Model 7 Hydrostatic Transmission - MTD Garden Tractors User Manual

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2. Input speed. Faster input speeds have the
capability to pump more fluid at the same
swashplate angle.
3. Axle ratio. A higher ratio will take less pres-
sure to make a vehicle move than one with
a lower ratio, assuming similar vehicles.
4. Ground conditions. On flat smooth con-
crete, a vehicle will have less rolling resis-
tance than on a rough or grassy surface.
5. Tire size, style, and pressure, along with
vehicle weight, will have an effect on the
coefficient of surface friction.
6. Temperature. A hydrostatic unit with oil at
room temperature will generally be more
efficient (less leakage) than one that is
running at normal operating temperatures.
6-2.

MODEL 7 HYDROSTATIC TRANSMISSION.

6-2.1 Fluid Flow.
1. Fluid flows through an internal closed loop
between the pump and the motor. The flow is
directed by the pump to the motor and then
back to the pump. Because of leakage, the
amount of fluid driven back of the motor is
slightly less than that required by the pump.
See Figure 6-2.
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2. Check valves on the inlet side of the pump are
open to the reservoir enabling the pump to
draw fluid as needed. Speed control is
achieved by changing the amount of oil deliv-
ered by the variable displacement pump to the
fixed displacement motor by moving the con-
trol lever. See Figure 6-3.
3. The Model 7 hydrostatic pump is not a self
purging pump and must be manually purged if
it gets air in the system or is repaired. To purge
the pump you would remove the air vent plug
that is next to the expansion tank and fill the
tank until fluid comes out of the vent opening.
Replace the plug. With the hydrostatic pump
cold, fill the expansion tank to the cold line.
Run the unit to see that it is operating properly.
If not, check the vent plug again for more air
and repeat the procedure.
6-2.2 Fluid Level. The transmission fluid level
should be checked prior to initial use. The level
should not be above the COLD mark which is
about 1/4 inch from the bottom of the reservoir/
expansion tank. See Figure 6-4.
Overfilling reduces the expansion area in the
reservoir/expansion tank and fluid will spill at
operating temperatures.
6-2
CAUTION