Harmonic Drive Reducers: Fundamental Knowledge for Precision Transmission

1. Working Principle of Harmonic Drive Reducers

• Wave Generator: Usually composed of an elliptical cam and an outer ring bearing, it is used to generate elastic deformation waves.

• Flexspline: An elastic thin-walled gear that can undergo elastic deformation along with the wave generator.

• Circular Spline: A rigid internal gear ring that meshes with the flexspline but does not rotate directly.

1.1 Transmission Process

1. When the wave generator rotates, its elliptical structure causes the flexspline to undergo elastic deformation, and part of the flexspline meshes with the circular spline.

2. With each full rotation of the wave generator, the flexspline moves relative to the circular spline by a number of teeth equal to the difference between the number of teeth of the circular spline and the flexspline, achieving a high reduction ratio.

3. Reduction Ratio Calculation: Reduction ratio = (Number of teeth of circular spline - Number of teeth of flexspline) / Number of teeth of circular spline. Typically, the flexspline has 2 fewer teeth than the circular spline (e.g., 100 teeth on the flexspline and 102 teeth on the circular spline), resulting in a reduction ratio of 50:1.

1.2 Kinematic Characteristics

• Zero Backlash: The elastic deformation of the flexspline can compensate for gear gaps, enabling backlash-free movement.

• High Precision: A large number of teeth mesh simultaneously (usually more than 30% of the teeth), leading to extremely small transmission errors (<1 arcminute).

• High Efficiency: Efficiency can reach over 90%, but it is affected by lubrication and load.

2. Design of Harmonic Drive Reducers

2.1 Key Component Design

• Flexspline Design

• Material: High-strength alloy steel (e.g., 40CrNiMoA) to ensure a long fatigue life.

• Tooth Profile: Involute or circular arc tooth profile to optimize stress distribution.

• Wall Thickness: Needs to balance rigidity and elasticity to avoid fatigue fracture.

• Circular Spline Design

• Material: High-hardness alloy steel (e.g., 20CrMnTi), with heat treatment to improve wear resistance.

• Tooth Ratio: Usually has 2 to 4 more teeth than the flexspline to achieve a large reduction ratio.

• Wave Generator Design

• Ellipticity: Determines the deformation of the flexspline and affects the meshing depth.

• Bearing: Flexible bearings are used to reduce friction loss.

2.2 Fatigue Life Analysis

3. Advantages, Disadvantages and Applications of Harmonic Drive Reducers

3.1 Advantages

• High reduction ratio (50:1 to 320:1)

• Zero backlash and high positioning accuracy

• Compact structure and high power density

• Stable operation with low vibration and noise

3.2 Disadvantages

• Limited fatigue life of the flexspline

• Complex manufacturing process and high cost

• Strict requirements for lubrication and sealing

3.3 Application Fields

• Industrial Robots

• Joint Drive: Harmonic drive reducers are widely used in the rotating joints of six-axis robots (e.g., brands like ABB, Fanuc, and KUKA) to provide high-precision and high-torque output.

• Collaborative Robots: Their lightweight design makes them an ideal choice for collaborative robots (e.g., UR).

• Aerospace

• Satellite Antenna Adjustment: Used in high-precision pointing mechanisms to ensure stable signal transmission.

• Spacecraft Solar Wing Drive: Harmonic drive reducers resistant to vacuum and radiation are suitable for space environments.

• Medical Equipment

• Surgical Robots: Systems like the Da Vinci Surgical System rely on harmonic drive reducers to achieve precise operations in minimally invasive surgery.

• CT/MRI Equipment: Used in precision rotating scanning mechanisms.

• Other Fields

• Optical Instruments: Precision focusing mechanisms for telescopes and LiDAR (Light Detection and Ranging).

• Semiconductor Equipment: High-precision positioning for wafer handling robotic arms.

4. Conclusion