Design a Cycloidal Drive

How to Design a Cycloidal Drive

Cycloidal drives are gear reducers which are utilized in mechanical systems, where large amount of reduction, accuracy, power density and virtually no backlash, is of great importance. These drives are used in robotics, industrial machinery and other things, where compared to the conventional gear mechanisms these offer much smoother, compact and efficient motion. However, it is not a case of adjusting individual attributes when it comes to cycloidal drive design. Every parameter of the system depends on another, and when modifying one, the others are impacted in one way or another.

Here, we will look at how the various factors of a cycloidal drive are related and how these relations determine the effectiveness of the system. It is necessary to understand these conditions to achieve the goal of constructing a drive that will meet the requirements of the intended use to have more torque, higher speed or to work in a more integrated manner.

Cycloidal Drive Simulator

Transmission Ratio and Lobes

The transmission ratio is the major characteristic of the cycloidal drive. This decides on how many turns the input shaft (the eccentric shaft) does for every full circle turn of the output shaft/disk. It has the function of regulating the assembly ratio of torque to speed. This ratio cannot be set as simply as an arbitrary number; the transmission ratio is directly tied to two critical parameters: the number of lobes (teeth) on the cycloidal disk and the number of external pins.

Lobes vs. Pins

It is must that the number of lobes in the cycloidal disk be slightly less than the number of external pins in the fixed outer ring. For instance, you can have nine lobes if you have ten external pins. This small difference is the essence that makes for cycloidal motion. In general, number of lobes are one less than the pins 

Effect on the Transmission Ratio

Additional lobes raise the transmission ratio as well; the input shaft will spin several times to make the output wheel complete one cycle. It is evident that as the transmission ratio rises the torque rises, and the output speed drops. Therefore, if torque is required, one opts for a higher number of lobes and numbers of external pins, but this results in slower output.

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This relationship between lobes, pins, and the transmission ratio demonstrates the first major interdependency. And as you add one factor to gain more torque, you also set an impact on the output speed.

In our Cycloidal Drive Simulator, we consider one less number of lobes than that of pins; so the transmission ratio is equal to the number of lobes.

Input Speed, Output Speed, and Torque

In any gear system, the speed of the part that the input shaft turns to determines how fast the output will operate.

  • Input Speed: The input speed is the rotational speed of the eccentric shaft of the shown in the Figure above. Increasing the input speed adds to the overall activity of the system but the output speed is also determined by the transmission ratio.
  • Output Speed: It depends on the input speed and the transmission ratio as to the speed of the output shaft. A higher ratio of transmission will slow the output shaft even though the input is very fast. That is why if you decide to raise the transmission ratio for higher torque it will be noticed that the output speed has gone down.

The correlation between the input and output velocity and the transmission ratio constitutes the basis for optimizing your cycloidal drive to fulfill precise performance specifications. It is fine if torque is more important than speed; you will have to be prepared for slower outputs and slow down your inputs.

Role of External Pins and Cycloidal Disk Lobes in Smoothing Motion

Cycloidal drive is well known for its ability to produce continuous motion free of backlash due to the interaction of the lobes of the cycloidal disk with the external pins of the fixed ring. The number of external pins defines the role in this process to a significant degree.

  • More Pins, Smoother Motion: In most cases, increasing the number of external pins is known to have positive effects on the smoothness of the drive. There is an increased number of pins that make contact with the lobes at any one time hence minimizing vibrations and play or backlash between gears. But more pins mean a bigger and more complex drive.
  • Lobes and Transmission: This is because the lobes of the cycloidal disk must match the pins in terms of the number. If for instance you decide to have many lobes you are increasing the transmission ratio, but this comes at the cost of larger torque but slower speed. So, introducing more external pins and raising the number of lobes, you get smoother motion and more torque, but it slows down the whole system and increases its size.

From this interdependence, one can see how necessary it is to maintain a balance between the flow and the physical constraints of the design. For example, if you are designing a small system, such as a robotic arm, you might need to accept roughness at the drive’s expense to maintain size and mass.

Eccentricity

Because of this eccentricity of the drive, which is the distance from the center of the eccentric shaft to its rotation axis, the cycloidal disk will wobble to engage the external pins. That is why eccentricity is a rather delicate though essential characteristic since it determines how much motion the cycloidal disk transfers to the other part of the system.

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More Eccentricity = More Movement

Growing radiality results in a larger cycloidal disk offset angle that provides for a greater extent of torque-coupling deviation. However, if the eccentricity is too excessive for the number of lobes, the system may be rendered unstable or the friction leading to wear out of the drive system may build up hence lowering the efficiency of the drive.

Interaction with Lobes

Just as the eccentricity must be balanced with the number of lobes, so the lobes must be balanced by the corresponding area. If you have a smaller number of lobes, then decreasing the eccentricity leads to poor disk engagement or poor interaction with the exterior pins. However, when it is low, the performance of the system will be low due to inefficiency of the motion transfer.

