Tracing through path taken by the ink disposal system in my Canon Pixma MX340 multi-function inkjet, I noticed a circular assembly that hinted at a peristaltic pump. My guess was quickly confirmed after I opened up that assembly. I lifted up the rotor and, yep, it moves a trio of rollers across those tubes.

This thing has some interesting differences from the basic textbook design for a peristaltic pump. The most obvious difference is that, instead of a single tube, this pump is working to move fluid through two tubes on opposite sides. There are three rollers spaced 120 degrees apart, so at any given point at least one roller is in contact with each tube. But it's rare for the third roller to have contact at the same time. There's barely time for any two rollers to work together before one of them departs for the other tube.

What might be the tradeoffs of jamming two tubes into one pump like this? I don't know how important it is to have multiple rollers in simultaneous contact. Always having one in contact should be enough to ensure things move along and nothing sneaks back. I guess perhaps this design would be less capable of moving thick (viscous) fluids or pushing against back pressure. But this pump is working with watery (low viscosity) ink and dumping them off into an disposal area that exerts no back pressure. So even if those tradeoffs were real, they would not cause any issues in this application.

A standard peristaltic pump works in both directions, but here we don't want to pump ink or even air back into the maintenance tray. This pump rotor is designed so it only pumps in one direction. The trio of rollers each sit in their own spiral track. When the rotor is rotating in the pumping direction (counter-clockwise in picture above) the rollers are pushed outwards so they press on the tubes and do their thing.

When the rotor turns the other way (clockwise in picture) the spirals retract all three rollers inwards. It looks like they still make contact with the tubes, but not exerting enough pressure to pinch off tube interior and thus no pumping takes place.

I thought this is a very clever way to modify the standard peristaltic pump design so it only pumps one way. Much simpler and less prone to failure than introducing, say, a gear engagement/disengagement mechanism. This peristaltic pump rotor is always coupled to the paper feed motor. When that motor feeds paper forward during printing, these rollers retract. When the motor spins backwards relative to printing direction, the rollers extend for pumping action. This pump is a good explanation for long backwards motions performed by the printer during its various startup/prep/shutdown procedures.

OK, enough distraction with the ink pump, back to the paper sheet feeding mechanism.

This teardown ran far longer than I originally thought it would. Click here to rewind back to where this adventure started.