Building a Custom Cycloidal Drive for Robotic Arm

This Instructables is about building an inexpensive, yet precise and robust cycloidal drive with the help of JLCCNC. This gearbox will be used as the 6th joint of my robotic arm IRAS. It therefore needs to have high precision and torque capabilities to carry more than 10kg continuously.

I used aluminium parts, machined by JLCCNC, which ensures a long lasting and precise gearbox. However, this gearbox can also be 3d printed, though you sacrifice a lot of torque and precision and the aluminium parts are not expensive.

2x 32910 50x72x15mm Single Row Taper Roller Bearing

4x ET2520zz-ZEN 20x25x4mm Deep Groove Bearing

2x 617032rs_ZEN 17x23x4mm Deep Groove Bearing

6x M4 x 30mm Shoulder Bolt

20x 4mm x 20mm steel dowel pins

I let the gearboxes output, the output pincarrier and two cycloidal disk be machined by JLCCNC who did a great job. The cycloidal disks are made of 7075 aluminium and the rest from 6061 aluminium.

I printed all of the other parts from Overture PLA Pro, but every stiff filament will likely work-

If you decide to print these parts, make sure to use thick walls and high infill percentage.

Begin by installing one 61703 2RS deep groove bearing and a 32910 tapered roller bearings to the output piece, repeat this for the output pincarrier.

If you have machined these two parts from aluminium, make sure they fit snuggly. If the 61703 bearings doesnt fit (which it should), you can heat the parts to 80°C and then install the bearing. If the tapered roller bearing doesnt fit, heat it up to 100°C and carefully install it (it needs to be very straight, or it might get stuck).

If the fit between the bearings and the parts are too loose, use bearing reataining compound to secure them. Loose bearings will lead to a unprecise drive.

If you've decided to print these parts (output and output_pincarrier), you should be able to press the bearings into positions, though they shouldn't come loose.

Repeat this for both cycloidal disks. For them, put two ET2520 bearings into the centre bore of each disk. Make sure they are secured very well.

Put each of the M4 x 30mm shoulder bolts through their corresponding holes in the output_pincarrier

On each of the M4 x 30mm shoulder bolts, put the components in the following order:

1. spacerring 1
2. MR85 bearing
3. spacerring3
4. MR85 bearing
5. spacerring2
6. MR85 bearing
7. spacerring3
8. MR85 bearing
9. spacerring2

Also make sure there is no grease or oil on the threads of the screws.

Put the first cycloidal disk onto the short side of the ecccentric shaft (see 1st image for reference), the orientation of the drive doesn't matter yet.

Then put the second cycloidal disk on the long side of the eccentric shaft. Make sure that the two slots roughly line up, and the disks are 180° out of phase (see 3rd image as reference). If the disks are out of phase, the cycloidal drive won't run smoothly and vibrates at higher speeds.

Without breaking the disk alignment, put the short side of the eccentric shaft into the 61703 bearing in the output part. This fit should be tight.

Put the outer race of the 32910 tapered roller bearing into the shallower recess in the housing (the side where screw heads will be, see first image). Make shure the race is fully seated and doesn't move, then apply some grease (I used white lithium grease). IMPORTANT: DO NOT INSTALL THE SECOND RACE YET.

Refer to the second image and place the housing over the installed cycloidal disks, do not break their alignment. As seen in the image, only place it a few millimeters, so that the installed roller pins don't fall out.

Apply a bit of grease into every roller pin recess and on the sides of the cycloidal disks. Now push each roller pin into place while not breaking the disk's alignment. You might have to move the housing a bit to get them in place.

The last pins require more force. The housing may also be lifted up to make installation easier, though all pins may fall out if not done carefully.

When all pins are in place and grease is applied generuously, push down the housing fully.

Make sure the eccentric shaft turns, though you might require some initial force, this is normal and will go away.

If all pins are inplace, cyclidal disk are in 180° out of face, their slots roughly match up and you can see through the drive, install the second race of the 32910 tapered roller bearing and make shure it is seated fully.

Apply some thread locker to the threads of the shoulder bolts, to prevent them from coming loose.

When all the output pins are installed in the output_pincarrier and have the bearings and spacers equipped, carefully line them up with the 6 holes of the cycloidal disk and the slots of the output_pincarrier line up with the others and lower the output_pincarrier into the assembly.

Screw the shoulder bolts down equally, otherwise the MR85 bearings may come loose and the drive would need to be taken apart.

Tighten the bolts very firmly.

Congratulations, you now have a cost effective, accurate, strong and long lasting cycloidal drive for many purposes with the help of JLCCNC and their great service.

After assembly, the drive might feel very hard to turn. This is normal, but after a few rotations the drive will get very efficient. With the the help of JLCCNC I managed to get over 20Nm of torque without the gearbox failing. With the metal parts the drive can easily handle more, though I haven't measured the maximum allowable torque.

If you have any questions regarding the design, help or need a custom design, feel free to contact me via my website.