Therefore no oil is being moved to provide either forward or reverse motion. 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. 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. The oil circulates through the housing and returns through the oil cooler and back to the reservoir. The charge pump pumps the oil past the high charge pressure control valve and into the main pump housing.
#Forward and reverse motor spool sdcf755m134lf1 free
6-16).- With the speed control lever in neutral, free oil flows from the reservoir through the oil filter to the charge pump. Then the relief valve closes and oil again flows to the cylinder block, moving the machine forward. This action continues until the load is reduced below the rated psi. This will either slow or stop the machine. If pressure exceeds the rated psi, a relief valve opens and oil bypasses the cylinder block in the motor. 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. This action keeps the charge circuit open to the low-pressure valve while the system is running. The only extra oil comes from the charge pump that maintains a given flow of oil through the system whenever the machine is running.Ī 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 is called a "closed system" because the oil keeps circulating between the pump and the motor. rotates the output shaft driving the machine forward.Īs 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. Since the fixed swash plate is always tilted, the pistons slide down the inclined surface and the resulting forces rotate the cylinder block. As the angle of the swash plate is varied so will the volume of oil displaced from the pump.Īs pressure oil enters the inlet port of the motor, the pistons that align with the inlet port pushes against the swash plate. The greater the angle, the greater the volume and the more oil flows from the pump. This determines the volume of oil displaced per revolution of the pump. The distance the pistons reciprocate in and out of the cylinder block depends on the angle of the swash plate of the pump. 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. 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. 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. The swash plate will remain titled until the operator moves the speed control lever. returns through the displacement control valve (FNR valve) to the pump case.Īs 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. Oil, expelled by the opposing servo cylinder.
This action allows pressure oil to flow into the upper servo cylinder forcing the swash plate to tilt. 6-15).- When the operator moves the speed control lever forward, the spool in the displacement control valve, also known as the FNR valve (Forward, Neutral, and Reverse), moves from its NEUTRAL position.
Now look at the complete system in operation- forward, neutral, and reverse.įORWARD (fig. The pump has a variable displacement, while the motor has a fixed displacement. The system we will use has an axial piston pump and motor which is the 'most common hydrostatic drive system. For you to understand how a hydrostatic drive operates, we will explain the operation of a typical system.