MK4S Bogie Idler & Main Plate

Updates

“Idler_6-7_R2” dimensions adapted to the Main Plate R3 and angled Swivel to minimize the screw tread friction in the heatsink.

“Main_Plate_9-10_R3” have tighter tolerance to the Gear.

Printing

Printing on MK4S. Material PETG. Nozzle 0,4 and layer 0,2. I use original presetting’s for Prusament PETG structural except for part 1-2 where I changed seam position to nearest.
The design requires fine tolerances. Check your x-y belt tension and use dry filament.

If you print in an enclosure with higher environment temperature consider using PCCF filament as recomended for original parts.

Idler assembly

The numbers refer to the pictures. No glue is needed.
Press bogie parts 1&2 together. Place original bearings 3 (693 2RS) into the bogie and press original pins 4 (3x8.5) in place. Fix a nut 5 (M3) in the idler lever 6. Place the bogie assembly between idler lever 6 and 7. Tighten firmly with screw 8 (M3x6). Don’t overtighten the screw! Push the original tubular spacer 9 (13.2x3.8x0.35) into 6 and 7.

Main Plate assembly

The numbers refer to the pictures. No glue is needed.
Press together part 9 and 10. Check that the total height of the filament guide is not exceeding 10 mm (see picture). The two parts will keep together when assembled in the Nextruder.

From the original Prusa MK4S STL files I made a reverse engineering of the Nextruder main-plate and the idler. When you look at the Idler you get the impression that the two bearings distribute the pressure evenly as they push the filament against the gear. In fact, only the lower bearing is pushing the filament against the gear. This causes very big pressure on the filament. For hard filaments this is not a problem but flex filaments is smashed. This mod is a “nice to have” for hard filaments like PLA and PETG and “must have” for soft Flex filaments.

I also noticed that the screws holding the Idler-swivel assembly rub against the screw holes in the heatsink and cause friction. In this latest update the Idler-swevel is angled down to reduce the screw friction in the holes.

The new bogie design of the Prusa MK4S idler distributes the pressure equal on the two bearings and the filament. The Idler only has guarantied function together with the modified Main Pate!

Regarding the Main Plate there are similar designs to improve guiding of the filament. This Main Plate have geometry adapted to the Bogie Idler. It consists of two parts that are put together witch makes printing of the parts easier and the quality of the Main Plate better.