# Steven Mosbrucker

## Forum Replies Created

Just be carefull if you have only 1 cutter blade engaged and it locks up the cutter will explode. Form my design of shredder my blades can withstand about 400 lbf at the tip. You would put down 935 lbf. That is enough to blow up 2+ blades if they lock up. and i believe that precious plastics blade design is thinner than mine. Just saying make sure that your shredder can witstand those forces if it locks up or if you overload it.

jorgegarcia96

i think that the number of blades is to high. Reason being if you have 12 blades on a shaft lets say the chances that almost all the blades are trying to cut the material is very high. Thus the force required is multiplied by the number of blades that are engaging. I see that this might be preliminary design as you have yet to design some way to hold the blades in the center and have some way to forcibly turn it. But other than that your blade shape should work. Usually they are pitched a little forward to get more of a shearing action. As that means the entire area of the material is not trying to be cut at once. This drops down the force a little bit. Other than that it looks pretty good.

Romero sorry it took so long.

With cross section of 619.28 mm^2 of cut which is circle of blade there. as max cutting of material

Using HDPE as material with 24MPA Ultimate Yeild Stress

Force to cut required is 3,340.5 Pounds of Force

thus torque with your blade radius length of around 1.1 you are left with 500 ft lbs of force. so in this case if you are 160 lbs you would need to put a handle on it that is 3 feet long. But the most important thing about this is this is per contact tooth to material. If you have 6 teeth engaging the material at once then that goes up by x amount of time per extra tooth engagement so if 6 teeth are engaging you have 3000 ft lbs required etc. from my studying of my own shredder cutting by hand just cant really be done unless you have massive amount of mechanical advantage. Pulley system. giant breaker bar (10 ft long). Or hydraulic or something.

However, with these same calculations at about 30 rpm of the motor you need approximately .45 hp or 340Watts of engine power per tooth engagement. so if you had a 3 hp motor geared down to 30 rpm then you can have almost 7 teeth engaged at once no problem. Now will you mechanical components survive 3hp that is probably not going to happen.

This calculation is again worst case scenario if you tried to cut a 1″ diameter piece of HDPE in your machine. So these calculations would be fore the worst possible scenario you machine will encounter.

josette: the design does come into account. Once you find out what force it requires you need to design the shaft such that it does not fail in torsion. Generally in a shredder if not designed properly this is the first place it will fail if you have a motor strong enough.

sail: this: Solve for Torque = (HP * 5252) / RPM = 10.5 Ft-lbs (pounds) is correct equation.

You control the diameter of filament by measuring the diameter live then pulling it to diameter where it is still hot after the nozzle. This stretches the filament to make it small.er. If it is to big then you don’t stretch as quickly.

i will have to run calculations on the internal stresses involved with plastic blades and if they will survive the abuse. With let alone shredding plastic bottles.

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.

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

I made a acrylic shredder concept of my metal shredder. It was glued together. I could shred like 1 sheet of newspaper with 2 n shreds it would pop the glue off the sides.

naw thats fine ppboys i look at cad drawings all day at work. It is good enough to make a rough estimate but usually you need both “blades” in the case of this design it uses stationary blades. It would be nice to see where those blades are relative to the rotating blades in the assembly.

Well for the hell of it lets say you have a 28mm cross section circular bar of HDPE that fits exactly in that scoop of the blade. That fits in that hook of that blade. First determine that area .014m * .014m * 3.14 = .00061544 m^3. ok and again lets say we are shredding HDPE. Lets say the 3500 PSI for HDPE Shear Stress just like i said in first post. Metric equivalent of pressure is pascal 24,132,000 Pa or N/m^3. lets use worst case scenario which is the line between blade tip and center or rotation which is the shaft in this case 60mm or .060m. So with a Cross section of .00061544 m^3 and 24,132,000 Newtons of Force Per M^3 of area we need to cut (shear) HDPE. so we must multiply the 2 together to get Newtons force so .0061544 * 24,132,000 = around 15,000 Newtons ok now for torque the units are Newton-meters so now we need to multiply our newtons by distance to blade tip so 15,000*.06m = 900 N-m. Ok now we need to pick rpm for our motor this is where you can come up with your own option here. My personal choice and the speed i like is 30 RPM reason being is 2 sided blade means at 30 RPM you have 1 cut per second that is plenty fast. Equation for power is <b>Power (kW) = Torque (N.m) x Speed (RPM) / 9.5488. the 9.5488 is some conversion factor</b>. It probably has to do to do with conversion of unit and turning minutes into seconds and such. so (900* 30) / 9.5488 = 2.83kW or 3.79 HP. that is massive.

I think that is because you have a cutter tip that is to long and a blade that is to big in diameter. thus increasing the torque required. If you gear down to 15 rpm your HP requirement goes down to 1.9HP or 1.41kW. Again alot of the calculations here are worst case. But HDPE is not the hardest plastic in the world.

ok i will perform calculations tomorrow morning. I go through forms once a week. so sorry about the wait.

I have a couple ways of calculating the torque needed on my calculation post.

I am working on my V2 of my filament extruder dubbed the FilaFactory. But i need to go to market with my shredder in a couple weeks first before i can continue working on it

2Hp at 30RPM should be more than enough to tear apart any 3d printed parts.

Hopefully you dont get to the point of taking a plastic to its melting point where heat of fusion comes into play. Basically what heat of fusion is is at this point the plastic does not change temperature but changes state from solid to liquid. You just want to find the temperature that with a specific extrusion speed will take the plastic right up to its glass transition temp. but if it is “melted” then heat of fusion and specific heat. if glass tradition specific heat use will be fine. cp*m*delta t basic equation. cp is specific heat of plastic and m is mass of plastic.

