tenbob

asked Dec 31, 2013 at 12:03pm
Hp HP DeskJet Plus

Laserjet 4 plus grinding, gears jumping

Hi. My faithful Laserjet 4 Plus has started making loud grinding noises when it powers up and when it prints. The paper feeds normally, but the image is distorted with horizontal bands where the rows of text are squashed together.

I have fairly conclusively traced this to the helical gears that drive the cartridge - one in the printer and one in the cartridge. These keep jumping out of mesh, causing the nasty noise and the image defects. Although I know where the problem is, I can't find anything obviously wrong and I'm unable to fix it. These are the things I've done so far,

I carefully examined all the gears in the train from the motor to the cartridge and to the fuser. I took off the black plastic piece that covers the gear train for better access. I looked for damaged teeth and, as suggested here, for radial cracks in the gears. Everything seems OK.

I examined the cartridge itself by inverting it and turning the white gear attached to the drum. It turns reasonably freely, but it's quite hard work on the thumbs. I also tried a different cartridge and that feels about the same. The other cartridge exhibits the same jumping fault when it's installed in my machine. Both are original HP, so should be good quality.

I then removed a lot of stuff that might be making it harder to turn. I completely removed the fuser, the black plastic cover over the gear train and the roller that presses the paper against the drum. None of these made much difference to the jumping.

I then tried to test what torque was available from the motor to turn the cartridge. With the cartridge removed I grabbed the drive gear through a piece of thick cardboard and tried to stop it turning as the machine powered up. I couldn't stop it, which shows there's lots of torque available. None of the other gears started jumping, so that seems OK too.

I'm left with the thought that it must be a positional thing that means the teeth are not engaging properly between the printer and the cartridge. Here's where the results get a bit strange. The cartridge doesn't locate as positively as I would wish. But I think that's fairly standard for this machine. It slides in down a sloping path and then drops down into its final position. After dropping it seems quite secure. It won't come out unless you raise it significantly first. Looking through the space where the fuser goes the gear teeth seem to be fully engaged.

I tried wiggling the cartridge while the motor was running. Raising the cartridge causes the teeth to unmesh completely. But you have to raise it quite a lot for that to happen, so it seems to work as expected. But an odd thing happens when you press down on the cartridge. This causes a continuous grinding noise with the gears jumping all the time. I would have expected this to reduce the jumping, but it doesn't. Possibly something is making the cartridge harder to turn when it is installed in the printer and it gets worse when you press down.

If anyone can suggest how to fix it or further tests to try, I would be very grateful.
I've done a bit more investigation. The gear wheel in the printer drives a gear in the cartridge which is attached to the drum. This in turn drives a smaller gear which attached to the primary charge roller inside the cartridge.

There are two places these gears could jump.
(1) Between the printer and the drum gear.
(2) Between the drum gear and the PCR gear.

Slipping at point (2) might occur because the cartridge is hinged in the middle and flexing it separates the gears. But I don't think this would cause the image defects I got.

Slipping at (1) seems to fit the symptoms better. It could be caused by a badly positioned cartridge. But I still can't find anything wrong.

I'm curious as to why the printer is designed the way it is. There's nothing actually holding the gears together at (1) or (2) apart from a bit of friction and springiness. Presumably HP built it that way so the the gears could be forced apart if something got stuck, avoiding more serious damage.

I'd still be grateful for any help to fix it.



by tenbob on Jan 14, 2014 at 4:28pm Add comment
I've worked on thousands of the 4/4+/5 series printers. Never experienced the problem you are having. Never replaced any gears except for the coupling gears that drive the fuser. Given that, it's really hard to diagnose the problem you are having without actually having my hands on it. The first thing I would blame is the toner cartridge. Try separating it into the two halves by taking out the 2 plastic pieces on top. They are held in by 1 screw. Next remove the metal axle holding the drum in and remove the drum. Put it somewhere out of the light or cover with a towel. Then put the 2 halves together and reinstall the cartridge. I'll bet the noise stops.
by moe on Jan 14, 2014 at 5:22pm Add comment
Hi Moe. Thanks for these helpful suggestions. As you say, the problem might be the cartridge. I've already tried two different ones and they both give similar problems. They are both HP originals, but they may have been manufactured a few years ago.

I will look around for another cartridge I can try, but I don't want to waste money buying a brand new one that I may not be able to use.

Meanwhile I will try a test with the drum removed as you suggest.

Thanks, Bob
by tenbob on Jan 15, 2014 at 8:59am Add comment