1、 Published by: Society of Automotive Engineers, Inc. 400 Commonwealth Drive Warrendale PA 15096-0001 U.S.A. Phone: (412) 776-4841 Fax: (412) 776-5760 All technical reports, including standards approved and practices recommended, are advisory only. Their use by anyone engaged in industry or trade or
2、their use by governmental agencies is entirely voluntary. There is no agreement to adhere to any SAE Standard or Recommended Practice, and no commitment to conform to or be guided by any technical report. In formulating and approving technical reports, the Technical Board, its councils, and committe
3、es will not investigate or consider patents which may apply to the subject matter. Prospective users of the report are responsible for protecting themselves against liability for infringement of patents, trademarks, and copyrights. Copyright O 1994 Society of Automotive Engineers, Inc. All rights re
4、served. Printed in the United States of America. ISBN 1-56091 -537-4 Permission to photocopy for internal or personal use, or the internal or personal use of specific clients, is granted by SAE for libraries and other users registered with the Copyright Clearance Center (CCC), provided that the base
5、 fee of $.50 per page is paid directly to CCC, 222 Rosewood Dr., Danvers, MA O1 923. Special requests should be addressed to the SAE Publications Group. 1-56091-537-4194 $.50. Preface The extensive and increasing use of pneumatic springs in automotive applications has made it appropriate that this m
6、anual be prepared to assist the engineer and the designer in understanding the basic principles and uses of pneumatic springs. This manual provides descriptions of the principles in- volved in pneumatic spring function along with details of the various configurations of pneumatic spring devices and
7、the unique characteristics of each. Pneumatic spring types have been defined correlating with SAE 54511 in the SAE (Oct. 1970) Handbook regarding pneumatic spring termi- nology. Background information tracing the evolution of pneumatic springs has been included, as well as some spe- cific examples o
8、f design calculations to serve as a guideline for the designer. Supplementary information regarding aux- iliary components and design considerations frequently en- countered in automotive pneumatic spring application is provided in an effort to make this manual useful to the auto- motive engineer. I
9、t is the intent of the authors that this manual will be used to guide the engineer in pneumatic spring design and apprise him of the general capabilities and limitations of pneumatic springs. NOTE: The curves, charts, tables, and calculations in the manual use only SI recommended units. c W It shoul
10、d be noted that this paper by the subcommittee was primarily directed under the chairmanship of Mr. Tho- mas A. Bank, retired from Firestone Tire and Rubber Com- pany in 1982, while h4r. Bernhard Sterne, of Bernhard Sterne Associates, provided many valuable editorial comments during the final compil
11、ation of this document. For the proof- ing prior to release, Mr. C. William Grepp, also retired from Firestone Tire and Rubber Company in 1982, is gratefully acknowledged for his efforts in making this paper as useful as possible to the design engineer. Table of Contents Chapter 1-Introduction and H
12、istory . 1 Chapter Basic Principles of Pneumatic Springs . 3 1 . General Discussion 3 2 . Compression Processes 3 3 . Constant-Volume Pneumatic Springs 3 4 . Constant Mass Pneumatic Springs . 3 5 . Basic Cylinder and Piston Springs . 3 6 . Reinforced Flexible Member Springs 4 7 . Flexible Member Ope
13、rating Life Considerations 4 8 . Effects of Low Pressure Operation 4 9 . Basic Calculation Considerations 4 11 . Spring Characteristic Features Peculiar to Constant Volume Pneumatic Springs 4 B . Vertical Supporting Force . 5 10 . Design Balance 4 12 . Gas Law Processes 5 A . Definitions - Units 5 C
14、 . Constant Pressure, No Gas Flow, Constant Effective Area 5 D . Constant Pressure, Constant Temperature, Gas Flow, Varying Displacement, Varying Effective Area, Varying Volume 5 E . Constant Volume, Non-Flow Process . 6 F . Constant Temperature, Non-Flow Process (Isothermal Process) 6 G . Non-flow
15、Processes, Adiabatic . 6 Chapter Types of Pneumatic Springs . 7 1 . Convoluted Types 7 2 . Rolling Lobe Types 8 3 . Hydro-Pneumatic Types . 9 4 . Bladder Types 9 5 . Pneumatic Spring/Shock Absorber 9 Chapter for a unit of deflection, the pressure and, therefore, the spring rate will be different for
16、 isother- mal, adiabatic, or polytropic processes. 2. Compression Processes For a specific spring design, the minimum pneumatic spring rate occurs under isothermal compression conditions, and the maximum spring rate occurs with adiabatic com- pression. The polytropic rate varies between the isotherm
17、al and the adiabatic. The isothermal rate results when all the heat of gas compression escapes so that the gas remains at a constant temperature. The isothermal rate is approached when the spring is deflected very slowly to allow time for the heat to escape, the gas temperature remains constant, and
