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HP Forum Archive 08

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An o-ring for my card reader
Message #1 Posted by Michael Reinold on 22 July 2002, 3:59 a.m.

The card reader of my HP-67 has the well-known "gummy-wheel-problem". For this purpose I bought numerous silicone fuel tubings to cut little o-rings from. But none of them has the right diameter! Can anyone tell me where I can get one with the right size? (I live in Germany).

Greetings Michael Reinold

      
Re: An o-ring for my card reader
Message #2 Posted by Ellis Easley on 22 July 2002, 4:21 a.m.,
in response to message #1 by Michael Reinold

I had good results with some silicone tubing I bought at a hobby shop specializing in radio controlled model cars and airplanes. The outside diameter measures .220 inch and the inside diameter measures .093 inch. There were no specifications available at the shop, they just had two sizes, large and small! When you say "O-ring", you might be thinking of the alternative method using actual rubber O-rings instead of a piece of tubing. I think the size of O-ring used is 1/4 inch outside diameter, 1/8 inch inside diameter. If my memory serves me, it is a standard #5 O-ring. Two O-rings are used to replace the wheel. There are two regulars on this forum who have sold the parts for very little cost on Ebay - one guy sells the O-rings, the other guy sells the tubing, both supply detailed instructions. Mike Davis's tubing method is described in this Museum Article. Mark Hoskins describes using O-rings to repair an HP41C card reader in this Museum Article.

            
Re: An o-ring for my card reader
Message #3 Posted by Tony Duell (UK) on 22 July 2002, 6:47 p.m.,
in response to message #2 by Ellis Easley

I've found it much easier to use the O-rings than the silicone rubber tubing. It's often hard to find tubing where the the bore is truely concentric with the outside. I use silicone rubber O-rings, size 005 (15/64" OD, 7/64" ID, 1/16" thick). Size 006 (1/4" OD, 1/8" ID) are rather too sloppy a fit for a card reader roller (although they are ideal for HP9100 keyboards). I've fixed both my 41 card readers, both my 67s, my 65 and my 97 that way with no problems at all (and yes, I do use all of them!).

                  
Re: An o-ring for my card reader
Message #4 Posted by Ellis Easley on 23 July 2002, 4:37 a.m.,
in response to message #3 by Tony Duell (UK)

I seem to have the #5 right but the diameters wrong!

The SmallParts.com catalog doesn't refer to the number sizes explicitly but they do have three digits at the end of their part numbers that are the same for O-rings of given dimensions - 1/4"OD and 1/8"ID, 1/16" thick is -006 in all materials - but the dimensions that they give corresponding to -005 are 7/32"OD and 3/32"ID, 1/16" thick. Do you think this is the same as you are using?

I have yet to do a card reader repair with O-rings. (I've only done one period!) I have replaced all the O-rings on my 9100 keyboard - the feel that results gives a satisfaction similar to that of a well-built car door closing. Transports me back to 1968 (or 1969 actually, mine is a B). I'm glad to learn about the silicone rubber O-rings. I think this property listed in the catalog: "Excellent resistance to compression set" will be very advantageous for the card reader wheel application.

The SmallParts catalog has O-rings in a variety of materials including silicone rubber, also teflon (I don't imagine that would be any good for this application!) and a material called Viton which has this stern warning:

"Great care should be taken in regard to burning Viton. When burned it gives off highly carcinogenic hydrogen fluoride. Merely handling just-burned Viton can be dangerous."

                        
p.s re: my tubing dimensions
Message #5 Posted by Ellis Easley on 23 July 2002, 5:25 a.m.,
in response to message #4 by Ellis Easley

I just multiplied the OD (.220") and ID (.093") (which I measured on my silicone rubber tubing with my trusty dial calipers) by 64 and got OD: 14.08/64" and ID: 5.952/64, surprisingly close (within 1%) to 7/32" and 3/32".

