I was trained as an electrical engineer. I've been working in the field of Xerography for many years at a well-known maker of printers in Rochester, New York. Some years ago we had a problem with toner hoppers jamming up in the final run and test area. It was an intermittent failure, only happening about once a month, but it created a big mess. Management wanted to know why and how to fix it. I have found that finding the why is usually the hard part. Once you know why, the fix is usually obvious. No one could reproduce the problem, so I got called in to investigate.
The system consisted of a round toner bottle that used an extraction auger to keep a toner hopper filled to the level of a piezoelectric toner sensor. When the toner level would drop in the hopper, the sensor would call for more toner and turn on the bottle extraction auger motor. A second gear-driven auger and motor would feed toner from the hopper to the developer housing as needed.
Measuring the level of powders can be difficult. Powders can be clumpy or free flowing. Fluffed-up they can flow like water and have very little density to sense. The sensor we were using consisted of an excited vibrating piezoelectric vane. When immersed, the toner would damp the vibration and indicate a full condition. I instructed the operators to call me the next time there was a failure.
A week later the phone rang. One of the telltale signs of failure was that the fine-pitched gears on the motor and auger would jump teeth and make a loud grinding noise from the stalled auger. When I arrived I got to hear the noise. The first thing I did was use a DVM to verify the sensor was indicating a full condition. It was, so why the jammed hopper? After much head scratching we tried running the jammed auger motor while monitoring the hopper signal.
It turned out that vibration from the jumping gear teeth was enough to fool the piezoelectric sensor into thinking it needed more toner. So we found an unstable condition where if the hopper overfills, the gear teeth jump and fool the sensor into calling for more toner. So why does it start in the first place?
We vacuumed out the hopper and tried running the system again. It turned out the auger gear mesh was marginal and would sometimes skip a tooth, even with a nominal hopper level. Every time a tooth jumped the sensor would be fooled and the bottle would feed a little more toner. The parts were hard tooled, and no one wanted to change the gear centers. The fix was to switch to much courser pitch gears, which could tolerate the variance in gear centers.
This entry was submitted by Bill Wayman and edited by Lauren Muskett.
Bill Wayman has a BSEE degree from RIT and has worked in Xerography for 37 years. He holds 80-plus patents and restores antique cars in his spare time.
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