Output Disk Diameter, Output Pins, and Load Distribution

The output disk which has the output pins is another important part of the cycloidal drive mechanism. The diameter of this disk, the number of the output pins also determines how the load is being distributed in the system depending on how this motion is transferred to the cycloidal disk to the output shaft.

Over Crowded

Less output pins

Balanced Drive

Lobes and Pins Working Together

The lobes of the cycloidal disk need to get along well with the output pins. Adding lobes gives smoother interaction with more output pins but both the diameter of the output disk and the number of pins must be altered to prevent crowding or mechanical interference.

Larger Output Disk, More Output Pins

When the output disk is bigger it can contain a greater number of output pins and thus the load is distributed. This makes individual pins wear out less while it also increases the life expectancy of the whole system if handling high torque loads. But for extra output pins, one needs a larger disk, and hence the drive will be bigger and heavier.

The last of these emphasizes the trade-off between the life cycle and the size of the system. Thus, if necessary, it is possible to increase the size of the output disk and the number of pins, however, this will require an increased load on the system, which worsens its compactness and speed. 

Pins, Thickness, and Friction

The outer and output pin diameters, as well as thickness of the cycloidal disk are also important factors that determine performance of a cycloid drive. It also indicates that the pins with larger diameter can bear more load, which also shows that a thicker cycloidal disk gives more stiffness and offers stronger structure.

Larger Pins Require a Thicker Disk

As you increase the pin diameter, one will need a thicker cycloidal disk to support them without deforming or breaking. A thin disk with large pins can lead to failure under load, while a thick disk can handle more force but also increases the system’s weight and friction.

More Friction, Less Efficiency

Greater pins and disks mass presents strength as well as durability but the negative side of the system is the high level of friction. This decreases the mechanical output and power to input ratio meaning more input power is needed to generate the same performance.

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The size of the pins and the thickness of the disk must be as close to each other as possible. In high-load uses, strength is what may be important, while for speed-critical uses, the importance of lowering the coefficients of friction and mass will be one’s paramount choice.
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Camshaft Hole

The camshaft hole is an important one because the cycloidal disk is fitted on the camshaft or eccentric shaft.
Mechanical Fit

This hole diameter must match the camshaft or mounting component in the system, so that it bears no risk of slipping during use.

Other dimensions that can be modified to suit the bearings which support the eccentric shaft include camshaft hole diameter. Bearings are used universally to support the load and ensure that the rotational motion is free of hindrance. Selecting the appropriate diameter for the camshaft hole to hold bearings guides the eccentric shaft while supporting it properly thus improving the drive’s efficiency.

A precisely sized camshaft hole makes the cycloidal disk rotate around the camshaft exactly, which is important for high speed or high torque equipment to avoid much vibration.
But when setting the camshaft hole diameter, we also guarantee compatibility with the camshaft and, naturally, can install bearings, which positively affect the stability and endurance of the cycloidal drive, reduce the coefficient of friction and increase the service life.
Cycloidal Drive Simulator

Fixed Ring Diameter

Spread across the entire drive the data that you need to determine its size. The increase in fixed ring diameter means that the external pins also increase in number which can improve smoothness and reduce backlash.

Export Options

Once the optimization of the cycloidal drive design has been done, the profile can be saved in two formats for use in other CAD applications or production.

2D DXF File

This format is convenient for two-dimensional designs by copying the cycloidal profile to CAD for laser cutting or CNC profile milling or any 2D manufacturing process.

3D STL File

In three-dimensional applications, for example in 3D printing and advanced manufacturing, the most used format is the STL format. Exporting to an STL format allows one to take the cycloidal profile into the 3D CAD space for modeling and testing of the drive before fabrication.

These export options facilitate the conversion from design to manufacturing and allow for the adoption of the improved cycloidal geometry into different CAD and manufacturing systems.

Conclusion

The conception of a cycloidal drive depends on the interrelated parameters which have a significant impact on the performance of the chosen equipment. The transmission ratio, determined by the number of lobes and external pins, sets the torque and speed balance: increasing the ratios gives more torque yet lesser RPM. The transmission ratios also govern the input and output speeds and, therefore, torques must always be traded off with speeds. Anti-backlash is achieved using external pins and lobes with more pins giving smoother motion at the cost of the size of the drive. With the eccentricity, output disk diameter, and cycloidal disk thickness that affect torque steadiness as primary factors, the load-carrying capacity is also highly influenced. Higher eccentricity benefits torque transfer but at the same time, must be aligned with the lobes in a manner that can reduce instability. The load distribution and durability of the components depend on the output disk diameter and pin size, larger components have strength but high friction and size. The camshaft hole, if carefully made to fit the camshaft and bearings, avails a rigid drive, decreased wear and tear and longevity. If properly adjusted, these parameters provide a cycloidal drive to fit a specific application of precision robotics, industrial machinery, and compact high torque systems.
Cycloidal Drive Simulator
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