# Hand cranked shredder

This topic contains 37 replies, has 10 voices, and was last updated by jecy 5 months ago.

Constructed the shredder utlizing a winch crank up the rejected 3D plastic prints. Drive shaft had a turned key that fit the spring loaded crank and testing is going on at the moment. Would prefer motor powered shredder for THICKER harder peices (ABS/PLA) but it does well on other less brittle materials.

@andyn he has some interesting videos and it’s handy for people wanting to start off without investing a lot of money the prob with the mincer is ya still have to cut the plastic up small to fit them in

I’ve been following your project on hand cranks. I am not a metal worker but I am very interested in raising money & distributing these in various countries thru a non profit organization to watch over & manage. When the kinks get worked out, many hands across the globe gathering & washing plastic could make a dent. What is your goal? I’m looking for help to get further down the road.

here are some calcs I did in another post for a bike powered shredder;

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.

here is the original thread. https://davehakkens.nl/community/forums/topic/bicycle-shredder-costs-work-in-progress/

With no flywheel or gearing, this looks like an easy way to make your biceps explode XD

I tried getting in touch with some company here in Uganda that I think build machinery but the amount they told me for making an extrusion machine was really so exagurated that importing could really be the only alternative either from Kenya or any other country if I can get contact in Kenya that would be great

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