徐州工程学院毕业设计附录附录一英文原文Hydraulic Conductors and FittingsEric Sandgren *, T.M. Cameronto account for uncertainty aMechanical Engineering, Virginia Commonwealth University, 601 West Main Street, P .O. Box843015, Richmond, VA 23284-3015, USA Received 19 October 2001;accepted 5 June 20021.1 INTRODUCTIONIn a hydraulic system, the fluid flows through a distribution system consisting of conductors and fittings, which carry the fluid from the reservoir through operating components and back to the reservoir. Since power is transmitted throughout the system by means of these conducting lines (conductors and fittings used to connect system components), it follows that they must be properly designed in order for the total system to function properly.Hydraulic systems use primarily four types of conductors:1.Steel pipes2.Steel tubing3.Plastic tubing4.Flexible hosesThe choice of which type of conductor to use depends primarily on the systems operating pressures and flow rates. In addition, the selection depends on environmental conditions such as the type of fluid, operating temperatures, vibration, and whether or not there is relative motion between connected components.Conducting lines are available for handling work pressures up to 10,000 Pa or greater. In general, steel tubing provides greater plumbing flexibility and neater appearance and requires fewer fittings than piping. However, piping is less expensive than steel tubing. Plastic tubing is finding increased industrial usage because it is not costly and circuits can be very easily hooked up due to its flexibility. Flexible hoses are used primarily to connect components that experience relative motion. They are made from a large number of elastomeric (rubberlike) compounds and are capable of handling pressures exceeding 10,000 Pa.Stainless steel conductors and fittings are used if extremely corrosive environments are expected. However, they are very expensive and should be used only if necessary. Copper conductors should not be used in hydraulic systems because the copper promotes the oxidation of petroleum oils. Zinc, magnesium, and cadmium conductors should not be used either because they are rapidly corroded by water-glycol fluids. Galvanized conductors should also be avoided because the galvanized surface has a tendency to flake off into the hydraulic fluid. When using steel pipe or steel tubing, hydraulic fittings should be made of steel except for inlet, return, and drain lines, where malleable iron may be used.Conductors and fittings must be designed with human safety in mind. They must be strong enough not only to withstand the steady-state system pressures but also the instantaneous pressure spikes resulting from hydraulic shock. Whenever control valves are closed suddenly, this stops the fluid, which possesses large amounts of kinetic energy. This produces shock waves whose pressure levels can be two or four times the steady-state system design values. Pressure spikes can also be caused by sudden stopping or starting of heavy loads. These high-pressure pulses are taken into account by the application of an appropriate factor of safety.1.2 CONDUCTOR SIZING FOR FLOW-RATE REQUIREMENTSA conductor must have a large enough cross-sectional area to handle the flow-rate requirements without producing excessive fluid velocity. Whenever we speak of fluid velocity in a conductor such as a pipe, we are referring to the average velocity. The concept of average velocity is important since we know that the velocity profile is not constant. As shown in Chapter 5 the velocity is zero at the pipe wall and reaches a maximum value at the centerline of the pipe. The average velocity is defined as the volume flow rate divided by the pipe cross-sectional area: In other words, the average velocity is that velocity which when multiplied by the pipe area equals the volume flow rate. It is also understood that the term diameter by itself always means inside diameter and that the pipe area is that area that corresponds to the pipe inside diameter. The maximum recommended velocity for pump suction lines is 4 ft/s (1.2 m/s) in order to prevent excessively low suction pressures and resulting pump cavitation. The maximum recommended velocity for pressure lines is 20 ft/s (6.1 m/s) in order to prevent turbulent flow and the corresponding excessive head losses and elevated fluid temperatures. Note that these maximum recommended values are average velocities.EXAMPLE 1-1A pipe handles a flow rate of 30 gprn. Find the minimum inside diameter that will provide an average fluid velocity not to exceed 20 ft/s.Solution Rewrite Eq. (3-26), solving for D:EXAMPLE 1-2A pipe handles a flow rate of 0.002. Find the minimum inside diameter that will provide an average fluid velocity not to exceed 6.1 m/s.Solution Per Eq. 3-35) we solve for the minimum required pipe flow area:The minimu