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Hydraulics and Pneumatics by Andrew A. Parr • ISBN: 0750644192 • Publisher: Elsevier Science & Technology Books • Pub. Date: March 1999 Preface Machines should work, people should think The IBM Pollyanna Principle Practically every industrial process requires objects to be moved, manipulated or be subjected to some form of force. This is generally accomplished by means of electrical equipment (such as motors or solenoids), or via devices driven by air (pneumatics) or liquids (hydraulics). Traditionally, pneumatics and hydraulics are thought to be a mechanical engineer's subject (and are generally taught as such in colleges). In practice, techniques (and, more important, the faultfinding methodology) tend to be more akin to the ideas used in electronics and process control. This book has been written by a process control engineer as a guide to the operation of hydraulic and pneumatics systems. It is intended for engineers and technicians who wish to have an insight into the components and operation of a pneumatic or hydraulic system. The mathematical content has been deliberately kept simple with the aim of making the book readable rather than rigorous. It is not, therefore, a design manual and topics such as sizing of pipes and valves have been deliberately omitted. This second edition has been updated to include recent developments such as the increasing use of proportional valves, and includes an expanded section on industrial safety. Andrew Parr Isle of Sheppey ea_parr @compuserve, com Table of Contents Preface 1 2 3 4 Fundamental principles 1 Industrial prime movers 1 A brief system comparison 2 Definition of terms 7 Pascal's law 17 Pressure measurement 21 Fluid flow 23 Temperature 28 Gas laws 31 Hydraulic pumps and pressure regulation 34 Pressure regulation 39 Pump types 42 Loading valves 51 Filters 52 Air compressors, air treatment and pressure regulation 55 Compressor types 58 Air receivers and compressor control 66 Air treatment 69 Pressure regulation 77 Service units 82 Control valves 83 Graphic symbols 86 5 6 7 Types of control valve 89 Pilot-operated valves 95 Check valves 97 Shuttle and fast exhaust valves 105 Sequence valves 106 Time delay valves 107 Servo valves 108 Modular and cartridge valves 113 Actuators 117 Linear actuators 117 Seals 130 Rotary actuators 133 Application notes 139 Hydraulic and pneumatic accessories 153 Hydraulic reservoirs 153 Hydraulic accumulators 155 Hydraulic coolers and heat exchangers 159 Hydraulic fluids 161 Pneumatic piping, hoses and connections 165 Hydraulic piping, hosing and connections 169 Process control pneumatics 171 Signals and standards 172 The flapper-nozzle 174 Volume boosters 176 The air relay and the force balance principle 177 Pneumatic controllers 179 8 Process control valves and actuators 183 Converters 192 Sequencing applications 194 Fault-finding and maintenance 199 Safety 199 Cleanliness 200 Fault-finding instruments 201 Fault-finding 204 Preventive maintenance 211 Index 219 I Fundamental principles Industrial prime movers Most industrial processes require objects or substances to be moved from one location to another, or a force to be applied to hold, shape or compress a product. Such activities are performed by Prime Movers; the workhorses of manufacturing industries. In many locations all prime movers are electrical. Rotary motions can be provided by simple motors, and linear motion can be obtained from rotary motion by devices such as screw jacks or rack and pinions. Where a pure force or a short linear stroke is required a solenoid may be used (although there are limits to the force that can be obtained by this means). Electrical devices are not, however, the only means of providing prime movers. Enclosed fluids (both liquids and gases) can also be used to convey energy from one location to another and, consequently, to produce rotary or linear motion or apply a force. Fluidbased systems using liquids as transmission media are called hydraulic systems (from the Greek words hydra for water and aulos for a pipe; descriptions which imply fluids are water although oils are more commonly used). Gas-based systems are called Pneumatic systems (from the Greek pneumn for wind or breath). The most common gas is simply compressed air. although nitrogen is occasionally used. The main advantages and disadvantages of pneumatic or hydraulic systems both arise out of the different characteristics of low density compressible gases and (relatively) high density 2 Hydraulics and Pneumatics incompressible liquids. A pneumatic system, for example, tends to have a 'softer' action than a hydraulic system which can be prone to producing noisy and wear inducing shocks in the piping. A liquid-based hydraulic system, however, can operate at far higher pressures than a pneumatic system and, consequently, can be used to provide very large forces. To compare the various advantages and disadvantages of electrical pneumatic and hydraulic systems, the following three sections consider how a simple lifting task could be handled by each. A brief system comparison The task considered is how to lift a load by a distance of about 500 mm. Such tasks are common in manufacturing industries. An electrical system With an electrical system we have three basic choices; a solenoid, a DC motor or the ubiquitous workhorse of industry, the AC induction