Application guidelines for selected fluid power components

Transcription

1 1 July 2010 Application guidelines for selected fluid power components Accumulators Cylinders Hydraulic valves Pneumatic filters, regulators and lubricators Selection of position transducers published by NATIONAL FLUID POWER ASSOCIATION, INC N. Mayfair Road / Milwaukee, WI USA PHONE: / FAX: / nfpa@nfpa.com

2 Copyright 2010 by the NATIONAL FLUID POWER ASSOCIATION Printed in the USA All standards, recommended practices, information reports, and bibliographies (collectively, NFPA Documents ) are advisory only. Use thereof by anyone for any purpose is entirely voluntary and in any event without risk of any nature to the National Fluid Power Association (NFPA), its officers, directors or authors of such work. There is no agreement by or between anyone to adhere to any NFPA Document. In formulating and approving NFPA Documents, NFPA and/or its councils and committees will not investigate or consider citations, references or patents which may or may not apply to such subject matter since prospective users of such NFPA Documents alone are responsible for establishing necessary safeguards in connection with utilization of such matters, including technical data, proprietary rights or patentable materials. The information and data contained in NFPA Documents has been obtained from sources believed to be reliable. However, it should not be assumed that all acceptable or applicable sources of information, procedures, methods or techniques are contained in NFPA Documents, or that additional measures may not be required under certain circumstances or conditions. NFPA Documents and/or policies and procedures are subject to periodic review and may be changed without notice. NFPA Documents are only current as of their publication date. NFPA Documents, after publication, may be revised or withdrawn at any time and current information on all NFPA Documents may be received by calling or writing NFPA. Additionally, the various codes and regulations referenced in NFPA Documents may be amended from time to time and it should not be assumed that the versions referenced therein are the most current versions of such codes and regulations. Please consult the appropriate regulatory authorities for the most up-to-date versions. NFPA Documents imply a consensus of those substantially concerned with their scope and provisions and are intended as a guide to aid the manufacturer, the consumer and the general public. The publication of NFPA Documents does not in any respect preclude anyone, whether they have participated in the development of or approved such NFPA Documents or not, from manufacturing, marketing, purchasing, or using of products, processes or procedures not conforming to the NFPA Documents. NFPA Documents do not constitute or indicate a warranty of any sort, express or implied, including but not limited to a warranty or representation as to quality, merchantability or fitness for a particular use or purpose. Participation by federal agency representative(s) or person(s) affiliated with the industry is not to be interpreted as government or industry endorsement of an NFPA Document(s). NOTICE NFPA Documents do not express or imply any judgment, certification or endorsement of or with respect to, the safety, design or performance of any product, component, or its use. NFPA does not examine, investigate, test, recommend, or certify the design, use or safety of any product or component, even those which may incorporate one or more NFPA Documents. NFPA Documents therefore have no application to and do not express or imply any recommendation, representation or warranty, with respect to the safety, design, use, performance, or functional interchangeability of components or products which incorporate NFPA Documents. This publication may not, in whole or in part, be reproduced, copied or disseminated, entered into or stored in a computer database or retrieval system, or otherwise utilized without the prior written permission of NFPA.

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4 Application guidelines Table of contents Application guidelines for selected fluid power components Table of contents Accumulators... page 1 Cylinders. page 9 Hydraulic valves page 21 Pneumatic filters, regulators, lubricators. page 31 Selection of position transducers... page 39

5 Application guidelines - Accumulators Application guidelines Accumulators Introduction In fluid power systems, power is transmitted and controlled through a fluid (liquid or gas) under pressure within an enclosed circuit. An accumulator is defined as a device for storing hydrostatic energy and subsequently releasing it to do useful work. The energy can be stored in the form of a compressed spring, lifted weight or compressed gas. By far the compressed gas (which must be an inert gas such as nitrogen) is the most common and is the type discussed in this document. This type is called a hydropneumatic accumulator. This is defined as an accumulator in which compressed gas applies force to the stored liquid. 1 Scope This recommended practice applies to bladder, diaphragm, and piston types of accumulators for their use in fluid power systems for industrial, marine, and mobile applications. 2 Normative references The following normative documents contain provisions which, through reference in this text, constitute provisions of the Application guidelines Accumulators section (pages 1-7) of this NFPA document. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this NFPA document are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the normative document referenced applies. NFPA maintains registers of currently valid NFPA Standards. Standards development organization contact information and links can be found on the NFPA website ( EN 982 (latest edition), Safety of machinery Safety requirements for fluid power systems and their components Hydraulics. ISO 4413 (latest edition), Hydraulic fluid power General rules relating to systems. ISO 5598 (latest edition), Fluid power systems and components Vocabulary. 3 Definitions For the purposes of this recommended practice, the terms and definitions given in ISO 5598 apply. 1

