Once I removed the horizontal X-axis (print carriage) actuator assembly from my Canon MX340 multi-function inkjet, I could see several mechanisms that were previously hidden. My attention first went to the lever that was pushed by the print carriage at a specific position. Now I can push on it with my thumb and turn the friction-coated paper feed shaft manually to see things move. Conclusion: this lever starts the process for feeding a sheet of paper from the stack in the paper tray into the print engine.

The other end of the lever sits within this T-shaped slot inside the gearbox. Normally it sits in this center position, which I will call "Neutral" because no rotational power is transmitted to the paper tray mechanism.

Once pressed, the T-slot can shift to one of two positions depending on which direction the paper feed motor is turning. I will call this position "Reverse" because the paper feed motor is moving opposite of the direction when printing.

In this mode, power is transmitted to the big rubber-coated roller in the paper tray, rotating it in the direction to feed paper. This is quite bizarre to me because, as already stated, this happens when the adjacent friction-coated paper feed shaft is rotating backwards, a recipe for a paper jam! I thought maybe the roller motion was just a side effect, but later gearbox teardown would confirm this is deliberate: a gear is specifically engaged in "Reverse" for this purpose, and it doesn't move anything else. Whatever purpose this counter-intuitive motion serves, it is an intentional part of the system.

Shifting into "Forward" gear, I see a sequence of events consistent with feeding a sheet of paper from the paper tray.

Here is a closeup of the paper handling mechanism at the base of the paper tray.

  1. This is the bottom of the paper tray. A spring-loaded and cam-operated mechanism pushes it forward and upwards so the top sheet of paper in the paper tray makes contact with the rest of the mechanism.

  2. I think this pair of claws usually keeps the stack of paper separated from the feed rollers. During this paper feed sequence, the claws retract.

  3. This mechanism has a smaller roller that receives no power, but its position can change to mesh with the big powered roller, or retracted so there's a gap.

  4. The big roller also receives rotational power in "Forward" via a different gear path, so now it turns in sync with the friction-coated paper feed shaft and there would be no paper jam.

A mechanical gear-and-cam system choreographs the timing and sequence for these actions. I look forward to seeing the details later in the teardown.

There's nothing beyond motor rotation holding the "Forward" or "Reverse" positions. So as soon as the lever is released (by moving the print carriage out of the special engagement position) a slight bit of paper feed motor movement in the opposite direction will cause the mechanism to snap back into "Neutral". This is a good candidate explanation for the small movement of 1800 encoder counts I saw repeatedly: the printer wants to make sure the paper feed mechanism is in "Neutral" before it does something else.

It should be fun to see how all this is implemented, so I freed the paper feed assembly to get at its gearbox... and immediately got distracted when I discovered an ink graveyard.

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