Cycloidal drive is a type of speed reducer commonly found in robotics. It reduces rotational speed of an electric motor whilst amplifying its torque, thus enabling it to power the joints of robotic arms. A striking feature of cycloidal drives is its mesmerising wobbling and meshing of its cycloidal disk.
Cycloidal drives are popular because of its high torque amplification ratio compactness, almost zero backlash and ease of manufacturing. Nonetheless, it has its drawbacks such as vibration during high-speed rotation and its performance is easily affected by slight imprecision in manufacturing and assembly.
Nevertheless, this article aims to provide a detailed explanation of the working principle behind a cycloidal drive and a design tutorial for designing a 48:1 cycloidal drive in Fusion360 Cad Modelling Software.
At the heart of a cycloidal drive, there is a disk with cycloidal profile mounted off-center from the input shaft and as the shaft rotates, it is forced to mesh against a series of cylindrical pins positioned equally along its peripherals (Figure 1b). There is one extra cylindrical pin (49 pieces) with respect to the number of teeth (48 teeth) on the disk such that the disk lags by one tooth for every one complete revolution of the shaft. For the disk to return to its original position, the shaft needs to revolve 48 times.
As the disk wobbles and slowly revolves, it pushes against another set of 5 pins that are connected to a load bearing plate and enable the load bearing plate to revolves concentrically with respect to the cycloidal drive (Figure 2a). The load bearing disk is also the output shaft that can be connected to other external structures inside robotic arm to transfer the amplified torque to the structures that needed it (Figure 2b).
Cycloidal Drive Design Tutorial
The two design goals for this 48:1 cycloidal drive is 1) to develop a highly compact cycloidal drive and 2) to develop for ease of manufacturing by a desktop CNC milling. A compact drive is lighter and improves payload capability and reach of a robotic arm whilst, designing for ease of manufacturing help keep costs low.
The first step in designing complex mechanism is to find a component that is difficult to change in dimension and then to begin design process from. The design process for this 48:1 cycloidal drive will begin from a NEMA17 stepper motor and its D-cut shaft.
Designing The Eccentric Nut
Based on documentations, a NEMA17 motor have screw holes positioned in a square with 31mm edge length. A 42x42mm body and a 23mm diameter step profile at the top. The D-cut shaft is 5mm in diameter with a flat cut at 1mm away from its circumferences. The D-cut shaft needs to push a cycloidal disk off centre by 0.5mm to force it to mesh against a set of cylindrical pins positioned around its peripheral. It can be accomplished with a simple eccentric nut mounted onto the shaft of NEMA17 stepper motor (Figure 1a).
The eccentricity is set at 0.5mm because there is insufficient space for a larger eccentricity (Figure 3a), given that the dimensions are also constrained by an 8x16x5mm ball bearing. A larger eccentricity is not encouraged because it meant larger movement of the cycloidal disk and a larger casing is required to accommodate for additional motions. It is acknowledged that a larger eccentricity enables tighter meshing and reduces the chance of disk slipping during high torque applications.
Designing the Casing
The casing for the cycloidal drive serves multiple roles such as 1) to hold 49 pieces of 3mm cylindrical pins in place, 2) attaches to a NEMA17 stepper motor, 3) allows space for cycloidal disk to wobble and 3) to allow a cover to be attached to the casing.
49 pieces of cylindrical pins are held in place equally along a circular path by the casing for the cycloidal disk to mesh against. Ideally, the pins ought to be placed closely le to keep the cycloidal drive as compact as possible. However, there need to be sufficient space between the pins to allow cycloidal disk to mesh in fully. As the eccentricity is set at 0.5mm, the amount of space between the two pins should be twice the eccentricity at around 1mm. Similarly, there need to be sufficient material between the pins to provide mechanical support during high torque scenarios. The position of cylindrical pins are derived through trial and errors until each pin is spaced around twice the eccentricity (Figure 4).
The design of cycloidal drive casing start with a flat cylinder and pocket are cut into it to create space for cover to be placed inside. There is a thin wall of 2.5mm thickness that will provide structural support for the final drive cover. 49 holes with diameter 3mm and depth 8mm are drilled into the casing for holding cylindrical pins in position.
Next, cut a pocket with diameter 62mm and depth 5mm to create space for cycloidal disk to wobble. Cut another pocket with diameter 39mm and depth 3mm to create space for torque pins as it is being pushed around by cycloidal disk. Create 4 counterbore holes for M3 screws for attachment to stepper motors.
Fillet the edges of counterbore holes with 1mm radius to enlarge it and to create sufficient space for CNC milling later. Cut a through hole with diameter 23mm to allow the stepper motor and its 23mm diameter stepped profile to fit inside. Lastly, we add threads to the thin wall for the final casing to be screwed in. Screwing in a cover helps reduce part counts as no additional screws are needed and it keeps the drive compact as there is no need to allocate space for screw holes.
