Very nice driving solution.
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Does plastic get stuck sometimes?
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A quick idea if there are problems with that topic:
You can add (or for future projects) a big rotating mass, similar to a diesel engine.
With that rotating mass you will have a bigger torque=shredd plastic more easily 
E.g. weights from the gym, welded together and welded to the connection bar.
I am not sure if i will build one bicycle shredder, but if i do i will let you know. At least to get some handy tips , hahha 
Best greetings from Bochum!
Your gearing situation looks interesting. do you know the gear ratio of that setup. or could you look at your input rpm vs shredder output rpm as easiest way to find gear ratio. I am curious how much torque is developed.
Going to need large reduction as average human can only put out about 130 Watts on a bike. ~.173 HP so in order to develop the same torque as an comparable electric motor you are going to need to drop your rpm drastically. Generally humans feel most comfortable with 50-60 rpm on a bike.
So at .173 Hp and 50 rpm a average human can do 18.2 Ft.lbs of torque
while with a 2hp motor at same rpm your can do 210 ft-lbs.
In oder to make comparable numbers of torque you would need to have down gear
210/18.=11.53:1
so ouptut gear will have to be 11.53 times larger than the input gear
Thus output rpm is 4.33 rpm
However, on a stock bicycle this is opposite
the input gear is larger than output gear for faster rotation speed but less torque
This would be incorrect for a shredder application
Hey @katharinaelleke your machine is looking great! Sorry to hear about those vertebrae though… what even happened?
I don’t know why I’ve only seen this post now… but nice write up. Its good to have more focus and results on tackling the “electricity’less” shredder.
What might help to get more from your current set-up is adding weight to the wheel driving the gears?
I’ve also noticed that after a lot of use and force – my shredder’s axle is pulled in the direction of the chain. This has caused the blades to miss-align and i’ve had to disassemble and recenter the bearings.
I haven’t changed much in terms of my design just yet… I’ve been focused on finishing the rest of the machines before my time in the workshop is overstayed.
I do have plans for a refined version it just requires some sourcing of parts and machine tinkering… I will definitely share it when complete
But I gave the whole thing a nice black finish to show off at our university exhibition hehe
It’s awesome how easy (or hard) is it to peddle whilst shredding plastic?
I’ve thought about having a peddle backup plan
But somehow it worked better with high speed (450rpm) than with 80rpm – with 80rpm pieces would get stuck and block the knives…?
How is the RPM adjusted? If the machine runs at 80rpm by reducing its power, that might be why the plastic pieces get stuck
Ideally you will get 80rpm by gear ratios, so you have full power and multiplied torque
And, as I had collected quite some plastic waste in my university and wanted to compress it (but had no motor yet), I simply attached the shredder to the milling machine in our workspace – another alternative to buying an extra motor!
Worked quite well.
But somehow it worked better with high speed (450rpm) than with 80rpm – with 80rpm pieces would get stuck and block the knives…?
I connected the hometrainer to the shredder and that’s basically it so far (it’s moving, but not really properly shredding by bicycle yet)
Then I had to stop working on it because I had an accident with fractured vertebraes as a result – workspace ban for Katharina.