from selection or rating tables).
4. Housing type (basic footed, hollow output shaft, output flange, or feet and flange).
5. Mounting position (from manufacturer’s catalog).
6. Input type (integral motor, scoop motor bracket, NEMA C-face adapter, top-mount bracket, or input shaft only).
7. Horsepower (for integral motor selections), or motor frame (with motor bracket and adapter selections), and speed (rpm).
8. Accessories or modifications (including brake, washdown capability, couplings, and guards).

Reynold Cioci is the manager of application engineering, Brook Hansen Drives Group, Philadelphia.

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