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Will it shred??
This topic contains 30 replies, has 19 voices, and was last updated by 
The questions often come up, “will this motor work?” “How many rpm do I need, how much power?”
There are several other threads about finding suitable motors and gearboxes
In truth there is no single answer to these questions and it can be quite bewildering to anybody not familiar with building machines.
I have created an Excel calculator into which you just enter as much information as you can, motor power, speed, gear ratio etc. and it tells you how likely the combination is to work, and gives advice on how to improve it. If you don’t know all the parameters it can calculate them from ones you do know. (Just type the numbers you know in the green boxes)
You probably don’t need to use this if you have a degree in mechanical engineering, but hopefully it will provide answers if you don’t know where to start.
I’ve based this on real world measurements taken by @timslab (thanks Tim) and also other people’s experiences in the ‘Share your Shredder motor experience’ thread. If you have built a shredder and can enter your parameters and the calculator gives a wildly different result, please let me know.
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Wow! THANKS for the hard work! I stayed up light into the night watching videos on youtube about basic motor design, and functions. I learned how to read the label plate! Well, at least I have a much better idea. Hopefully, I can figure this out.:)
I have a question : the plastic’s type, thickness and flexibility affect the ability of a motor to shred it, don’t they ? If so, it would be interesting to know what thickness for what material you could put in a shredder made from the V3 blueprints.
Will it shred ? the answer might be in both shredder characteristics (speed + torque) and material properties (thickness, hardness, etc …)
I think the current shredder design is limited in plastic thickness because of the diameter of the blade, the larger it is, the bigger the stress zone, the better it can capture plastics, an then more force or less speed would be required to provide the same torque… We can’t get a clear idea unless somebody checks this in practice or has enough knowledge in this field 🙂
Can anyone who has a working shredder try and explore the limits of these parameters to enrich our open source knowledge database ?
Hi @armbouhali
Yes, yes and yes, all three matter! Unfortunately it is impossible to post such information about the maximum thickness per material etc. because there are too much variables playing a very big part here…
some of th variables:
material group (PP, HDPE, PS,…), thickness, bigger structure of the material to shred (sphere, bar, block,…), other objects shredded the same time (needs some force of the motor too), motor power, speed, material of the blades, exact position of the material relative to the blades, sharpness of the blades, additives of the plastic (can vary the properties a lot!), level of efficiency of your motor,…
to name only a few :’D
So yes, it would be nice to have a sheet saying e.g. if you have that torque you can shred PP up to 6mm, etc, but it is just too complex of a system with too much variations to simplify it like that.
The only thing I can tell you: My motor (2,2kW, 56 rpm, 280Nm) got stuck at a 24*24*70mm injected PP block. Everything well below a constant thickness of 10mm should work with that configuration of mine but as I said, it’s not that easy to generalize.
I hope that helps anyways,
all the best
Flo
found this motor and according to the calculator its looking superior, just posting here for anyone else looking.
If anyone sees any issue with this motor choice let me know
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yeah i have went for the flanged version of the motor with the same spec for the worm gear.
This is great, thanks @andyn
Something that would be useful to add in the future is another field to enter the worm gear efficiency. Most reducers I have seen are around 90%-95% efficient.
Regards
@xxxolivierxxx There actually is a field for that, but it’s hidden, I thought it best to keep it simple. I’ve set the efficiency to 75%, a little on the conservative side perhaps, but that’s better than giving an overoptimistic result which might not pan out in reality.
90% efficiency is quite high for a worm reduction, and also as the ratio goes up the efficiency goes down, you could probably only get that with a very low ratio, not between 20 and 50:1 that most people are using. You might get 90%+ using all spur gears, or a 2 stage planetary, and still get down to the right ratio. But as you know, worm reductions are by far the most common.
Hey @andyn Thanks for providing the community with such a handy tool thanks for your hard work we need more of this stuff
Great timing, i had a feeling some bright spark would have utilised Excel for this purpose! Many thanks to andyn and everyone contributing data and experiences to the forums.
Hi Andyn,
Will your calculator work for both monophase
7 triphase or only for triphase?
Thks!
I have just gotten round to thanking you for this tool!
We used it in the very beginning when looking at shredder motors, we finally settled on a 2.2kw 4 pole single phase with a 25:1 reduction after consulting our supplier and your handy dandy xcel tool.
Great job, great tool! Thank you
Man, this thread and this tool just saved me in more ways than one. I was just looking at what I thought was a great deal on a “5 HP” motor (“3.72 kw”), but at 320 RPM. If that was accurate I would have to have a 4:1 reduction ratio in order to shred anything without damage to the equipment. Something about it seemed off, though, so I left “power” fields blank, and sure enough, it was a sham. It’s not “5 HP;” it’s .5 HP (a mere 0.372 kw) just as you can barely see in the listing image.
I can’t thank you enough for all of this information, @andyn!
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No Problem, thanks for bumping this thread, the calculator is quite buried in the forum and a lot of people might not have seen it.
Spent all this time looking for affordable motors locally and online and it turns out my stepfather has two broken air compressors, each with a functional 5HP motor. Now I’ve just got to hope we can figure something out for a proper speed reduction. He thinks he can build a pulley system but I’m also looking into gearboxes. IDK, but at least I’m finally moving forward.
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The weird thing is that two of these say 5 HP and one says 5.5 HP but they all have the same info on the panel. Still turns out decent on the calculator and it’s free so even if it isn’t ideal it will hopefully be good enough to start with.
Do air compressors have a different measure of horsepower or…?
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Welp, a quick Google search answered my own question.
Ah, well, gotta start somewhere, I guess.
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Where does it actually say 5HP?
As S2019 said, 15 Amps (“Full Load”) at 120v is not 5HP, actually less than 2.5 and that will be electrical input power, the mechanical output is probably around 2HP, but at a 50:1 ratio looks good enough (the calculator does take into account the mechanical inefficiency of the motor/gearing).
Where does it actually say 5HP?
One of these actually says it on the nameplate of the motor, but all of the compressors say “5HP” on the top of the compressor itself.
A little research suggests that the “5 HP” rating from the manufacturer comes from the maximum amps drawn for a few seconds before it shuts itself off to keep it from blowing up or tripping a breaker). Basically when the compressor has almost done its job it might need a little boost to reach its maximum pressure output. I don’t really understand this all that well and I’m paraphrasing what I’ve read, idk.
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