plate to tilt. Oil, expelled by the opposing servo cylinder. returns through the displacement control valve (FNR valve) to the pump case.

As the swash plate reaches the tilt set by the speed control lever, the displacement control valve (FNR valve) spool returns to a NEUTRAL position, trapping the oil to both servo cylinders and holds the swash plate in its titled position. The swash plate will remain titled until the operator moves the speed control lever.

With the pump drive shaft and cylinder block rotating clockwise and the swash plate is titled to the rear, it is now time to start pumping. As the cylinder rotates past the pump inlet port, the inlet check valve opens: oil is then forced by the charge pump into the piston bores that align with the inlet port under low charge pressure. As rotation continues, oil is forced out of the outlet port at high pressure by the pump pistons when they align with the outlet port. This flow of oil drives the motor.

The distance the pistons reciprocate in and out of the cylinder block depends on the angle of the swash plate of the pump. This determines the volume of oil displaced per revolution of the pump. The greater the angle, the greater the volume and the more oil flows from the pump. As the angle of the swash plate is varied so will the volume of oil displaced from the pump.

As pressure oil enters the inlet port of the motor, the pistons that align with the inlet port pushes against the swash plate. Since the fixed swash plate is always tilted, the pistons slide down the inclined surface and the resulting forces rotate the cylinder block. This, in turn. rotates the output shaft driving the machine forward.

As the cylinder block continues to rotate clockwise, oil is forced out the outlet port at low pressure and returns to the pump where it is recirculated through the pump and back to the motor.

This is called a "closed system" because the oil keeps circulating between the pump and the motor. The only extra oil comes from the charge pump that maintains a given flow of oil through the system whenever the machine is running.

A shuttle valve, located in the motor manifold and controlled by high oil pressure, prevents high oil pressure from entering the low-pressure side of the system. This action keeps the charge circuit open to the low-pressure valve while the system is running.

The high-pressure relief valve, located in the motor manifold, monitors the pressure of the forward flow of oil and protects the system from too high pressures. If pressure exceeds the rated psi, a relief valve opens and oil bypasses the cylinder block in the motor. This will either slow or stop the machine. The bypassed oil returns to the pump. This action continues until the load is reduced below the rated psi. Then the relief valve closes and oil again flows to the cylinder block, moving the machine forward.

NEUTRAL (fig. 6-16). - With the speed control lever in neutral, free oil flows from the reservoir through the oil filter to the charge pump. The charge pump pumps the oil past the high charge pressure control valve and into the main pump housing. The oil circulates through the housing and returns through the oil cooler and back to the reservoir.

Trapped oil is held in the cylinder block of the pump, in the motor, and in the connecting lines between the pump and motor by two check valves in the pump end cap.

When the control lever is in neutral, the swash plate in the pump is also in neutral and the pistons within the pump are not pumping. Therefore no oil is being moved to provide either forward or reverse motion.

The cylinder block in the pump rotates in a clockwise direction and is driven by the engine of the equipment. Rotation is viewed from the drive shaft end of the pump. Because the oil is not being pumped to the motor, the cylinder block in the motor is stationary and the output shaft does not move.

NOTE

With the drive system in neutral, the high charge pressure control valve, (located at the charge pump) controls pump pressure. When the system is activated for reverse or forward, the low charge pressure control valve located in the motor manifold controls the charge pressure at a lower psi.

REVERSE (fig. 6-17). - As the speed control valve is moved to reverse, the spool in the displacement control valve (FNR valve) moves out of neutral allowing pressure oil to flow into the lower servo cylinder, tilting the swash plate forward.

When the swash plate reaches its desired tilt, which is set by the control lever, the displacement control spool returns to neutral. This action traps the