Lesson 3 : How a Strain Wave Gear Work
Summary of the lesson:
Welcome to the third class of the Archimedes Academy by IMSystems. In this series, we are exploring the three most common drive types used in robotics. In the previous lesson, we discussed the cycloidal drive. Today, we will focus on the strain wave gear, also commonly known as the harmonic drive, named after the company that pioneered its commercialization.
> Part 1: How Does a Strain Wave Gear Work?
The strain wave gear has been in use for over 50 years and consists of three main components:
Wave Generator (Input): This elliptical component deforms the flexspline to engage with the circular spline.
Flexspline: A thin, flexible cup with external gear teeth that deforms to fit into the circular spline.
Circular Spline (Output): A rigid ring with internal teeth that meshes with the flexspline.
Working Principle
- The wave generator is placed inside the flexspline and is elliptical in shape.
- As it rotates, the flexspline is deformed in a way that forces its teeth to engage with the circular spline at two opposite points.
- Because the flexspline has fewer teeth than the circular spline, each full rotation of the wave generator results in a slight rotational movement of the flexspline relative to the circular spline.
- This results in a high gear reduction ratio with minimal backlash.
The gear ratio of a strain wave gear is calculated using:
Typically, the difference is two teeth, leading to very high gear ratios such as 360: 1 or more.
> Part 2: Terminology - PRECISION VS. ACCURACY, LOST MOTION, BACKLASH
Understanding the terminology is essential when discussing precision in robotic drives.
Precision vs. Accuracy:
- Accuracy refers to how close a system’s motion is to a desired target position.
- Precision refers to the repeatability of achieving the same position over multiple attempts.
- A system can be highly accurate but not precise, or highly precise but not accurate.
- Lost Motion: The deviation between the commanded motion and actual motion due to various factors such as backlash and flexibility.
- Backlash: The small gap between mating gears, leading to undesired play in movement. It is one of the primary contributors to lost motion.
- Rotational Stiffness: The resistance of a component to deformation under load, affecting motion accuracy.
The strain wave gear is nearly backlash-free, making it one of the most precise drive systems available.
> Part 3: Advantages and Disadvantages of a Strain Wave Gears
Advantages:
High Torque Density: Offers a high torque-to-weight ratio, especially in low torque applications.
Extremely High Precision: One of the most precise drive systems available due to minimal backlash.
High Gear Ratios: Can achieve very high reduction ratios (e.g., 360:1), surpassing most other drive types.
Backdrivability: Can be backdrivable at low gear ratios, allowing energy efficiency in certain applications.
Disadvantages:
Expensive: More costly than most other drive types, including cycloidal drives.
Limited Durability: Prone to wear and tear due to the flexible spline.
Higher Maintenance Requirements: Requires regular maintenance for optimal performance.
Lower Efficiency: Generally lower mechanical efficiency (~70-80%) compared to other gear types.
Noise Generation: Produces high-pitched noise, which can be bothersome in environments where workers are present.
Conclusion:
The strain wave gear is a widely used drive in robotics due to its unmatched precision and high gear ratios. However, it comes with trade-offs in durability, cost, and efficiency. In the next lesson, we will explore planetary gears and their applications in robotic systems.
If you have any questions or comments, please leave them below. See you in the next class!