While the results I got with the tubing are satisfactory, there is a certain amount of "lugging" that is probably due to the irregularity of the wall thickness. I had planned on building a fixture with a dial indicator to select uniform bits of tubing before rebuilding my other units. But now I think I'm going to get the silicone rubber O-rings from SmallParts.com. For one thing, they are burnt orange in color, and I attended UT for a while (to everyone's dismay).

                              
Re: p.s re: my tubing dimensions
Message #6 Posted by Katie on 24 July 2002, 2:37 a.m.,
in response to message #5 by Ellis Easley

If you're going to order form Small Parts, you should get their silicone rubber tubing 1/4" OD x 1/8" ID -- part # 210005, 10' for $24. It's perfectly symetric, works much better than o-rings (in my experience) and doesn't need to be glued in place. I've done zillions of card reader repairs using this tubing and it always works and has no noticeable wear after 2+ years of use.

Lots of other collectors here have tried it too with very good resutls.

                                    
Re: p.s re: my tubing dimensions
Message #7 Posted by Ellis Easley on 24 July 2002, 2:49 p.m.,
in response to message #6 by Katie

I might order both silicone tubing and O-rings. The silicone tubing I have now works OK although it "sounds" like it's not extremely concentric. The 1/4" OD is slightly larger than the tubing I have now, which seems to be big enough to grip the card. Have you by any chance measured the diameters (ID and OD) of your tubing? Might rubber tubing be made slightly smaller than the nominal dimension, since it needs to be stretched a little to do its job?

Also, since reading an HP Journal article Dave Hicks pointed to, about the development of the HP65 card reader, I've been wanting to try polyurethane tubing. The Journal article said a type of polyurethane is what they settled on for the wheel because its operation wasn't affectd by oil on the card. SmallParts has polyurethane tubing in 1/4" OD/1/8" ID, P/N B207005. It costs half as much as the silicone. I've never seen polyurethane tubing (that I know of for sure) but I think it might be what comes with the "MityVac" vacuum pump. That tubing is extremely transparent. One of the qualities of polyurethane mentioned in the SmallParts catalog and the HP Journal article is resistance to oil which would be important for working on cars. The tubing that comes with the pump feels like a material that would be too hard for the purpose, although if the thickness and uniformity is just right, the hardness of the material shouldn't matter much. The pump tubing might be PVC. The pump booklet doesn't say anything about it.

                                          
Re: p.s re: my tubing dimensions
Message #8 Posted by Katie on 25 July 2002, 3:05 a.m.,
in response to message #7 by Ellis Easley

The Small Parts silicone tubing measures a true 1/4" x 1/8" (as far as I'm able to read on calipers without squishing the tubing). It seems to be impervious to most oils and solvents that I've tried on it and it is very close to the original HP tubing as far as transparency goes. I've tried lots of other tubing before finding this -- it's really the perfect stuff to use.

                                    
Re: p.s re: my tubing dimensions
Message #9 Posted by Achim on 25 July 2002, 7:34 a.m.,
in response to message #6 by Katie

>I've done zillions of card reader repairs ...

Then you and your calcs are quadrillions of years old, right? I ever dreamed of a stone-age HP with flintstone display and marble card reader ;-))

            
Re: An o-ring for my card reader
Message #10 Posted by Dave the "lostrabbit" on 23 July 2002, 12:25 a.m.,
in response to message #2 by Ellis Easley

SELlING the tubing and O rings on ebay? Geez, I have sent a bit of tubing and a couple of O rings all around the Planet for nothing but a postcard in return! It's not like an O ring is worth the price of a share of worldcom stock, ya know!

                  
Re: An o-ring for my card reader
Message #11 Posted by Ellis Easley on 23 July 2002, 3:42 a.m.,
in response to message #10 by Dave the "lostrabbit"

Since I brought it up, I need to point out that when I saw their auctions for these items, they had listed them for about $1 (for a few inches of tubing or several O-rings) and filled their auction pages with instructional information. To me, it looked much more like "getting the word out" than profiteering. Someone searching for a replacement card reader or calculator on Ebay might have seen these auctions and then might have enjoyed the great pleasure that comes from fixing something instead of throwing it away. Also, it can be hard to find the right size, as the original post illustrates.


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