The Hobart cast iron wash arm was the wash arm used in commercial machines. They did not fail. The larger bronze bearing held much better in cast iron than the smaller one holds in the Bakelite. The wash arms at the top of the tank in the commercial machines had a hole in the hub for the bolt and nut assembly that held them in place. The cast iron also developed a film of oxidation and minerals that protected the cast iron from further oxidation where it was not coated. The 4 Way Hydrosweep which debuted in the 15 series has a much smaller and different style bearing and support system. The Bakelite hub wears as this bearing fails and usually the wash arm support and the wash arm have to be replaced. When the bearing wears, it not only allows water to spray out under the hub, but the wobbling arm destroys the support. If people put dishes with enough food soil in the dishwasher, don't over use detergent and don't have their water heater set to 160F, the wash arm and support can last for decades. The test to see if the wash arm needs replacing is to give it a flip of the hand while it is dry and watch how it spins. If it spins smoothly and freely and coasts to a stop, it is fine. If it makes noise and shudders instead of spinning, it needs servicing. Finally, the taller tub of the 15 series mandated a shorter motor & pump. The 14 series had to pump with greater force to make the larger sprays forceful enough to wash well and it used more water to keep the sump full enough to supply that powerful pump that was moving a lot of water. If you remember the first GE wash arm machines had a wash arm with the raised bumps surrounding the slits in the wash arm similar to the Hobart design. They were absolutely miserable at washing because they did not have a filter like the KitchenAids and their cheaper motor ran at half the speed of the Hobart motor and powered a much less efficient pump. The 15 had less space under the tank and made up for the design by having the pump force water through smaller holes to give more power. Hobart did give the smaller holes a design tweak in that each hole is surrounded by a slight dimple that forces the column of water to fan out into a wider pattern than the hole from which it emerges
As to performance, the 4 Way Hydrosweep was needed to wash in the larger tank. The finer holes allow water to be directed in precise patterns. The Big Blue Hobart Wash Arm produced "6 Moving Walls of Water" which were very effective in the smaller tanks up to the 15 series, although the water distribution did not give the more surgical precision of the Hydrosweep. The fan spray had advantages. For instance, I would place the wire rack from my Farberware Open Hearth Broiler on the center row of loops in the lower rack of the 14 and prevent it from tipping by putting the body of the broiler over it. In a normal wash cycle with the two detergent washes and three rinses, the 14 with the fan-shaped spray pattern cleaned the rack uniformly while the 18, with the rack in the same position in the lower rack and the more precisely directed spray pattern left some small places that needed further cleaning. Perhaps if I were to bother to rest it over saucers to hold it a bit higher, the water pattern would even out to be more like the fan spray of the 14. I use one of the Jenn-Air ceramic fake briquette plates under the heating element of the Farber to give a small amount of smoke flavor to the generally very lean foods cook on it and that washes well in both machines, just standing in the rack, but angled to that the soiled side faces down, the reverse of the way plates would face in the rack.
