Geared stepper motors are known for their precise positioning and ability to transmit high torque at low speeds, but require careful sizing to ensure the motor is matched to the load and application parameters to minimize the chance of step loss or motor stall . Adding a gearbox to the geared stepper motor system can reduce the inertia ratio of the load to the motor, increase the torque to the load, reduce the vibration of the motor, and thus improve the performance of the motor.

1. The inertia ratio of the deceleration load to the motor

Inertia is what causes geared stepper motors to lose synchronization in applications. The ratio of the inertia of the load to the inertia of the motor determines the ability of the motor to drive and control the load, especially during the acceleration and deceleration parts of the motion. If the inertia of the load is significantly higher than that of the motor, it will be difficult for the motor to control the load. And there may be overhead (more steps than the instruction requires) or underpulsing (out of sync). The very high inertia ratio of the load motor can cause the gear motor to draw too much current and stall.

One way to reduce the inertia ratio is to use a larger motor with higher inertia. However, this means higher cost, more weight, and also has a trickle-down effect on other parts of the system, such as couplings, cables, and drive components. But adding a gearbox to the system can reduce the load inertia ratio, which is the square of the reduction ratio.

2. Increase load torque

Another reason to use a geared stepper motor with a gearbox is to add more torque to drive the load. When a load is driven by a motor reduction gearbox combination, the gearbox multiplies the torque from the motor which is proportional to the reduction ratio and the efficiency of the gearbox.

However, although the transmission can increase torque, it will reduce speed. (This is why they are sometimes called "reducers" or (gear reducers).) In other words, when a gearbox is added to a motor, the motor must spin faster to provide the target speed for driving the load.

Geared stepper motors have the same ratio of torque reduction to speed increase, which is due to the loss of starting torque. The inverse relationship between speed and torque means that the only real thing to do is to increase the speed until the motor can't provide the required torque (even multiplied by the reduction ratio).

3. Reduce resonance and vibration

However, increasing the speed of the motor does have benefits. After the motor is overspeeded and installed in the gearbox, the motor runs outside the resonance frequency range, and the jitter and vibration may cause the motor to lose steps or even stall. In order to ensure that the gearbox has the correct torque, speed and inertia values, it is very important to choose a gearbox with high precision and low backlash, especially when connecting the gearbox to a geared stepper motor.

Recall that geared stepper motors operate in an open-loop system, with backlash in the gearbox reducing the positioning accuracy of the system, with no feedback to monitor or correct for set-up errors. This is why stepper applications typically use high precision planetary gearboxes with backlash as low as 2 to 3 arc minutes. Some manufacturers can offer geared stepper motors with harmonic gears that can exhibit zero backlash in many application scenarios.

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