Lubrication in pump and motor components is vital to the operation of the hydrostatic system. (Fig. 7) illustrates the design of a typical piston. A slipper is crimped to the end of the piston and works as a ball and socket keeping a flush contact with the swash plate, regardless the angle. The center of the piston and slipper is drilled with a small hole to allow a metered amount of oil to flow to the ball and slipper contact areas so these parts always float on a thin cushion of oil. As oil pressure behind the piston increases, flow through the port increases to provide more oil to maintain the oil cushion. Additionally, the piston and cylinder walls are finished to very close tolerances which allows the hydraulic oil to lubricate and seal this critical area.
VARIATIONS ON HYDROSTATIC TRANSMISSIONS
The stationary center hub contains the passages necessary to transfer the oil between pump and motor cylinder blocks. The ball piston positions are determined by a bearing like raceway encircling the cylinder block. When the pump race is positioned so the stationary hub is at its center, no movement of the ball pistons occurs in their cylinders.
Oil pressure and flow to the motor block begins when the pump race is moved away from its centered position by the tractors control apparatus. This causes the ball pistons to stroke in and out of their cylinders which are filled with oil, thus creating the necessary pressure. The motor race is permanently positioned off center to provide the fixed angle which results in the motor piston block rotation when the pistons react to the pressure applied to their cylinders. It should be noted that efficiency of the ball piston hydrostatic depends upon the close fit of each ball to its own cylinder. Extreme care must be taken when making repairs to these units to assure the original position of those parts are maintained.
Lubrication of the ball piston increases as system pressures rise, providing more oil when lubrication requirements rise. Oil that is utilized for lubrication would soon cause the hydrostatics to cease functioning if there were no way to replenish that volume of fluid œlost™ to lubrication. We have seen that two passages are required between pump and motor rotating groups for flow to take place. Check valves leading to each passage allows additional oil to enter either passage when in a negative pressure (vacuum) condition.
[Source: Simplicity publication, Hydraulic Systems Training Information, #840172, Principles and Operation of Tractor Hydraulic Systems]