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Speed reducers

By Clarence Klassen  
 
​We would prefer to operate web-handling drives without speed reducers. The drive and tension control see each tooth engage. Additionally, the speed reducer introduces measurable losses. In fact, if the drive train has an Adjustable Speed Drive (ASD), who needs a speed reducer?
Web Handling applications are constant torque applications. That is to say, the drive needs as much torque at thread speed as at full speed (a slight exaggeration). Motor nameplates show the motor power rating at a given RPM and Torque. The rated torque for a motor can be calculated from these values but is not provided on the nameplate. The motor’s nameplate power is only valid if the motor runs at its rated base speed (RPM) or above at the design line speed (MPM or FPM).

If the web roller must turn at a low RPM a speed reducer is normally used. The speed reducer reduces RPM. At the same time, it also increases torque at the roller. This allows the motor to be “right-sized” to the application.
Without the torque increase from the speed reducer, the motor rating must be increased to provide adequate torque.

This can be shown with an example. Consider a line speed of 30 MPM (98.4 FPM) and a roller diameter of 0.10 m (3.93 in.). The rated tension is 264 N/m (1.50 PLI). The web width is 2 meters (78.7 in.). 0.264 KW (0.354 HP) is required to produce rated tension at rated speed. This requires 26.4 N*m (19.5 ft*lb) torque at the roller.

At design speed, the roller will turn at 95.5 RPM. With an 1150 RPM motor, an ideal gear ratio of 12.0 would be used. A 0.3 KW (0.4 HP) motor capable of producing 2.50 N*m (1.83 ft*lb) torque could be used.

If my reluctance to use a speed reducer prevailed, I would be forced to buy a 12X larger rated 3.6 KW (4.8 HP) motor. This is the smallest motor capable of producing the required torque. The motor will never run above 1/12th of its rated power or speed. This motor is definitely not “right-sized.”

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