Bosch Rotak 32R Lawnmower Pulley/Fan - Version 2.3

My Bosch lawnmower stalled and melted its drive pulley - I designed and printed a replacement

Update 21June 2016
It finally died - the mower ate a chunk of wood which jammed it and caused grievous harm to the pulley and tension bearing assembly. These parts have kept the old mower running for a couple of years but I now have a new one, so don't expect further updates to happen automatically. If you have specific things that you'd like to see changed I might still be able to help, please let me know in the comments.

Update 4 June 2016
Lots of bad luck with this over the last week or so, I made errors with installation and assumptions about design changes to "improve" the part that did the opposite so I've gone back to interference fit on the fan with provision for tapped holes to mount the pulley but I've added a fan blade protection ring to avoid losing blades if the fan touches the casing. Just finished printing, going to test - wish me luck...

Update 29 May 2016
Second failure, pulley melted, fan's fine but discovered the design is not brilliant for replacing the pulley only. Redesign should make it easier to only replace the bit that seems to fail. Photos not updated but SCAD and STLs are new.

Update 11 August 2015:
I'm actually quite excited: first sign of failure! Last week, around5 months after I made this part (!) the friction fit of the pulley on the motor shaft started to loosen and let the pulley slip a bit. It only happens on start up of the mower and it's still usable for now but is an indication of what to re-work on the next version (I'll probably go with a clamped shaft next time)

I've been astonished at how long this 'temporary' repair has lasted, I'll try to redesign the part as soon as I can and I'll re-upload the new part when I've printed and fitted it.

Update 1st September 2015:
The new version of the pulley is printed and working - this improves the friction fit between the pulley and the shaft and adds clamping bolts in case it's not enough.

Instructions

The lawnmower is a couple of years old, no problems at all until it made a funny noise while my wife was using it, it was stopped straight away but too late - the motor shaft had slipped in the pulley and friction had melted it, belt tension pulled it off-centre and it was ruined.
I looked for a replacement part but they seem not to be available (you can buy whole motor assemblies for £130 - twice the price of a whole lawnmower).
I designed the replacement part in OpenSCAD based on measurements from the original, it's designed to print in two parts - a pulley and fan which are then bolted together.
I printed the part in PLA at 50% fill with four perimeters on the pulley, you might need to tweak the extrusion width to get a strong enough pulley.
It's bolted together using three M3 bolts. Ream the three bolt holes to 3mm on the pulley only and tap threads into the fan section after reaming to 2.5mm. You will need to ream the centre shaft hole to ~9mm - ensure that this leaves an interference fit for at least part of the shaft.

If you need to tap the part onto the shaft gently using a soft-face (wooden or plastic) mallet to provide a solid friction fit but be careful not to split the plastic.

Category: Outdoor & Garden