114 6 Hydraulic Fluid The Energy Transmitting Medium 6.1 Functions of a Hydraulic Fluid 6.2 Performance Characteristics of a Hydraulic Fluid 6.2.1 Lubricating Power 6.2.2 Resistance to Flow 6.2.3 Viscosity Stability 6.2.4 Low-Temperature-Operating Ability 6.2.5 Resistance to Oxidation 6.2.6 Reaction to Condensation and other Water 6.2.7 Resistance to Foaming 6.2.8 Resistance to Fire 6.3 Commonly Used Hydraulic Fluids 6.3.1 Petroleum-Based Fluids 6.3.2 Biodegradable Fluids 6.3.3 Soluble-Oil Emulsions 6.3.4 High-Water-Content Fluids 6.3.5 Fire-Resistant Fluids 6.4 Hydraulic Fluid Additives 6.5 Hydraulic Fluid Specifications 6.5.1 Viscosity 6.5.2 Viscosity Index 6.5.3 Specific and API Gravity 6.5.4 Pour Point 6.5.5 Flash and Fire Points 6.5.6 Rust Prevention 6.5.7 Corrosion and Oxidation 6.5.8 Demulsibility Characteristics 6.5.9 Foaming Characteristics 6.5.10 Lubricity and Wear-Prevention Characteristics 6.6 Handling and Maintaining Hydraulic Fluids 6.6.1 Storage and Handling of Fluids 6.6.2 System Cleanliness 6.6.3 Fluid Operating Temperature 6.6.4 Maintaining Fluid Cleanliness and Operating Temperature Chapter Outline 115 absolute viscosity additives anti-wear agents API gravity biodegradable fluids capillary viscometer catalyst emulsion extreme-pressure agent film fire point flash point Four-Ball Method friction high-water-content fluids (HWCF) inverted emulsion kinematic viscosity lubrication lubricity oil-in-water emulsion oxidation oxidation inhibitor phosphate ester polyglycol pour point rust inhibitor Saybolt viscometer sludge spontaneous ignition synthetic fluids Timken method viscosity viscosity grades viscosity index number Key Terms In hydraulic systems, energy is transmitted through a liquid, rather than through shafts, gears, cables, and belts, as in a mechanical power transmission system, or the wires and complex control components used in electrical systems. Although hydraulic systems may sound less complicated, they are still complex devices requiring consideration of many elements. One of the fundamental elements that must be considered is the liquid to be used in the system. This chapter deals with the many factors relating to liquids that must be considered when selecting a hydraulic fluid for a particular application. After completing this chapter, you will be able to: • Describe the various functions a hydraulic fluid performs in a fluid power system. • Identify and explain the general properties of a liquid that would make it suitable as a hydraulic fluid. • Name and describe the general categories of materials that are commonly used as hydraulic fluids. • Explain the terms used to describe the basic characteristics of hydraulic fluids. • Explain procedures to follow for the selection and performance monitoring of hydraulic fluids. • Describe appropriate procedures for handling, storage, and disposal of hydraulic fluid. Learning Objectives Guided Tour Copyright Goodheart-Willcox Co., Inc. Chapter 10 Controlling the System 271 10-41 shows the internal structure of these The passageway through the valve contains orifice. This orifice may be simply a hole machined the valve body or a separate part that can be eas- replaced. The needle has a tapered point and is mounted on a shaft with fine threads attached to an handwheel. Turning the handwheel allows an to adjust the valve opening from fully closed fully open. This type of valve produces fluid turbulence, in increased pressure drops at higher flow In addition, the small diameter of the orifice pro- a pressure drop as the fluid is metered through needle valve. A more detailed discussion using a needle valve as a metering device is found in the Flow Control Devices section of this chapter. NOTE 10.5.2 Check Valves A standard design of check valve has the primary pur- pose of allowing free flow of fluid through a line in one direction, while preventing reverse flow in that line. More complex check valve designs also are available that include restriction and pilot-operated units. Both of these versions allow free flow in one direction, but may also allow reverse flow in selected circuit operat- ing situations. from the fully open to the fully closed position. When the valve is in the fully open position, there is unre-- stricted movement of through valve.. Spool valves A spool valve consists of a valve body with a preci-- sion-machined bore and passageways connecting inlet and outlet A precision-fit spool is placed in the to provide a means by which the inlet and outlet can be connected and disconnected. Figure 10-40 shows the internal structure of a spool valve. Differ--r ent spool configurations and shifting methods may be used to provide different operating characteristics. These valves can be designed to with minimal flow resistance, resulting in a low pressure drop across the valve even at maximum flow capacity. Needle Needle valves are very commonly used in fluid power systems as both shutoff and metering devices. Outlet Spool Inletn Openn ClosedC Inlette Goodheart-Willcox Publisher Figure 10-40. The internal structure and flow path through a basic spool valve. Linear movement of the valve handle provides rapid adjustment of this valve from fully open to closed. Goodheart-Willcox Publisher Figure 10-41. The internal structure and flow path through a basic needle valve. This design is commonly used for both shutoff and metering of fluid in hydraulic circuits. Chapter Outline provides a preview of the chapter topics and serves as a review tool. Objectives clearly identify the knowledge and skills to be obtained when the chapter is completed. Objectives are tied to the chapter outline, to the text, and to our end-of- chapter materials. Key