BICYCLE SHREDDER [costs] – work in progress

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.


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)

Hello @thetank , do you have an instagram account or facebook? I couldnt open the video in vimeo and Im interestes in making the bicycle shedder here in Peru and maybe with a solar panel attached to it.

@s2019 Thank you for your comments, yes I tried changing the gear ratio, with better results. We are working now on an old shredder design (double shaft) we think it could work better.

Yes we use a flywheel.

we are also thinking to add a washing machine, well is more like a big liquidizer. I will upload a video, when ready.

Thank you

@thetank The shredder looks great. Have you tried different gearing to slow the shredder down and maybe a flywheel?

I think you just need to change the marketing. Here in the US, there is no shortage of people willing to pay good money for the pain and exhaustion of a stationary bike in a health club spin class. I think if you add some web connectivity, a new fad of competitive plastic shredding will be sure to follow.

Great work

This is what we’ve been working on, very satisfied with the result and power, but our conclusion is that is to tiring for a big production.

Yea the best weight would be to use a pipe of some kind. One of my side passions is cars. I have been thinking about making a inertia dyno where you detect the rpm of the rollers and the change of rpm and the duration. Knowing these nad the polar moment of inertia of the whole system will tell you the cars hp.

Anyways at my local metal supplier they have some big diameter thick wall steel pipe you might also want to look at that. Then have someone machine some supports in center for bearings and weld in supports and push bearings in.

The problem with concrete is that it is not dense compared to steel. The wheel in your second link is interesting in that filling the spoke area provides a lot of volume (and weight). If I was going to do that, I might add some wire mesh for a little bit mode reinforcement in case you drop the wheel during handling. You can also combine it with the metal weights on the perimeter or use a mold frame to make it bigger or thicker. The formulas above let you compare that case with the metal options. There isn’t enough volume in a tire to make filling it with concrete worthwhile, I don’t think (maybe the fat tire or motorcycle wheel).
To place the individual weights on the rim, the inertia formula does not care but you would want to balance it just for smoothness. How you attach them to the rim depends on shape of your weights. You could bolt them radially by drilling holes like your valve stem. You can also attach them to the side of the rim.
If you have access to scrap lead, you could make yourself a mold and cast weight segments that fit the rim.
I think any approach that puts on sufficient weight, is reasonably balanced, and does not fly apart should work.
Looks like a great experiment, good luck.

@thegreenengineers, thank you for your interest, hope to see some explanations about my questions A,B,C.

, thank you for your answer, if I understand well, you mean it is better to keep the bicycle rim empty inside and just put the weight around. Is that right?
If yes, for sure I can keep the original rim. Easier.
Then how should I put the weights ?
Distributed in a balanced or unbalanced manner?
If unbalanced i will not put the weights all around.
Continuously or discontinuously?
If continuously, I can fill the inner tube of the tire with concrete.
If discontinuously, I can fix some weights 1kg around following your way.
A quick sketch could be very useful for me.

Below a link to the new inertia flywheel using concrete (which is very good in compression and not good in traction) enclosed in a shell.

And a link to an interesting machine : the pedal powered mill

@benjamin50 , For your flywheel, the moment of inertia is the key parameter . For a solid disk like the 20 Kg plate, the inertia is 1/2 mr^2 whereas for the bicycle wheel with the mass at the perimeter, it is mr^2. The radius of the plate is around 205mm and a typical bicycle rim is around 300mm. This all means that the mass is about 4.3 times more effective on the bicycle rim. So if you found a way to attach a number of 1 kg weights around the perimeter of the rim (should not be that hard to do) you will have a more effective flywheel and you don’t need to figure out hoe to mount the plate to the hub.
Predictions of required torque are useful but recognize that they are for a unique cutting condition. You probably want as much flywheel inertia as you can get.

@thegreenengineers , It is funny, I noticed that for posts where I use Google Translate, it flips the day-month to the US sequence….

woops read months and day backwards so it was less than 2 years ago.

o man i talk about this 2.5 years ago. i will look back at the calculations.

Hi @thegreenengineers,
I would like to continue the idea Katharina started.
Hope you can see my message.

A- So you confirm that a flywheel with a gym 20kg cast iron lifting plate would be enough?

B- I also want to have the shredder in front of me so I can put some plastics during I do exercises. And also higher, so I will need to have a second set of sprocket and chain. Could you tell me which diameters should I use for the wheel sprockets 1,2,3? I know that there is some rule about small then big to get better torque. But with 3 sprockets I don’t know.

C- I will cut the rear frame and keep the whole shift mechanism to mount it to the front, so I could increase the speed progressively. Do you think it is necessary to keep the shift mechanism with all different diameters of sprockets?


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.

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

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.

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.

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.

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.