When did Hydro drives come into play and get perfected?

[MHarryE]

As mentioned above, the IH 656 in 1967 was the first major hydro. It had problems and did not really catch on. They aren't as efficient as straight gear drives but they make up for it in versitility where frequent changes in speed and/or direction are required. They do demand clean, high quality oil that is well cooled. Cooling - you are at the mercy of the manufacturer.