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Oil the hydraulic circuit?

Hydraulic oils

Oil supplies fluid to the hydraulic circuit. It moves through the circuit to make it work. The hydraulic circuit needs a fluid, called hydraulic oil, to transmit energy from the pump to the receivers. The oil therefore has a power transmission role. It must be incompressible and viscous in order to lubricate the metal components.

Open-centre hydraulic circuit: the most common diagram.

Let's take the example of an open-centre hydraulic circuit to illustrate. The main equipment pump will draw oil from the reservoir and send the flow through the main safety valve to the central part of the auxiliary distributor. It will then pass through the centre of the other two spool valves to the reservoir. The steering circuit pump (in neutral position) draws oil from the tank and sends the flow to the steering circuit hydraulic supply port and to the pressure reduction valve. This valve will adjust the maximum pressure in the steering system. The oil will flow into the pressure reduction valve via the non-return valve and the accumulator, before flowing into the pilot shut-off valve. The accumulator will act as an emergency pilot control oil supply when the pump stops. When the valve is in the "off" position, the oil is blocked and prevents the hydraulic pilot circuit from operating. When the valve is in the "on" position, oil will flow through the valve to the auxiliary spool valve pilot control, lift and tilt valves.

The circuit in "neutral" mode.

When the circuit is in "neutral" mode, the control oil flows through the supply port and is blocked by the control rods. The oil remaining in the main valve lines is then drained to the reservoir port through the centre of the control rod. Using the pilot drain control lever, force is applied to the swivel plate to move the upper and lower plungers, control springs and retainer downwards. Oil from the pilot pump will flow through the orifice and centre of the control rod to the main tilt valve. The return oil from the main tilt valve will then pass through the drain control rod, the centre of the control rod and to the reservoir port. The purpose of the control rod is to move the spool valve in proportion to the movement of the pilot valve lever. Pilot oil trapped in this spool will cause an increase in pilot pressure, exceeding the spring force of the main tilt valve spool, forcing it to move; it will then direct oil from the main hydraulic circuit to the hydraulic tilt cylinder. When the increase in pressure exceeds the force applied, the control rod will compress the control spring. By limiting the flow of oil, the pressure in the spool of the main tilt valve can be controlled. The regulating spring will therefore adjust the pressure of the main tilt valve spool in proportion to the movement of the pilot valve lever. To finish this circuit, we must not forget to bleed (which we can also call "decompression" in this particular case) the oil from the hydraulic circuit, as the cylinder's internal circuit must only contain oil, and its temperature must remain unchanged. To bleed, simply open the circuit and move the rod back and forth in the cylinder, collecting the fluid as it drains. If the cylinder does not have a bleed screw, the cylinder must be moved slowly back and forth to expel the air in the oil.

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