HP67 repair squelch

[teenix]

(01-17-2020 02:55 PM)burkhard Wrote:  It's not a clutch, but a coupler. If it acts like a clutch, it has failed. There should never be slippage.

The job of this item is to transfer the motor shaft rotation to the worm gear drive, ie - a coupler.

I suspect some head scratching went on when the first motors arrived to test in HP's labs. With such a small diameter motor shaft, I'm guessing it would be difficult to manufacture screw mount couplings or machine it in any way for keyways and the like. The end solution was simple and elegant and does its job well.

The contact area between the motor shaft and the coupler is very small so it can slip under high load. However, if the coupler is in good condition, a card jam can cause motor stall without slippage. The microcode senses jams (CRC for 65) and hopefully turns off the motor before damage is done.

The drive wheel can slip on the card if worn. This is also allowed to occur as it is written up in the service manual. For example, holding the cards while testing for motor current. In this case the drive wheel is most likely slipping otherwise it has to be the coupler. I personally wouldn't do this test as the coupler may let go on the shaft and cause wear.

Vibration may be an issue but I don't think it is quelled by the coupler as it is made of quite dense material and as mentioned, the contact area is very small and it is in tight contact with the shaft. The shaft turns at 10,000rpm and the data transfer rate is about 1KHz. Unless the vibration is large I can't see much of a problem. I think the main source of vibration is not from the motor itself, but from the rpm reduction mechanism which is actually on the card side of the coupler. The chosen worm gear drive is the simplest reduction method with just one helical gear and therefore reasonably quiet with minimal backlash.

These mechanical marvels have been around for 46 years now and I'm sure some vibrate a lot more than when originally manufactured, but they still work. I've even fitted a dodgy coupler and it vibrated quite a bit, but still worked.

Worst case vibration might cause card flutter against the head but that would have to be pretty bad.

I think card azimuth alignment would be a worse condition, and could be caused by weak or missing card side load springs.

Just an opinion :-)

cheers

Tony