Belts vs Planetary vs Harmonic vs Cycloidal

Choosing the Right Reducer for Your Robot

In this blog, we’ll explore the role of reducers in robotics, covering how we select, design, and apply them in our robots. We’ll describe the most common types of reducers, provide examples of their use in popular robots, and discuss which ones can be produced using DIY methods like 3D printing or low-cost CNC machining. We’ll also highlight those that may be out of reach due to cost or complexity.

Cycloidal Reducers

Cycloidal reducers are known for their high torque density, compact design, and low backlash, making them ideal for robotics applications. These reducers can be easily 3D printed, and we used them in our Faze4 robotic arm. In the industry, they are commonly found in large industrial robots and are becoming increasingly popular in humanoid robots.

However, cycloidal reducers are inefficient for small gear ratios, making them unsuitable for dynamic robots that require rapid movements. Typical gear ratios range from 30:1 to 300:1.

Harmonic Drives

Harmonic drives are the industry standard for most industrial robotic arms. They offer high reduction ratios and near-zero backlash, which are essential for precise control and positioning. However, due to their intricate design, they are difficult to produce with 3D printers.

While the cost of harmonic drives has decreased in recent years, they remain relatively expensive and less accessible for budget-conscious projects. Gear ratios for harmonic drives are similar to cycloidal reducers, typically ranging from 30:1 to over 300:1.

Gear Reducers: Compound and Planetary

Gear reducers, including compound and planetary types, are widely used in robotics due to their simplicity and reliability. Modern planetary reducers offer low backlash and are available at a reasonable price, making them a great choice for many applications.

We use small planetary reducers in our PAROL6 robotic arm, and they perform exceptionally well. However, as the size increases, so does the price—often exponentially. For larger robots, harmonic or cycloidal reducers may be necessary due to performance and safety requirements.

Unlike harmonic drives, gear reducers can be easily produced using a 3D printer, making them accessible for DIY projects. Gear ratios can range from 1:1 to virtually any value, though for planetary systems, multiple stages are typically required to achieve higher reductions.

Belt Reducers

Belt reducers are often used in combination with other reducers to relocate mass closer to the robot's base, reducing inertia and improving performance.

Belts offer minimal backlash, but they can stretch under load, introducing some degree of backlash in practical use. Despite this limitation, belts are a cost-effective solution for reducing weight and increasing efficiency in robotic arms.

One major drawback of belt reducers is that due to the required tooth contact and slippage, they are typically limited to gear ratios from 1:1 to 6:1.

Capstan Drives

Capstan drives use a rope, cable, or tendon wrapped around a drum to transfer motion. This design enables high reduction ratios with minimal backlash, making them suitable for lightweight, precise applications.

Capstan drives are relatively simple to manufacture using basic machining techniques, making them an attractive option for DIY robotics projects. Recently, they have been used in dynamic robots like quadrupeds and humanoids, though they have been outclassed by other reduction types in many applications.

Conclusion