Air Motor Applications for Shredders and Choppers

[CristianEc]

TL;DR Summary

Need advice for considerring an air motor to rotate a shaft

Hi everyone!
My name is Cristian and I would like to ask you for advice. I am trying to build a shredder for solid recycled plastics (particularly PP and PE) and I am seeking and motor to rotate the shaft of this device.
Specifically, I require a motor with a 75 RPM speed and over 20 Nm torque and searching in the internet for alternatives to electric motors, I found that certain air motors can match these requirements. However, when going through usual applications of this kind of motors, I could not see any suggestions on using them for shredders or choppers.
So, I would like to know your opinions on the possibility of using an air motor for this purpose.
Thanks in advance!

 

[jrmichler]

Air motors are specified by torque and RPM. It is your responsibility to decide if a particular motor is appropriate for your application. The exact application is not important. The only concern is the desired torque and RPM, the motor rating, and the ability to supply enough compressed air at the correct pressure.

Some reasons to specify an air motor:
Hot environment. Air motors tolerate hot environments because they get cooler when they run.
Overloads. Air motors can be stalled without being damaged.
Exhaust air is cold: You may have a need for a jet of cold air.

Factors to consider:
Air motors are noisy. You may need to specify a better muffler to protect the operator.
Efficiency. Air motors are low efficiency. It takes roughly 4 compressor horsepower to drive a 1 hp air motor.
Don't forget to make sure that the air supply is dry.
Remember that air pressure drops when flowing. Make sure that you will get the pressure you need at the flow rate you need.

 

[Asymptotic]

This shredder must be very tiny. Knife granulators I've worked on seldom were smaller than 50 horsepower.

In addition to what @jrmichler outlined,
Air motor are high speed devices, and to get 75 RPM out of one it must have an integral gearbox of some sort.

Air supply must be filtered to at least 40 micron (or smaller) particle size as well as being dry.

All granulators in my experience have flywheels to keep the cutter spinning during transient overloads typical in this application. Motor rotor mass adds to the flywheel effect, but I don't know whether the same is true of an air motor rotor. If this isn't taken into account, an air motor may be stall prone even though capable of satisfying nominal torque demand.

Take a look through several manufacturer's catalogs, carefully study motor performance curves, and their compressed air recommendations/application notes. Not an endorsement, but Parker-Hannifin's catalog is written clearly.

https://www.parker.com/Literature/Pneumatics%20Division%20Europe/PDE-Documents/Cylinders/Parker_Pneumatic_P1V-M_Air_Motors_Catalogue_PDE2539TCUK.pdf

 

[berkeman]

Some reasons to specify an air motor:

Slightly off-topic, but not too far: Why do dentist drills use air motors? Is it because they need high RPM and not much torque?

 

[Dullard]

Slightly off-topic, but not too far: Why do dentist drills use air motors? Is it because they need high RPM and not much torque?

I can't speak to the intent of the people who design dental equipment, but 1 (sometimes) important advantage of air tools is that they are small/light compared to their equivalent electric bretheren - As long as you don't include the compressor in the comparison.

 

[CristianEc]

Air motors are specified by torque and RPM. It is your responsibility to decide if a particular motor is appropriate for your application. The exact application is not important. The only concern is the desired torque and RPM, the motor rating, and the ability to supply enough compressed air at the correct pressure.

Some reasons to specify an air motor:
Hot environment. Air motors tolerate hot environments because they get cooler when they run.
Overloads. Air motors can be stalled without being damaged.
Exhaust air is cold: You may have a need for a jet of cold air.

Factors to consider:
Air motors are noisy. You may need to specify a better muffler to protect the operator.
Efficiency. Air motors are low efficiency. It takes roughly 4 compressor horsepower to drive a 1 hp air motor.
Don't forget to make sure that the air supply is dry.
Remember that air pressure drops when flowing. Make sure that you will get the pressure you need at the flow rate you need.

Many thanks for the explanation!

 

[RonL]

Here is a guide to air motors that might offer you the information you need.

https://www.atlascopco.com/content/...guides/9067 01 Pocket Guide to Air motors.pdf

 

[CristianEc]

Great!
Thanks