6 Application guidelines - Accumulators 4 Accumulator types The following is a description of commonly used hydropneumatic accumulators. For their official definitions see ISO a) A bladder accumulator is an accumulator in which the liquid and gas are separated by an elastic bag or bladder. b) A diaphragm accumulator is an accumulator in which the liquid and gas are separated by a flexible diaphragm. c) A piston accumulator is an accumulator in which the liquid and gas are separated by a floating piston. d) A nonseparator accumulator is an accumulator in which the compressed gas operates directly on liquid within the pressure chamber. This type of an accumulator is not normally recommended because of safety reasons. Standards such as ISO 4413 state that circuits incorporating accumulators shall vent the accumulator liquid pressure or shall positively isolate the accumulator when the equipment is shut off. 5 General 5.1 A hydropneumatic accumulator works by storing energy in the form of a compressed gas. Normally this is an inert gas such as nitrogen. The accumulator is initially charged with some amount of gas pressure (the actual value of this pressure depends on the application); see Figure 1. At some point in the machine cycle, a pump will typically push hydraulic fluid into the hydraulic fluid side of the accumulator. At this point, the separator (a bladder, diaphragm or piston) will compress the gas to the same pressure as the hydraulic fluid; see Figure 2. Separator Gas precharge As charged Figure 1 Accumulator empty of hydraulic fluid and containing some amount of gas precharge Separator Hydraulic fluid Gas pressure Maximum operating pressure Figure 2 Accumulator charged with hydraulic fluid at maximum system pressure 5.2 When hydraulic fluid flow from the accumulator is required, a valve in the hydraulic system will direct the fluid to the component or components while the expanding gas pushes on the separator, expelling the hydraulic fluid from the accumulator. The pressure of the hydraulic fluid coming from the accumulator will follow the thermodynamic laws as applied to the expanding gas on the other side of the separator. Since the gas volume is not infinite, the hydraulic pressure will drop as an accumulator is discharged; see Figure 3. 2

7 Application guidelines - Accumulators Separator Hydraulic fluid Gas pressure Minimum required system pressure NOTE All of the hydraulic fluid has not been expelled. It is considered a good practice to maintain some fluid in the accumulator at the end of the discharge cycle for longer accumulator life. Figure 3 Accumulator, after it has been discharged and system is at minimum required system pressure 5.3 The general gas law for expansion or compression of a gas is described by the following equation: P 1 V 1 n = P 2 V n 2 where P 1 is the initial pressure; V 1 is the initial volume; P 2 is the final pressure; V 2 is the final volume; n is the polytropic exponent. 5.4 If an accumulator is discharged very slowly (this is seldom the case), the polytropic exponent is the isothermal exponent and is equal to 1.0. However, if the discharge is rapid, this value will be closer to the adiabatic exponent and is greater than 1.0. For nitrogen this factor will vary depending on several operating conditions. Consult your accumulator supplier for specific applications. The gas pressure may also vary when an accumulator is charged based on the rate at which it is charged. Again, consult your accumulator supplier for specific applications. 5.5 On occasions, especially when the operation requires the maximum and minimum operating pressures to be close to each other a large amount of gas volume may be required. An auxiliary gas bottle may be used as shown in Figure 4. 3

8 Application guidelines - Accumulators Auxiliary gas bottle Gas pressure Separator Gas pressure Hydraulic fluid Figure 4 Illustration of an auxiliary gas bottle used with a hydropneumatic accumulator. 5.6 Hydropneumatic accumulators are considered pressure vessels and, as a result, they may be subject to certain laws. Each state in the U.S. as well as each province in Canada has laws concerning pressure vessels. Foreign countries also have laws concerning pressure vessels. 5.7 Several national and international standards cover the application of an accumulator such as ISO 4413 and EN 982. Other industries may have their own standards. 6 Applications 6.1 General Accumulators are used to store or absorb energy. Typical applications are: a) energy storage; b) pulsation dampening; c) compensation for thermal expansion; d) hydraulic shock suppression. 6.2 Energy storage Supplementing pump flow In cases where relatively high actuator velocities are required with dwell time between cycles, it is possible to use an accumulator in conjunction with a pump to make a more efficient system. During the dwell time in the cycle, the pump charges the accumulator. Then when the actuator is activated, both the accumulator and the pump supply fluid under pressure to the actuator Minimum information required for this application: a) maximum hydraulic fluid pressure required at actuator; b) minimum hydraulic fluid pressure required at the actuator; 4

9 Application guidelines - Accumulators c) hydraulic fluid flow rate and volume required by actuator or actuators; d) pump displacement (volumetric); e) dwell time Leakage compensation Hydraulic valves and cylinder pistons may have some internal leakage. In some applications, it may be desirable to make up this leakage by using an accumulator. This is possible as long as there is some dwell time in the machine cycle so that the pump can recharge the accumulator Minimum information required for this application: a) maximum hydraulic fluid pressure required at actuator; b) minimum hydraulic fluid pressure required at the actuator; c) expected leakage volume; d) pump displacement (volumetric); e) dwell time Emergency (auxiliary) power source Certain equipment may require some sort of stored energy to be used in an emergency. In such cases, an accumulator is charged with hydraulic fluid that can be relieved manually or electrically in an emergency Minimum information required for this application: a) maximum hydraulic fluid pressure required; b) minimum hydraulic fluid pressure required at the actuator; c) hydraulic fluid flow rate and volume required by actuator or actuators. 6.3 Pulsation dampening Positive displacement pumps, especially piston type, generate hydraulic flow that may exhibit pressure ripple, which is not always desirable. The exact amount of ripple depends on pump displacement, pressure and number of pumping chambers. Accumulators can be used to smooth out these pressure ripples Minimum information required for this application: a) pump displacement (volumetric); b) number of pumping chambers in pump; c) rotational speed of pump; d) system pressure. 5

