Problem(s) Encountered:

At closing time, the telescope would not move to the zenith using the TCS GUI. We moved the telescope to the zenith using the hand paddle.  The rest of the shutdown procedure went as normal.  No time was lost.

Solution:

With all the proverbial dust settled:
Found that the 4 1/4-20 bolts that hold the RA drive preload plate to the roller had failed.  Three of the 4 bolts had sheered.  The 4th bolt was pulled through the preload plate.  This culminated in the roller that is physically connected to the secondary drive wheel walking out (south) roughly 1.5cm.  The secondary drive wheel came into contact with the assembly holding the RA drive motor and pulleys, leading to greatly increased friction and reduced mobility.  This led to torque-limited draw on the motor and possible damage to the motor encoder (although there’s also a chance it was ok and I damaged it when we were focused on the issue being with the encoder).  Regardless, the motor encoder has been replaced.  Thankfully the sheered bolts were able to be removed from the end of the roller.  The telescope fork was lifted (two hydraulic jacks pumped to ~9,000 lbs per), releasing contact between the primary drive wheel and the roller.  The roller was then properly repositioned and tested to verify that the bearings are in good shape.  With the telescope lowered and contact reestablished between the primary drive wheel and the roller, the preload plate was reconnected using stronger bolts in a temporary configuration—upside down with 3 washers per bolt.  This was required since one of the bolt holes was pushed through.  Telescope has been successfully back in service since then.I believe the problem occurred due to those 4 bolts holding the preload plate failing.  The bolts that failed were grade 2 instead of the demanded grade 8 in strength.  It is my belief that when the bearings were replaced coincident to the TCS upgrade in October2017, the wrong bolts were used to attach the preload plate and that they have been slowly failing over the last 18 months.  With the proper bolts installed, it is my belief that the system is now fixed.  Still, I will check things periodically to watch for any more walking, which may indicate skew misalignment.  The last thing left to do, which will be done today, is to drill a new bolt pattern into the preload plate so it can be mounted in the correct orientation.