Why use a gearbox with a stepper motor?

Image credit: Global Electric Motor Solutions, LLC

Stepper motors are known for their accurate positioning capabilities and high torque delivery at low speeds, but they require careful sizing to ensure the motor matches the load and application parameters, to minimize the possibility of lost steps or motor stalling. Adding a gearbox to a stepper motor system can improve the motor’s performance by decreasing the load-to-motor inertia ratio, increasing torque to the load, and reducing motor oscillations.

Decrease load-to-motor inertia ratio

One cause of missed steps in stepper motor applications is inertia. The ratio of the load inertia to the motor inertia determines how well the motor can drive, or control, the load — especially during acceleration and deceleration portions of the move profile. If the load inertia is significantly higher than the motor inertia, the motor will have a difficult time controlling the load, and overshoot (advancing more steps than commanded) or undershoot (missing steps) can occur. A very high load-to-motor inertia ratio can also cause the motor to draw excessive current and stall.

J_L = inertia of load

J_M = inertia of motor

One way to reduce the inertia ratio is to use a larger motor with higher inertia. But that means higher cost, more weight, and trickle-down effects on other parts of the system such as couplings, cables, and drive components. Instead, adding a gearbox to the system reduces the load-to-motor inertia ratio by the square of the gear ratio.

i = gear reduction

Increase torque to the load

Another reason to use a gearbox with a stepper motor is to increase the torque available to drive the load. When the load is driven by a motor-gearbox combination, the gearbox multiplies the torque from the motor by an amount proportional to the gear ratio and the efficiency of the gearbox.

T_o = torque output at gearbox shaft

T_m = torque output at motor shaft

η = gearbox efficiency

But while gearboxes multiply torque, they reduce speed. (This is why they’re sometimes referred to as “gear reducers” or “speed reducers.”) In other words, when a gearbox is attached to a motor, the motor must turn faster — by a factor equal to the gear ratio — to deliver the target speed to the load.

N_o = speed output at gearbox shaft

N_m = speed output at motor shaft

And stepper motor torque generally decreases rapidly as speed increases, due to detent torque and other losses. This inverse relationship between speed and torque means it’s only practical to increase speed by a certain amount before the motor is unable to deliver the required torque (even when multiplied by the gear ratio).

Notice that stepper motor continuous torque (green) falls off rapidly as speed increases, unlike servo motor continuous torque (blue), which is relatively flat even at high motor speeds.

Reduce resonance and vibration

But speeding up the motor does have a benefit. The additional speed required by the motor when a gearbox is installed means the motor operates outside its resonant frequency range, where oscillations and vibrations can cause the motor to lose steps or even stall.

The stepper motor with a gearbox (top) shows significantly less vibration than the non-geared stepper motor (bottom), especially at low speeds in the resonant frequency range.
Image credit: Oriental Motor USA Corp.

Image credit: Oriental Motor USA Corp.

In addition to ensuring the gearbox has the correct torque, speed, and inertia values, it’s important to choose a high-precision, low-backlash gearbox — especially when connecting the gearbox to a stepper motor.

Recall that stepper motors operate in an open-loop system, and backlash in the gearbox degrades the system’s positioning accuracy, with no feedback to monitor or correct for positioning errors. This is why stepper applications often use high-precision planetary gearboxes, with backlash as low as 2 to 3 arcminutes. And some manufacturers offer stepper motors with harmonic gears that can exhibit zero backlash under most application conditions.