Need help with shredder calculations -- Plastic bottle shredder forces....

[Hammad]

this is my design

i want to calculate its various stress and forces. since the bottle is being cut, i think there is perpendicular force upwards, how do i calculate this cutting shear force?

1) force and stress applied by cutting blade on the plastic bottle
2) force moment and stress applied on the rotating shaft with blades.
3) force required to exert on the hand to shred the plastic bottle
4) diameter of shaft for safe design
5) forces, stress and moment at the bearings.
6) what boundary conditions i need to apply for shaft and the handle?
etc

these are the following calculations i have done

Link: https://www.pdf-archive.com/2018/05/01/shredder-calculations/

can you please guide me on this

thanks

 

[anorlunda]

Wow, that is a lot of questions.

Is this a do-it-yourself home project, or are you designing a commercial product? If commercial, there may be legal and safety codes you must meet.

Nothing could be computed without specifications on the bottles to be shredded and any foreign object contamination.

In a professional setting, such calculations would be done with structural analysis and finite-element software tools. What tools do you have available?

 

[Hammad]

Wow, that is a lot of questions.

Is this a do-it-yourself home project, or are you designing a commercial product? If commercial, there may be legal and safety codes you must meet.

Nothing could be computed without specifications on the bottles to be shredded and any foreign object contamination.

In a professional setting, such calculations would be done with structural analysis and finite-element software tools. What tools do you have available?

it is my university assignment, i have also now attached the calculation i have done, can you please help me with this?

i have already done finite element analysis but i need to verify the design with calculations.

thanks

 

[anorlunda]

This could be moved to the homework forum, but I'll leave it here.

We'll see if it attracts answers. It would take a lot of effort to answer all your questions.

 

[Dr.D]

I suggest that you run some experiments to get data on which to base your design.

 

[Asymptotic]

What size bottle?

Why wouldn't the bottle ping around on top of the cutter wheels instead of being caught in their nip?

What would this device produce? Is it intended to reduce bottles to particles suitable to feed into an extruder for recycling?

 

[Hammad]

What size bottle?

Why wouldn't the bottle ping around on top of the cutter wheels instead of being caught in their nip?

What would this device produce? Is it intended to reduce bottles to particles suitable to feed into an extruder for recycling?

yes bottle is meant to be reduced in size for further recyling process.

the bottle is a typical soda 1.5 liter bottle size is 35 cm length and 8 cm in diameter, the bottle would be inserted vertically not horizontally.

i didnt actually understand you second sentence, but i presume you mean why the cutters are intersecting into each other so that the bottle is squeezed into it?. that the typical design of a twin shaft shredder.

 

[Asymptotic]

I don't have any experience with shredders, but the ones I've seen were reminiscent of hob cutters, except with cutter plates offset from one another, or large offset cams with removable cutting teeth like the one shown below (image from http://www.seekpart.com/product/Plastic-Crusher-Blade-2222065.html ).

This isn't to say your cutters won't work, but one potential problem I'd be thinking about is how to sharpen them once they've become dull, and how removing metal during the sharpening process affects cutter diameter and shredder performance.

This detail figure in your PDF is of a rotating knife granulator. It isn't clear to me how you've incorporated this into the force calculations, but it doesn't matter because your shearing mechanism is completely different.

A hand crank speed of 200 RPM (3.33 revolutions/second) seems a bit optimistic to me.

Cutting through the bottle's sides aren't too big a problem, but it takes quite a bit more force to chomp through the bottom, and especially through the neck area. You may consider adding a flywheel to the design.

 

[Hammad]

I don't have any experience with shredders, but the ones I've seen were reminiscent of hob cutters, except with cutter plates offset from one another, or large offset cams with removable cutting teeth like the one shown below (image from http://www.seekpart.com/product/Plastic-Crusher-Blade-2222065.html ).

View attachment 225010

This isn't to say your cutters won't work, but one potential problem I'd be thinking about is how to sharpen them once they've become dull, and how removing metal during the sharpening process affects cutter diameter and shredder performance.

This detail figure in your PDF is of a rotating knife granulator. It isn't clear to me how you've incorporated this into the force calculations, but it doesn't matter because your shearing mechanism is completely different.

View attachment 225011

A hand crank speed of 200 RPM (3.33 revolutions/second) seems a bit optimistic to me.
View attachment 225013

Cutting through the bottle's sides aren't too big a problem, but it takes quite a bit more force to chomp through the bottom, and especially through the neck area. You may consider adding a flywheel to the design.

