How to Select a Gearmotor

AC & DC Powerdrives wishes to make the selection of a gearmotor for your specific application as easy as possible yet with detailed accuracy… all intended to provide the right answers to your questions and ultimately reduce your time-to-market.

The exhibits below entitled "Selecting the Right Gearbox" and "Selecting the Right Motor" will get you started in selecting the right gearbox based on the torque requirements of your application, and the selection of motors available for that gearbox. The information contained on this web site will provide details on Merkle-Korff products, applications and specifications.

The remainder of this section is a overview of the application considerations which are the basis for the specification form. Providing as much information as possible about the application will allow us to provide the best motor or gearmotor for your application.

MAXIMUM RATED GEARMOTOR TORQUE

GEARBOX / MOTOR COMBINATIONS

ELECTRICAL CONSIDERATION:
Determine the nominal or rated voltage and frequency. Also, the high and low voltage conditions must be addressed, as there is a direct relationship between voltage, motor performance and winding temperature.

TORQUE:
Torque performance is normally rated in inch pounds (or inch ounces) of starting, running, or breakdown torque. It is important to determine if the torque load is immediate, gradual, or of shock nature. This decision is essential for determination of the proper motor, gearing and gearcase configuration. All variable loads and worst case scenarios of your application must be considered.

TORQUE MEASUREMENT:
There are two commonly used methods of measuring torque if you are unsure of your torque requirements:

1) String and Pulley Method: Affix a pulley to the shaft of the machine to be driven (see fig. A). Secure one end of a cord to the outer surface of the pulley and wrap the cord around it a number of times. Tie the other end of the cord to a spring scale. Pull on the scale until the shaft turns. The force, in pounds indicated on the scale, multiplied by the radius of the pulley (in inches) gives the torque or twisting rate in inch pounds. Depending on the application, and if used carefully, this method is reliable in determining both starting and running torque.

When the pulley begins to turn, starting torque is indicated. The average running torque can be determined if a long enough string is used.

When the torque characteristics of the machine vary in different parts of the operating cycle, the starting torque must be determined at the point where the motor or gearmotor will “see” the highest resistance (torque) to starting.

Simple “string and pulley ” method of torque measurement. (Torque = Force reading on spring scale x radius of the pulley)

2) Wrench Method:
A simple torque wrench can be applied to the shaft of the machine to be driven. Turn the wrench as you would an ordinary pipe wrench and, when the shaft begins to rotate, read the value (in inch pounds) on the torque wrench gauge. The observed value represents the torque required to start the machine. This method is normally limited to obtaining starting torque or peak torque values, as it is unsafe and difficult to attempt to continuously rotate a torque wrench.

Whether AC or DC drives are used the wrench method is one of experimenting with an “oversize” drive at reduced power levels, recording the experimental readings, and subsequently bench-testing the drive to determine the torque that was being produced at the recorded readings.

OUTPUT SPEED:
Speed (RPM) is rated in a full load or free running condition. The rated frequency, if applicable, of the associated voltage will affect overall output speed. The performance of the gearmotor can be effected with the use of dual frequency AC coils.

ENVIRONMENT:
Environmental areas of cercern should be storage, condition when not running and maximum usage. Numerous applications involve caustic, dusty, humid or extreme temperature environments. Please consider all conditions within the specific environment.

DUTY CYCLE:
This specification is critical in determining the endurance of the gearmotor as it relates to the total product and application. The most important criteria are “time-on” and “time-off” in actual seconds, minutes, or hours. Also important is the total number of cycles when considering gearmotor design.

BRAKING:
Various brakes are available as options. (Please see accessories section).  Brake types include positive stop, ratchet style and friction type (coil-spring or cone). Consider all loading and inertia criteria of the application in both forward and reverse directions.

POWER SUPPLY:
In DC applications, the type of power supply can effect the performance and life of the motor. Other than a battery, a heavily filtered, Full-Wave Rectified power supply is the best power supply to use with our motors.

FORCES ON SHAFT:
An extensive radial load may dictate a larger shaft diameter for increased bearing surface. Extreme condition(s) may dictate needle bearings.

Light axial loads require the use of appropriate thrust washers. The gearmotor output shaft may not support excessive axial loads. Consult Merkle-Korff for guidelines on gearcase strengths.

SHAFT ROTATIONS, DIMENSIONS AND MACHINING:
Shaft Rotations must be considered carefully especially when reversing motors are being used. Reversibility is more costly and involves additional circuitry. With uni-directional gearmotors the direction of rotation is specified from the shaft end of a single shaft gearmotor. With dual shaft gearmotors, the direction of rotation is specified from either the cover of the gearcase or the motor side of the gearcase (please indicate).

Output shaft diameter, length and machining should be determined with the advice of your Merkle-Korff application team (see output shaft section for additional information). Many design problems are solved with the use of creative output shaft machining. The most cost-effective output shaft configuration is 5/16" diameter, 3/4" long from the mounting  surface.

COUPLING-LOAD TO SHAFT:
Consider the type of coupling with the configuration and machining of the output shaft. Rigid shaft coupling, for example, should be avoided. Specify if a pulley is to be used as this may affect the type of output shaft bearings to be used.

FEEDBACK REQUIREMENTS:
Our DC motors are well suited for applications requiring regulated speed and positioning control. We offer a variety of tachometers and encoders for many of our motors. Please see the accessories section of the web site for more detailed information.

MOTOR PROTECTION:
From a mechanical standpoint, first determine if a stalled condition is a normal mode of operation, occasional or unintentional. Gearcase and motor designs are greatly influenced by this criteria.

There are two basic types of recognized locked rotor AC motor protection, recognized by the U.L. and C.S.A.:

1. Thermal Protection:
Utilizes a thermal protector (Merkle-Korff specifies an automatic resetting version) located on the motor coil. Should a coil reach a maximum temperature as prescribed in agency standards (Class A, Class B, etc.) the thermal protector will open, shutting off current to the motor winding. Tripping of the thermal
protector should be considered unusual.

2. Impedance Protection:
This type relies on the inherent total impedance of the stator/coil combination to dissipate excessive winding temperatures.

EMI/RFI:
Any switching of current can cause electrical interference, including the type of power supply being used. Merkle-Korff offers a variety of filtering solutions to meet your EMI/RFI requirements.

MOUNTING STYLE AND POSITION:
Design considerations will be effected by the position of the gearmotor in the application. The mounting position may affect shaft loading or the performance of the optional motor brakes. The optimal position for peak performance is a horizontal shaft-mounting configuration. Consult the mounting positions as depicted on the "Basic Specification Form".