My thoughts on this are super heating water then using it on steam engine. Steam engines are not 100% efficient on using all the heat into steam engine. so send that remaining heat across the barrel for the plastic to absorb and send it around to be heated again in the solar exchanger to be sent through the steam engine after. Now you have a 40-60% efficient machine.

Ok so i had one question asked. How do you calculate the area for a bottle. i have a image that i made for that there is a average amount for calculation. basically you want to take this length that is in black which is where the blades pinch and the furthest down a water bottle can go (i suggest crushing bottles before putting them into shredder). This is the same for single axel shredder. just measure from there the blade starts cutting to the furthest down the material can go. Then multiply this length by 2 thicknesses of the water bottle skin. As you have 2 sides because it is basically a flatten rectangle. then procceed with the calculations at the beginning of the post.

https://youtu.be/LJ9OspwEA5A

Above is the video i made about shear stress hopefully that helps.

I replied inside my original post that you read the calculations required for shredder. I will make a video to explain but the blade thickness has nothing to do with the calculations. Check out my reply ask me any questions you have.

Hey guys I have been in and out of the forum. I am going to try to be around more often as I am trying to move into this space and away from my day job hopefully soon. So in shear mode of failure the thickness of blades makes no difference in the determination of the cut area. The thickness of the blades determines if the blades will survive the stress they are putting on the plastic without exploding. If the blades are to thin and you try to put to much force on them by trying to cut to big of a piece of plastic the blades will rip off. That is a separate issue when it comes to these calculations this is assuming that you are making the blades adequately thick and out of steel thus that regardless of the force applied they will not fail. That is the assumption you are making you will have to check this later on.

The area you are checking and thus the cutting area that you are looking for is the cross section of the plastic that you are cutting. not the cross section of the blade but the cross section of the plastic. Why I say that it is a triangular cross section as this is the worst case scenario for the shredder. This is the maximum size of plastic that you could possibly put in there. so lets say you produce a design of the shredder and you do not know what size material will be thrown in there you can design for the biggest possible piece that can fit into the shredder. A triangle is not necessarily the biggest geometry that will fit inside the shredder the shape will be way more complex but the area of a triangle is easy to calculate by hand.

To figure out what motor to get I would go from the end to then size the motor. what is the biggest cross section of the plastic that can possibly be cut inside the shredder. What is the worst case stiffest plastic that that could be. Then solve forward to figure out the motor and the gear box.

However be aware that sometimes the specifications of motors are of 2 different clasifications either max power they are capable of which is 60% efficiency and 50% maximum rpm or they are classified by maximum efficiency or rated power which is 80% rpm and 80-90% effieciency so from there you will have 2 motors to look for one that could have ____ rating for max power or a ____ rating if it uses rated power.

I will probably make a video about this later.

So basicly these cross section are regardless of blade thickness. The cross section is with respect to the parrallel contact of the blade the cross section goes top to bottom not along material. Where blade thickness comes into play is the resistance to torsion at the hex/square shaft input.

My perticualr and precious plastics design has only 1 blade contacting at a time so you only have to take that 1 force into account. Again the question was not can it cut a waterbottle it was maximum cross section it can cut. Now about is theoretical as in your blade may not even have the spwept area to even cut that much area of a given plastic. These are just assumming that whatever you throw in may be cut. So basicly lets say you can cut .375 cc of PET but in actuallity your blades can only sweep .25cc then you are fine until you go with a harder material. If you still have a question on it i will make a video for you explaining it. Sorry about the long time between responses.

This is the correct link.

As long as you get it small enough it is not that much different. I will probubly have a different hopper as shavings dont really roll as well into barrel compared to the virgin resins. as the current hopper design is for pellets.

I am also still working on opening up my first version of the extruder (Multistruder) as open source the idea is that it is supposed to be a injection machine and a high quality filament extruder all in one machine. I am adding the feedback loop for diameter control and the option to work as an injection moulder currently. But that one you should be able to build in most oversea’s country pretty easily and cheaply.

https://studio.youtube.com/video/-4ELb3uyVSI/edit

This is the current one that i am working on. I need to add production grade screw however i lost access to my 4th axis add on to make a custom screw for it so i am going to use modified wood auger for now. Currently i have the winder and spooler working. I am now working on getting my working dimensional control to drive the spooler wrapping speed using PID loop then i am done. This is a test stand the final version will be 12″ x 12″ x 5.5″ box super tiny.

thanks. Yea i have been away for a while because of universtiy. But i actually just graduated yesterday for mechanical engineering so i do plan on being on here more often and spedingmore time on stuff such as this and The Green Engineers.

so that goes to about .25″ x .25″. thats not that bad. most of everything that is shredded is waterbottles and such which have like a .040″ or up to 1/16″ (.0625″) thickness but are really tall which does not really cause any problem. But yea i would reccomend dropping the rpm if you want thicker stuff. more than 1 revolution per second is a little high. My own shredder product that i am working on mass manufacturing now will be operated by hand with a breaker bar(3 foot) first. with with me 170lbs will put down 170 * 3 = 510 ft lbs. ~710 Nm 3.5 times the torque of the electric motor with 2 hp. Then the next expansion would be a 15 rpm motor. with chained gear drive.

Another thing that i might do that i just thought of is just increase the teeth and drop if you double the teeth and drop the rpm in half then you will still have the same through put. my current verionhas 2 teeth i have been thinking about adding more.