10 Application guidelines - Accumulators 6.4 Compensation for thermal expansion Thermal expansion, if not compensated for in a closed loop hydraulic system, can increase pressure beyond acceptable limits. Accumulators absorb the volume increase and return fluid to the line when the temperature falls Minimum information required for this application: a) type of hydraulic fluid; b) fluid conductor material; c) volume of contained fluid; d) normal operating temperature; e) maximum operating temperature; f) normal operating pressure; g) maximum operating pressure. 6.5 Hydraulic shock suppression When valves open and close quickly, they can generate shock waves in a hydraulic system. Both shock and vibration can generate noise and also lead to premature wear and tear on machine components. Accumulators have relatively good damping characteristics over a wide frequency range that effectively reduces shock and vibration Minimum information required for this application: a) flow rate in hydraulic lines; b) specific gravity of hydraulic fluid; c) length and size of hydraulic lines; d) valve closing time; e) operating pressure; f) maximum operating pressure. 7 Ports It is suggested that the port at the hydraulic end of the accumulator be of a type that is sealed by an elastomer rather than an interference fit, such as a pipe thread port. It is also suggested that the port size be sufficiently large for the required fluid flow rate out of the accumulator. 8 Environment Accumulators contain static seals as well as dynamic seals in the separator. These seals, which are available in several compounds, are exposed to the operating condition of the accumulator as well as the hydraulic fluid. Seal selection 6

11 Application guidelines - Accumulators should be done together with the accumulator manufacturer, taking into account the maximum and minimum operating temperature and hydraulic fluid used. 9 Corrosion Accumulators are considered pressure vessels. As a result, corrosion, both internal and external, should be taken into account. Internal corrosion that can be caused by the hydraulic fluid will both weaken the pressure vessel and damage the separator. External corrosion will weaken the pressure vessel. Therefore, the environment to which the accumulator is exposed must be considered. Some codes also require that a corrosion allowance be addressed in the stress calculations related to the pressure vessels. 10 Mountings An accumulator should be securely mounted to a machine frame. The hydraulic plumbing should not be the sole method by which the accumulator is secured to a machine frame. Care should be taken not to squeeze the body of piston accumulators by overtightening U-bolt type brackets. Your accumulator supplier should be consulted for the proper method of securing the accumulator to the machine frame, as well as its orientation. 11 Precharge pressure 11.1 Each specific application shall be accomplished most effectively with a specific value of precharge pressure. The specific value shall be determined by the system operating conditions. Variation from this pressure will cause the system to degrade in performance. Since the precharge pressure is gas pressure in a contained vessel, it will vary as a function of temperature, i.e., if the gas temperature increases, the pressure will increase, and if the gas temperature decreases, the pressure will decrease In a typical hydraulic system, the operating temperature of the accumulator is usually higher than the temperature of the surrounding environment. When checking the precharge pressure, it is important to determine the temperature of the gas. It is easiest to check the pressure when the equipment has been shut down for some time and the gas temperature has stabilized to the temperature of the surrounding environment. Also, shutting down the equipment may remove all hydraulic fluid from the accumulator, which will affect the gas precharge pressure value. Therefore, it is important to know how the precharge pressure in this condition will translate to a gas pressure at operating conditions. The accumulator supplier can help to determine these two values. 12 Maintenance 12.1 An accumulator should be charged only with an inert gas, such as nitrogen Charging of an accumulator or the periodic checking of proper precharge pressure should be done only in accordance with the accumulator manufacturer s instructions and using only components specified by the accumulator manufacturer Any repair to an accumulator shall be done after it has been relieved of liquid and gas precharge pressure and removed from the hydraulic system. The discharging, removal and repair shall be done by a properly trained person following procedures specified by the accumulator manufacturer. 7

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13 Application guidelines - Cylinders Application guidelines Cylinders Introduction In fluid power systems, power is transmitted and controlled though a fluid under pressure within an enclosed circuit. The application of hydraulic and pneumatic cylinders requires precise communication between manufacturer and user. It is the intent of this recommended practice to provide basic guidance in cylinder applications and to promote communication. Users of this recommended practice are cautioned, however, to review each clause for applicability and to utilize sound engineering judgement 1 Scope This recommended practice will apply to hydraulic and pneumatic fluid power cylinders for their use in hydraulic and pneumatic systems for industrial, marine, and mobile applications. 2 Normative references The following standards contain provisions which, through reference in this text, constitute provisions of the Application guidelines Cylinders section (pages 9-20) of this NFPA document. At the time of publication, the editions indicated were valid. All documents are subject to revision, and parties to agreements based on this NFPA document are encouraged to investigate the possibility of applying the most recent editions of the documents indicated below. NFPA maintains registers of currently valid NFPA standards. NFPA/T3.6.7 R1-1996, Fluid power systems and products Square head industrial cylinders Mounting dimensions. ISO 5598:1985, Fluid power systems and components Vocabulary. 3 Definitions A description of different types of cylinders is given in this document several times. For their official definition as well as definition of other terms see ISO Operation and cylinder concepts Cylinders are used when linear force and motion are required. Cylinders are broken down into two main categories: pneumatic and hydraulic. Pneumatic cylinders can be operated by several types of gases, however, compressed air is by far the most common. Hydraulic cylinders can be operated with a very large range of fluids. By far the most common is petroleum based hydraulic fluid. Fire-resistant fluids are also common, they may be synthetic or water based. 9