Sir i cannot chnge the design unfortunately because we have already submitted this design in our assignment, so we have to work on it only

i just want to know that in this arrangement what is the free body diagram of this system to calculate the shear stress and bending moment of the shaft

i have two ideas

1) i have included the handle with the shaft to calculate the bending moment, but I am stuck around here, i don't know if i apply the shear force in this diagram, the only forces I am considering is the loaded generated by the weight of the cutters and the bearings, here is the diagram i have made

i have made one pin and one fixed joints, both these joints represent two bearings the bigger rectangle represents the handle
how do i deal with the loads, shear force and the torque which i will apply on the handle to rotate the shaft? (this is the idea i took from a belt pulley system so it is not very close to my system except i replace pulley with handle. The guy who used with diagram doesn't consider the cutter shear force to evaulate the moment etc. in this method, the guy has used loads of the shaft as the Force on the pulley but no cutter shear force.

2) i exclude the handle from the free body diagram of the shaft all together and calculate the shear stress using only load and not even use shear force of the cutter, i apply the load in the centre. i can do that but if i include shear force in the diagram, how do i calculate the bending moment and where do i apply the shear force?, here is an example diagram i made for the free body diagram of the shaft, i have only used pin pin support for the two end of the shaft which are held by the bearings, I am conflicted if i include hand in this diagram, I am also unsure what forces to use and where to apply the loads and the cutter shear force

(please ignore the values)

can you please guide on this free body diagram and shear stress/bending moment diagram calculation issue

thanks

 

[CWatters]

I think you can probably ignore the weight of the parts in the load calculations. The cutting forces are likely to dominate. This isn't my field but I don't think they are trivial to calculate. Measuring them on a prototype might be easier and more accurate?

If I understand your drawing in #1 correctly it will probably squash the bottle and cut it into about 8 strips each roughly the width of one "gear". Is that correct?

Equation here (imperial units sorry) tells you how to calculate the force required to shear metal and that should work for sheet plastic as well.

https://www.engineersedge.com/sheet_metal_pierce.htm

However it assumes you know the simultaneous length of the cut and your cutter appears to work "at point" like a pair of scissors rather than something like a wood chissel that cuts across its whole width at the same time. Hope I'm explaining that ok. The length you need to use in this calculation will be something like the length of the cutter in the material while it's cutting. That will depend on the material thickness and the dimensions and geometry of the cutting edge.

Then there is the issue of the direction the force acts. At which point I'm running out of ideas.

 

[Dr.D]

Equation here (imperial units sorry) tells you how to calculate the force required to shear metal and that should work for sheet plastic as well.

What CWatters describes as "imperial units" are known in the American engineering world as "US Customary Units" (USC) and work very nicely. They got us to the moon and back, among other things.

 

[anorlunda]

The picture in post #1 makes me think that is is a small version of this car shredder.

 

[Hammad]

i actually finally found a perfect example calculation for shredder

the only different is there is a pulley rotating the shaft, but my system consists of a handle, and this system is single shaft.

 

[CWatters]

It's hard to read that paper but it appears the approach they take assumes the cutter teeth crush the rock (eg They calculate the force required to push a tooth through the rock). I think you could take that approach and it would give you a worse case figure. I think shearing forces should be less than crushing forces.

 

[Hammad]

hi how much force and torque do i need to be able to rotate the two twin axles connected by spur gears of equal gear ratio and moving counter to each other.

the maximum force i can apply on the handle in the given direction is 400 N and the torque generated by 19.4 cm by applying 400 N is 77.8 Nm

 

[CWatters]

As I suggested above, if both sides are same diameter the total load torque is (1269+1015)*0.04 = 91.4Nm

The handle has to supply the same torque 91.4Nm.

The force required at the handle is the torque divided by length of handle = 91.4/0.194 = 471N

So if you only have 400N available it looks like this won't quite be enough.

 

[CWatters]

As I suggested above, if both sides are same diameter the total load torque is (1269+1015)*0.04 = 91.4Nm

The handle has to supply the same torque 91.4Nm.

The force required at the handle is the torque divided by length of handle = 91.4/0.194 = 471N

So if you only have 400N available it looks like this won't quite be enough.

Oops I made a mistake with the diameter of the gears. Will correct and repost in a moment.

 

[CWatters]

The gears are 16cm in diameter not 8cm so the torque is (1269+1015)*0.08 = 183Nm

The handle has to supply the same torque 183Nm.

The force required at the handle is the torque divided by length of handle = 183/0.194 = 943N

So if you only have 400N available it looks like this won't be enough. Make the handle longer?