What we think of as new has probably been done before if you take time to search for it. Google patents is the easiest way to search the USPTO content. We use patents as a time stamped proof of design activity which is of course traceable and a legal document.
Some Skid Steer Loader History
Brothers Louis J Keller & Cyril N Keller both inducted into the National Inventors Hall of Fame.
https://www.invent.org/inductees/cyril-keller
https://www.invent.org/inductees/louis-keller
If you know about this machine then you know what an incredible invention this was, it changed small agricultural and construction loaders for ever.
The single armed skid steer loader was a JCB invention to help with operator safety when entering or exiting the skid steer. At the time around two fatalities a year were taking place when operators were entering or exiting the skid steer loader and they accidentally actuated the loader arm trapping them between the loader arm and the cab front entrance.
The JCB skid steer loader design had a single sided loader arm (like the JCB Si-Draulic single sided Loader tractor attachment you may have seen from around 1953) which allowed a cab with side door entry.
Originally all skid steer loader had radial loader arms (i.e. the arms rotate around a fixed pin axis). This type of loader arm is very simple and easy to manufacture, and moves the bucket in a path expected by the operator.
Vertical lift path skid steer loader are very popular today as they give the bucket more forward reach at height than a radial arm thus helping the loading of a high sided trailer. The downside is to get the vertical lift path a 4 bar link mechanism is needed to allow the loader to follow this motion. The path is not linear but a series of curves approximated as close as possible to a straight line.
Frank Moeller invented the mechanism shown below applied to a single armed skid steer loader.
