Shredder axis protection

Hi everyone !

I present you our freshly built shredder, nothing particular unless a specific coupling system.

I saw some return about guys breaking their axis if too much torque was involved. I also saw the subject with an automatic motor inversion when the axis is stopped, but either I am better with mecanics than electonics and I could imagine that a brutal stop could still be a thing.

So the general idea is that the coupling is made by tightening the two axis together, with a nylon washer,to allow it to slip if the torque become to high.

So here are the drawing idea and some pictures.

Also a youtube link if some people want to see the first tests !

Next week I should be able to press my first plastic plate, to built a bodysurf handplane as a test, keep in touch !

Next level stuff 🙂 Creative and ingenious!

No matter what to use at the end, to me, axis protection should be be built together with the auto-reverse functionality.  I couldn’t get my head around yet but that’s what i figured so far :

– all above cheapest VFDs have a overload protection (over torque)
– such VFDs have also frequency output, 0 – 10 V, this is used for CNCs to set the RPM in the software
– pretty much all the VFDs direction (forward/backward) can be controlled via input (0-10 V), that somehow also includes the way of stopping the motor

if somebody here could make a board for this, I’d be very happy because shipping an arduino based controller (easy indeed) to non technical users makes me quite nervous, considering what could happen…

anyways, since a shredder will be more likely 3phase and the user has single phase, we need really a standardized solution which can work with most VFD, using 0 – 5/10 V ins & outs

Good topic here.
Another option (easier and cheaper) is to use a hall effect sensor and an arduino to measure the RPMs of the axis, and if the RPM suddenly slow down, then the arduino can power off the motor immediately to avoid/reduce damage.

Datum has a torque sensor (Datum M425) that can measure from 10nm to 60.000nm, but costs between $1500 and $3500 (USD)

@lagrenouille, f3d, yup, now i see it. my bad, sorry.

36mm hexbar : i think 32 mm is enough, that would enable  a 25 or 30 mm driveshaft. 32 mm is widely available in metric countries. When asking for a 27 mm we always get laughter and big eyes 🙂 However, I made also excessive tests with the dial indicator after shredding bigger amounts of PET bottles: 28 mm tends to bend (basically anything below 30 mm) for 2.2 Kw, and worst : with 4Kw.

32mm seems to be just right on the money and suited to the limits of the shredder-box, also enabling replacing the motor for the more suited 4Kw (efficiency wise), I recommend also to use UFCL206 bearings (30 mm dia) and an additional enforcement plates to mount the bearings it as seen below. The noise, vibrations and skewing of the shredder box are within acceptable levels then compared to the 20mm shaft/27 mm hexbar.

btw. those 3mm sheets have to be welded lot’s more than just one point per edge. if you put a level indicator on the box whilst shredding PET you will be amazed.

about the jaw-couplings, as this was about ‘sustainable projects’, thie design below in picture turned out to be more sustainable (It’s hard to get good couplings in more crucial countries, here ins spain a typical coupling which can stand the forces are around 60 – 100 Euro). As said, there was no significant wear – out  of the couplings, bolt of shaft, even with 4Kw 1500RPM/1:20, after inspecting user machines.

anyways, let me try your design, just for fun 🙂

@pporg I think I added the .f3d file, it doesn’t work ?

I will modify it, it seams that the 36mm hex bar is an easy one to get a bit everywhere around the world. Motor side is very depending on what you find.

A real motor alignment is something close to impossible if you don’t have a kick ass milling machine where you can put the entire frame on it, so yes, the tricky part is to have a misalignment tolerant coupling.

I would be interested to see you DIY aluminium jaw coupler.

@lagrenouille would be nice if you can upload the real Fusion 360 file, makes things so much easier.

just for reference: here another broken shaft :

the guys assume mis-alignment and/or vibrations. that also won’t happen when using the DIY jaw spider coupling, it takes 3 hours to machine those. 70 mm  diameter of 6070 alu.

i tried also one day those commercial jaw spider couplings, 70 mm diameter for  whopping 80 Euro the set. During the installation, i figured it’s indeed really error prone procedure, motor alignment isn’t exactly easy, even for pros. With 15 Kg on the shredder, 30 Kg for a motor, mount on a few beams – mostly unusable as reference – it’s pretty hard to align that for real within the coupling’s tolerances. Tightening all up will also move things around. It’s so easy to make this one wrong and end up with a broken shaft. So that’s why, a DIY jaw spider without the filling will compensate all that pretty good 🙂

thanks a lot for sharing ! I will try your design, just for fun 🙂

honestly, i wouldn’t use anything below a 32 mm hexbar and a 28 – 30 mm axle.

btw, producing your coupling in a normal machine shop seems pretty expensive to me, just looking at it tells me I am busy for 2 days, using massive amounts of Kw to get this done

However, I was thinking to switch to a aluminum pin which breaks just with massive force. For now using a simple M10 bolt to keep a DIY jaw spider coupling in place works well for us, even with abusing a 5Hp motor. it wears a little out but thats it.

people can also use just better VFDs and set the overload protection parameters right, job done

Finally made the 3d for this coupling, I share the link for fusion 360 and .step file under.

It is made for a 25mm axis in shredder side and 35mm in motor side. I will try to modify it for a parametric system soon, but I have to learn how to do it.

Whoa @lukashatihatala ! Nice work !!

Can you share the 3d when finished, I might try to build a proper one in the current workshop !

Nice solution. I’m planning to make a couple from plastic (using the injection machine). My logic is that I make the coupler’s thinnest point about as thick as I want to be able to shred so if I exceed this the coupler should break. Then I uder the mould to make a new one.

Good evening gentlemans

Perfect idea @lagrenouille. It is very useful.  I am working with your idea and adjusting on my dimension of shaft diameter. 3D model is nearly done, see attached picture 🙂

Hi @chelbig, on the top I posted a youtube video, if you go to 1’11, you will see the coupling system doing it’s job.

Also I made a short video about the plastic shredding : Precious plastic shredder - rotation speed - YouTube

You will see that it is extremly low, I wanted to use the less energy possible, so this is a 0.22kW motor rotation at 8rpm. Maybe to slow, it would be interesting to see is going 4 time faster would make me shred 4 time faster also ?

I have a second motor, I will do some comparison.

Hi @lagrenouille i thing your idea is great. Is it posible upload a video with the mecanism working and viewing how the plastic is shreddering?

Thanks a lot!