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Keep the hydraulic pressure on

Updated: 09/08/2014 @ 10:23am

A hydraulic system works under three key principles: A liquid can’t be compressed; resistance to flow is the only way to create pressure in a system; and energy created under pressure will yield either work or heat.

The heart of a hydraulic system is a positive-displacement pump that is either of a fixed-displacement or a variable-displacement style. Either of these pump styles can be a gear, a vane, or a piston design.

A fixed-displacement pump moves the same volume of hydraulic oil with each cycle. Only the speed of the pump modifies the output.

On the other hand, a variable-displacement pump can alter the volume of oil it moves with each cycle even if the operating speed stays the same. This design is employed in applications where a specific pressure or flow must be maintained.

It’s important to remember that hydraulic pumps cause oil to flow but they do not create pressure. Pressure is the result of a restriction to flow.

Flow-control design in a hydraulic system can be an open-center, a closed-center, or a load-sensing system.

Most early hydraulic systems used on tractors were open-center designs. As farmers grew more dependent on hydraulics, their systems advanced to a closed-center system and finally to a load-sensing system.

Open and closed designs

With an open-center system, the pump produces a continuous flow of oil that must return to the reservoir when the cylinder or other actuator is not being moved. When flow is directed via a control valve to a cylinder, the oil volume stays constant. However, the oil pressure is increased to the level necessary to perform the work.

When the control valve is released, the fluid remains trapped in the cylinder, and the workload is supported. The pump pressure goes down and flow increases.

In contrast, with a closed-center system, a central source of fluid power is used to operate multiple functions.

Pump displacement and, thus, flow, changes to meet the demand required. When no function is required, oil flow is blocked at the control valve. When one (or more) control valve is opened, the pump automatically adjusts the delivery rate (volume) to satisfy the demand. Pressure to the valves will be maintained as long as the pump volume is sufficient to meet the demand.

Today, it is common to find a load-sensing system in use on tractors, particularly high-horsepower models. It is a modification on the closed-system design. This design permits stand-by pressure to be low when the control valve is in neutral.

When you move the control valve, flow is designed to maintain a pressure slightly higher than the highest pressure needed in the system. It regulates flow based on the pressure required to move the load rather than based on the pump output.

Oil viscosity

It’s been estimated that 70% of hydraulic failures are rooted in the use of either the wrong hydraulic fluid or contaminated oil.

One reason for this is that this oil does more than perform work. It must lubricate moving parts, be chemically stable at high temperatures and pressures, protect parts from rust and corrosion, resist foaming and oxidation, and be capable of separating itself from air, water, and other contaminants.

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