We've just launched our map. Add yourself by clicking here!


BICYCLE SHREDDER [costs] – work in progress

This topic contains 18 replies, has 11 voices, and was last updated by  Patryck Shepard 3 months ago.

Katharina Elleke katharinaelleke

BICYCLE SHREDDER [costs] – work in progress

17/09/2017 at 22:17

Finally taking the time to share my own precious plastic story/progress.
If interested, also have a look at my post on our project website.

Background „story“
I spent 4 months doing a research semester about plastic waste and recycling in Kenya, where plastic shredder are also an essential part of most of the recycling concepts/businesses. But they are mostly imported from China, super expensive, super noisy and not easy or even impossible to maintain by the locals themselves. So when Michela Consiglio (recycling artist) told me about her plan to build the precious plastic shredder, I got very excited about this project because it can really help bringing recycling even to the small villages in Kenya.
Now, another issue – even with the small-scale precious plastic machines – is power. First of all, buying a motor in Kenya is really expensive and then many villages have no access to power at all or if they have, power cuts belong to the daily routine.
So I came back to Munich with the aim to work on a construction for an efficient bicycle drive for the plastic shredder.

// Photo: Dandora, Kenya’s biggest dumpsite.

18 replies
7 subscribers
2 saved
sort on date
17/09/2017 at 22:47

But I gave the whole thing a nice black finish to show off at our university exhibition hehe 🙂

17/09/2017 at 22:18

Getting started
Feels like I started ages ago, but I realized that most of the hours I spent were for searching, comparing and finding out about manufacturers and conditions for cheaper laser cut parts. The time and effort was worth it! And the group orders I ended up organizing are slowly even coming to a professional level haha!
Anyways, as soon as I ordered the laser cut parts, I bought all the other materials for the shredder and started building.

Costs (always interesting to see and compare, right?)
95€ – Lasercut parts + shipping
18€ – 2 Bearings
25€ – Angle + Tubes
9€ – 330mm Hexbar (1m = 28€)
3€ – 180mm mesh (1m = 14€)
5€ – 2m threaded bars
25€ – Screws, nuts, washers
The rest (for example the metal sheet for the hopper) I „borrowed“ from our university workspace.

Total costs
Only counting the material (more or less) I really used: 180€
(No motor included.)

For the bicycle drive, I started with getting some old bikes and a home trainer.
10€ – 2 Old bikes (eBay)
5€ – Hometrainer (eBay)

02/11/2017 at 11:37

Very nice driving solution.

Does plastic get stuck sometimes?

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!

17/09/2017 at 23:51

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…?

17/09/2017 at 22:39

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.

03/11/2017 at 20:44

I did some of the math for a 20Kg cast iron lifting plate
that would get you pretty close to cutting that 1 inch by 1 inch cross section
however that is only 1 tooth so you would need a 20kg plate per tooth /2
so if you have 12 i would do 6-8 20kg plates so 160kg of weight for flywheel.
reason why you would need a plate per tooth is that your flywheel would stop after only one tooth cut but you want to keep going indefinitely so you will need to have energy to spare so that you just spend energy keeping the flywheel up to speed.

so a 20kg cast iron lifting plate would be a good idea.

03/11/2017 at 20:04

Well there are a few governing equations to sizing flywheels for this application
there are a few variables that are listed bellow:
1. Material that is being cut properties
2. Material that is being cut cross section
3. Flywheel design
4. Flywheel Material

So onto the equations:
First rotational kinetic energy units (Joules)
I = rotational inertia (moment of inertia) has to do with flywheel design
W = angular velocity
to find I just design a flywheel in a standard cad program and it will usually tell you what it is at center of mass
W is found by taking the tangential velocity on the outer rim of the flywheel and dividing it by the radius of the flywheel W = v/r

Next value that is important is the Tensile Impact Strength value of the material.
This is looked up from a table of your material for HDPE it is around 243KJ/m^2

example problem:
i designed this bowl on cad (not designed for flywheel)
and looked up its highest moment of inertia on cad which = .0004647 Kg m^2
r = .109
i need to find how fast i must rotate the flywheel to cut the material
impact strength of HDPE of 243KJ/m^2
and a cutting cross section of 1 inch x 1 inch or .00064516 m^2

impact strength needed is 243,000 * .00064516 = 156.77388 Joules of KE needed
so solving KE equation for velocity
1/2(W^2)(I)=KE W= V/r 1/2(V^2/r^2)(I)=KE ((KE(r^2)(2))/I)^(1/2) =V
plugging in
((156.77388(.109^2)(2))/(.0004647)))^(1/2) = 89 m/s or 200 mph lolz

a more important problem would be to solve for I as most people can pedal at 10mph
because you are solving for I you can determine what you need to model and using what material
to solve for I
however as you see you would also need to know the radius of your part as well so this equation has 2 unkowns.
basically the r would be limited by the machine that you are going to use to make the flywheel or the material you can get your hands on
you would then design around that.

02/11/2017 at 13:48

For a flywheel, it’s much better to have the mass concentrated around the circumference. Mass in the centre just makes it heavier and doesn’t contribute to it’s effect. The larger the diameter the more torque it will provide. Something like a bicycle wheel with the tyre filled with concrete would provide much better assistance whilst also being lighter then a barbell plate. The flywheel can also be connected to the ‘spare’ end of the shaft that sticks out on the other side of the shredder, it doesn’t have to go on the drive side.

26/10/2017 at 08:57

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
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

17/10/2017 at 21:05

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

03/11/2017 at 10:45

hey andyn,
You are right!
Its much better to put the most weight to the biggest diameter.

The idea with the bicycle wheel is interesting, but maybe hard to make the mass of the cement evenly distributed. I dont know if the spokes would be strong enough to transfer the torque, because they experience only vertical forces on a bicycle.

A gym weight does contribute to this effect, but it depends on what (additional) torque is needed. Since its already working, it doesnt have to be that big.
The gym weight was an example that came quickly to my mind,.. easy to add and easy to buy.

31/10/2017 at 00:57

nice idea

25/10/2017 at 17:17


26/10/2017 at 09:00

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.

03/10/2017 at 16:02

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

28/09/2017 at 00:50

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

25/04/2019 at 22:33

Hey @katharinaelleke great looking design! How did you couple the bike sprocket cassette to the shredder shaft? I can see there is a sleeve that fits over the shredder shaft with the bolt to secure it, but how is the sprocket cassette secured to that sleeve? I’m trying to design a gearbox made of bicycle sprockets to provide enough torque from the bike-drive of my shredder.

18/04/2019 at 04:22

Congratulations on your work and helping the world turn trash into useful things!

Viewing 18 replies - 1 through 18 (of 18 total)

You must be logged in to reply to this topic.