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LUBRICATION
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.
Click for a closer view
VARIATIONS ON HYDROSTATIC TRANSMISSIONS
Another transmission using the same hydrostatic principles utilizes radial
ball piston pump and motor rotating blocks. In this design, the cylinder
bores are positioned like spokes around a central hub, and the pistons are
large
hardened steel balls instead of cylindrical in shape. (See Fig. 8)
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
“lost’ 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]
