Gearing in Motion-Control Designs
What are the Advantages of Using a Gearhead?
Estimates say that only about one-third of the motion control systems in service use gearing, although there are good reasons for doing so. For example, when your motion control system must run at 1,000 rpm or less, the use of a gearhead is advantageous. It’s also wise to use gearing when designing systems that have size constraints. Pairing a gearhead with your servo motor or using an integrated gearmotor can let you employ a smaller motor, thereby reducing the system size.
Advantages of using a gearhead with your servo in a motion control system include:
- Torque multiplication. Gearheads can provide a mechanical advantage when mounted to the motor output shaft. That’s because the number of gears and teeth on each gear provide a mechanical advantage defined by a ratio. Say a motor can generate 100 in.-lbs of torque, and a 5:1 ratio gearhead is attached. Depending on the gearhead’s efficiency, the resulting torque will be close to 500 in-lbs. The use of a gearhead generates a higher torque output in a smaller envelope.
- RPM reduction. Gearheads are often referred to as gear reducers because they increase torque output while decreasing motor rpm. For example, when a motor is running at 1,000 rpm and a 5:1 ratio gearhead is attached, the speed at the output is 200 rpm. Such speed reduction can improve overall system efficiency. In a recent example, a stone-grinding mechanism required the motor to run at 15 rpm. The slow speed made turning of the grinding wheel difficult because the motor tended to cog. Here, using a 100:1 gearhead caused the motor to run at 1,500 rpm, providing a smooth, continuous rotation.
- Inertia matching. The past fifteen years or so have seen servo motor manufacturers introducing lightweight materials, dense copper windings, and high-energy magnets. Servo motors are therefore generating more torque relative to frame size than in the past, resulting in greater inertial mismatches between servo motors and the loads they control.
Recall that inertia is a measure of an object’s resistance to any change in its motion and is a function of the object’s shape and mass. The greater an object’s inertia, the greater the amount of torque needed to accelerate or decelerate the object.
When the load inertia is much larger than the motor inertia, it can cause excessive overshoot or increase settling times. Both conditions 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 necessary for the particular application. This increases costs because you’re paying more for a motor that’s larger than necessary and the increased power consumption means higher operating costs. The solution is to use a gearhead to match the inertia of the motor to the inertia of the load. Choosing the appropriate gearhead lets you use a smaller motor as well as develop a more responsive system.
Help cut system cost
The upshot is that torque multiplication, rpm reduction, and inertia matching help cut system cost because the use of a gearhead allows the uses of a smaller size motor and drive.
For example, say an application requires 200 in-lbs of torque at a speed of 300 rpm. To drive the load with a servo motor alone (at standard performance attributes found in the industry), a servo motor with a 142 mm frame size and a drive that can supply 30 A continuous is necessary. The system costs about $6,000. But using a gearhead for the application lets you use a 90 mm servo motor and a correspondingly smaller drive.