Pneumatic Can Crusher

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(This page is a work in progress, and will be updated as I modify the can crusher).

This is my take on a pneumatic can crusher. I've wanted to build one for several years, but never got around to it. After dealing with the new recycling laws that when into effect on July 1st, 2012, I decided I wanted to make fewer trips to the recycling center because of the headache. One way to do this is to crush the cans so that after a month or two, we don't have a half-dozen 33-gallon garbage bags of cans sitting on the back porch. Sure, I could get one of those manual can crushers, but where's the fun in that? This was a perfect excuse to start shopping on eBay.

Lots of people have built pneumatic can crushers, and it's hardly an original idea. Most of the videos on YouTube just show people operating them, and don't discuss the construction techniques and parts list. Googling around shows a lot of hits, but then most of them are from the metal working sites, and these folks have MIG welders and metal working capabilities (something I minimally have).

Ultimately I'll do a metal version, but I wanted to get something up and running. I'm pretty sure that 3/4" plywood should hold up, and will let me get the dimensions correct, and get the pneumatics and controller worked out.

Here's a link to the video on YouTube. I'll get into more details in the explanation below.

After doing some research, and using this calculator, I pretty much settled on using a 63mm or 80mm bore air cylinder with a 5" stroke (a little more than the height of a can). I'm not exactly sure how much force it takes to crush a can, but I'd rather error on the side of excess than build something only to discover it's under-powered. The calculator tells us that a 63mm bore size at 100 PSI should generate about 490 pounds of push force, which would provide more than ample crush pressure.

Searching on eBay, I found 2 SMC NCDGBN63-500 pneumatic cylinders for $32.40 (including shipping). The SMC part number tells us it's a double-action, basic cylinder with built-in magnet, rubber bumpers, 63mm (2.5") bore, and a 5" stroke. Double-action means there's a port on either end of the cylinder so that air pressure can be used to move the ram to either position. The other type are single-action cylinders, where a spring or other external force is used to move the ram from the extended to the retracted position. The magnet, in conjunction with some external sensors, is useful for determining if the ram has reached the desired position. These particular cylinders had been mounted in some equipment, but (supposedly) never used. That sounded good enough to me to buy them.

I then fabbed up a frame using 3/4" plywood, sheet-rock screws to hold it together, and 12" threaded rod stock with fender washers and NyLock nuts to provide the strenth. Obviously the sheet-rock screws would rip loose after a few compression cycles, and they're just there to hold the shape. The threaded rod is where the actual strength comes from.

My first attempt at the ram head was a piece of 3/4" plywood cut to a 2.5" diameter. I epoxied a nut to it that would thread to the end of the ram on the air cylinder. This fell apart on the first actuation, which didn't really surprise me. The next version of the ram head is a piece of 1.25" thick nylon that I've had in my parts bin for years. Again, I cut it to 2.5", and then tapped it with a 1/2 x 20 die. After threading a 1/2 x 20 nut on to the ram to reinforce it, I then threaded the nylon circle on to it. There's no force on the threads when the ram is retracting, so the threads in the nylon are sufficient. The nut behind it keeps the ram from pushing through the nylon when the ram is extending.

Besides the air cylinders, I need pneumatic valves, fittings, a flow rate adjuster, air filter, tubing, and some other miscellaneous parts. I ordered the following parts from Mettler Tools. These folks have an eBay shop, good prices, and they'll optimize your shipping costs. I paid $7.50 for shipping on the following items.

P/N Description Quantity Price
AF2000-N02 Air filter 1/4 NPT 750L/min Water Trap 1 $16.51
MTL 1/4-N02 One-Touch Fittings 1/4" OD 1/4" NPT 90 Deg Swivel Elbow Male 5 $6.19
MTC 1/4-N02 One-Touch Fittings 1/4" OD 1/4" NPT Straight Male 10 $7.80
MTC 1/4-N01 One-Touch Fittings 1/4" OD 1/8" NPT Straight Male 10 $7.80
MSC 1/4-N02 Quick-Push 1/4" OD 1/4" NPT Air Flow Speed Controller 2 $13.80
BSC-N01 1/8" NPT Pneumatic Muffler 2 $11.64
PU 1/4-30C 1/4" OD 30 Meters Polyurethane Tubing 1 $22.35

The 3 Airtac 5-port 2-position air valves came from Ebay seller marcmart.home, and cost $23.29 (free shipping). They're out of China, and the China post is kind of slow.

There are a number of different configurations of air valves, with different combinations of ports and positions. I chose the 5-port 2-position valves because of cost. A 5-port 3-position valve would have been a better choice, but I could get three of the 2-position valves for the cost of a single 3-position. The 2-positions are more common, and give me a more flexibility since I don't know what I'm doing.

(I'll insert a schematic and explanation of a 2-position vs 3-position valve here).

In a number of the can crusher videos on the web, you'll see people using 2-position valves with double-action cylinders. This means that there is always pressure applied to the air cylinder. I only want pressure applied when moving the ram to either position, rather than keeping pressure applied after the ram has reached the desired position. This is primarily in case I need to manually move the ram, and partly so that when the system isn't in use, there's not pressure constantly present.

Using three valves, one will be used to supply pressure from the compressor to the other two valves, and then a valve for each side of the ram. When the ram is to be moved, the compressor-side valve will be energized, supplying air to the to two air cylinder valves. To extend the ram, the valve on the extension side will be energized, and air will flow into the cylinder. The other valve, which is de-energized, will allow the air from the other side of the cylinder to vent. To retract the ram, the extension-side valve is de-energized, and the retraction-side valve energized. Once the ram has reached either of the positions, the compressor-side valve can be de-energized. Since there's no external force trying to move the ram once it's in either position, there's no point in keeping pressure applied to hold keep it in that position.

This page will be updated after I get the Airtac valves in and plumb them up.