Terms list the important terms to be learned in the chapter. Caution features point out safety- related issues to help you avoid potentially dangerous materials and practices. FFigure vvalves. aan iin iily m eexternal ooperator tto rresulting rrates. dduces tthe 1 A p d M t o m i f rom the full y open to the full y closed position. When the valve is in the fully open position, there is unre stricted movement o f flfluid uid throu ghtothethe the valve Spof ool valves A sp oo l va l ve consists o f a valve bod y with a preci sion-machined bore and passageways connectin g inlet and outlet pports. orts. A p recision-fit sp ool is p laced in the bbore ore to provide a means b y which the inlet and outlet pports orts can be connected and disconnected. Fi gure 10-4 0 shows the internal structure of a sp ool valve. Diffe ent spool config urations and shiftin g methods ma y b e used to provide different operating characteristics. T hese valves can be designed to ffunction unction with minimal fl ow resistance, resultin g in a low pressure drop across the valve even at maximum flo w capac ity. N eed le vvalves al ves N eedle valves are very commonly used in fluid power sy stems as both shuto ff and meterin g devices. O utle t Spool I let bblockeddlo cke O pe lo s ed I nl Copyright Goodheart-Willcox Co., Inc. 402 Fluid Power A receiver-sizing formula suggested by the Compressed Air and Gas Institute is: V = T × C × P A P 1 – P 2 where: V = Capacity of the receiver (ft3) T = Time to reduce receiver pressure from P 1 to P 2 (min) C = Air requirement (ft3/min) P A = Atmospheric air pressure (psia) P 1 = Initial receiver air pressure (psig) P 2 = Final receiver air pressure (psig) This formula assumes a constant air temperature in the receiver and a standard air pressure of 14.7 psia. The formula also assumes that no additional air is being supplied to the system. In other words, the compressor is stopped or unloaded using a start-stop or inlet valve capacity control system. If air is being added system, that additional volume needs to be subtracted from variable C the formula. Sizing the Receiver An air receiver must supply air to a pneumatic system for 10 minutes while the compressor is unloaded. The system is consuming air at 10 cubic feet per minute (cfm) during this time and the receiver air pressure drops from 100 psig to 50 psig during this time. Temperature is constant and the atmospheric air pressure is 14.7 psia. V = T × C × P A P 1 – P 2 Substituting in: T = 10 min C = 10 cfm P A = 14.7 psia P 1 = 100 psig P 2 = 50 psig V = 10 min × 10 cfm × 14.7 psia 100 psig – 50 psig = 1,470 ft3 50 = 29.4 ft3 EXAMPLE 16-1 16.2 Air-Distribution System The purpose of the air-distribution system is to carry high-pressure, conditioned air from the system receiver to workstations. Delivery must be done with output inherent in the operating cycle of reciprocat- ing compressors and some other designs. The receiver also acts to remove additional water vapor from the air as the air temperature continues to drop after passing through the system aftercooler. In addition, the stored volume of air in the receiver reduces the frequency of compressor cycling in capacity-control systems using start-stop operation or inlet valve unloading. Receiver design and construction A receiver is typically cylindrical with domed ends, although other designs exist. The domed, cylindrical con- figuration provides an easily constructed unit that can withstand the pressures used in consumer and industrial systems. Standardizing organizations provide construc- tion specifications and pressure ratings that are often listed as part of consumer and industrial safety codes. Care should be taken to be certain codes are followed. A large volume of highly compressed air can present serious safety issues. CAUTION The receiver is constructed with inlet and out- let ports, fittings to allow the connection of various associated components, and, in some designs, a hole to facilitate cleaning. An internal baffle may also be incorporated to ensure liquefied water, pipe scale, and rust particles that enter from the connecting compres- sor line fall out of the airstream. Receivers are also fit- ted with feet for vertical or horizontal installation. In smaller units, they may include fittings to allow for the easy attachment of compressors and prime movers. Accessory components often directly connected to the receiver are an electrical pressure switch for operat- ing the prime mover, pressure gauges, liquefied-water drain valve, and maximum-pressure safety valve for the system. The size and complexity of the pneumatic system has a great deal to do with the included acces- sories and if they are located on the receiver or in some other location in the system. Sizing the receiver An adequately sized receiver must be provided if a pneumatic system is to operate efficiently. Formulas are available that can be used to provide a reasonable estimate of the needed size of receiver for a system. The variables involved in the operation of a pneumatic system make the strict application of a receiver-sizing formula questionable. It is often suggested that a fac- tor based on operating experience be used as a part of a recommended mathematical formula. For example, one major manufacturer suggests a factor of 1.5 to 3.0 this is based on experience alone. Examples demonstrate the mathematical concept that has just been presented, showing the math skills required to solve real-world problems. Note features present additional information related to the topic being discussed to deepen understanding. Illustrations have been designed to clearly and simply communicate the specifi c topic.