In this article, we explore how cycloidal gearbox design is implemented and introduce a case study involving testing with an actual robot.
Cycloidal Gear
Unlike conventional gears, cycloidal gears transmit force in a slightly different way. Traditional gears operate by the interlocking of gear teeth, whereas cycloidal gears use a unique rolling motion to deliver rotational force. This motion resembles how a bicycle wheel rolls along a surface and is based on a distinctive shape known as the cycloidal curve. Thanks to this structure, cycloidal gears can significantly reduce friction during rotation.
A cycloidal curve refers to the path traced by a point on the circumference of a circle as it rolls along a straight line without slipping. For instance, if you mark a point on the side of a bicycle wheel and roll the wheel forward, the path that point traces illustrates a cycloidal curve. This concept provides an intuitive understanding of how cycloidal motion works.
In terms of structure, the pin-type configuration is the most widely used in cycloidal gears. In this design, small pins are arranged around the outer edge of a gear disk, and the disk rotates by engaging with these pins. Since the disk moves as if it is rolling between the pins, friction is minimized during operation.
In a cycloidal gearbox, the way power is transmitted differs from the typical tooth-meshing mechanism found in conventional gears. First, the rotational force generated by the motor is transferred through the input shaft to the reduction mechanism.
An eccentric shaft is connected to the input shaft, and a bearing is placed between the eccentric shaft and the cycloidal disk. As the name suggests, the eccentric shaft is mounted slightly offset from the central axis. This means that when the input shaft rotates, the rotational force is delivered to the disk via the eccentric shaft. However, the disk does not rotate around a perfectly central path. Instead, it follows a slightly offset trajectory. This motion is known as eccentric motion.
When eccentric motion occurs, the disk rolls smoothly over the pins, as if rolling on a surface. Through this process, the disk reduces the high-speed rotation received from the input shaft and transmits it to the output shaft in a stable, decelerated form.
Key Features of Cycloidal Gearbox Design
Why is the cycloidal mechanism gaining attention among so many types of gear reducer designs? A cycloidal gearbox is capable of delivering high torque despite its compact size, and it excels in achieving high reduction ratios. Because it operates based on a rolling contact mechanism rather than direct tooth engagement, it significantly reduces friction during motion.
Thanks to these advantages, cycloidal gears are being increasingly adopted across various industrial applications. They are especially effective in fields that demand high torque, such as robotics and automation systems.
In recent years, the robotics and automation industries have seen a sharp rise in the demand for components that offer both miniaturization and high torque output. This reflects not just a desire for improved performance, but a fundamental shift in the way systems are being designed. Modern industrial equipment and robots are expected to be smaller and more efficient, yet powerful enough to perform reliably within confined spaces. In this context, the cycloidal gearbox has emerged as a key solution for compact, high-torque drive systems.
In application areas such as production lines, logistics systems, collaborative robots, and mobile platforms, the physical space available for installing equipment is often limited. In such constrained environments, there is a growing demand for drive systems that are not only compact but also high-performing. Given these limitations, applying large gear reducers or complex assemblies becomes impractical. Therefore, miniaturization of drive modules must be considered from the earliest stages of design.
When designing robots or automation equipment, the system must be able to move freely and perform diverse tasks within a limited space. To achieve this, the size and shape of core components like motors and gearboxes must be adaptable to the design intent. It is also critical to deliver the required output without increasing the overall dimensions of the system. A flexible layout of the drive components is essential to accommodate complex internal structures. If the drive system is too large or heavy, it can impose serious design constraints, making it difficult to implement the desired functions.
Moreover, flexibility in how the drive unit is positioned, connected, and integrated with other components is just as important. This design freedom becomes especially critical in devices like mobile robots or collaborative robots, which require a wide range of postures and complex movements. In these cases, the degree of design freedom directly affects system performance, making it a key consideration in high-functionality robotic applications.
To address these challenges, Bon Systems has developed a proprietary cycloidal gearbox design technology. Traditional pin-type reducers require extremely precise engagement between the cycloidal disc and the roller pins. As a result, each component must be manufactured with very high precision, and even slight deviations during assembly can significantly affect overall performance. This has posed a major limitation in industrial environments where repetitive production and large-scale assembly are necessary, as it increases the burden of precision machining and assembly, impacting both productivity and cost.
The most distinctive structural feature of our cycloidal gearbox design is the complete elimination of fixed roller pins, which are essential in conventional pin-type reducers. Instead, our cycloidal discs are designed with curved tracks that are precisely machined along the path of the cycloidal motion. These tracks naturally guide the rolling motion, allowing the same reduction principle to be achieved without the need for separate roller pins.
