Case

Case Highlights

A Korean startup developing a bipedal humanoid robot needed very precise, low-backlash joint drives for hips, knees, shoulders, and elbows. Early prototypes used off-the-shelf gearboxes that left too much backlash and torsional compliance, making balance control and smooth walking difficult. Backlash is widely recognized as one of the most critical error sources in robot joints.

DD Gear engineered small-module spur and helical gears for the joint actuators’ internal reducers (planetary + right-angle stages). Combined with the customer’s strain-wave final stage, the upgraded gearsets delivered lower backlash, higher torsional stiffness, and smoother motion, helping the robot achieve more stable walking and arm trajectories. High-precision, low-backlash planetary and strain-wave reducers are standard in modern robotics and humanoid joints.

Customer Background & Project Scope

• Customer industry: Korean startup focusing on humanoid robots for logistics and R&D.

• Application: Integrated joint actuators combining a frameless torque motor, multi-stage gear reducer and encoder—similar to common humanoid joint modules that use zero-backlash precision gears and high-resolution encoders.

• Joints covered: Hip pitch/roll, knee, ankle, shoulder and elbow.

• Project goals:

  • Reduce mechanical backlash and increase torsional stiffness to stabilize balance and foot placement. Backlash and stiffness strongly influence motion accuracy and stability in robotic joints.

  • Keep actuators compact and lightweight for whole-body dynamics.

  • Support fast prototype iterations (small batches, 2–3 week sample lead time).

Challenges

• Backlash causing wobble and tracking errors

  • Early drives used standard low-cost planetary gearheads with several arc-minutes of backlash.

  • When the robot shifted weight or swung its arms, control commands were partially “lost” in the mechanical play; the robot swayed and required frequent controller retuning.

• Limited torsional stiffness in joint train

  • Under dynamic walking and push-recovery tests, joints twisted more than expected.

  • Insufficient stiffness in the gear stages amplified oscillations at the extremities—an issue also highlighted in recent research on robot joint stiffness.

• Small-module, high-accuracy gears needed

  • Joint actuators relied on small-diameter, small-module gears to fit within compact housings.

  • The customer needed ISO Grade 4–5 level accuracy on key gears, similar to other high-precision robotics applications where ISO 1328 Grade 4–6 is used to minimize vibration and wear.

DD Gear Solution

1. Joint Reducer Gear Design

• Low-backlash planetary & spur/helical gearsets

  • DD Gear co-designed small-module sun, planet and ring gears for the first reduction stage, targeting very low backlash when assembled with the startup’s preloaded bearings.

  • A compact helical or bevel stage was used where right-angle layouts were required (e.g., hip roll joints), similar to precision robotic gear reducers that combine planetary and bevel stages.

• Micro-geometry tuning

  • Applied profile and lead modifications so multiple teeth share the load evenly and to minimize loaded transmission error, an approach widely used to improve smoothness and reduce noise in precision gearboxes.

  • Target backlash at the planetary output was kept within a few arc-minutes, so that, after the strain-wave final stage, overall joint backlash was close to the near-zero levels typical of high-end humanoid joints.

2. Materials & Heat Treatment

• Carburizing alloy steels for high torque density

  • Selected 20MnCr5 / 18CrNiMo7-6 case-hardening steels for sun and planet gears—steels widely used in high-load planetary and robotic gear applications because they combine deep case hardness with a tough core.

  • Case depth tuned (~0.8–1.2 mm) for high contact fatigue strength without excessive distortion.

• Heat treatment & finishing

  • Carburizing + quenching followed by precision gear grinding on critical gears to reach ISO 4–5 flank accuracy and low roughness—key for low noise, high efficiency and long life in robotics gearboxes.

3. Manufacturing & Quality Control

• Small-module precision capabilities

  • Modules in the m 0.5–1.5 range machined using dedicated small-module hobbing and shaping processes; similar machines are marketed specifically for robot small-module gears.

• Inspection & testing

  • 100% check of key dimensions; sampling of tooth profile, helix deviation, and runout on a CNC gear measuring center.

  • Paired gearsets tested for backlash and torque transmission; assembled joint prototypes tested for torsional stiffness and lost motion before shipment.

Results

After integrating DD Gear’s components into its joint actuators, the startup reported:

• Lower backlash & improved stiffness

  • Measured joint lost motion significantly reduced, bringing behavior close to the near-zero backlash levels achieved by precision planetary and strain-wave systems used in advanced humanoid and collaborative robots.

  • Increased torsional stiffness at the joints reduced oscillations at the robot’s extremities, improving positional accuracy and stability.

• More stable walking & smoother manipulation

  • Balance controller tuning became easier; the robot showed less sway during single-support phases and better recovery under pushes.

  • Arm trajectories became smoother with fewer overshoot corrections, which matched the team’s expectations from high-precision, low-backlash drives.

• Fast iteration & scalability

  • DD Gear supplied small prototype batches in 2–3 weeks, enabling the startup to iterate mechanical and control designs quickly.

  • The same gear families are now used across multiple joint sizes (hip vs. wrist), simplifying BOM and future volume scaling.

Typical Technical Specifications

Representative for humanoid hip/knee/shoulder joints; DD Gear customizes per project.

Case Summary

By combining small-module high-accuracy gears, low-backlash planetary stages, and EV-grade case-hardening steels, DD Gear helped this humanoid-robot startup:

• Cut joint backlash and raise torsional stiffness, enabling more stable walking and manipulation.

• Maintain compact actuator size while increasing torque capacity and durability.

• Iterate fast from prototype to pilot builds with reliable small-batch production.

This approach is ideal for humanoid robots, collaborative arms, service robots and exoskeletons that need compact, precise, and durable joint gear solutions.

Working on humanoid or advanced robotic joints?

Contact DD Gear’s engineers to co-develop small-module, low-backlash gearsets tailored to your joint actuators.