@anris and i are thinking about making the shredder human powered, and it would be a huge improvement to have a flywheel, flywheels are unfortunately rather difficult to come by here in Denmark. this is why we were thinking about using a car tire as a flywheel. does anyone have a formula to calculate the angular momentum of a given tyre and wheel?

We got the idea efter some correspondance in this topic about powering the shredder with your hands:

http://onearmy.world/community/forums/topic/hand-cranked-shredder/

By adding a flywheel and some gears, the momentum from the flywheel should be enough to shred the plastic.

**Picture one:**

The desired flywheel; an old wheel made from casted iron (these are the ones that are hard to come by)

**Picture two:**

A drawing of the hand powered shredder we have in mind

*Red* = Gears from a bike

*Green* = The chain from a bike

*Yellow* = The handle

*Pink* = The flywheel with 12 – 20 kg, to create momentum

*Blue* = axis

*Light blue* = Plastic

**Picture three**

What we are trying to figure out; if we want to use a tire from a car or a bike, with or without added weights

A bit more info can be found on the “Hand cranked shredder”-topic but this is the short lines.

Can anyone help us out with some calculations or guidance to what kind of flywheel that is the best?

@agarciare the idea i have in mind is to have an increase in speed between the crank and the flywheel and a very deep reduction between the flywheel and the shredder itself. but the idea of making your own is definitely on my mind. thanks for the idea

Why dont you try this

here are some posts from a bike powered shredder thread that i posted to:

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)

1/2(I)(W^2)

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

(KE(r^2)(2))/(V^2)

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.

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.

the thread could be found here: http://onearmy.world/community/forums/topic/bicycle-shredder-costs-work-in-progress/