Plastic-gear micro servos (most commonly the ubiquitous blue 9g SG90) strip their teeth for one of three reasons: mechanical leverage exceeding the gear teeth's shear strength, sudden back-driving when an arm is moved by hand or dropped, or prolonged stalling against an unyielding mechanical limit. The fix is not always to buy bigger motors—it is understanding torque arithmetic and knowing precisely which joints demand metal gears.

The Arithmetic of the Stripped Gear

Most beginners assume that if an SG90 is rated for 1.5 kg·cm of torque, it can lift a 100-gram gripper. That assumption ignores the lever arm. Torque equals force multiplied by distance from the pivot point:

• A 100 g load at the end of a 5 cm forearm requires 0.1 kg × 5 cm = 0.5 kg·cm of static torque.
• Add the weight of the structural acrylic or 3D-printed link (roughly 40 g with its centre of mass at 2.5 cm): that adds another 0.1 kg·cm.
• Extend that same arm horizontally to 15 cm with a shoulder joint, and the same 100 g payload now demands 0.1 kg × 15 cm = 1.5 kg·cm before factoring in the weight of the elbow servo, the forearm, or the bracket.

At 15 cm, the shoulder joint is already operating at 100% of the motor's absolute stall limit simply holding the arm horizontal. The moment the software commands an upward move, the acceleration spike exceeds the plastic teeth's physical shear rating. The output spline slips, the tiny plastic teeth on the intermediate gear shear off, and the servo begins buzzing helplessly without rotating.

The Shock Load Problem (Back-Driving)

Plastic gears rarely strip while moving slowly through open air. They strip when something stops them abruptly. In school classrooms and maker labs, the primary killer of 9g servos is manual manipulation. When a learner physically pushes a robot arm into position while the servo is powered off (or worse, powered on), the gear train acts as a step-up multiplier in reverse. The large output horn forces the tiny driving pinion to spin at extreme speed. The shear resistance of injection-moulded nylon is low; a quick manual nudge generates enough shock torque to strip the first reduction gear instantly.

When to Use SG90 vs MG90S vs MG996R

You do not need to build your entire robot out of heavy, expensive standard servos. The practical approach is joint-by-joint selection based on the moment arm and impact risk.

The Gripper: Stay with Plastic (Usually)

An SG90 is perfectly adequate for opening and closing a light claw. The moment arm is short (under 4 cm), so torque demands are minimal. In fact, plastic gears here act as an inexpensive mechanical fuse: if a student commands the claw to close tightly around an immovable wooden block, the motor stalls. While prolonged stalling will overheat the motor coil, it gives you a buffer before drawing dangerous current spikes.

The Elbow: Drop-in Upgrade to MG90S

If your design uses 9g micro servo brackets, the MG90S is a direct drop-in replacement. It shares the identical physical footprint and mounting holes as the SG90, but replaces the plastic gear cluster with brass and aluminium. It delivers roughly 1.8 to 2.2 kg·cm of torque and can withstand the routine bumps, balance wobbles, and minor stall conditions that instantly shred an SG90. If you are ordering replacement parts for classroom arm kits, stocking MG90S units from the Sheen Store is the most cost-effective way to stop maintenance churn.

The Shoulder and Waist: Step Up to Standard Size

For any 4-axis or 6-axis arm with a reach exceeding 12 cm, micro servos (even metal-geared ones) fail mechanically at the bearing level. The small 4.8 mm output shaft cannot support the bending moment (radial load) of a heavy arm. Here, you must step up to standard-size metal servos like the MG996R (rated at ~9–11 kg·cm) or digital coreless servos. These feature dual ball bearings that isolate radial bending forces from the internal gear train.

Three Practical Rules to Protect Any Servo

• Add software soft-starts: Never command a servo to jump instantaneously from 0° to 180°. Step the target position incrementally in a loop (e.g., 1° every 15 milliseconds). Sudden angular acceleration is what shears gear teeth.
• Calibrate mechanical end-stops: If your acrylic frame physically stops at 160°, but your code commands 180°, the servo will draw its maximum stall current continuously until either the plastic gears strip or the motor driver shuts down. Calibrate min/max angles in software before bolting the horns on.
• Power servos independently: A stalling servo draws between 500 mA (SG90) and 2.5 A (MG996R). Powering them directly off a micro:bit or Arduino 5V pin causes voltage brownouts, resetting the microcontroller and creating erratic jitter that hammers the gear teeth. Always use a dedicated 5V/6V power supply with a shared ground.