Robot Joint Precision Planetary Gear Case Study

Client Background#

An industrial robot company builds six-axis robots. It requires precision planetary gears for the joint reducers of these robots — the gear sets that sit inside each joint and largely determine how accurately and how rigidly the arm moves. Because every cycle of the robot passes through those joints, the gear set is one of the parts that governs the overall behaviour of the machine.

Requirement & Challenge#

• Module M1.5, number of teeth Z12 / Z36 / Z84 — a small module, which means small parts and tight manufacturing tolerance.
• Backlash ≤5 arcmin.
• Service life ≥10,000 hours.

Backlash in a robot joint directly affects positioning behaviour, so it has to be controlled at the level of the gear set rather than only at the level of the individual part. In a planetary arrangement the sun gear and the planetary gears work together as one unit; if each part is inspected only on its own, the backlash measured after assembly can still fall outside the target. The parts therefore have to be checked as a working pair. At the same time, the material has to combine surface hardness with a core that can carry the repeated load, and heat treatment deformation has to be controlled so that the profile produced by grinding stays within tolerance instead of being distorted afterwards.

Solution & Process#

1. Gear Grinding Process: All gears are ground, with accuracy DIN 5, so that the final tooth profile is produced after heat treatment rather than before it.
2. Material: 20CrNiMo, carburized and hardened, with deformation control during heat treatment.
3. Matched-pair Inspection: The sun gear and the planetary gear are inspected as a matched pair rather than as separate parts.

Result & Feedback#

• Backlash ≤4 arcmin, better than the design requirement.
• Batch of 500 sets, with good consistency.
• The customer has made volume purchases.