HP card reader drive coupling

[GreyUser]

I don’t want to hijack the OP’s thread so I’m starting this one to continue discussion. With respect to Geoff’s comment over in the HP-25 keyboard thread:

(09-26-2021 10:41 PM)Geoff Quickfall Wrote:  As GreyUser accurately points out, the ‘pores’ are vias. I just didn’t want to assume everyone here including me is an electronics engineer ;-)

Just as the card reader does not have a clutch but rather a ‘dampening couple’ used to prevent vibration from the motor transferring to the card as it is read.

Cheers, Geoff

Well, it’s certainly not a clutch but practically speaking, it does not pass muster as a vibration dampener. It’s just a coupling. There is no dampening going on there.

If we want to call it vibration dampener, what vibration does the arrangement dampen? More importantly, how does it dampen vibration? The end of the lead screw fits directly onto the tip of the motor shaft, the urethane inside the aluminum sleeve is there to add friction and to help center the screw on the motor shaft. The lead screw was cemented onto the motor shaft, likely an with cyanoacrylate adhesive or similar.

If the designers were concerned about vibration, it would not be hard coupled, it would float in some way. Have you ever seen a true vibration dampening coupling? They are also known as flexible couplings. They are typically three pieces. Two shaft hubs with something in between to transmit the torque through the medium. Soft plastics are used in low torque applications, flexible metal grids are used on motors into the megawatt range.

Vibration is created from misalignment between a motor and the shaft it is driving. A vibration dampening coupling allows for that misalignment. But, that’s not what we have in a card reader. It’s hard coupled.

I suppose if you wanted to torture the verbiage, one could say it is a vibration dampener - because it centers the driven shaft on the driving shaft, thereby reducing vibration. I’m not sure but that sounds like recursive thinking to me.

It took years to dispense with the notion it was a clutch. Perhaps it will take a few more to bury the vibration dampener canard. That is of course if you agree with me. If I’m just being pedantic, feel free to tell what you think. If not, let’s call this thing by its proper name. It’s a coupling.

[teenix]

I mentioned this previously and agree it is not a dampener or a clutch , it is simply a shaft coupler. Considering the small diameter of the motor shaft, it would be difficult to make it any other way. The coupler is also small and quite hard so is more likely to transfer motor vibration.

I made some couplers to replace some degraded ones and if the center hole is off a bit, the shaft does obviously vibrate. However, if not too great the cards still read/write fine. The material I make it out of is softer than the original coupler, but still doesn't dampen anything. Vibration is bad of course as it is likely to damage the aging motor bearings, coupler, end float bearing and increase gear tooth wear.

HP specifies a test to adjust the pressure applied from the drive wheel to the cards and idler roller, where you have to stop the card feeding into the reader and measure and adjust the motor current. I gather this makes the drive wheel slip on the card while the adjustment is being made. Hence, why I don't consider the coupler a clutch otherwise it would slip.

Probably not a wise test nowadays as the original coupler is degrading into a crumbly material and it may fail.

I don't think HP decided that the motor would be in a stalled scenario in normal card use, so a clutch is probably not warranted. I don't think the sense chip has motor high current detect so some overheating and damage might result.

My 2 cents, I can't afford anymore ;-)

cheers

Tony

[Geoff Quickfall]

We can all agree it is not a clutch, never was. The use of the word dampening coupler was anecdotical from the last 10 years at HCC meetings. Including a discussion with people associated with the design.

I thought I saw it in a manual but having perused the HP-97 and 41C card reader service manual I do not see it referred to except as the drive mechanism.

Since it is made out of the same material as the original pinch roller the thought was, prior to it decomposing that it would take out some vibration including at a higher frequency. As it decomposes, I have found that it self destructs in two ways: it hardens, then crumbles or it turns into goo. In both cases it slips and does not transfer the torque from the motor to the worm gear. The assumption was that in its pristine shape (New from the factory) that it had dampening qualities due to it being more pliable. It certainly did not age well as seen in so many HP applications such as the 97 printer, 9825a tape drives, all card readers and etc. Again only anecdotal from conversations at HCC.

None of this is explained in the manual but only referred to as a couple, never a clutch.

Cheers, Geoff

[Divasson]

I have found that this coupling's failure is a frequent problem (once solved the gummy wheel issue). In some cases it is seen as cards not being drawn; but at others, it is just enough to have small speed differences, resulting in a "MALFUNCTION" message despite passing the card.