14 Application guidelines - Cylinders Cylinders can be broken down into two main components. The pressure containing envelope and the piston and rod assembly. Typically the pressure containing envelope is fixed on the machine and the piston and rod assembly is attached to the machine member on which motion and force need to be applied. In rare occasions the opposite is done. See figure 1. Pressure containing envelope Piston and Rod assembly Motion 4.1 Force. Figure 1 Typical cylinder The amount of force applied to the machine member is the product of the fluid pressure multiplied by the effective area of the piston. Most cylinders contain a single piston rod. Assuming a constant value for the applied pressure, a cylinder would normally exert more force in the push stroke than it will in the retract stroke. The amount by which the retract force is less than the extend force depends on the cross-sectional area taken up by the piston rod. See figure 2. For example, a 100 mm bore (10 cm) cylinder housing a 45 mm (4.5 cm) diameter piston rod operating at 1,000 kilopascals (1,000 kpa) will deliver 7,854 neutons (N) of force in the push direction and 6,264 N in the pull direction. Example: one neuton (N) of force = one pascal (Pa) applied to an area one meter square (M 2 ). N=Pa M 2 Calculation for push stroke surface area = Π 4 D2 bore Calculation for pull stroke surface area = Π 4 (D2 bore - D 2 rod) An area of cm 2 = M 2 In push direction force (N) = 1,000 kpa x M 2 =7,854 neutons In pull direction force (N) = 1,000 kpa x M 2 =6,264 neutons F p ull F p ush 10

15 Application guidelines - Cylinders 10 cm Dia piston 4.5 cm Dia. Piston rod Area = cm 2 Area = cm Velocity Figure 2 Effective areas The velocity at which the piston and rod assembly moves is dependent on the rate at which the fluid is introduced into the cylinder. Normally a cylinder is connected to a directional valve and sometimes flow control valves. In a pneumatic system the maximum speed of the piston and rod assembly is dependent on the rate at which the air can flow through the valving. For a hydraulic cylinder, the maximum piston rod velocity depends on the ability of the hydraulic system to provide the hydraulic fluid to the cylinder. In most applications piston rod velocity is held in the.3 to.9 m/s. However, velocities up to 3 m/s have been achieved. See figure 3. 11

16 Application guidelines - Cylinders Hydraulic circuit Pneumatic circuit Figure 3 Hydraulic and pneumatic circuits 5 Basic construction The following three types of construction are the more commonly used types of cylinders. 5.1 Square head tie rod This is a very common type of cylinder used in industrial applications. Head and cap enclosures are secured by tie rods that run the length of the cylinder. See figure 4. 12

17 Application guidelines - Cylinders Figure 4 Square head tie rod cylinder 5.2 Bolted end These cylinders have flanges that are attached to the barrel. The end caps are attached to these flanges with bolts. See figure Nonbolted end Figure 5 Bolted end This type of cylinder has end caps that are usually retained by welding or with a locking ring. In many cases these are used for mobile applications. Some pneumatic cylinders crimp the tube onto the end caps. See figure 6. Figure 6 Nonbolted end 6 Types of cylinders Double acting, single piston rod cylinders are by far the most common types of hydraulic and pneumatic cylinders used in industry. However, there are other configurations which are used in special situations. 6.1 Double acting By far, most cylinders are considered double acting. That is, when the cylinder is required to extend, pressure is applied to the cap end of the cylinder while pressure is exhausted from the rod end. When the cylinder is required to retract, pressure is applied to the rod end and the cap end is exhausted. See figure 7. 13

18 Application guidelines - Cylinders Fluid flow Fluid flow Fluid flow Fluid flow Thrust Thrust Figure 7 Double acting 6.2 Single acting In these types of cylinders pressure is applied only to one side of the piston, and some other force is used to move the piston and rod assembly in the opposite direction. In some cases it may be gravity; however, in most cases this is accomplished by a spring. Springs can be mounted on either side of the piston. However, most commonly a spring is mounted on the rod end of the piston and is used to retract the cylinder. Springs can be mounted in either hydraulic or pneumatic cylinders. Normally the mechanical force provided by a spring is rather small in comparison to the potential force which a hydraulic cylinder can apply. Therefore, most single acting cylinders tend to be pneumatic. See figure 8. Fluid flow Spring 6.3 Double rod Figure 8 Single acting Double rod cylinders can be used so that the applied load to the machine member is the same in both directions. Sometimes a smaller rod is used on one end to trip limit switches. See figure Ram cylinders Figure 9 Double rod end The rod and piston are the same diameter. This type of cylinder is usually single acting, with an external force used to retact the cylinder. A typical application is a lift in a service station. See figure

