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I'm looking at a vintage aerospace computer that has strange nuts on the PCB connectors, but I can't figure out how they work. The nut is mostly split in half, with locking pins (?) sticking out of the shaft. Does anyone recognize these nuts? The nuts seem way too far from the PCB to be useful fasteners and I don't see how they would be threaded. Maybe they aren't nuts at all, but the ends of concentric ejector shafts?

PCB connectors with mysterious nuts

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    \$\begingroup\$ They might not be threaded. I've never seen these, but they look easy enough to make custom at a machine shop (drill out the threads of a nut, cut that notch in it), and if there was sufficient need they could surely be mass-produced. I couldn't guess at the reason for using them though. \$\endgroup\$ Commented Apr 12 at 17:19
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    \$\begingroup\$ it looks like a screw with a shear pin to prevent overtightening \$\endgroup\$ Commented Apr 13 at 18:19
  • \$\begingroup\$ Would a bicycle chain breaker fit on to push the pin out? However, I suspect the value of the equipment is too great to chance non-reversible disassembly. Is the other end of the fastener visible? \$\endgroup\$ Commented Apr 17 at 17:36

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Does the slot we can't see go all the way through the nut?

If so then it's basically a bayonet connection: press against spring pressure and turn 180 degrees, and the nut either comes off or is captive but in either case allows the pin to pass through and the PCB to be ejected from the front.

Assuming that my interpretation is correct... I've never actually seen anything like this, and it clearly isn't intended for field repairs or any situation in which the back and front of the rack can't both be accessed with ease. It has the advantage that once unlocked boards can be swapped without continually unlocking/locking, but the disadvantage that a harassed technician could potentially forget to lock the boards before returning the assy to a high-vibration environment.

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  • \$\begingroup\$ Given the space between the nut and the casing that seems quite possible. The nut may not even need to come of, as long as the pin can slide through the slot. \$\endgroup\$ Commented Apr 13 at 14:12
  • \$\begingroup\$ @jcaron yes, that occurred to me after I wrote and I will edit appropriately. I was originally thinking that the pin was turned from the other side by a special (long) tool, but as somebody else points out one can see marks from the nuts having been turned against the backplane. \$\endgroup\$ Commented Apr 13 at 17:15
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    \$\begingroup\$ That's an interesting suggestion. Unfortunately, I checked and this doesn't work. The nut does not move inward and there are pins on both sides, so the nut doesn't get released. \$\endgroup\$ Commented Apr 14 at 0:03
  • \$\begingroup\$ That's weird. But there's a mark on the LH nut which suggests that it /has/ been pushed and turned, and- possibly irrelevant- it's possibly not at the same angle as the RH one. \$\endgroup\$ Commented Apr 14 at 6:50
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From the marks on the backplane PCB I'd guess that if you unscrew them they'd bear on the back of the backplane PCB and force the module out of the connector.

The construction might just be an artifact of how they were made-- the inner shaft is assembled into the outer tube, the nut dropped over it and a dowel pin pressed into the cross-drilled hole to retain it.

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  • \$\begingroup\$ Since Ken tells us that the pin protrudes on both sides etc., I'm beginning to lean towards the pressed-in-pin option, although that wouldn't explain the scuff mark which looks as though the nut has been depressed against spring pressure and turned. \$\endgroup\$ Commented Apr 14 at 10:56
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Is it a castle nut? I've seen castle nuts used with safety wire on helicopters to prevent loosening of the screws under vibration:

Castle nut with safety wire

Alternatively you can use a castle nut and cotter pin:

Castle nut with cotter pin

Images from Safety wiring and cotter pinning, general practices for.

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  • \$\begingroup\$ Any post about safety nuts on helicopters isn't complete without a reference to the aptly named en.wikipedia.org/wiki/Jesus_nut \$\endgroup\$ Commented yesterday
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could be some sort of reverse lock nut? try putting a pin through the hole in order to lock the nut then try to turn the nut

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