This structural innovation greatly simplifies the overall design of the reducer. Without the need to assemble or align pins, assembly becomes much easier and more efficient. In addition, the manufacturing process no longer demands ultra-high precision, allowing the system to maintain stable performance even within a broader tolerance range.
Another structural advantage is that our design uses a pair of cycloidal discs working together. This allows the transmitted force to be distributed between two contact zones, resulting in a more stable and powerful torque output. It also makes the structure well-suited for achieving high reduction ratios while maintaining reliability and strength.
How Much Torque Can It Actually Deliver?
Once you understand the structural design and fundamental principles of a cycloidal gearbox, the next question is what kind of performance it can deliver in real-world applications. To verify this, we built and tested a leg module for a quadruped walking robot using our cycloidal actuator (BCSA) with a 49:1 reduction ratio.
Details of this test can be found in the project [Make a Robot Dog! A Special Day with Students from Kyungnam University]
Initially, the goal was to conduct a functional test, so we adopted a timing belt mechanism for the drive. This structure offers easy assembly and greater flexibility when making design adjustments. However, during the test, we encountered unexpected issues.
During testing, we observed that the timing belt slipped under high torque, causing the leg of the robot to lose precise control. This revealed a critical limitation: the belt-driven structure was not able to withstand high torque loads reliably. As a result, we concluded that a more robust drive mechanism was necessary to achieve stable performance.
To address this, we designed and implemented a new link mechanism optimized for high torque transmission. The leg of the robot we developed was not just a prototype, but rather an experimental platform created to push the limits of our actuator’s performance. Naturally, the development involved multiple rounds of trial and error, but each iteration helped us refine the system further.
As full-scale testing began, the newly designed robot leg equipped with a link mechanism and metal gears successfully lifted a load of over 44 lb using just a single leg. This means that a single actuator was able to bear the entire load and operate stably. The primary goal of this test was not high-precision control, but rather to evaluate the actuator’s ability to deliver raw mechanical power. The results exceeded our expectations, confirming that our product is both powerful and stable, and can be trusted even in high-load environments.
Due to its complex operating principles and unique design, the cycloidal gearbox can be difficult to understand for those encountering it for the first time. This makes it especially important to fully grasp the advantages that cycloidal reduction technology offers when selecting a reducer or actuator for a project.
The unique properties of cycloidal technology are not just technical features they are key drivers of performance. When applied appropriately, this technology can do more than simply fulfill a functional requirement. It can become a foundation for differentiating your product and gaining a competitive edge in the market.
Bonsystems Cycloid Smart Actuator
The BCSA is an all-in-one smart actuator developed by Bon Systems, built around our core technology: a cycloidal gear-based reduction mechanism. Unlike conventional setups where the gearbox, motor, and controller must be assembled and connected separately, the BCSA integrates all three components into a single compact unit. This significantly simplifies both the design and assembly processes.
The greatest strength of this actuator lies in its ability to reliably deliver high torque output despite its slim profile. By adopting a compact cycloidal drive core (CDC type), it combines ease of assembly with high production efficiency.
What truly sets the BCSA apart is its all-in-one module design, which combines the gearbox, motor, and controller into a single system. This allows for power transmission, motion control, and actuation without the need for additional components or complex cabling. As a result, the overall system layout becomes more efficient, and wiring tasks are significantly reduced.
Because the BCSA handles all key functions within a single unit, it enables more efficient use of internal space when designing robots or automated equipment. The system structure becomes cleaner and simpler, which is especially valuable in robotic joints or space-constrained devices. In these applications, the design flexibility that the BCSA provides becomes one of its most valuable advantages.
In this post, we introduced the design principles and structure of cycloidal gearboxes. While the topic can be quite complex when explored in depth, we have aimed to explain it in a simplified and accessible way. We hope this overview has been helpful in enhancing your understanding.
Bon Systems provides a range of specialized services including the development of cycloidal gear reducers, actuator manufacturing, and robotic hardware production based on our proprietary cycloidal reduction technology. In addition to the model introduced in this post, we also offer a wide variety of products, all of which are developed in-house to meet the specific needs of our clients.
We are committed to leading the next generation of drive modules in the robotics industry and to becoming a reliable technology partner for your business. For product inquiries or development consultations, please visit the quotation request section on our official website. When submitting your inquiry, we kindly ask that you include details such as the intended application, target quantity, and required specifications. This will help us propose the most suitable cycloidal gear and reducer solution for your needs quickly and accurately.
