Re: So it was the clutch in the end



#7

Thank you all very much for your counsels. I think it was the clutch after all.

I obsereved the worm gear closely and it appeared to make little stops when rolling, so the clutch was worn and the motor was sliping inside it.

I've thought of how to reinforce the clutch without using the glue and finally I have found it:
I've taken a piece of film plastic --the one used in the kitchen to cover aliments, etc. and I've cut it to the size of the little shaft that comes out of the motor (great patience). I simply have interposed it between the motor shaft and the worm gear. The resulting clutch is suficient so that the motor does not slip, but as there is no glue it may slip (I think) in case the motor is forced. (The plastic piece must be sufficiently small so that when the worm gear is attached there are no traces of plastic to be seen).

I've reassembled the card reader and it works properly --WITHOUT noises!!

Rafa.


#8

You might like to post an article in the articled forum describing what you did in more detail.

I'm sure there will be others with this problem, and many (myself included) prefer easily reversible fixes wherever possible.


#9

Thank you Steve, I'll post an article in the article forum explainig what I did in more detail. I also will ad some more tips that I'm learning repairnig this card reader and that may too be interesting.


#10

Thank you Rafa, great and thoroughly clear post in the Articles Forum.

I had just been messing with an HP-97 card-reader the day before I got to your article, and thanks to dried gummy marmalade-like stuff in the worm gear, my "clutch" had given up, too.

My solution was probably a bit more drastic than yours: after cleaning all parts in alcohol, I punched out the remaining plastic fragments inside the aluminum sleeve of my clutch, filled it with clear RTV silicone, and slid it onto the motor, then pushed the worm gear home... a bit of positioning and it seems to work just fine, but only time will tell if it has longevity and adequate "give". Your fix Rafa, with the cling-wrap, seems very sensible. Wish I'd thought of that...

My card-reader appears to be a little strange. The little nylon balls and single nylon roller-wheel have mysteriously gone quite rough and porous; I can scratch powder off them with my fingernail. Never seen nylon DO this before. I may have to change them out soon, though for now they will still work marginally. Katie W. has said the balls are available from SmallParts.com; I may have to lathe a new little wheel myself, though.

For the rubber capstan, I epoxied on a small piece of latex surgical tubing. I had not intended to glue it at all, but the "tire" sometimes stalled on the moving shaft as a card met it. I am waiting for the epoxy to finish its set right now, in fact.

For assembly: I put the balls and wheel on the one side of the sandwich, cut a strip of paper a bit less wide than a mag-strip and wrapped it around the ball-laden carriage-piece, securing the ends on the other side of the piece with tape. Then on the other side of the sandwich, I installed the springs. I laid the piece with the balls in it on top and snapped them together, then cut off the tape and slid the paper out from between. I wonder how they did this at the factory?

Repairing the mag-card reader seems to be the initiation, the rite of passage that HP users must go through. This is my first; I just got the '97 a few days ago and thanks to the articles on this site, I think I shall now have an operable machine and figure I can learn 67/97 programming. (I think a nametag or badge that looks like a mag-card, with the MoHPC logo, would be a fun memento for the gift shop, Dave...) :-D


#11

Thanks Glynn for reading it. What a pity you had glued yours by then. Now it is true that old HP calculators have proved to be very robust except for the card reader system!, but well, after 25 years or so…it could be expected!

I don’t know how the HP-97 looks like, but in my HP-65 reassembling the card reader seemed to be relatively easy. I just put the nylon balls and idler roller in its hole-places in one part of the sandwich, and then placed the two springs carefully in the other one part. The springs appeared to hold on, so I put this sandwich part up-side down and reassembled. All seemed to fit properly in its place. I then adjusted the screws.


It's very strange the case of the nylon balls. I also don't know nothing about nylon physic properties, but when reading your problem the idea came to me that you may perhaps apply a synthetic varnish to them. You may repeat the operation many times so that the balls get well protected. I suppose this would stop the disintegrating process (as long as the varnish endures, of course). I’m just supposing, I’ve never done this before and don’t know if it will work. To varnish a so small pieces may prove to be a very complicated process! –it may so happen too that the varnish doesn’t get properly stuck to the nylon.


#12

Thanks, Rafa!

Since the replacements are quite cheap, I think I'll opt right now for the new balls. Well, as far as that one that is the shape of a roller wheel, I might just try some light coating, if I can figure out what will stick to a porous nylon without becoming sticky or flaky after drying.

An immediate thought was a thin coat of cyanoacrylate glue, which dries shiny and non-sticky. But I felt it possible that the highly-active cyanoacrylate might just attack the nylon further, fracturing it or making it more brittle.

My experiences with enamel paints and plastic sprays are that they tend to be "sticky" or frictive, even when dry, especially in contact with other plastic surfaces. I wonder about my sister's nail polish or nail hardening compound, but haven't an appropriate "test" area of porous nylon on which to experiment safely. Not to mention having to ASK for the nail polish... ;-)

Whatever attacked my nylon balls left the nylon bearing shaft (on which the capstan turns) alone. It is not deteriorated in any way like the balls are. Very odd.

As new nylon stock is not terribly hard to find, and as it should not be difficult to turn a small rod of it into the roller-wheel's shape and size, I figure that is probably my best bet, if I haven't found a coating I can count on in the next week or two.

As an update, my card-reader is not yet ready to write/read cards without error. Seems that now my latex surgical tubing is too smooth to reliably grab and pull the card through. I am taking it apart again so I can rough up its surface with some fine sandpaper. I'll just order and replace those nylon balls while I'm at it-- maybe they are just not smooth enough to LET the latex capstan do its job.


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