Motor and coupling selection 

When selecting what motor to use, the team chose the desired specifications based on the process followed by similar shredding operations. Specifically, a high torque, slow speed motor with the ability to go in reverse. The motor acquired is pictured below from Dr. Mick Peterson, who told the team, "Torque and speed is all you need," therefore it was determined it would do the job given its specifications on those parameters. The motor has a speed reducer; therefore, it can get to a slow speed that is optimal for shredding plastic efficiently, which should be determined during testing. 

Motor Selection Justification

The majority of guidance in deciding if the provided motor would be sufficient was based on its specifications next to preferred motors of the shredder manufacturer (Action Box). Specifically, the Nema 34 and the Lichuan stepper motors were used in their set up tutorials. By looking at their publications we found the Lichuan motor held a torque of 4.0 N*m as well as a speed of 3000 rpm before being run with a 50:1 planetary gear box, bringing the speed to about 60 rpm. 

In contrast, the motor selected here is well above their torque at around 25 N*m, with a slightly lower speed of 31 rpm after reduction. With this motor we are able to eliminate the need for any additional gear boxes, while still meeting the criteria we are looking for. This difference is not an issue, rather an opportunity for a wide variety of plastics to be run  without major concern of the machine clogging. 

However, when testing began, it was found that the motor was not powerful enough for the plastics. It did not have enough torque. For future designs, a different motor with higher torque should be used.

Motor Mounting 

The motor is mounted next to the shredded using an aluminum sheet metal base plate that was machined so that the motor can set horizontal parallel to the shredder and can drive the shredder with a sprocket and chain. The base plate can be interchanged to mount a different motor. Both the motor and shredded are mounted at the top of the device under sheet metal shielding.

Sprocket and Chain 

One sprocket is attached to the motor shaft that has a speed of 31 RPM. The roller chain connects the motor shaft sprocket to the shredder shaft sprocket. A size 40 roller chain was selected and corresponding size 40 sprockets. The length of chain is dependent on the final location of the motor in relation to the shredder, but the motor shaft should be no more than 1 ft away from the shredder shaft, so 2.5 ft of chain should give us plenty of room to work with. This specific selection also provides a 2:1 ratio for the speed of the motor shaft to the speed the shredder shafts are turned. This allows a slower speed and higher torque for the shredder.

Gear Selection

Coupling gears with a pitch diameter of 55 mm (2.165”) were implemented on both driving shafts of the shredder to allow the one DC motor to turn both shredder shafts in opposite directions. The pitch diameter needed for the gears on the shafts was calculated to be 2 3/16." However, this pitch diameter is not available. Sizing up could prevent the gears from interlocking correctly, so sizing down is necessary. The next smallest size available was 2.165 ", which is what was chosen.