19 Application guidelines - Cylinders 6.5 Telescopic cylinders Figure 10 Ram Telescopic cylinders utilize multiple sections to achieve long strokes when short collapsed lengths are required. This type of configuration is sometimes used in cylinders for dump trucks or lift trucks. These are usually single acting. The net force is a function of the smallest section. See figure Tandem cylinders Figure 11 Telescopic Tandem cylinders consist of two pistons attached to a common piston rod. These cylinders are used in situations where space requirements are very tight and the combined areas of two pistons can be applied to one piston rod. These are used to increase force. In some cases such as a press, a tandem cylinder is constructed of a large press cylinder with a small cylinder attached to the back. This configuration is used where the smaller bore cylinder is pressurized to rapidly advance the larger cylinder. See figure Duplex cylinders Figure 12 Tandem Duplex cylinders consist of multiple piston and rod assemblies which are not connected to each other. This configuration is often used where more than two distinct stopping points are required. See figure 13. or 6.8 Rodless cylinders Figure 13 Duplex These cylinders are typically used when limited space prohibits the use of a cylinder with a piston rod. 15

20 Application guidelines - Cylinders Direct coupled The carriage and piston are directly connected. See figure 14. Carriage Piston Figure 14 Direct coupled Magnetically coupled The carriage is and piston are magnetically coupled. See figure 15. Carriage Piston Cable connected Figure 15 Magnetically coupled The carriage is connected to the piston by a cable or flexible band. See figure 16. Carriage Motion Cable/band Fluid flow Piston Fluid flow Figure 16 Cable connected 16

21 Application guidelines - Cylinders 7 Mounting 7.1 Fixed and pivot The pressure containing envelope is normally mounted to the machine. This is normally done with one of two types of mounting configurations. A fixed type mounting or a pivot type mounting. If the machine member moves in a straight line type motion, a fixed mount can be used. If the machine member is required to move in an arc a pivot mounting must be used. See figure 17. Fixed mount Pivot mount 7.2 Types Figure 17 Fixed and pivot mounts Fixed mounts are available in many configurations. Typical mountings may be front or rear flange mountings, tie rod extended mounts, or foot mounts. Pivot mounts can be head, cap or center trunnions, or a rear cap pivot mount. For a more complete list of the mounts, consult the manufacturer or NFPA/T See figure 18 for some examples. Figure 18 Typical mounts 17

22 Application guidelines - Cylinders 7.3 Mounting enhancements Selection of any mounting style depends primarily upon the operating specifications of the application. Cylinder mounting directly affects the maximum operating pressure at which the cylinder may be used. Whether the cylinder is used in thrust (push) or tension (pull), its stroke length, piston rod diameter and method of connection to the load must be considered when selecting a mounting style. Fixed mountings that absorb the force on the centerline of the cylinder are considered the best for straight line force transfer. The symmetrical mountings allow the thrust or tension forces to be distributed uniformly within the mounting framework. Flange mounts are considered the best mounting styles within this category. Front flange mounts are ideally suited when the piston rod is in tension (pull), while cap flanges are recommended for thrust (push) applications. Fixed noncenterline mountings do not absorb forces on their centerline. The offset thrust introduces bending stresses and additional loads on the mounting bolts. This is especially true in short stroke applications. This type should be very well aligned for maximum service life. Use of shear pins or keys should be considered when these cylinders are subjected to high pressures or shock loads. These mountings are among the easiest to use for mounting and replacement ease. If the path of the load is curved or misalignment is a problem, a pivoted centerline mounting should be used. These mounts offer compensation for non-linear travel in only one plane. Pivot mounts require a pivot type rod attachment. Threaded rod end attachments should be torqued tightly against a threaded shoulder to minimize bending and reduce fatigue stresses. 8 Design considerations 8.1 Column strength Piston rod column failure (buckling) can occur if the rod diameter is not sized to match the stroke and load. The manufacturer should be consulted for application assistance. Reference figure 19 for an example of piston rod column failure. 8.2 Bearing load Figure 19 Piston rod column failure Cylinders are normally intended to push and pull without excessive side load. If a side load exists, the manufacturer should be consulted. Usually bearing load can be reduced with the addition of a stop tube. The use of a stop tube increases the distance between the piston bearing and piston rod bearing. See figure 20. Piston bearing Stop tube Piston rod bearing Figure 20 Stop tube 18

