To understand the basics of hydraulics, it is best to start off learning about the standard power unit. Once a person knows how this integral system operates, it will be easier to learn about more complex hydraulic systems. Further study can help engineers troubleshoot and validate system designs.
A power unit converts mechanical power into fluid power, which can be used to drive an actuator. An actuator is generally a cylinder, motor, or rotary actuator that is controlled by a directional valve. A power unit must meet the power demands of the system by producing enough force or torque to move the load.
Six basic components, illustrated in the schematic below, make up a power unit. They all work together to transmit power to the actuator and keep fluid clean.
The first component in a power unit is the prime mover. Electric motors are commonly used as prime movers in industrial hydraulics, while gas and diesel engines are more commonly used in mobile hydraulics.
The prime mover supplies mechanical power to the pump via shaft connection. This transfer of energy allows the pump to overcome hydrostatic pressure in the closed circuit, and creates a vacuum relative to the atmospheric pressure, which pushes fluid from the reservoir into the pump inlet, and generates flow throughout the system. There are three basic types of pumps. They are gear, piston, and vane pumps.
Upon exiting the pump, the hydraulic fluid has gained energy, and is exposed to a relief valve and a pressure gauge. The relief valve acts as a safety device to limit the maximum pressure to the power unit. It ensures energy is directed and under control at all times.
If the system pressure gets too high, the relief valve will save the system simply by sending the high-pressure fluid back to the reservoir where it came from. The pressure at the relief valve is indicated by a pressure gauge to provide operational pressure readings of the system.
The fluid proceeds to travel through high-pressure line to transmit hydrostatic pressure to the actuator, which often contains a directional valve. The pressure must be higher than the resistance of the actuator and the load to create work.
Finally, fluid runs through a return filter before flowing back into the reservoir. Clean fluid is necessary in any hydraulic system, so filters are important components in power units.
Content has been updated as of Oct. 27, 2021.
Four different levels of analytical schematics of pump and motor models exist, arbitrarily numbered as Type 0 through Type 3, each one progressively more complex and more inclusive. Type 0 Models are ideal because they contain no losses. They are perfectly efficient, having 100% volumetric, mechanical, and overall efficiencies. They are referred to in industrial jargon as models for calculating the “theoretical” performance of pumps and motors. More-practical models include the theoretical imperfections encountered in the real world.
In analytical schematics, the idealized portions of the circuit are identified with an I inside the pump or motor symbol. The elements associated with losses are characterized as being external to the idealized elements. As more of the real performance of real machines is incorporated into the models with more and more theories, the model actually becomes more practical, not more theoretical, assuming, of course, that the modelers have applied the various theories correctly.The Role of a Model
The purpose of mathematical models is to present a set of mathematical expressions that can be used to derive values for the variables that emulate the real machines they were designed to represent. Comparison to actual machine test data verifies the validity and usefulness of the model. The process of model verification with real data is called correlation, which provides a quantitative measure of the goodness-of-fit for the data.
A model can occur in at least two basic forms. The first is an analytical schematic, so called because the schematic is in a form that facilitates analysis of the circuit. That is, it facilitates the writing of descriptive equations. The second form is the set of describing equations themselves. The use of schematic models requires that the writer and user of the models agree on how, very specifically, the equations are to be written. Schematics are attractive to technical people because we like illustrations that convey details that mere equations sometimes cannot.
The equations themselves are always the more precise and unambiguous way to convey the exact nature of the model. The user will have less interpretation with the equations. However, equations present less opportunity for creative enhancements. On the other hand, analytical schematics facilitate non-mathematic interpretations better than equations do. The non-mathematical model will be covered here. However, the schematic models have undergone evolutionary improvements. They differ slightly from basic versions because details have been added.
Selecting the appropriate filtration system when designing a hydraulic circuit is critical for maintaining fluid cleanliness and preventing premature wear, both of which contribute to optimal system operation and service life.
Contamination of the hydraulic system can occur during assembly and during operation. Contamination can come in many forms, including water or other fluids, air, solid particles, or corrosive agents and heat.
Dirt is the greatest enemy of hydraulic systems, since it generates wear that results in shortened service life of components. The cleaner the system, the higher its service life expectancy. Therefore, it is imperative that only clean fluid enter the circuit. In addition, a filter capable of maintaining fluid cleanliness to ISO 4406 class 22/18/13 or better, under normal operating circumstances, is recommended.
Between open- and closed-circuit filtration designs, there are additional factors to consider as well.Closed-Circuit Designs
A closed-circuit filtration design will typically follow into either suction line filtration or charge pressure filtration (both partial- and full-flow).
In suction line filtration, a filter is placed in the circuit between the fluid reservoir and the inlet to the closed-circuit pump. Follow the manufacturer’s recommendations for bypass versus non-bypass filter in the suction line. A vacuum gauge can be used to show when the inlet pressure exceeds the manufacturer’s requirements. A contamination monitor will indicate when a filter change is needed, once a maximum vacuum level is reached.
Examinations have revealed that a filter in the suction line with a β35 – β45 = 75 (β10 ≥2) at a differential pressure of 0.25bar achieves the required cleanliness of 18/13 under normal operating conditions. In some applications even better cleanliness levels are achieved.
In charge pressure filtration, the filter is designed into the charge circuit of the closed-circuit pump. Placing the filter in the charge circuit can mitigate high inlet vacuum during cold start-up conditions. Fluid filtration is provided immediately ahead of a pump’s control and the hydraulic loop.
Examinations here, too, have shown that filter elements with a β15 – β20 = 75 (β10 ≥ 10) at the differential pressure occurring in the application are recommended. A strainer with a mesh of 100 µm – 125 µm must be used to protect the charge pump against course contamination. However, the actual filtering is done by the filter in the charge circuit.Partial Versus Full Flow Charge Filtration
The charge pressure filtration design can be further broken down into either partial flow or full flow.
Partial flow filtration refers to the charge pressure relief valve located upstream of the filtration. A portion of the charge pump oil flows through the filter while the rest flows over the charge pressure relief valve back to the reservoir. Only the fluid needed by the closed loop system and the pump control is filtered.
Maintaining a Filtration System
As this article serves only as a guideline, it is imperative to work closely with the filter manufacturer when selecting a filter. Every open- and closed-circuit hydraulic system is unique, and the filtration requirements and performance must be determined by test.
It is essential that monitoring of prototype systems and evaluation of components and performance throughout the test program be the final criteria for judging the adequacy of the filtration system.
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