That is an uncommon failure for a TPDD. I have seen that same type of failure on scads of other plastic parts pressed onto metal shafts. The plastic shrinks as it ages, the metal does not and eventually the plastic hub cracks.
It is hard to tell, but it looks like the hub is a separate piece from the pulley and staked in place. Perhaps they are two different materials? I suspect the only way to fix this would be to bore out the hub at least 1-2mm and glue in a compatible piece of plastic with an ID suitable for pressing onto the shaft and an OD that is a tight fit into the bored hole. Jeff Birt -----Original Message----- From: M100 <[email protected]> On Behalf Of Brian K. White Sent: Thursday, March 21, 2024 3:17 AM To: [email protected] Subject: [M100] unexpected tpdd failure mode Encountered an unexpected way for a tpdd to fail. I have a Purple Computing drive I got a couple years ago that never worked. But it almost worked. The cpu and sensors and motor control actually all worked fine. You can send it commands and it responds correctly, you can read the state of the sensors, you can make both motors run on command. The only problem was actually reading or writing data never worked. I always assumed that it was going to be some exotic problem with the head read amplifier circuit or something like that that will require the scope to hunt down. I was in the process of swapping the guts out from a parts-only busted up tpdd1 from ebay and along the way the big pulley and main shaft basically fell out. So the problem all along was the disk grabber hub was spinning freely loose on the main shaft. Turns out the disk grabber hub is plastic and just pressed onto the shaft, and the plastic had cracked around the shaft. Overall the hub is still whole, just the bore is no longer an interference fit, it's now merely an exact fit. It's hard to see this problem because all the parts you can see are working fine. There's almost no way to see any of the hub or the disk when it's working to see that it's not spinning when the big pulley is spinning. The way you can tell is if the big pulley can be pulled out at all. Normally it can spin but not pull in/out at all. I don't know what's a good way to glue the hub back to the shaft. It's a smooth polished bearing surface shaft, so I tried super glue but I expect it'll just break free as soon as I try to use it, but I'll let it sit for a day before I try. Here's a couple pics of the parts since you can't normally see these parts. The main spindle has a single ball bearing and a plain bushing or possibly sintered bearing. The brass part in the middle is both the holder for the ball bearing on the disk side, and the plain bushing bearing on the belt side. https://photos.app.goo.gl/QbirJowcGEEdV3xb9 -- bkw
