Accumulator power drive systems are the most common hydraulic power drives used in ordnance equipment. Instead of having the output flow of the pump of the system being put directly to use operating a mechanism, it is pumped into an accumulator flask. The prime function of the accumulator is to store a volume of fluid under pressure. As the system demands fluid, it is supplied from the flask. The pressure regulator we just described monitors the pressure in the flask, keeping it within prescribed limits. Systems that require a high volume of fluid under pressure will be equipped with several flasks.

Figure 4-19 shows the most common type of accumulator-the gas-operated, bladder type of flask. This accumulator flask uses a nitrogen-filled bladder inside a steel cylinder. The cylinder has a poppet valve to keep the bladder from being pushed into the output line by its nitrogen pressure when the system is not energized. The bladder, filled with nitrogen to a certain pressure, is compressed by the fluid as the cylinder fills. When the upper level pressure is reached, fluid flow from the pump to the accumulator is stopped. The bladder pressure then continues to cause fluid flow, maintaining system pressure. Without the accumulator flasks, the pump of the system could easily fall behind system requirements during times of peak demand.

Pumps and Power Drives

The purpose of a hydraulic pump is to supply a flow _{of fluid to a hydraulic system. The pump does not}

Figure 4-19.-A gas-operated, bladder type of accumulator.

Figure 4-20.-A rotary gear pump.

produce pressure. The pressure is developed in the system due to the resistance of the system to fluid flow. Pumps can be of several different types, but the most common are the rotary gear pump and the axial piston pump.

The rotary gear pump (fig. 4-20) operates by trapping fluid at the inlet (suction) port and forcing it through the discharge port to the system. The gear pump is used throughout ordnance systems to supply fluid flow at a variety of pressures. They are most efficient, however, at pressures around 500 psi. There are several configurations and modifications of the gear pump. These will not be discussed in this text.

The axial piston pump is used in most power drives that require either a variable output or a high-volume output. First, let's describe the principle of operation, then we can show how they are used.

Figure 4-21 shows how the output of an axial piston pump is varied. The pump uses pistons attached to a movable tilt plate. The pistons and cylinder barrel rotate together, driven by an electric motor, between the tilt plate and the valve plate. The tilt plate is moved either left or right by stroking pistons. The larger the tilt angle, the longer the piston stroke and the larger the quantity of pump output. The direction of flow is reversed by reversing the direction of tilt. When the

Figure 4-21.-Operation of the axial piston pump. 4-15

Figure 4-22.-A hydraulic CAB unit.

piston stroke is on zero stroke, there is no pump output, even though the pump is still rotating.

This configuration of the axial piston pump is used primarily for the train and elevation power drives, magazine ready service rings, and loaders on gun mounts and missile launchers. The pump output is used to drive a hydraulic motor. The only difference between the pump (A-end) and the hydraulic motor (B-end) is that the tilt plate is set at a permanent angle. These combination A-end pumps and B-end motors are referred to as CAB units. Figure 4-22 shows a CAB unit setup. The pump output is transmitted to the motor, which operates just the reverse of the pump. The pump takes a mechanical input and turns it into a variable bidirectional hydraulic output. The motor takes a variable hydraulic input and converts it to a variable bidirectional mechanical output.

Another common configuration uses the same pump but with a fixed angle tilt plate. The pump has a constant output that is supplied to an accumulator system with a pressure regulator. This type is used to power most gun mount and missile launcher loading systems.