Copyright Goodheart-Willcox Co., Inc. 186 Fluid Power appearance, construction, and application. However, a number of basic factors are related to items in all of the categories. Part of these factors are concerned with safe system operation, such as physical strength and shock resistance. Other factors deal with the effi ciency of system operation, such as resistance to fl uid fl ow and achieving correct fl uid velocity. Still other factors deal with ease of installation and service. Adequate strength Conductor strength must be considered a critical fac- tor when selecting a conductor for a system. The most obvious reason for this concern is system operating pressure. Strength must also be considered from the standpoint of pressure surges or shock caused by sud- den, unexpected load changes during the operating cycle. In addition, other strength aspects that must be considered are mechanical stresses produced by ther- mal expansion, physical abuse, and even the weight of other system components or machine members the conductor is expected to support. A conductor must have a continuous operating pressure rating equal to the expected system operating pressure plus an adequate safety factor. A factor of 4:1 is often stated as a minimum. The actual factor selected is based on experience with specifi c applications. Safety factor is a margin of error used to provide pro- tection from hydraulic shock pressure. Shock pressure is a momentary high-pressure surge that exceeds the system relief valve setting, resulting from machine- member or workload kinetic energy when a directional control valve is centered or reversed and the load sud- denly stops or reverses. Shock pressures must be con- sidered when selecting a conductor, as they are often higher than most circuit designers and machine opera- tors realize. It is possible to measure shock pressures using electronic test instruments. These devices are fi tted with appropriate accessories to convert the pres- sures into electrical pulses that can be displayed as a waveform on a screen or printed out for future analysis. The pressure rating of all conductors depends on the tensile strength of the construction materials, conductor wall thickness, and inside diameter of the conductor. Formulas exist for calculating wall thick- ness for conductors when the required inside diameter and material tensile strength are known. However, in most practical system design situations, manufacturer data sheets are used to select the conductors that have adequate strength to safely function. Low fl ow resistance It is desirable to have the fl uid in the lines of a sys- tem move with as little resistance to fl ow as possible. This requires conductors that are properly sized with an inside diameter large enough to allow the needed volume of fl uid to move through the lines within the tank. This ensures increased heat dissipation and separation of dirt or water. Many times, the fi nal selection of the reservoir size is a compromise infl uenced by space and weight limi- tation. Fixed installations, where weight is not a factor, often have a ratio of pump output to reservoir capac- ity above 3:1. On the other hand, mobile installations, where increased weight can be a problem, have ratios below that guideline. 8.2 Conductors The individual component parts of a hydraulic system cannot function alone. They must operate as an inte- gral part of a system. This is accomplished by mov- ing fl uid from the reservoir to the pump and on to the control valves and actuators. In most system designs, it is not practical to directly connect these components. This is because many of the components must oper- ate machine members as well as interact with other hydraulic components in the system. Conductors ensure distribution of system fl uid to all components, no matter how widely they are dispersed, Figure 8-7. The proper selection, installation, and maintenance of these conductors is of utmost importance to proper operation of a hydraulic system. 8.2.1 Inherent Design Requirements A variety of different types of conductors and asso- ciated connectors and fi ttings are available for use in hydraulic systems. Pipe, tubing, and fl exible hose are considered basic conductor categories. Of course, the items in each of these groups vary considerably in Goodheart-Willcox Publisher Figure 8-7. A wide variety of conductors, connectors, and fittings are available for use in hydraulic systems, as can be seen on this piece of earthmoving equipment. Selecting appropriate components requires familiarization with each of the types available.