NEMA-17 1:4 step-up gearbox for remote direct drive

Introduction

Remote direct drive extruders like the Flex3Drive (and its clone the Zesty Nimble) use large reduction gearing at the toolhead side to keep high torque from the extruder hob off the flex shaft. This has a side effect of seriously limiting the speed/acceleration of retraction and Linear Advance/Pressure Advance moves. For me, this pretty much defeated the purpose of switching to direct drive. So, I designed and built this gearbox to bring the net reduction down to 1:10. This has made it possible to do 1 mm retractions in under 50 ms (32 mm/s, 2000 mm/s² acceleration), and to use a very low smooth_time of 10 ms with Klipper's Pressure Advance, for accurate extrusion at high speed and acceleration.

The gearbox design is modular, consisting of an extender bracket to get the gears to the end of the motor shaft and to allow assembly on the motor in-place, a shaft coupler that engages the planetary carrier with a square drive, the gears themselves including a split ring gear, and spacers which can be used if needed.

The motor end of the extruder's flex drive shaft inserts directly into the sun gear. The square hole in the sun gear is nominally 3.2 by 3.2 mm, and is designed for the flex shaft that ships with the Flex3Drive G5. Before printing and assembling the rest of the gearbox, make sure your shaft fits the sun gear. It should be easy to modify if it doesn't.

There are two versions of the planetary gears drive. One is designed for press fit with 683ZZ bearings for smooth motion and distribution of force across the gear, but requires more hardware for assembly. The other has 2 mm holes in the gears, for use with metal shaft. The version with bearings should run smoother and not develop slop as it wears, but the no-hardware version is nice if you want to test before buying components, and seems viable for long-term use with TPU gears.

I'm using this on an Ender 3 with the 22 mm extruder motor shaft length and D flat cut in the shaft, but installation should be possible on any NEMA 17 motor with a 5 mm shaft diameter, using appropriate spacers and positioning of the coupler.

Non-printed parts needed for assembly

Shaft pinch coupler:

• 1x M3 nut
• 1x 8mm M3 bolt

Mount to motor:

• 4x 8mm M3 bolts (possibly longer depending on bracket and spacers)

Main assembly:

• 4x M3 nuts
• 4x 20mm M3 bolts (possibly longer if using spacers)

Planetary gear set (only if building the version with bearings):

• 3x 683ZZ bearings
• 3x 8mm M3 bolts, button head preferred
• 3x M3 nuts
• 3x M3 washers (can be printed if you don't have any)

Print material

Gears: PETG or TPU is ideal. If using TPU, it should be a hard variety, and printed with proper wall settings so that the part comes out solid. PLA works but is not recommended; it will be very noisy to operate.

Structural parts: PETG or PLA.

ABS should not be used for any parts unless you have a compatible (non-oil-based) lubricant.

Lubrication

Gears should be lubricated before attempting to operate. Any automotive grease is good; red high-temperature STP was what I had on hand. Operate the gears by hand, or with a drill or rotary tool, before installing to ensure they can turn freely.

Recommended print settings

Gears: 0.10-0.12 mm layers, 3 walls, recommend reducing to 0.35 mm outer wall to allow a contiguous second wall in teeth. Use 5 walls for the no-bearings version of the planet gears to get a solid part. This is especially important if printing in TPU.

Carrier and coupler: 4 walls, 0.20 mm layers.

Extender: 3 walls, supports, 0.20-0.25 mm layers.

Spacers (if needed): 3 walls, 0.20-0.25 mm layers.

Infill percentage can be low (e.g. 10%) for all parts since the strength comes from walls not infill.

All parts require fairly tight dimensional accuracy, and are designed for 0.1 mm clearances (most places, in both directions, so 0.2 mm total). If your printer is not well tuned, you may need to apply some horizontal offsets to get parts that fit together. I recommend 0.4 mm nozzle/line width (except as mentioned above) but nothing in the design depends on single-line walls, so other sizes close to 0.4 are probably okay to use.

Planetary gears assembly

Bearings version

1. Mount bearings to the carrier with washers so that the outer race does not rub on the carrier body when it turns.
2. Press 2 planet gears just slightly onto their bearings.
3. Hold sun gear in place.
4. Fit third planet gear to sun gear and press it onto its bearing enough to hold it.
5. Press all three planet gears the rest of the way onto their bearings.

No-bearings version

1. Cut 3 10mm shafts from 2mm diameter metal rod (commonly sold for toy car axles, etc.).
2. Ensure shafts fit in upper and lower carrier holes. If they don't, drill the holes slightly by hand or with a power tool, but be careful not to make a hole all the way through.
3. Insert one shaft into each hole of lower carrier.
4. Lubricate the shafts.
5. Sit 2 planet gears on the end of their shafts.
6. Hold sun gear in place.
7. Fit third planet gear to sun gear and place it on the end of its shaft.
8. Press all three planet gears the rest of the way down.
9. Clip upper carrier to lower carrier, fitting the shafts to their corresponding holes.

After assembling the planetary gears, lubricate them well.

Installation

1. Press fit 4 M3 nuts into the extender bracket.
2. Attach the exender bracket to the motor where it's mounted using 4 M3 screws.
3. Press fit 1 M3 nut into the motor shaft coupler and place an M3 screw through it, but don't tighten yet.
4. Press the coupler into the square on the planetary gear assembly's carrier.
5. Slide the coupler onto the motor shaft.
6. Place half of the ring gear on top of the bracket and hold it in place temporarily with screws.
7. Line up the planetary gear assembly to the ring gear to determine the height to mount the motor shaft coupler at. This should put the top of the coupler's square output drive roughly 1mm below the top of the extender bracker. (If the shaft extends above that such that it would interfere with the sun gear, you'll need to add spacers above or below the bracket.) Tighten the coupler down in place using a hex key to reach it through the openings in the bracket.
8. Remove the temporary screws holding half of the ring gear and install the full ring gear, placing a spacer and the part provided with your extruder to hold the flex cable housing on top, and fasten the whole assembly with M3 screws.
9. After ensuring the gears turn correctly without the extruder connected, press the square end of the flex shaft into the sun gear.

If you need to remove/disassemble the gearbox, it can be separated from the coupler to lift it off by applying pressure to the carrier from below, or using needle nosed pliers to grab the carrier and pull it up. Pulling by the gears is not recommended and will probably make the assembly fall apart.

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