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Keep the hydraulic pressure on

09/04/2013 @ 4:12pm

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.

Hydraulic oil must also maintain a designated viscosity while operating in a wide temperature range. Viscosity is a fluid’s resistance to flow. It is the thickness at a defined temperature set by the Society of Automotive Engineers.

All petroleum-base oils tend to thicken when they are cold, and they become thinner when heated. If the viscosity is too low (or thin), it can cause leakage past the seals. But if the fluid is too thick (high viscosity), sluggish operation of the hydraulics occurs along with an additional power drain on the engine.

Oils are ranked with a viscosity index that reflects the fluid’s ability to change thickness with temperature.

For example, an oil will be given a low viscosity index if it becomes very thick at low temperatures and very thin when heated. A high viscosity index describes fluid that remains relatively stable in thickness as it is heated or cooled.

Farm equipment hydraulic systems are fitted with components that have very tight and exacting tolerances. As a result, they require hydraulic oil that has a high viscosity index and also has lubricating qualities paramount to long life. Good oil will be able to cling to close-fitting parts even under high temperatures. Many tractors use the hydraulic oil to lubricate the transmission. Low-quality hydraulic oil will provoke excess wear in the hydraulics and transmission.

Hydro fluids wear out

Ensuring a smooth operating hydraulic system is quite basic. You need to remember that hydraulic oil does wear out over time and needs to be changed. Often the additives in the oil (which are essential to its performance) become consumed. Plus, oil also absorbs dirt and moisture over time, compromising its ability to perform, let alone prevent corrosion of key components, seals, and gaskets. One sign of worn-out oil is components that stick when operating. This is especially true of control valves.

Cheap oil is a mistake

When purchasing hydraulic fluid, make sure the brand meets or exceeds the requirements for your machine as dictated in the owner’s manual. Equipment manufacturers have application-specific requirements for the oil. Even though you may save a few dollars selecting a cheaper oil, it may cost you in the long run. That same advice is true when selecting hydraulic filters.

When buying fluid, only purchase what you need for that season, because hydraulic oil can get old (their additives can precipitate out of the oil with time). Be sure to always store fluids in a shop that has minimal temperature variation to avoid condensation from forming in the storage container and polluting new oil.

Listen and watch

From time to time, listen to the hydraulic system operation and watch how well it performs. These efforts can tell you that something is going wrong long before a major problem occurs.

Proper service intervals are meaningless if a hydraulic system isn’t kept clean. Always use the dust caps on coupler valves and wipe off any fitting or service port before opening up or closing.

Keep the hydraulic system’s exterior clean by simply washing with a pressure wash, as dirt left around seals and dipsticks eventually work into the fluid.


Editor's Note: Ray Bohacz is an automotive expert and contributor to Successful Farming magazine.