motor. Of these, the solenoid produces a linear stroke directly but its stroke is normally limited to a maximum distance of around 100 mm. Both DC and AC motors are rotary devices and their outputs need to be converted to linear motion by mechanical devices such as wormscrews or rack and pinions. This presents no real problems; commercial devices are available comprising motor and screw. The choice of motor depends largely on the speed control requirements. A DC motor fitted with a tacho and driven by a thyristor drive can give excellent speed control, but has high maintenance requirements for brushes and commutator. An AC motor is virtually maintenance free, but is essentially a fixed speed device (with speed being determined by number of poles and the supply frequency). Speed can be adjusted with a variable frequency drive, but care needs to be taken to avoid overheating as most motors are cooled by an internal fan connected directly to the motor shaft. We will assume a fixed speed raise/lower is required, so an AC motor driving a screwjack would seem to be the logical choice. Fundamental principles 3 Neither type of motor can be allowed to stall against an end of travel stop, (this is not quite true; specially-designed DC motors, featuring good current control on a thyristor drive together with an external cooling fan, c a n be allowed to stall), so end of travel limits are needed to stop the drive. We have thus ended up with the system shown in Figure 1.1 comprising a mechanical jack driven by an AC motor controlled by a reversing starter. Auxiliary equipment comprises two limit switches, and a motor overload protection device. There is no practical load limitation provided screw/gearbox ratio, motor size and contactor rating are correctly calculated. Raise II .. II - - ~ r 3~,,,V ' - - - ~ - 415 | ~___...J Ovedoad Lower LS1 Lower [~l Raise LS2 ~--o'-'~ Raise I"-] I o--t p I i Lower Raise __--o -13"Lower (a) Electriccircuit ~~ Electric motor LS1 Top limit switch LS2 o-~ Bottom limit switch Screw jack (b) Physical layout Figure 1.1 Electrical solution, based on three phase motor 4 Hydraulics and Pneumatics A hydraulic system A solution along hydraulic lines is shown in Figure 1.2. A hydraulic linear actuator suitable for this application is the ram, shown schematically in Figure 1.2a. This consists of a movable piston connected directly to the output shaft. If fluid is pumped into pipe A the piston will move up and the shaft will extend; if fluid is pumped into pipe B, the shaft will retract. Obviously some method of retrieving fluid from the non-pressurised side of the piston must be incorporated. The maximum force available from the cylinder depends on fluid pressure and cross sectional area of the piston. This is discussed further in a later section but, as an example, a typical hydraulic pressure of 150 bar will lift 150 kg cm -2 of piston area. A load of 2000 kg could thus be lifted by a 4.2cm diameter piston. A suitable hydraulic system is shown in Figure 1.2b. The system requires a liquid fluid to operate; expensive and messy and, consequently, the piping must act as a closed loop, with fluid transferred from a storage tank to one side of the piston, and returned from the other side of the piston to the tank. Fluid is drawn from the tank by a pump which produces fluid flow at the required 150 bar. Such high pressure pumps, however, cannot operate into a dead-end load as they deliver constant volumes of fluid from input to output ports for each revolution of the pump shaft. With a dead-end load, fluid pressure rises indefinitely, until a pipe or the pump itself fails. Some form of pressure regulation, as shown, is therefore required to spill excess fluid back to the tank. Cylinder movement is controlled by a three position changeover valve. To extend the cylinder, port A is connected to the pressure line and port B to the tank. To reverse the motion, port B is connected to the pressure line and port A to the tank. In its centre position the valve locks the fluid into the cylinder (thereby holding it in position) and dead-ends the fluid lines (causing all the pump output fluid to return to the tank via the pressure regulator). There are a few auxiliary points worthy of comment. First, speed control is easily achieved by regulating the volume flow rate to the cylinder (discussed in a later section). Precise control at low speeds is one of the main advantages of hydraulic systems. Second, travel limits are determined by the cylinder stroke and cylinders, generally, can be allowed to stall at the ends of travel so no overtravel protection is required. Fundamental principles A i ; ,, 5 w B Raise (a) Hydraulic cylinder / / / / t/ ,'t , I' I _ \ \\ motor \ "', H Filter H ~] I ,~ t=%2,, Pump I \ Electric ('M) // // /- - Off Raise q 9 9 Lower \\ Pressure regulation ~ =xoe I r "\, ~ "'. _ g _ ,......?;;J. ! fC,I~ f~ /w\ = I1 ~ NEd va've ~ ' I I I L_ . . . . . . . . . . . . . . . . . . . . I I~- Components common J to many motions (b) Physical components Figure 1.2 Hydraulic solution Third, the pump needs to be turned by an external power source; almost certainly an AC induction motor which, in turn, requires a motor starter and overload protection. Fourth, hydraulic fluid needs to be very clean, hence a filter is needed (shown in Figure 1.2b) to remove dirt particles before the fluid passes from the tank to the pump.