I've had several skid steers, all hydro-static. Never had a chance to dive into one before, then the Allis 720 came along with a blown Hydro. I figured since it was gimped already no harm in taking it apart. My only regret was cleaning it up before disassembling, from what the previous owner told me thought it was toast.
After disassembly found a few brass slippers had broken free from the pistons, the broken bits jammed into the rotating internal parts seizing the unit solid. Inside the hydro are two cylinder blocks, the nine pistons fit parallel. The brass slippers work against the swash plate.
A small gear pump called a charge pump supplies oil to the Hydro pump, oil from there goes to the motor section, both the hydro pump and motor share the same piston assembly.
Note: The charge pump also supplies oil for aux hydraulics, but the hydro has first demand on the oil supply.
Pistons on the motor cylinder run against a swash plate, when the hydro is at rest ( neutral ) dead centre of plane there is no action against the piston slippers. When the Hydro is set to revers the swash plate over centres on its pivoting point to say the left, the hydraulic oil circuit is reversed, go forward the swash plate over centres to the right.
A little to the right or left give slow mobility, fell right on the swash plate will give you full speed ahead.
Ok, now we know how the hydraulic oil is regulated, the motor end pistons run against a fixed swash plate, oil fed to the motor cylinder will respond to the volume of oil supplied from the hydro pump depending on the angle of the swash plate.. The motor piston cylinder is splined to accept the output shaft, for tractor use the shaft has a pinion gear.
If you attempt a hydro rebuild chances are there's a youtube to assist.
Below are motor housings with fixed swash plates, the surface on mine was damaged from the broken brass piston slippers. I could have probably used it but I'm a bit to critical.
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