I have used successfully E6000 glue on several samples recently (after cleaning the old material residue). Its elasticity avoids fragility, and it "sounds" nicer than normal.

[Geoff Quickfall]

I take it then, that you leave the old urethane couple in place to centre the shaft?

So what are your steps?

Remove the old urethane couple and glue shaft in place (how do you centre the shaft)?

Leave the old urethane couple in place to centre the shaft. Place glue in the centre of the old couple then insert the shaft?

Or…

Cheers

[Kees Bouw]

(09-29-2021 05:25 PM)Divasson Wrote:  I have used successfully E6000 glue on several samples recently (after cleaning the old material residue). Its elasticity avoids fragility, and it "sounds" nicer than normal.

Very interesting. I recently repaired a HP-67 coupling (after removing all the old material) with a combination of wire insulation, shrink tubing (without shrinking it) and cling film to achieve a tight enough fit while still being somewhat flexible.
But using something that resembles the original construction, like elastic glue would be a much better solution. However, as Geoff mentioned, the big problem is how to center the shaft.

[Divasson]

Hi,

I first get rid of the old material, making sure to get all residue out, and also trying not to deform the small Aluminium cylinder. This is the difficult part for the clumsy operator I am. However, perfect shape is not critical.

Then I put a good drop of E6000 glue in the hole of the cylinder. Then I assemble the motor and the shaft together into the cylinder, making sure of eliminating all excess of glue on the motor side (to avoid gluing the motor body to the shaft!).

In a moment it takes enough “body” to keep shape. Then I align it by sight. I wait for some more time and then I ensure straightness rotating the shaft and making sure it rotates well. Then I let it dry for at least one day.

Any micro-deviation from total straightness is absorbed by the E6000 elasticity. Also remember that the other extreme of the shaft is captured by the small hole in the card reader head’s body, so you know it will work fine - and it does!

[Kees Bouw]

(09-30-2021 03:34 PM)Divasson Wrote:  Any micro-deviation from total straightness is absorbed by the E6000 elasticity. Also remember that the other extreme of the shaft is captured by the small hole in the card reader head’s body, so you know it will work fine - and it does!

Do you think that the motor can still keep running in the event of the worm gear getting stuck for some reason? I think that was part of the original purpose of the coupling.

[Divasson]

With the E6000 glue the motor will not be able to run. That may be a risk- I don’t know if there is an electrical or firmware provision to avoid burning the motor. However it the shaft is stuck you already have a problem and you need to open and repair anyway.

So far I haven’t had any problem with this kind of repair yet - on a sample of admittedly just 6 members!

[Sylvain Cote]

(09-30-2021 07:25 AM)Kees Bouw Wrote:  I recently repaired a HP-67 coupling (after removing all the old material) with a combination of wire insulation, shrink tubing (without shrinking it) and cling film to achieve a tight enough fit while still being somewhat flexible.

Similar to yours and Geoff solutions, I have successfully repaired several HP-41 card readers with two different wire insulation gauge.

[Geoff Quickfall]

It is not a clutch! It was not supposed to run with a jammed card. Cards were not supposed to jam ;-) Strictly a couple. HP decided that a jammed card would not burn out the motor as the user is right there inserting the card and could turn the machine off.

Off course I have not tested this info!

Again anecdotal in conversation with HP designers of the 41C version.

Rinsing with IPA: I use a lab plastic wash bottle with nozzle and also IPA in a plastic spray bottle which produces a jet for those hard to get places. Also the IPA is excellent for removing the gummy residue if your pinch roller and couple decomposed the gummy way and not the hardened crumble way!

Cheers, Geoff

Example of extreme gummy wheel:



Motor assembly and couple; Label C is the original couple which in this exampled hardened and no longer gripped the motor shaft. This caused slippage as the card passed through the pinch roller. It is a good idea to repair both at the same time. Some have added a dab of cyanoacrylate glue (super glue) to the motor shaft and reinserted into the old hardened couple. I don’t what the long term efficacy of that solution is. So far all my repairs with the insulation method (15 HP41C readers, 12 HP 65 and 9 HP 67 readers) have not failed over the last 12 years. However they do not get a lot of use. I did check my versions and they all work perfectly:



Example of insulation method for couple with no glue; the inner red insulation is half the length of the outer white insulation and is inserted into the white insulation. This is due to the fact the worm gear shaft and the motor shaft are different diameters.



Completed assembly: