Pairing Gearheads with Servo Motors

Machine designers are increasingly turning to gearheads to take advantage of the latest advances in servo motor technology. Essentially, a gearhead converts high-speed, low-torque energy into low-speed, high-torque output. A servo motor provides highly accurate positioning of its output shaft. When pairing gearheads with servo motors, they enhance each other’s strengths, providing controlled motion that is precise, robust, and reliable.
As servo technology has evolved, with manufacturers producing smaller, yet more powerful motors, gearheads are becoming increasingly essential partners in motion control. Finding the optimal pairing must take into account many engineering considerations. Before addressing those, here is a short review of gearhead basics.

Gearhead Basics

So how does a gearhead go about providing the power required by today’s more demanding applications? Well, that all goes back to the basics of gears and their ability to change the magnitude or direction of an applied force.

Gearheads can accomplish this in a few different ways:

Torque multiplication. The gears and numbers of teeth on each gear create a ratio. If a motor can generate 20 in-lbs. of torque, and a 10:1 ratio gearhead is attached to its output, the resulting torque will be close to 200 in-lbs. With the ongoing emphasis on developing smaller footprints for motors and the equipment that they drive, the ability to pair a smaller motor with a gearhead to achieve the desired torque output is invaluable.

A motor may be rated at 2,000 rpm, but your application may not require that. Trying to run the motor at 50 rpm may not be optimal based on the following;

If you are running at a very low speed, such as 50 rpm, and your motor feedback resolution is not high enough, the update rate of the electronic drive may cause a velocity ripple to have a measurable count at every .357 degree of shaft rotation. If the electronic drive you are using to control the motor has a velocity loop of .125 milliseconds, it will look for that measurable count at every .0375 degree of

shaft rotation at 50 rpm (300 deg/sec). When it does not see that count it will speed up the motor rotation to find it. At the speed that it finds the next measurable count, the rpm will be too fast for the application and then the drive will slow the motor rpm back down to 50 rpm and then the whole process starts all over

again. This constant increase and decrease in RPMs cause velocity ripple in an application.

• A servo motor running at low rpm operates inefficiently. Eddy currents are loops of electrical current induced within the motor during operation. The eddy currents produce a drag force within the motor and have a greater negative impact on motor performance at lower RPMs.

• An off-the-shelf motor’s parameters may not be suited to run at a low rpm. When an application runs the aforementioned motor at 50 rpm, it is not using all of its available rpm. Because the voltage constant (V/Krpm) of the motor is set for a higher rpm, the torque constant (Nm/amp)—which is directly related to it—is lower than it needs to be. As a result, the application requires more current to drive it than if the application had a motor specifically designed for 50 rpm. A gearhead’s ratio reduces the motor rpm, which is why gearheads are sometimes called gear reducers. Using a gearhead with a 40:1 ratio, the motor rpm at the input of the gearhead will be 2,000 rpm and the rpm at the output of the gearhead will be 50 rpm. Operating the motor at the higher rpm will allow you to avoid the concerns mentioned in bullets 1 and 2. Bullet 3 allows the design to use less torque and current from the motor based on the mechanical advantage of the gearhead.

As an example, consider a person riding a bicycle, with the person acting as the motor. If that person tries to ride that bike up a steep hill in a gear designed for low rpm, he or she will struggle as

they attempt to maintain their balance and achieve an rpm that will allow them to climb the hill. However, if they shift the bike’s gears into a speed that will produce a higher rpm, the rider will have

a much easier time. A constant force can be applied with smooth rotation being provided. The same logic applies to industrial applications that require lower speeds while maintaining the necessary torque.

• Inertia matching. Current servo motors generate more torque relative to frame size because of dense copper windings, lightweight materials, and high-energy magnets.

This creates inertial mismatches between servo motors and the loads they are trying to move. Using a gearhead to better match the motor’s inertia to the inertia of the load allows the use of a smaller motor resulting in a more responsive system that is easier to tune. Again, this is achieved through the gearhead’s ratio, where the reflected inertia of the load to the motor is decreased by 1/ratio2.

 Recall that inertia is the measure of an object’s resistance to change in its motion and its function of the object’s mass and shape. The greater an object’s inertia, the more torque is needed to accelerate or decelerate the object. This means that when the load inertia is much larger than the motor inertia, sometimes it can cause excessive overshoot or increase settling times. Both conditions can decrease production line throughput.

On the other hand, when the motor inertia is larger than the load inertia, the motor will need more power than is otherwise necessary for the particular application. This increases costs because it requires paying more for a motor that’s larger than necessary and because the increased power consumption requires higher operating costs. The solution is to use a gearhead to match the motor’s inertia to the inertia of the load.

System Cost Savings

Gearheads allow using smaller motors and drives, which can help lower a system’s cost. Because smaller servo systems draw fewer amps they reduce operating costs. Power savings are the greatest when applications demand high torque and low speed because direct-drive servo motors need to be considerably larger than servo motors coupled to gearheads.

Gearheads often drive long mechanisms, such as material-feed systems that move lengths of wire, wood, or metal, where high speed is not essential but high torque and highly repeatable accuracy are critical. Pairing gearheads with servo motors in this kind of application can provide unmatchable flexibility with a traditional direct-drive motor. The servo-gearhead combination will cost less to operate, take up less space, and provides an inertia match for better motion control.