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HP-97 Printer Gear Replacement

Posted by Katie Wasserman on 15 Nov 2000, 2:07 a.m.

A common problem that causes the premature death of the printer in the HP-97, is the deterioration of the off-white colored spur gear in the drive train. This can be replaced by using the right parts and a little effort.

If your printer is not working, you can easily determine if it's because this gear has deteriorated:

- Remove the 6 recessed screws on the bottom of the calculator and lift off the bottom cover.
- The bottom of the printer assembly is immediately visible, locate the motor and gear train.
- Inspect the middle of the three gears. The pinion gear at the end of the motor shaft is metal and drives the plastic spur gear, which in turns drives another plastic gear that moves the carriage back and forth.
- If your printer problem is due to a damaged middle spur gear, you should be able to see some missing or damaged teeth on the gear.

If this is your problem, you'll need to replace it with a gear that ideally matches all of the following specs:

- 80 pitch - measured by the pitch diameter method (not the circumferential pitch method, my first mistake!)
- 30 teeth
- 1/10" gear face
- 1/4" hub diameter
- 5/64" hub projection
- 3/32" bore
- made of Delrin or nylon

I have not been able find this as a stock item with any manufacturer and I've had no luck with getting this custom made. But I did find a gear that can be made to fit with just a little bit of work. The gear is available from PIC Design as a stock item part number G46-30 for about $7.00 in single quantities (gears are very expensive items!). Its specs are:

- 80 pitch
- 30 teeth
- 1/8" gear face
- 3/8" hub diameter
- 3/16" hub projection
- 1/8" bore
- made of aluminum

These specs don't look too close on the surface of it, but the things that really count (pitch and teeth) are the same and the other stuff is easy to get around. Here's how.

(1) Cut off the hub entirely so that it's just a flat gear with no hub projection. Since the gear is aluminum, it's easy to do this with a fine blade hacksaw or a course blade jeweler's saw.
(2) Adapt the bore from 1/8" to 3/32" by inserting a nylon screw insulator into the hole in the gear. In particular you should buy part number U-IN-2/2 from Small Parts, Inc.. It's a size #2 x 1/8" length insulator that has exactly the wall thickness that we need. A package of 10 of these sells for $1.23 (much more reasonable!).

To finish the gear replacement:

(1) Loosen the 3 screws holding the printer assembly to the plastic frame so that you can lift up the printer the 1/2" or so needed to access the end of the gear shaft.
(2) Remove the "C" external retaining ring from the end of the shaft and set it aside. If you lose this part (it's very small) you can buy a replacement from Small Parts, Inc as part number U-RSE-1-1/2. A package of 10 sells for $0.74, so you might as well buy them when you buy the screw insulators. Note, these are functionally the same but are "O" rings not "C". However, they are an exact fit and work just as well.
(3) Pull the old gear straight off.
(4) Slip the new gear with the screw insulator inside of it onto the shaft keeping the flange on the end of the screw insulator facing out.
(5) Put the retaining ring back on. The "O" ring takes a bit of force to push it on, the "C" ring goes on easily.
(6) Rotate the newly installed gear back and forth with your finger. It should take some force to do this but not a huge amount. If it doesn't move or requires a lot of force remove the gear and try a different screw insulator from the same pack of 10 that you bought, they seem to vary slightly in wall thickness.
(7) Put the back cover on and tighten the screws.
(8) Test it!

Here's where to find the needed parts:

Small Parts, Inc.
Address: P.O. Box 4650 - Miami Lakes, FL 33014
Voice: (800) 220-4242
Fax: (800) 423-9009

PIC Design
Address: 86 Benson Road - P.O. Box 1004 - Middlebury, CT 06762
Voice: (800) 243-6125
Fax: (203) 758-8271


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