As in the UK, the mechanism was not without its problems. Some of the Union Pacific 9000 class locomotives were converted to a "double Walschaerts" valve gear, while later examples were built with roller bearings for the moving parts of the Gresley mechanism. In Australia, the VR S class Pacifics avoided many of the middle cylinder problems that beset Gresley by placing a 'set' in the axle of the leading driving wheels, thus allowing all three cylinders to be in the same horizontal plane. Later VR and NSWGR three cylinder locomotive designs used alternative mechanisms to the Gresley system in an effort to overcome its high maintenance overhead. The Victorian Railways H class of 1941 was fitted with a German Henschel und Sohn conjugated valve gear mechanism which was judged to be superior to the Gresley system, while in New South Wales the D58 class of 1950 utilised a rack and pinion system which while in theory an improvement over the Gresley system, proved in practice to be even more problematic.