High Precison Cycloidal Drive for Robotic Arm

I designed this cycloidal drive for my high strength robotic arm IRAS.

However, this design can easily be adapted or changed for your personal needs. The reduction is 1:43 and the drive has zero backlash and is torsionally stiff.

For best precision I chose to let JUSTWAY machine the cycloidal disks from 4340 steel. Their dimensions are perfect and the surface finish is great.

Supplies

Step 1: Printing and Ordering Parts

The parts to be printed are:

1x cycloidalDrive_eccentricShaft
1x cycloidalDrive_eccentricShaft_end
1x cycloidalDrive_eccentricShaft_endsupport
1x cycloidalDrive_eccentricShaft_endsupport2
1x cycloidalDrive_greaseCover
1x cycloidalDrive_housing
8x cycloidalDrive_outputpin_spacer1
8x cycloidalDrive_outputpin_spacer2
8x cycloidalDrive_outputpin_spacer3
1x cycloidalDrive_spacerring_1
1x cycloidalDrive_spacerring_2

Parts to order:

2x cycloidalDrive_disk (4340 steel) from JUSTWAY
1x RU66 cross roller bearing
44x 4mm x 20mm hardened steel pins
48x MR85 2RS 5x8x2.5mm bearing
8x M4 x 20mm Shoulder Bolts
4x 6704 20x27x4mm Bearing
2x 6703 17x23x4mm Bearing
24x M4 threaded inserts
10x M2 threaded inserts
4x M2x8mm cap head screws (DIN912)
8x M4 x 18 cap head screws (DIN912)

Step 2: Installing Threaded Inserts Into Printed Parts

To install the 24 M4 threaded iserts into the housing I used a soldering iron.

Be carefull to not deform the plastic of the housing when inserting the 8 inserts on the bottom side, otherwise the drive will not run smoothly.

Also put in the 10 M2 inserts into the corresponding holes in the eccentricShaft.

Step 3: Mounting the RU66 Bearing

Using the 8 M4 x 18mm bolts, attach the RU66 cross roller bearing to the cycloidalDrive_housing. The head of the screws go into the recessed holes in the bearing.

Make sure to tighten the screws, but not strip the inserts or pull them out of the plastic.

The RU66 bearings also functions as the output face of the cycloidal Drive. The cross rolle bearing allows for very high bearing loads in all directions.

Step 4: Building the Eccentric Shaft Endsupport

Install one 6703 bearing each into the cycloidal drive endsupports (cycloidalDrive_eccentricShaft_endsupport and cycloidalDrive_eccentricShaft_endsupport2). Make sure the bearings are fully seated.

Then press the endsupport (the one with the shallower hole) into the centre bore of the RU66 bearing on the side of the housing.

Afterwards do the same for endsupport2 (deeper hole), but put it on the output side of the drive (3rd picture).

Step 5: Installing Spacerring and Grease

Put the spacerring_2 (the thicker one) into the middle of the bearing, inside of the housing.

Then spread a healthy amount of grease into the ridges, in which the roller pins will be located. Also put some grease on the bearing, where it is in contact with the end of the roller pins (I used white lithium grease).

Step 6: Installing the First Cycloidal Disk

Both cycloidal disk are the same and there is no specific orientation the FIRST on has to go in.

Start off by pressing in two 6704 bearings into the middle of the disk. If they are 3d printed, adjust the hole diametre, so they fit tightly, but rotate freely.

If you have metal disk, you can heat up the disk and put in the bearings carefully, if the hole is to tight. If the hole is a bit to loose, glue the bearings in using bearing retaining compound.

Either way, the bearings should be located tightly in the bore and not move. This is important for precision.

Next, put the disk onto the eccentric shaft, from the side with the 4 inserts (see picture for reference). This fit should be tight as well and require some force, but the disk should spin freely.

Make sure the the spacerring from the 5th step is in place, then push the eccentric shaft with the cycloidal disk in the bearing from step 4. Push in all the way and check if the disk and the eccentric shaft move freely.

Step 7: Installing the Eccentric Shaft End

For this step, you need to turn over the drive. Be careful, that the eccentric shaft and disk don't fall out. However the fit between the previously installed eccentric shaft and the 6703 bearing should be so thight, that this doesnt happen.

Press the cycloidalDrive_eccentricShaft_end into the 2nd 6703 bearing and fix it to the eccentric shaft using four M2 x 8mm screws. Do not overtighten the screws. The eccentric shaft needs to rotate freely.

Step 8: Installing the Second Cycloidal Disk and Roller Pins

On the already installed cycloidal disk, place the cycloidalDrive_spacerring_1.

Turn the eccentric shaft, so that the installed cycloidal disk is moved to the most left position. While holding in place the eccentric shaft, rotate the disk, so that the lobe on the disk is over the left most roller pin "bay" (see image for reference).

Only after the step above is done proceed.

Rotate the second cycloidald disk, so that the "hill" of the disk is touching the roller pin "bay", when the disk is moved 0.6mm offcentre to the right. And the 8 holes need to align. Please refer to the first image of this step.

The disk need to be 180° out of phase to compensate wobble and ensure smooth operation.

You can now put more grease into the gap between the cycloidal disk and housing. Be careful, not the turn the disks out of alignment. You then need your 44 roller pins (4mm x 20mm). Push them into the grooves in the housing.

This should require not much force, but also not to little. I find the backlash and performance best, if I have some friction when inserting them.

In case some pins won't go in all the way, push them down lightly, while rotating the eccentric shaft.

Step 9: Building the Output Pins

In this design, the output pins for the drive are eight M4 x 20mm shoulder bolts. They screw directly into the output bearing (RU66).

Begin by putting 8 outputpin spacer 3 on each screw, followed by 3 MR85 2RS bearings each, then the outputpin spacer 2, 3 MR85 2RS each and finally the outputpin spacer 1.

The order from top to bottom is (see first image):

outputpin_spacer1

MR85 2RS

outputpin_spacer2

MR85 2RS

outputpin_spacer3

If you have all 8 bolt populated, begin to screw them in to the output bearing though the cycloidal disk. For this, rotate the the RU66 bearing manually until you see the threads through the cycloidal disk.

Make sure there is no grease on the threads or the bolts might come loose in the future. You may even use thread locker.

Tighten all bolts very tight.

Step 10: Closing the Drive

In my case I have mounted a HTD 3M pulley on the input shaft. You can of course use whatever you want. The 6 M2 inserts are 12.5mm spaced apart.

Eventough the design is open on one end, as i had geometric constraints, you should always close at least the part where the rolling pins are located and the grease is. This prevent dirt from getting in and mostimportantly the pins from coming out.

I have included all the file to modify for your needs.

When initially turning the drive, it might feel very difficult to do so, I encounter the same thing and this is normal. The grease I used is very viscous and needs to spread evenly. Also there is always a bit of wearing in.

After rotatting it a few times with a motor (about 120 times) the input is very free to move. If this isn't the case,

you need the reprint the housing with an increased rolling pin diameter (Cut-Extrude2 in the Solidworks model).

I hope you have fun using the drive. If you have any problems assembling, using, getting technical drawing or adapting the drive, feel free to contact me via my website jacobutermoehlen.com/contact . Also special thanks to JUSTWAY for machining the parts, they are driving factor for the precision of the parts.

Introduction: High Precison Cycloidal Drive for Robotic Arm