spray arm would not spin

Even though there was plenty of water pressure from the pump, the lower arm would not spin. I could move it with my hand but there was too much friction for it to spin. Turns out an item named seal ring[38] (I would call it a slip ring) had become dislodged somehow at the interface between the rotating parts and the stationary parts but I did not learn that until I was able to disassemble it. I could see from the diagram that there were threads at the top of the SPRAY ARM SUPPORT[39] that engaged with the threads in the CENTER SPRAY NOZZLE EXTENSION[33] so I held the arm and tried to unscrew [33]. CRACK! The 4 thin plastic arms in [39] snapped right off. I then disassembled and found the dislodged seal ring[38]. I concluded that I would never have been able to disassemble without breaking [39] due to the calcium in the threads (even with repeated applications of vinegar or lime away): the plastic arms are just too wimpy. But the price of a new [39] was a bargain, I was amazed. Came in 2 days, I re-assembled, now the spray arm spins freely and the dishwasher works again. I will try to remember to use vinegar periodically incase the slip ring dislodges again. This has been a great dishwasher otherwise IMHO( and a bargain price).