Axle spring seats are attached to the axle using a U-bolt. The bolt loops over the top of the axle and is attached to a plate underneath by a pair of nuts. The axle spring seat, which sits under the axle, is held in place by the plate and has holes that allow hooks on the leaf springs to be attached to them. Can you install the seats yourself? Yes, but in most cases it's unnecessary. Axles for trailers are typically sold with the seats already welded in place.
But hat if something goes wrong with the axle spring seats? The results, obviously, would be disastrous. Without the seats serving as an attachment for the springs, the trailer would begin to vibrate wildly every time it hits even a small bump. The trailer would begin to shake apart and the axles would be in danger of falling out.
Should you worry about this? Probably not. Trailer service technicians say that they rarely, if ever, see problems with axle spring seats. When problems occur, usually in older trailers, they tend to involve the U-bolt. The ends of the bolt can wear down, causing the alignment of the axle to slip. There is some risk in this situation that the spring seat will become unattached. It is possible to buy a new U-bolt (and a new axle spring seat, if necessary) and attach it yourself. However, a better -- if more expensive -- option is to buy a completely new axle, complete with preinstalled spring seats. Unless you have absolute confidence in your ability install the new seat and to weld it in place, going with the new axle is safer.
For more information on axle spring seats and all things auto, please see the links on the next page.