A stub axle is an axle that is connected to an assembly that mounts on one side of a trailer. It does not go all the way across the trailer like a typical straight axle does. The stub axle typically is part of an assembly that includes the spring and shock mounts and is connected so that it can move freely as an integral part of the trailer's suspension.
By using a stub axle assembly on a trailer, it is able to traverse rough roads without disturbing its load. The axle works much like an independent suspension on a sports car and allows each tire and wheel assembly to move independently. This drastically improves cornering as well as braking characteristics, especially on slippery roads.
Most units are cast into the axle assembly and the entire piece is one single part, although others are independent assemblies that fit into a receptacle on the trailer and are a separate component. In either case, the axle typically has a single wheel and tire bolted to it. Occasionally, this axle can be used in a dual-wheel application as well.
The stub axle, in most cases, is easily replaced if broken. Most axles are bolted into a mounting assembly and can be simply removed by loosening a few bolts and sliding the broken stub out of the housing. In the cast units, the entire unit with stub must be replaced. This typically requires the removal of the springs and shocks as well.
While some axle failures can be welded, this is not advised in most cases. The stress and leverage on a stub axle is so great that many welded ones can break during a typical turn. As the trailer is turned, the axle has to slide the tire throughout the entire turn. This causes the repaired stub to strain and pull against the weld. In most cases, failure will occur with the axle breaking off at the weld.
In the case of a single cast piece, the cast iron is not a good candidate for welding either, and this type of repair should never be attempted. The porous nature of the cast iron creates many weak spots in a weld area. Catastrophic failure is sure to arise from any attempted welding. If in doubt of which type of axle assembly is in place, contact the trailer manufacturer or read the owner's manual.