Yard Force Classic 500 wheel gearbox

The original gearbox uses plastic gears that are prone to wear after long term use and no replacements are available as spare parts. The new design uses a planetary gearbox and a timing belt to offset the motor and drive shaft so that the position of the wheel and motor are the same as on the stock gearbox. 

Bearings, motor, drive shaft seal, o-ring, hexagon wheel adaptor, pins and screws are used from the original gearbox.

Printed parts:

• Base plate 1x
• Cover 1x
• Spacer 2x
• Wheel weight spacer 1x
• Cover seal 1x (two parts)
• Template for drive shaft (optional)

I printed everything out of PETG, except the seals that are from soft TPU, which can also be made using the filament as a string since the diameter of the seal is 1,75mm. If you need more weight on the wheels you can also print the case with 100% infill. 

Together with the printed parts the components bellow are also needed:

• Planetary gearbox (1 Ratio 50, 8MM Shaft Diameter): Diameter 36MM Planetary Reducer Gearbox For RS555 545 550 540 3650 Brushed Geared Motor To Gear Motor - AliExpress 13 
• Timing pulley (10 mm, 11 mm): Pulley Wheel T5-24T Bore 5/6/6.35/7/8/10/12/14/15/16/17/19/20 mm Alloy Pulley Teeth Pitch 5mm For T5 Width 10/15 mm Timing Belt - AliExpress 13
• Timing pulley (8 mm, 11 mm): Timing Pulley T5-15T Bore 5/6/6.35/7/8/10/12/14mm Alloy Wheels AF Shape Teeth Pitch 5mm Match With T5 Width 10/15 mm Timing Belt - AliExpress 13
• Timing belt (10mm, 250mm): 1PCS T5 Timing Belt Width 10/15/20/25mm Rubber Closed Loop Synchronous Belt Length 165/185/190/200/220/225/240/245/250/255/260mm - AliExpress 13
• Drive shaft (D10xM8, 80mm): Shaft 12.9 Grade Hexagon Socket Head High-Precision Roller Bearings Shoulder Screw Bolt M5-M12 - AliExpress 13
• Drive shaft nut (M8 5pcs, Nickel Plated): M3 M4 M5 M6 M8 M10 M12 Black White Carbon Nickel A2 Stainless Steel Hex Flange Nylon Insert Lock Nut Self-locking Nylock Locknut - AliExpress 13

Optional:

• Drive shaft seal (TC 10X22X7 NBR): 10pcs TC 10X22X5 10X22X6 10X22X7 10X22X8 NBR Skeleton Oil Seal 10*22 Seals MOCHU Seals Radial shaft seals Nitrile rubber - AliExpress 13
• Bearings (ABEC 5, 6900-2RS): 10pcs ABEC-5 6900-2RS RS 10x22x6mm Deep Groove Ball Bearing Double Seal Thin Section 6900 2rs Ball Bearings - AliExpress 13

Instructions:

1. Remove the original gearbox from the mower
2. Remove the big O-ring seal on the outside of the gearbox, at the motor side
3. Disassemble the gearbox. There are plastic tabs that also hold the case together. They can also be broken off. 
4. Remove the motor with the two screws from the inside. 
5. Remove the hexagon nut by removing the two pins.
6. Remove the shaft seal and both bearings.
7. Make a new drive shaft from the shoulder screw. You can also order a stainless steel screw which will be harder to drill but the shaft seal will last longer. The screw needs to be cut to length and 3mm holes for the pins drilled. There is a printable template which can be used to get the exact positions of the holes. I recommend drilling from both sides to not be too far off if you only have a standard drill press or hand drill. The two holes for the gear bolts just need to be tapped using a 4mm drill. This is to prevent the gear slipping on the shaft. You could also grind the shaft on one side to have a flat spot to lock the gear. Just be careful to not also drill through these two holes or make them too deep. They only need to be drilled by 1-2mm. 
8. Bearings need to be pressed into the new housing and into the cover.
9. The shaft seal needs to be pressed in the cover
10. Remove the original gear from the motor. Since the gear will not be used anymore the simplest way is to clamp it into a vise and punch out the shaft. Just be careful to not drop the motor. 
11. Press the new gear that is supplied with the gearbox to the motor. The distance between the motor and end of the gear is X. The rest of the motor shaft needs to be cut off or grinded. You can press the gear using a vise and a small socket. 
12. Mount the motor and gearbox together.
13. Mount the motor with the gearbox on the new printed gearbox using 4 screws. Orientation of the motor is not relevant. 
14. Install the small 15 tooth timing gear onto the exit shaft of the gearbox. lightly tighten one screw in the gear so that it sits on the flat side of the shaft and push it towards the motor as far as it goes. Then tighten both screws. Use loctite on screws. 
15. Install the hexagon wheel adapter on the new shaft and install both pins. 
16. Insert the shaft in the cover and use grease on the shaft seal. 
17. Install a spacer on the shaft on the inside, onto the bearing. 
18. Put the belt on the bigger 24 teeth gear and install the gear with the belt in the cover. Align the screws with the tapped holes on the shaft and tighten the screws. Use loctite on screw. 
19. Install a spacer on the drive shaft towards the gear. 
20. Install both printed seals on the cover.
21. Take the other part of the gearbox and slide the belt over the small gear and close the gearbox. 
22. Screw the gearbox together using black screws from the original gearbox. There will be one extra. 
23. Install the big O-ring on the gearbox on the motor side.
24. Install the gearbox on the mower
25. Put on the wheel and put the spacer in the wheel weight. Tighten the nut. Install the wheel cover
26. Go for a test drive!

I don't have any long term results yet since it's winter time. It does run without any software modifications although I have to measure exact ticks/meter. And it is also quieter than the stock gearbox.