It was reported on Dec. 20, 2022 that when Operations attempted to run the target ladder motor, it was unresponsive. A fault report was created: 15477.

During winter shutdown 2023, the T2 area was uncovered and the motor was inspected on 2023-01-11 by Adam Newsome and Maico Dalla Valle. The connectors and cabling appeared OK.

Upon removing the motor from its mounted position, it became clear that the coupling part between the motor and the gearbox had failed. The motor was running properly but not actually connected to the drive shaft and therefore nothing happened when ops tried to move the ladder. We confirmed that the motor runs properly in no-load condition when it is isolated from the rest of the assembly. We were able to move the rest of the drive shaft by hand and it does not appear to have any significant resistance, so it is suspected that this coupling failed due to radiation exposure over a long period of time rather than overload. The coupling part is plastic and designed intentionally to break under high loads (it is thinned out in the middle, where it snapped). Hopefully it did not actually see any high loads (i.e. there is no other root cause of this issue) and it simply degraded so much that it eventually broke.  

To resolve this issue, we will fabricate/purchase and install another one (Albert, Maico) and work with operations to test that all target ladder positions are reachable once it is installed. We will also confirm at that time that the limit switches and potentiometer readback function as expected.

It is recommended to consider the following changes to the drive system going forward to prevent this from happening in the future:

• Replace these plastic coupling parts with metal versions
• Replace the plastic coupling part that is thinned out to intentionally fail under high load with another method of system protection. The same end result could be achieved using a clutch or perhaps a fuse/circuit breaker which would effectively inhibit motion if the motor experienced overload. This would be a significant improvement over breakage because it would not require replacing the part (dose-intensive) and would be more easily recognizable.

For now, this drop-in replacement should suffice because it is not likely that the part will degrade so significantly within the next few years that this would happen again (unless there is a different root cause aside from radiation that we are not aware of!).

[update 2023-01-01] The broken coupler has been replaced by Albert Kong and Adam Newsome. The replacement part was tested - DCR operated the target ladder motor, moving it up and down through all of its positions multiple times. Everything mechanically and electrically appeared OK during motion. DCR confirmed they are able to reach each position and the proper readbacks are present - everything looks OK on their end. The issue is considered resolved and the fault is returned.