23 Application guidelines - Cylinders 8.3 Cushions The purpose of a cushion is to decelerate the piston and rod assembly as it nears the end of stroke, preventing excessive mechanical stresses. Cushions may be of either a fixed or adjustable design. Both designs function by providing a bypass passage to remove the pressurized fluid trapped between the piston and the cylinder head when the cushion sleeve has entered the cushion bore. Varying the orifice opening with an adjustable cushion screw allows the user to select the best cushion rate for the system. A check valve in the cushion assembly allows for the free flow of fluid back to the piston face for quick acceleration when the rod is withdrawn. Heavy loads may also require an external device to stop the motion. Cushions are available on either or both ends of the cylinder. Consult manufacturer for the conditions under which cushions should be used. See figure 21. Fluid flow Piston rod motion Figure 21 Cushions 8.4 Eccentric Loads A very important general consideration is to keep the cylinder thrust as close as possible to the centerline of the piston rod and free from misalignment or side thrust. Off-center thrust or side loads can substantially reduce the service life expected from the rod bearing and seals. Off-center thrust and side loading can be caused by cylinder deflection under load, machine frame deflection, rod bending or sagging, as well as by poor design of the machine. See figure 22. Piston rod centerline Applied load Figure 22 Eccentric Loads 9 Compatibility The temperature and fluid type must be considered in any application to ensure proper seal effectiveness and life. The manufacturer should be consulted for assistance. 10 Corrosion Surfaces of either hydraulic or pneumatic cylinders can be corroded by water or other substances. Although paint may be suitable protection against corrosion for most indoor industrial uses, the cylinder specifier should be aware of corrosion potential in the operating environment. The material content and surface preparation of the piston rod must be considered in any application where a corrosive environment exists. Consult the cylinder manufacturer for corrosion protection recommendations. 19

24 Application guidelines - Cylinders 11 Maintenance Cylinder maintenance primarily involves replacing seals and/or bearings. Consult the manufacturer for maintenance guidelines Visual inspection Degradation of cylinder thrust or erratic motion can indicate seal leakage. Prior to disassembly, visual inspection can be performed on the rod seal and tube end seals. This is accomplished by inspecting these locations for either oil or escaping air depending on whether the cylinder is hydraulic or pneumatic. Inspection of the piston seals and bearing require disassembly Disassembly Prior to performing any maintenance that requires the disassembly of the cylinder, the pressure within the cylinder must be zero. The machine manufacturer should be consulted for any lock-out requirements. Follow the cylinder manufacture s recommended disassembly and reassembly procedures, paying particular attention to prescribed fastener torque values and internal springs. While the cylinder is disassembled, inspect all components for evidence of unusual wear. Signs of internal corrosion may indicate the presence of water or lack of lubrication. Signs of contamination may indicate improper filtration. Proper preventative maintenance will extend seal and bearing life. Wear can be caused by misalignment of the cylinder and load, or other problems that should be remedied before the cylinder is returned to service Preventative maintenance. Preventative maintenance such as hydraulic oil filtration and compressed air filtration/drying will extend cylinder service intervals. 20

25 Application guidelines - Hydraulic valves Application guidelines Hydraulic valves Introduction In hydraulic fluid power systems, power is transmitted and controlled through a liquid under pressure within an enclosed circuit. The application of hydraulic valves requires precise communication between manufacturer and user. It is the intent of this recommended practice to provide basic guidance in valve application and to promote communication. Users of this recommended practice are cautioned, however, to review each section for applicability and to utilize sound engineering judgment. 1 Scope 1.1 This recommended practice is applicable to hydraulic valves used in industrial, marine and mobile equipment. 1.2 This recommended practice provides basic guidance for the selection, installation, operation, and maintenance of hydraulic valves. 2 Normative references The following normative documents contain provisions which, through reference in this text, constitute provisions of the Application guidelines -- Hydraulic valves section (pages 21-29) of this document. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. However, parties to agreements based on this NFPA document are encouraged to investigate the possibility of applying the most recent editions of the normative documents indicated below. For undated references, the latest edition of the normative document referenced applies. NFPA maintains registers of currently valid NFPA Standards. Standards development organization contact information and links can be found on the NFPA website ( NFPA/T2.6.1 R (R2005), Fluid power components Method for verifying the fatigue and establishing the burst pressure ratings of the pressure containing envelope of a metal fluid power component NFPA/T (latest edition), Hydraulic fluid power systems Methods for preventing external leakage. NFPA/T3.5.1 (latest edition), Hydraulic fluid power Valves Mounting surfaces. NFPA/T (latest edition), Hydraulic fluid power Directional control valves Method for determining the metering characteristics. NFPA/T (latest edition), Hydraulic fluid power Valves Method for determining the internal leakage characteristics. NFPA/T (latest edition), Hydraulic fluid power Flow control valves Method for measuring and reporting regulating characteristics. NFPA/T R (R2005), Hydraulic valve Pressure rating supplement to NFPA/T2.6.1 R Method for verifying the fatigue and burst pressure ratings of the pressure containing envelope of a metal fluid power hydraulic valve 21

