# Where could I find a low RPM high torque motor?

Dec 12, 2013
5
Hi i am building a machine that shreds plastic bottles and containers for recycling and reuse. I assume it would have to very low RPM/ high torque but i am not sure if thats what i need due to the fact i know very little about motors or electronics. the original builder used a ( ? hp) 2 kW/70 RPM motor. But i was hoping to find a similar motor to use that i could just plug in and go. Any help anyone?

#### Minder

Apr 24, 2015
3,378
The most economical/efficient way is to use a reduction G.B.
You need to do an empirical test of the torque required or at least an estimate.
Torque increases at the ratio of the reduction.
Was the original direct drive?
M.

Dec 12, 2013
5
The most economical/efficient way is to use a reduction G.B.
You need to do an empirical test of the torque required or at least an estimate.
Torque increases at the ratio of the reduction.
Was the original direct drive?
M.
please forgive me for my ignorance but how would i perform an empirical test of the torque? And whats the original driect drive - i may not know it because the original builder gave limited info on the motor - just 2kW/70rpms

#### duke37

Jan 9, 2011
5,364
2kW is quite a chunky motor. There are 746W in one horsepower (UK).I assume it is driven from the mains and the motor will probably run at 2800 rpm in the UK. It will therfore be geared down a lot to reduce the speed and increase the torque.
Power is torque times speed so you can work out the torque the 2kW motor will give at 70rpm. You must however use radians/second for the speed.

Can you do the sums?

#### Minder

Apr 24, 2015
3,378
On low RPM applications I have used a automotive torque wrench in cases where a thread was on the end of the input shaft, alternatively if it has any other means of winding a string around a pulley etc, and use a (fishing) spring gauge and measure Inch/Lbs etc, based on the radius of the pulley. This gets you in the ball park.
Did the original have gearing on the end of the motor or was it direct drive.
M.

Dec 12, 2013
5
Like i said i have beginners experience with motors and electronics, so im having trouble keeping up with what duke37 was talking about. But maybe this will help with getting a picture of what im talking about. This is the projects that im working on and here is what was given to me.

Project Source
Project Files

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#### Minder

Apr 24, 2015
3,378
That is using a worm and pinion gear box, if it is a four pole motor then the ratio is ~25:1.
If 2 pole then 12:1.
Motor 2hp - 2.5hp. Looks a little oversized for that application?
M.

Dec 12, 2013
5
That is using a worm and pinion gear box, if it is a four pole motor then the ratio is ~25:1.
If 2 pole then 12:1.
Motor 2hp - 2.5hp. Looks a little oversized for that application?
M.
So what kind of motor would be appropriate for the shredder? After all, its only shredding plastic and not even thick plastic at that.

#### Colin Mitchell

Aug 31, 2014
1,416
Our shredder (granulator) runs a quite a high RPM.

#### Minder

Apr 24, 2015
3,378
So what kind of motor would be appropriate for the shredder? After all, its only shredding plastic and not even thick plastic at that.
As I said, for accurate sizing the input torque to the shredder operating at its highest load would need to be known in order to accurately size a motor, if you want to use the existing design pictured, that appears to use an ordinary induction motor, if so, I would tend to go with a 4 pole motor, rpm is dependent on the country you are in 50Hz or 60Hz and the supply you have available. You mention 2hp and the picture bears it out.
You will need both motor and attached G.B.
You may not need that HP.
M.
.

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#### Colin Mitchell

Aug 31, 2014
1,416
You need at least 2HP. The RPM needs to be high and the cutters need to be on a very heavy armature as it takes an enormous amount of energy to cut through plastic and bottles and if you jam up the machine it will take hours to open it up and unblock it.
The grill at the bottom determines the size of the granules.
The granules are worthless because you get paper, glue, sugar, etc in the mix.

#### Jeremy Simanjuntak

Dec 14, 2016
1
So what kind of motor would be appropriate for the shredder? After all, its only shredding plastic and not even thick plastic at that.
What did you do in the end? I'm making the same precious plastics project and am stuck as to where to buy the motor

#### Colin Mitchell

Aug 31, 2014
1,416
Our shredder (granulator) runs a quite a high RPM.
The secret is the flywheel.
You need a very heavy base to the blades otherwise your shredder is going to jam.

DO NOT USE A GEARBOX - obviously from someone who does NOT have a clue what he is talking about.

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