Service Manual

BDP-10A/16A/21L

Figures 4-7 represent hydraulic schematics of

the standard charge and auxiliary charge pump.

Figure 8 provides a graphical illustration of the

hydraulic oil circuit.

The input shaft and pump cylinder block are turned

in one direction only by the engine/ drive belt/

pulley combination or direct drive gear box.

The oil is drawn through an external filter that pre-

vents contaminants within the reservoir from en-

tering into the charge pump gerotor.

The charge pump supplies fluid to keep the

closed loop pressurized, preventing cavitation

and providing cooling oil flow for the system.

The charge relief valve is used to maintain charge

at a predetermined pressure.

Output of the system oil flow is controlled by the

direction and amount that the swashplate is

angled. As the pump pistons compress, they

force oil into one of two passageways (“A” or “B”)

in the system hydraulic circuit. Oil is supplied ex-

ternally under pressure to an external load, (e.g.,

a vehicle wheel motor). As the angle of the pump

swashplate is increased, the amount of oil being

pumped will increase and cause a higher speed

output of the wheel motor. Reversing the angle

of the swashplate will reverse the direction of the

oil flow. During the operation of the pump, fluid

is “lost” from the hydraulic loop through leak paths

designed into the product for lubrication purposes

(around pistons, under the rotating cylinder

block, etc.). This “lost” fluid returns to the reser-

voir through the case drain. This fluid must be

made up in the loop. The charge pump makes

up this fluid loss.

The makeup flow is controlled (or directed) by

the system check valves. The check valves are

used to direct makeup fluid into the low pressure

side of the closed loop. Each check valve will

either be held open or closed, depending upon

the direction of the vehicle operation: Closed in

a pressurized system passage, open in a low

pressure, “charged” system passage.

Figure 8. BDP-10A,16A and 21L Standard Charge Pump Graphical Schematic

GRAPHICAL SCHEMATIC