26 Application guidelines - Hydraulic valves IEEE/ASTM SI 10 (latest edition), Standard for Use of the International System of Units (SI): The Modern Metric System. ISO 1000 (latest edition), SI units and recommendations for the use of their multiples and of certain other units. ISO (latest edition), Fluid power systems and components Graphic symbols and circuit diagrams Part 1: Graphic symbols for conventional use and data processing applications (second edition) ISO (latest edition), Fluid power systems and components Graphic symbols and circuit diagrams Part 2: Circuit diagrams ISO 4401 (latest edition), Hydraulic fluid power Four-port directional control valves Mounting surfaces. ISO 4406 (latest edition), Hydraulic fluid power Fluids Method for coding level of contamination by solid particles. ISO 5598 (latest edition), Fluid power systems and components Vocabulary. ISO (latest edition), Connections for hydraulic fluid power and general use Ports and stud ends with ISO 261 metric threads and O-ring sealing Part 1: Port with truncated housing for O-ring seal (second edition) ISO (latest edition), Connections for hydraulic fluid power and general use Ports and stud ends with ISO 261 threads and O-ring sealing Part 2: Dimensions, design, test methods and requirements for heavy-duty (S series) stud ends (second edition) ISO (latest edition), Connections for hydraluic fluid power and general use Ports and stud ends with ISO 261 threads and O-ring sealing Part 3: Dimensions, design, test methods and requirements for light-duty (L series) stud ends (second edition) ISO (latest edition), Hydraulic fluid power Flange connectors with split or one-piece flange clamps and metric or inch screws Part 1: Flange connectors for use at pressures of 3,5 MPa (35 bar) to 35 MPa (350 bar), DN 13 to DN 127 ISO (latest edition), Hydraulic fluid power Flange connectors with split or one-piece flange clamps and metric or inch screws Part 2: Flange connectors for use at pressures of 35 MPa (350 bar) to 40 MPa (400 bar), DN 13 to DN 51 ISO 7368 (latest edition), Hydraulic fluid power Two-port slip-in cartridge valves Cavities. ISO (latest edition), Hydraulic fluid power Electrically modulated hydraulic control valves Part 1: Test methods for four-way directional flow control valves. ISO (latest edition), Connections for general use and fluid power Ports and stud ends with ISO 725 threads and O-ring sealing Part 1: Ports with O-ring seal in truncated housing ISO (latest edition), Connections for general use and fluid power Ports and stud ends with ISO 725 threads and O-ring sealing Part 2: Heavy-duty (S series) stud ends ISO (latest edition), Connections for general use and fluid power Ports and stud ends with ISO 725 threads and O-ring sealing Part 3: Light-duty (L series) stud ends SAE J518 (latest edition), Hydraulic flanged tube, pipe, and hose connections, 4 bolt split flange type. SAE J1783 (latest edition), Selection of hydraulic directional control valves for marine vehicle applications. 22

27 Application guidelines - Hydraulic valves 3 Terms and definitions For the purposes of this NFPA document, the terms and definitions given in ISO 5598 apply. 4 General 4.1 Valve classifications Hydraulic valves can be classified into four general categories based on their function. These categories are described in through Valve assemblies or subassemblies comprise components from more than one valve category, such as a directional valve incorporating an internal pressure control valve. See ISO and ISO for hydraulic valve graphic symbols and their use in circuit diagrams Directional control valves Directional control valves connect or isolate one or more flow paths. They are commonly used to control speed and direction of a hydraulic actuator. They may range from a simple two-port check valve to a multiple-port directional valve. Application guidelines for these valves are contained in clause Pressure control valves Pressure control valves are commonly used to limit or control pressure excursions of the pump or of portions of the hydraulic circuit. They may range from direct-acting relief valves to pilot-operated counterbalance valves. Application guidelines for these valves are contained in clause Flow control valves Flow control valves are used to limit or control flow with the circuit or portions of the circuit. They may range from a simple fixed orifice to a pressure- and temperature-compensated flow control valve. Application guidelines for these valves are contained in clause Load holding valves Load holding valves are used to prevent drift of a loaded actuator and to prevent uncontrolled movement of an actuator due to overrunning loads. Application guidelines for these devices are contained in clause Fluids Hydraulic fluids may range from petroleum oils to synthetics, fire resistant fluids and biodegradable fluids. Select a fluid that is appropriate for the system and machine type and environment. Fluid types and equipment manufacturer s recommendations may vary widely among mobile, industrial, and marine systems. Ensure that valves, including seals and electrical wiring, are compatible with the system fluid. 4.3 Fluid conditioning Fluid filtration requirements depend upon several factors, including the type of system and components, the severity of the application, the operating environment, and the level and frequency of maintenance. Filtration should be provided to maintain the system fluid within the cleanliness level recommended by the valve manufacturer and system designer. The system design and installation should minimize entrained air and other contaminants, such as water. The fluid temperature and water content should be maintained at levels consistent with the fluid and component manufacturer s recommendations. Acceptable fluid temperature ranges will differ among mobile, industrial, and marine systems. 23

