Tech Tips: Back-Drive & Braking

This Tech Tips discusses the advantages of back-drive and brakes and the types of applications for which they are best suited.

Video Transcript: Groschopp Tech Tips – Backdrive & Braking

Hello!  This is Heather with a Groschopp Tech Tip.  Today we will be discussing Back-drive and Braking for electric motors.

Back-drive-ability is defined as the degree of ease of which a motor or gearmotor can be driven by its attached load when power is removed from the motor.

As you can see motors are easy to back-drive by turning the output shaft by hand.  While gearmotors are more difficult to back-drive — depending upon the gear reduction ratio and the efficiency of the reducer.

Easy back-drive-ability can be useful when moving high-mass loads, where the motor “coasts” as the heavy load comes to a rest.

Easy back-drive-ability can also prevent the load from causing damage to the gearmotor in some applications. On the other hand, if a gearmotor can be back-driven by its application, it could be a safety hazard.

Most applications require back-drive to be controlled, holding a load at an intended position. A brake can be mounted to the end of the motor to accomplish this.

There are many types of brakes, today we will look at two styles of Electro-Magnetic brakes.  This brake consists of an electro-magnet, a spring, a friction disc coupled to the motor shaft, and two clutch plates, all in a small housing

In applications where back-drivability needs to be minimized when the motor is not working, a “power-off” brake is used.    When the power is removed from the motor and brake, the electro-magnet releases causing the friction disc in the brake to be clamped between the clutch plates which are forced together by the spring.  When power is applied to the motor and simultaneously to the brake, the electro-magnet in the brake is energized pulling one of the clutch plates away from the friction disc, releasing the brake and allowing the motor shaft to turn freely.

Be aware this system can still be back-driven above a certain amount of torque, depending upon the torque rating of the brake.

Some applications, may require the system to be back-drivable when power is removed. In this instance, a “power-on” style of brake is used. This style works opposite of the previous style brake, as it requires power to engage the brake. For example, if we have a crossing gate in the vertical position. The brake on the gear motor operating the arm is energized; locking the gear motor and holding the crossing arm up even though power is removed from the motor. In the event of a power failure, the brake would disengage, and the weight of the crossing arm would back-drive the gear motor to the horizontal position: a matter of safety.

This has been a Groschopp Tech Tip.  For more information about any of our products or to view other Tech Tips please visit us online at www.groschopp.com.

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