Design the Cycloidal Disk
To begin designing the cycloidal disk, 4 parameters need to be defined beforehand. Those 4 parameters are a) amplification ratio, b) eccentricity, c) cylindrical pin diameter and d) cylindrical pin positions. They were earlier defined in this article:
- Amplification ratio = 48 to 1
- Eccentricity = 0.5mm
- Cylindrical pin diameter = 3mm
- Cylindrical pin position = diameter 62mm
Step 1: Draw a circle with radius of 31mm since the pins are placed 31mm away from shaft.
Step 2: Draw a horizontal line from the centre to the edge. Divide the horizontal line into 48+1=49 portions where the long segment takes up 49 portion at 30.367mm while the short segment takes up 1 portion at 0.622mm.
Step 3: draw another big circle from the centre to the edge of long segment so that its radius is at 30.367mm.
Step 4: Duplicate long segment around the centre point. Each long segment is separated exactly 0.5deg from one another. There should be 16 long segment and the last one is at 7.5deg. The last one is at 7.5 deg because 360deg/48 = 7.5deg.
Step 5: These duplicated long segment are now known as primary lines. Each primary lines had its own angle between 0deg to 7.5deg and it will be used to draw secondary lines. The angle for secondary lines are simple the angle of primary line multiply by 48+1= 49 times. For example, secondary line stretching out from primary line #7 with an angle of 3.0deg will have an angle of 3.0*49=147deg. Repeat it till all the primary lines are connected to secondary lines.
Step 6: Connect the ends of all secondary lines with spline line.
Step 7: offset the spline profile by 1.5mm which is half the diameter of pin at 3.0mm.
Step 8: duplicate the offset profile around the center 48 times to enclose an area. Select the area and extrude 5mm.
Step 9: The disk will wobble with an eccentricity of 0.5mm and will be pushing again 5 pieces of load pins. The load pins have a diameter of 9.0mm and to allow the disk to wobble on all size, 10.0mm holes are added to the disk. For fitting of ball bearing, a hole of 16mm is created.
Step 10: This is an optional step to reduce the weight of the cycloidal disk by cutting out excess materials. This reduces the weight of cycloidal disk and minimize vibration while the cycloidal drive wobbles during operation. However, excess removal of material will reduce the stiffness of the cycloidal disk and cause it the bend end slip during high torque scenarios.
Designing Load bearing Plate and Pin
The cycloidal disk wobbles as it rotates and not suitable for bearing load. A secondary structure in the form of load plate and load pins is needed. It is a simple mechanism with 5 load pins attached to the underside of the load plate via threaded screw holes (Figure 6a). Together with the load plate, the load pins are inserted into 10.0mm holes on the cycloidal disk and pushed by the cycloidal disk in a circular path. Additional threaded screw holes allow external structures to be attached the cycloidal drive.
The load pins are simple cylinder with a 9mm external diameter and a 3mm through hole. Additional chamfers are added to the through hole to allow a counter sunk screw to sit flush and align the load pin in precise location. Smaller chamfers are added to the external rim to allow the load pin to slide into cycloidal disk easily (Figure 7).
The load plate provides anchorage for external structures and need to be constrained in laterally in X, Y and Z directions (Figure 8a) and rotational around X and Y axis whilst allowing free rotation motion around Z axis (Figure 8b). A small pin with external diameter of 8mm and internal diameter of 5mm is used to constrain the load plate and ball bearing laterally in X and Y axis. A thrust bearing placed on top of load plate helps to constrain it laterally in Z axis against the top cover and rotation around X and Y axis as well. The thrust bearing allows rotation around Z-axis to continue hence allowing the cycloidal drive to perform as intended.
Load plate consists of threaded screw holes arranged 15mm away from the center to allow load pins to be attached to it. Another set of threaded screw holes allow external structures to be attached to the cycloidal drive. A thin disk along the peripheral is for holding a thrust bearing with external diameter of 52mm and internal diameter of 35mm (Figure 9).
The top cover is the final piece for the cycloidal drive. It consists of small pocket at the top to help reduce weight as well as anchor point for tools to screw in the top cover (Figure 10a). There are thread at its side to allow it to be screwed into the cycloidal drive casing. Small ledges below help elevate thrust bearing to allow it to rotate freely inside the casing (Figure 10b).
After the thrust bearing are place on the load plate, the top cover can be placed over the thrust bearing and screwed into place. By screwing the top cover in, it helps to keep all the components together whilst preventing dust from falling inside the cycloidal drive (Figure 11).
Download Design Files
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STEP, STL and Fusion 360 design and assembly files are included.
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List of components needed:
1. NEMA17 stepper motor x1
2. 8x16x5mm ball bearing (688) x 2
3. M3x8 countersunk screws x 9
4.Thrust Bearing (3552) 2mm x 1
5. M3x8 dowel pins x 49
6. 5x8x10mm nylon spacer x 1
Watch detailed assembly guide below: