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A New Life for Old Cardboard

Below is a report from Plant Chicago intern Tyler on our ongoing project to convert our old gas-powered chipper/shredder to electric, and repurposing its function from shredding straw for mushroom growing to shredding cardboard. Cardboard is a material that is in great abundance at The Plant, and being able to do something with it other than just recycling (such as chicken bedding, vermicompost, etc) would mean saved energy and a more direct path back into the biosphere.

The shredder we are repurposing is a Dr Power model 14.50, which uses a 10 HP, 2700 rpm gas engine. Using this machine as is has always posed safety concerns when used indoors: noise, exhaust fumes, dust, etc). Converting the machine to electric would reduce the noise and eliminate fumes, leaving us to then tackle the issue of dust generated from the actual shredding process.


First Steps

To begin converting the gas engine to an electric motor I applied a general conversion for relating horsepower generated by gas engines to horsepower created by electric motors. The common conversion is about 3 HP electric for every 5 HP provided by the gas engine. Using this conversion it is safe to assume that to get the full potential out of this shredder the largest required horsepower provided by the electric motor is about 6 HP. This size of motor unfortunately would be very large, heavy, and quite expensive to purchase. Fortunately, the original chipper shredder was intended for use on branches/wood, while our new intended use is to shred cardboard, so there was not as much need to meet the maximum specifications of the original engine.

One constraint of this project was that I needed to repurpose an old motor that The Plant’s owner Bubbly Dynamics had in storage. After sifting through all the available electric motors (that didn’t need an excessive amount of cleaning or modification), the closest fit we could find was a 2 HP Dayton motor operating at 1725 rpm. Although this was much lower than the optimal replacement, it would be enough to operate the shredder with sufficient torque to shred cardboard.


Upon choosing a replacement motor, I disconnected the original gas engine from the shredder so that we could begin mounting the new motor. In order to ensure the new motor would line up with the flywheel shaft of the shredder, a hard plastic High Density Polyethylene (HDPE) sheet was cut to fit where the old engine was located, and drilled with mounts to attach the electric motor. Once the motor was attached, this assembly needed to then be attached to the shredder in a way that would allow for the tension on the driving belt to be adjusted (the motor assembly needed to be able to slide back and forth). It was important to be able to easily adjust the position of the motor relative to the flywheel shaft because the belt could stretch with prolonged usage and cause a variety of problems. There were already existing cutaways in the frame of the shredder that we were able to use to secure the assembly while still allowing for easy adjustments. (Using these cutaways also allowed us to more easily install the motor assembly.


Once we had everything mounted and adjusted, the next step was to get power to the motor and test out the shredder. Since we were using a repurposed electric motor we couldn’t be certain that it would work until we tried it out. We built a new plug for the motor and installed a 3 phase power outlet on the wall near where intended to use the shredder. We then powered it up and fortunately it all ran smoothly… backwards!

Unlike regular 120v power you get from a standard outlet, 3 phase power uses 4 wires, and the order of them determines which way the motor spins. Luckily, switching two of the wires around where the power cord entered the motor reversed the direction of rotation and all was well.

Figure 4. Installing new power cable.


Next, we tested the shredders capabilities by inserting cardboard pieces of varying sizes and consistencies.  The shredder employs 24 ‘free swinging hammers’ that pulverize the gathered materials before they are ejected from the machine. The size of the material ejected after shredding can be set by adjusting the location of a screen on the bottom of the machine – the closer the screen to the hammers, the longer the material stays in contact, and the smaller the ejected pieces. Our best results took place when we used long pieces of cardboard that were folded to increase their stiffness. When using thinner or previously wet cardboard the material would wrap around the hammers and eventually get ejected as long pieces of pulverized cardboard with inconsistent shredding.

One output we did not anticipate was the dust created by the shredder. This is undesirable due to its location near the aquaponic garden and the aquatic life. The dust is created by the fact that the hammers inside the machine are basically flat pieces of steel that are designed to pulverize and fracture dense, stiff objects (ie tree branches) fed into the machine. Soft, fibrous materials like paper and cardboard get ripped apart by the hammers, creating fine dust.

What’s next?

Our goal moving forward is to minimize the dust created by the shredder so that it does not interfere with nearby projects when in use. The plan is to disassemble the machine and remove the hammers inside. We will then sharpen these hammers so that they will cut through the cardboard instead of pulverizing it, which will ideally lessen the amount of dust created by the shredder and should bring the cardboard to the desired consistency much faster than with the unsharpened hammers.

Stay tuned for future updates on this project!


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