Wear and tear!
Having worked on hundreds of Hobart commercial units sharing the same parts as the 15 thru 20 series domestic machines, I have seen my share of worn out arms.
Basically, the bushings will wear away inside and the arm will develop a pronounced wobble when you spin it. You won't hear anything when it is running but it will slow down a bit which can affect washing performance. The black plastic of the hub will wear out from rubbing against the top of the support and many times it will become so worn that the bushing will begin to be pushed out of the wash arm hub itself and extend above the top of the wash arm.
Hobart began installing a stainless steel washer over the shaft on the top of the support which really slowed down the wear on the wash arm hub and if you were to find a NIB support for your machine, it will likely have that washer as well as the white plastic top piece instead of the bronze top. But you will still find some of the original style ones out there as well.
I will try to find some of the more worn out arms and take a picture of them to illustrate the wear pattern I mentioned above. I have plenty of used,good condition arms as well as a brand new one if you are interested. The problem is finding a properly sized and not oversized carton to ship them in. Nowadays, shipping can cost more than the item you buy!
