Startup Torque in Harmonic Drive Gearboxes for SCARA Robots: Definition, Key Factors, and Selection Guide

In SCARA robots, 3C automation equipment, and precision assembly systems, harmonic drive gearboxes have become the preferred transmission solution thanks to their high precision, near-zero backlash, high reduction ratios, and compact design. When selecting a gearbox, engineers typically focus on parameters such as rated torque, reduction ratio, backlash, and torsional rigidity. However, one critical specification that is often overlooked is startup torque.

In many cases, issues such as low-speed vibration, jerky startup, servo alarms, or heavy manual teaching are not caused by the motor or control system, but by a mismatch between the gearbox's startup torque and the application requirements. This article explains what startup torque is, the factors that affect it, and how to select the right harmonic drive gearbox for SCARA robot applications.

What Is Startup Torque?

Startup torque is the minimum input torque required to overcome the internal static friction of a stationary harmonic drive gearbox and initiate rotation under standard operating conditions. It is an important indicator of the gearbox's internal friction characteristics and directly affects low-speed motion performance.

Startup torque can generally be divided into two categories:

Forward Startup Torque refers to the torque required when the motor actively drives the gearbox during normal robot operation. It is the primary parameter for evaluating motion performance during automatic operation.

Reverse Startup Torque refers to the torque required when an external force drives the output shaft in reverse, causing the input shaft to rotate. This parameter influences manual teaching, collision detection, and human-robot interaction.

For SCARA robots, stable startup torque contributes to smoother low-speed motion, higher repeatability, and more accurate positioning.

Why Is Startup Torque Important?

Harmonic drive gearboxes transmit motion through the elastic engagement of the flexspline, circular spline, and wave generator. This unique mechanism provides near-zero backlash and exceptional positioning accuracy but also creates higher static friction compared with other reducer types.

Excessive startup torque may lead to:

• Low-speed vibration, creeping, or jerky motion

• Increased motor starting load and potential servo alarms

• Higher resistance during manual teaching

• Increased friction losses during frequent start-stop cycles, reducing gearbox service life

For applications such as 3C electronics manufacturing, precision inspection equipment, medical devices, and other systems requiring smooth micro-motion, startup torque is often just as important as rated torque.

What Factors Affect Startup Torque?

Several factors influence the startup torque of a harmonic drive gearbox.

1. Component Manufacturing and Assembly Accuracy

The machining accuracy and assembly precision of the flexspline, circular spline, and wave generator directly affect internal friction. Higher manufacturing quality results in smoother startup and more stable motion.

2. Lubricant Selection

Lubricant type, viscosity, and filling volume all influence internal resistance. Low-friction grease specifically designed for robotic applications can significantly reduce startup torque and improve motion smoothness.

3. Reduction Ratio

Within the same product series, different reduction ratios produce different contact conditions and friction characteristics, resulting in variations in startup torque. Engineers should evaluate this parameter together with the overall application requirements.

4. Operating Temperature

Ambient temperature has a significant impact on startup torque. At low temperatures, lubricant viscosity increases, raising internal resistance and making startup more difficult. As the gearbox warms up, friction decreases and motion becomes smoother.

How to Consider Startup Torque When Selecting a Gearbox for SCARA Robots？

When designing a robotic joint, the servo motor must overcome not only the external load but also the gearbox's startup torque. Therefore, motor sizing should never be based solely on load torque calculations.

For example, consider a SCARA robot used in 3C electronics manufacturing with a payload of 3 kg, an arm length of 0.2 m, and a 50:1 harmonic drive gearbox with a startup torque of 0.15 N·m.

The total motor startup torque can be estimated as:

Total Startup Torque = Gearbox Startup Torque + Equivalent Load Torque

The selected servo motor should provide a rated torque higher than this calculated value while maintaining an appropriate safety margin. This ensures smooth startup, stable operation, and long-term reliability.

For this reason, startup torque should be treated as a key selection criterion alongside rated torque, backlash, and torsional rigidity when designing SCARA robot joints.

Conclusion

Rated torque determines how much load a harmonic drive gearbox can handle, while startup torque determines how smoothly the robot can start and perform precise low-speed motion.

For high-precision applications such as 3C electronics manufacturing, precision assembly, inspection equipment, and collaborative robots, selecting a gearbox with optimized startup torque can improve motion quality, reduce motor load, minimize equipment downtime, and extend service life.

At Honpine, we specialize in the research, development, and manufacturing of high-performance harmonic drive gearboxes for robotics and industrial automation. Our products are designed to deliver high precision, smooth motion, reliable performance, and long-term durability for SCARA robots, collaborative robots, medical robots, and other advanced automation systems.

Contact us today to learn more about our harmonic drive gearbox solutions or speak with our engineering team for expert product selection and technical support.