Читать книгу Hydraulic Fluid Power - Andrea Vacca - Страница 20

As mentioned before, the hydraulic fluid can be seen as a “distributed” component of the system. This concept can be described with the help of Figure 2.1, which shows a very simple hydraulic circuit that moves a linear actuator (CYL). The circuit is composed by a pump (P) driven by an electric motor (EM). The pump displaces fluid from the tank (T) to a directional control valve (DCV), which determines the direction of the flow from the pump to the actuator. Depending on the DCV configuration, the actuator rod (CYL) can move to the right, to the left, or remain at rest. A relief valve (RV) is used to limit the maximum pressure at the pump outlet, protecting it from overpressurization. The circuit also has a filter (F) that ensures the proper cleanness level of fluid, removing the solid contaminants coming from component wear or entering the circuit through the cylinder lip seals. Finally, an HE removes excess heat from the fluid. The hydraulic lines connecting all the components mentioned above represent pipes or hoses used to connect the different parts of the circuit.

A more detailed description of each component will be provided in the following chapters of the book; the reader at this point should not worry too much about having a full understanding of the operation of the above circuit. In general, each component has a certain function in the system operation, and its functionality can be studied by considering either its ideal behavior or its actual behavior. The ideal behavior represents the best possible scenario, i.e. it excludes the presence of undesirable – yet unavoidable – phenomena such as leakages, frictions, etc. Instead, the actual behavior is representative of the real‐world operation, which accounts for the presence of these undesired effects. For example, the motion of the actuator (CYL) ideally occurs without frictional losses or leakages between the piston and the cylinder bore. In reality, these undesirable aspects are present during the operation of the system. Because of that, the energy required to generate the piston motion is higher than the energy that would be involved in the ideal case.

Figure 2.1 Example of simple hydraulic circuit. The hydraulic fluid can be imagined as an additional “distributed component” of the circuit.

In the analysis of engineering problems, the use of ideal models greatly simplifies the circuit analysis. The ideal case is also useful to define a reference behavior that can be used to quantify the relative performance of an actual system or component. Important parameters such as energy efficiency will also be defined in this book by comparing the ideal use of energy of a system (or a component) with respect to the actual behavior.

Imagining the working fluid as a physical component of a hydraulic system might not be an obvious assumption. However, from a very high level, the hydraulic fluid can be seen as a physical element present everywhere in the circuit. Similar to other components of the entire system, the hydraulic fluid accomplishes specific functions, and its behavior can be described by either an ideal or a realistic model. In particular, the hydraulic fluid is a distributed “imaginary” component that accomplishes the following functions:

 Energy transport. With reference to Figure 2.1, the energy in the system flows from the prime mover (EM) through the pump (P) to the rest of the hydraulic system and finally to the end user (CYL). This transport occurs thanks to the fluid particles that travel and vary their energy level throughout the hydraulic system. This is the main function of the working fluid in a fluid power system. The lines dedicated to the transport of the energy are represented with continuous lines (see example in Figure 2.1).

 Lubrication. Many hydraulic components have internal parts in relative motion. The hydraulic fluid can lubricate these parts, avoiding solid‐to‐solid contacts that can lead to wear and energy dissipation. On the other hand, the fluid should be selected to avoid excessive leakages through the clearances between the solid part.

 Heat removal. The functioning of hydraulic systems often involves high levels of power. The energy dissipation present in the system, caused by mechanical friction, fluid shear, or throttling losses, often represents a significant portion of the overall power exchanged by the system. This energy dissipation is mainly converted into heat. Fortunately, the hydraulic fluid can carry most of the generated heat. The ability of the fluid to carry heat permits the use of only one HE (in Figure 2.1), which can cool the working fluid to maintain its temperature within an acceptable range.

 Signal transfer (pilot lines). In addition to the working lines where the flow rate is significant, a hydraulic system often contains lines dedicated to the transmission of the pressure signals. Most of control strategies are implemented with components that sense the pressure at certain remote locations of the system. Dedicated lines are required to transmit this pressure information (pilot line). In these lines, the flow rate can usually be neglected, at least for a first understanding of the system operation. These pilot lines take advantage of the property of the fluid to transmit the pressure information rapidly and without significant energy consumption.

The ideal fluid accomplishes the above functions by:

 avoiding irreversible interactions with the component materials, such as oxidation or erosion;

 having an infinite life; and

 having constant physical properties, particularly as pertains to fluid compressibility and viscosity.

Instead, an actual hydraulic fluid:

 can be chemically reactive and interact with the material used to build the components;

 has physical properties that degrade with time; and

 has physical properties that strongly depend on the operating conditions, particularly on pressure and temperature.

Significant advancements in the formulation of hydraulic fluids have been made in recent years. This has led to the development of hydraulic fluids whose properties are very close to the ideal behavior. The features usually desired in a hydraulic fluid are:

 limited influence of pressure and temperature on fluid viscosity;

 high lubrication capacity;

 high bulk modulus (meaning low compressibility);

 long‐life resistance and chemical stability;

 high flash point value (limiting risks of explosions and flammability);

 limited toxicity;

 high compatibility with the component material; and

 limited tendency to induce material corrosion.