the pumps are designed to operate at a speed much
slower than that of the motor.
The replenishing pump is a spur gear pump.
Its purpose is to replenish fluid to the active
system of the power drive. It receives its supply
of fluid from the reservoir and discharges it to
the B-end valve plate. This discharge of fluid from
the pump is held at a constant pressure by the
action of a pressure relief valve. (Because the
capacity of the pump exceeds replenishing
demands, the relief valve is continuously allowing
some of the fluid to flow back to the reservoir.)
The sump pump and oscillator has a twofold
purpose. It pumps leakage, which collects in the
sump of the indicator regulator, to the expansion
tank. Additionally, it transmits a pulsating effect
to the fluid in the response pressure system.
Oscillations in the hydraulic response system help
eliminate static friction of valves, allowing
hydraulic control to respond faster.
The control pressure pump supplies high-
pressure fluid for the hydraulic control system,
brake pistons, lock piston, and the hand-
controlled clutch operating piston. The control
pressure pump is a fixed-displacement, axial-
piston type. An adjustable relief valve is used to
limit the operating pressure at the outlet of the
For the purpose of this text, control constitutes
the relationship between the stroke control shaft
and the tilting box. The stroke control shaft is one
of the piston rods of a double-acting piston-type
actuating cylinder. This actuating cylinder and its
direct means of control are referred to as the main
cylinder assembly (fig. 12-6). It is the link between
the hydraulic followup system and the power drive
In hand control, the tilting box is mechanically
positioned by gearing from the handwheel
through the A-end control unit. In local and
automatic control, the tilting box is positioned by
the stroke control shaft. As shown in figure 12-6,
the extended end of the control shaft is connected
to the tilting box. Movement of the shaft will pivot
the tilting box one way or the other; which, in
turn, controls the output of the A-end of the
transmission. The other end of the shaft is
attached to the main piston. A shorter shaft is
attached to the opposite side of the piston. This
shaft is also smaller in diameter. Thus the working
area of the left side of the piston is twice that of the
area of the right side, as it appears in figure 12-6.
Figure 126.Main cylinder assembly.
Intermediate high-pressure fluid (IHP) is
transmitted to the left side of the piston, while
high-pressure hydraulic fluid (HPC) is transmitted
to the right side. The HPC is held constant at 1000
psi. Since the area of the piston upon which HPC
acts is exactly one-half the area upon which IHP
acts, the main piston is maintained in a fixed
position when IHP is one-half HPC (500 psi).
Whenever IHP varies from its normal value of
500 psi, the main piston will move, thus moving
the tilting box.
Assume that a right train order signal is
received. This will cause the pilot valve to be
pulled upward. The fluid in the upper chamber
of the amplifier piston can now flow through the
lower land chamber of the fine pilot to exhaust.
This will cause the amplifier piston to move
upward, and the fluid in the right-hand chamber
of the main control valve can flow into the lower
chamber of the amplifier valve.
The main control valve will now move to the
right, IHP will drop below 500 psi, and the stroke
piston will move to the left. Movement of the