Ray tech can correct me if I am mistaken, but in narrow aisle and 3 wheel trucks, the "radial ring" is the support bearing races* and/or the large support bearing the drive unit rides upon (in 3 wheel trucks think 'the steering support bearing') and "cluster cylinder" are the multi-cylinder packs that do the lifting.

I say the races, since the ball bearings themselves can often be replaced without removing the race (but _I_ almost never do it that way, if the **** fell out, something else is wrong too), and removing the race requires lifting the truck off the drive unit, which safety dictates requires something to stabilize the top of the truck on reach and order pickers, since the max permitted height to jack up the truck without support for the upright is only about 2 inches, if you read the manual.