If the gearbox is the weakest point, then teeth may break off.
If the shredder is the weakest point, then the axles may get bent.
If the motor is the weakest point it may burn out.
If the bones are the weakest point you may get sharp splinters.
I have 3 gearboxes: 1) 1:160 planetary from a 2000lbs winch 2) massive planetary from a transfer case (could do 1:100+ with modification) 3) 1:8 transmission (?) from a go kart or something, it has a reverse lever - could be modified to very high ratio and takes belt drive from the motor.
Axle will probably be 5/8 or 1" shaft.
I've never seen a shredder, no matter how cheap, that doesn't have automatic stall detection to turn off the motor if a jam occurs.
The other reason for a reverser. whether automatic or not, is to guide the object to be crushed into the jaws. Things can get stuck floating at the edges sometimes, and I cannot emphasize enough that you absolutely, positively do not ever want to be inserting your hands in there to move something while it's on -- nor a tool, because then the tool can get crushed into the grinder, too!
With that said, I suspect you'll need to do multistep grinding if you're starting with beef femurs: a coarse grind through your shredder, then a finer grind through a grinder. Otherwise I'd be concerned about shards that are juuust the right size that the dog (cute dog, by the way!) can swallow them without chewing, which might be sharp and cause problems.
The other concern is how you clean the thing. The last thing you want is bits of meat and bone rotting inside some inaccessible crevice of the device!
Adding jam detection and auto reversal adds a lot of complexity to the project
I'm motivated to learn how since I find it interesting but can't put time investment into that right now. It's something I'll add later down the line.
I think if I make the torque ratio super high and have a beast of a motor no bones can ever jam it
^ This is what I want to make a cheap version of, single shaft and less blades, rotating at 1/20th of the speed.
Single phase AC motor isn't good for reverse. DC or 3 phase motors are better. You can run either DC or 3 phase AC motor with single phase AC power by using a drive.
What specs you recommend for a DC motor?
Can't run 3 phase at the moment and VFD are out of budget.
WARNING: You need guarding, safety switches and an emergency stop, to prevent you from shredding your arm, or worse.
Yea will do
Yep reversal feature is as much of a convenience as it is a safety thing.
Shredders are designed to pull material trough them so if something gets lodged in there it is often impossible to get back out without running the shredder in reverse. They always have a ton of gear reduction because they need a LOT of torque and not much speed, while motors like to run at low torque and high speed. So you usually can't move the shredder mechanism even when it is off since the gearbox has too much mechanical advantage (takes a huge amount of force to backdrive). As a practical result this means that trying to shred something too tough will stop the shredder and make it very difficult to dislodge the problematic piece out of it. This also encourages you to poke around inside of it in an attempt to free it up. Then as a bonus if you do end up getting your fingers sucked into the shredder you will not be able to get your hand back out without cutting your own fingers off.
That being said one of the best places to look for a drive motor is an electric winch. Those motors will already have a reduction gearbox on the motor that turns it into really low RPM at a ridiculous amount of torque, exactly what you want for driving a shredder. As a bonus these motors also come with a reversal switch wired in already, so you get a shredder manual reverse feature already included.
Yea winches have beautiful gearsets and are definitely well suited to grinding. What capacity winch should I use though? My current one is rated for 2000lbs which maybe too low.
I definitely never want anything getting stuck. I can just imagine how annoying that will be without reverse. I think if the motor and torque ratio are good then it should blast through everything I throw at it and never jam. I could add jam detection and auto reverse in the future just as a precaution.
If you manage to put the tip of your tie in your self built shredder with the emergency stop just out of reach I'll give you a darwin award.
Ok but that's a big IF. And right now you're currently up for the nagging nancy award
I don't understand the problem. You have a functioning shredder in the form of a dog. Not only does it apparently enjoy the job, but if you take that job away then it will just need to find something else to do.
If I give my dog a lamb shank she will eventually eat all the meat but only 20% of the bone / knuckle. When she was young she'd eat the whole thing no problem. If I don't break up the bones sometimes she won't even touch it. She definitely prefers it broken up now that she's old.