28 Application guidelines - Hydraulic valves 4.4 Ports Valve ports should be of the O-ring sealed straight thread type conforming to ISO or ISO , or of the flange type conforming to ISO , ISO or SAE J518. Ports with pipe threads should not be used because of the potential for external leakage. See also NFPA/T for additional information on port types and their characteristics. 4.5 Mounting Valves are available in many different construction and mounting configurations, including: a) independently mounted, of monoblock, sectional, or combination block construction b) subplate or manifold mounted, in accordance with ISO 4401, NFPA/T3.5.1, or custom interface design c) cartridge design and installation, of the screw-in type in accordance with ISO 7789 or NFPA/T3.5.50, or of other custom design. d) cartridge design and installation, of the slip-in type in accordance with ISO 7368, or of other custom design e) actuator installed or direct mounted The function or at rest mode of some valves may be influenced by the mounting direction. Consult the manufacturer for recommendations regarding preferred mounting. 4.6 Location In most cases, valves should be mounted as close to the actuators as practical. Valves should also be installed so that they are accessible for maintenance and removal. In the case of manual valves, ease of actuation shall be taken into account. 4.7 Pressure rating and reliability Valve selection should take into account the maximum pressure excursions in the system, the operating duty cycle, and the expected service life. The potential for overloads, external mechanical loads, and other abnormal conditions should be considered. Fatigue rating of the pressure containing envelope(s) of valves should be in accordance with NFPA/T2.6.1 and NFPA/T Repair and maintenance Specific instructions for maintenance and repair should be contained in the equipment manual(s) and/or on equipment decals. If a repair involves disassembly of a valve or its hydraulic connections, care shall be taken to ensure that the cleanliness levels recommended in 4.3 are maintained before the valve is returned to service. Replacement fluid shall be of the proper type and shall meet the cleanliness requirements of Other application considerations Other system and application characteristics that may affect the performance or reliability of valves include: a) excessive vibration or shock, particularly if valves are cantilever mounted or if the system could be unstable under certain conditions; b) intermittent temperature extremes, such as cold startup conditions or temporary overheating; c) exposure to unusual or extreme environments, such as water spray or immersion, corrosive chemicals, fine dust, abrasive materials, etc. See SAE J1783 for marine application guidance; d) hazardous locations that may require explosion-proof or intrinsically safe devices. 24

29 Application guidelines - Hydraulic valves 5 Directional control valve application guidelines 5.1 Check (non-return) valves Unpiloted two-port check (non-return) valves are one of the simplest forms of directional control valves, blocking flow in one direction and allowing essentially free flow or flow at a prescribed pressure in the opposite direction. The check valve shall be sized to provide an acceptable pressure drop at maximum flow and shall be designed to provide acceptable internal leakage control in the blocking condition. Check valves normally are spring-biased to assist in rapid closing, stability, and leakage control. The spring bias adds to the pressure drop in the open condition. Three-port check valves, also known as shuttle valves, are utilized to connect the higher or lower pressure of two lines to a third connection. Pilot-to-open and pilot-to-close check valves are also available for situations in which the check valve must respond to an external pilot pressure. See clause 8 for a more detailed discussion of load holding valves. 5.2 Directional control valves Directional control valves may be actuated mechanically, hydraulically, pneumatically or electrically. Electrical actuation can be on/off or proportional. In the case of hydraulically, pneumatically or electrically piloted valves, the pilot system shall be designed to provide satisfactory response and acceptable characteristics in the event of failure. Directional control valves are normally spring-centered but may also contain mechanical detents, electromagnetic detents or other devices. Valves shall be sized to provide acceptable pressure drops at maximum flow rates, recognizing that individual ports may have different flow rates. If the internal leakage characteristics of the directional control valve are such that actuator creep is a concern, the use of pilot-operated check valves or other blocking valves should be considered Two-port valves Two-port spool, poppet, gate, or other mechanical valves are used to connect or disconnect two-ports. They differ from check valves in that they are typically bidirectional when open. Internal leakage characteristics may differ from check valves, depending upon the design Three-port valves Three-way valves provide a means to connect and disconnect three ports. A typical application of a two-position, threeport valve is to bypass pump flow to tank in one position and to connect pump flow to an actuator in the other Four-port valves Four-port valves provide a means to connect and disconnect four ports. A two-position, four-port valve can be used to control an actuator in forward and reverse directions by connecting opposite actuator ports to pump flow or to tank. Intermediate or transition characteristics of the valve may have a significant impact on pump and actuator pressures and metering, and should be selected based on the requirements of the application. Four-port valves are often applied in a three-position configuration, with the third (center) position providing a neutral condition for the pump and a holding or freewheeling condition for the actuator. A holding or blocked actuator port, center position is normally used with cylinders in order to maintain a load in position. The valve shall have internal leakage characteristics that result in acceptable drift rates for the loaded cylinder. A freewheeling, or vented-to-reservoir actuator port, center position is normally used with motors in order to allow the motor to coast to a stop after pump inlet flow is disconnected, avoiding large pressure spikes. With regard to the pump, the three-position valve can be open center, in which the pump flow is bypassed to the reservoir, or closed center, in which the pump flow path is blocked. An open center valve is normally used with a fixed displacement pump, and a closed center valve is typically selected for use with a variable displacement pump or a fixed displacement pump with an integral unloading device. Closed center valves may also be load sensing, controlling a pressure and flow compensated pump s displacement by means of a sensing line downstream of the valve s metering element. Special four-position options are also available with four-port valves. A relatively common fourth position option with cylinders is float, in which both sides of the cylinder are connected to the reservoir (unlike the center, or holding, position in which both ports are blocked). Regeneration, in which the supply side of the cylinder is connected to both the pump and the return (rod) side of the cylinder, is another option. This provides faster cylinder speed, but at the expense of available force, since the pressure is effectively only acting on the rod area of the cylinder. 25