Welding shaft and wheel assembly for universal wheel, and universal wheel

The welding shaft for a universal wheel includes a big wheel welding shaft and a small wheel welding shaft, wherein the big wheel welding shaft includes a big wheel welding shaft body, a big wheel welding shaft retainer ring and a big wheel welding shaft fixing head which are sequentially assembled into a whole. The small wheel welding shaft includes a small wheel welding shaft body, a small wheel welding shaft retainer ring and a small wheel welding shaft fixing head which are sequentially assembled into a whole. The wheel assembly for a universal wheel includes a wheel shaft fixing plate, the welding shaft, a big wheel, and a small wheel. The universal wheel includes an outer protection plate, an outer fixing plate, a wheel shaft drive connector, multiple wheel assemblies, an inner fixing plate, an inner protection plate, and multiple shock absorption assemblies.