18、 the gas pressure rise is minimum. Adiabatic rate occurs when all the heat of compression is retained within the gas. This condition is approached dur- ing rapid spring deflection when there is insufficient time for the heat to be dissipated. The higher temperature of the gas results in a higher gas
19、 pressure and, therefore, a higher spring rate. When the heat of compression is partially retained within the gas, a polytropic rate results. This occurs during most normal spring deflections and produces neither iso- thermal nor adiabatic rates, although in normal use it is much closer to the adiab
20、atic situation. 3. Constant-Volume Pneumatic Springs Pneumatic springs which maintain a relatively constant volume at a given operating height regardless of static load or gas pressure are referred to as constant-volume pneu- matic springs and are the more common type in use at this time. At a given
21、 height, the load-carrying ability and the spring rate are varied by changing the pressure of the con- fined gas. With this type of spring, an external source of compressed gas is needed to maintain the spring height as the load on the spring is changed. The natural vibration fre- quency of the cons
22、tant volume pneumatic spring remains more uniform with changes in load than does the natural frequency of the constant mass pneumatic spring. 4. Constant Mass Pneumatic Springs Pneumatic springs which use a fixed mass of gas as the elastic medium are constant mass pneumatic springs. Agiven amount of
23、 gas is sealed in the system and remains constant for all conditions of load or deflection., As the load on a constant mass pneumatic spring is in- creased, the gas volume is reduced and the spring rate in- creases. Conversely, when the load on the spring is reduced, the gas expands and the increase
24、d gas volume results in a reduced spring rate. Thus, the natural vibration frequency of a suspension system using a constant mass pneumatic spring generally increases as the load on the system in- creases. Pneumatic springs which are not connected to a gas source with height control or other valve a
25、rrangements, of- ten cailed“1ocked in“ systems, are classed as constant mass pneumatic springs, as are most hydro-pneumatic springs. 5. Basic Cylinder and Piston Springs A. Cylinders with pistons can be used as pneumatic springs, but they have several major drawbacks: (1) Sliding friction transmits
26、significant forces through the spring. Short impulses are espe- cially detrimental. (2) It is difficult to maintain zero gas leakage past the piston and rod seals for the desired life of the unit. (3) Clevises are required at the top and bottom for most mountings. (4) The effective area cannot be ma
27、nipulated. (5) The piston and rod guide present wear prob- lems. B. An advantage is that high operating pressures may be used, and the unit can combine load-carrying and damping functions. 3 6. Reinforced Flexible Member Springs The use of a reinforced flexible member in conjunction with rigid struc
28、tures overcomes many of the deficiencies found with basic cylinder and piston springs. Careful de- sign, however, is required to prevent high local stresses and severe fluctuations in stress which result in poor life. The flexible member structure carries only a portion of the de- veloped spring for
29、ce, with the remainder being transmitted directly through the gas column to the rigid supporting mem- bers. With some designs the circumferential stress created by the internal pressure, not the direct load stress, is the principal stress the flexible member encounters. Gauge pres- sures with curren
30、tly regularly used materials are generally limited to 700 kPa for 2-ply and 1,200 kPa for 4-ply rein- forcement. Some severe operational conditions limit this still further. 7. Flexible Member Operating Life Considerations Pneumatic spring designs which have the lowest maxi- mum stress and low stres
31、s variation with cycling will achieve the best durability. Durability is also directly coupled with imposed stresses which are the result of the suspension de- sign. In applications encountering repeated severe stresses, the design maximum static pressure should be less than one- third the normal bu
32、rst pressure at the static design height of the spring. For moderate- and light-duty service, the maxi- mum pressure may be increased up to one-half the normal burst pressure. However, more conservative operating pres- sures will generally result in increased life. 8. Effects of Low Pressure Operati
33、on To maintain their correct shape, pneumatic springs must have at least slight positive internal pressure under all de- flection conditions. This means that springs with long re- bound must have higher design position pressure than springs with short rebound. Failure to meet this requirement may ca
34、use girdle hoops to slip out of proper position with multiple convolution designs. Rolling lobe springs may pinch extra folds between the piston and the top mounting metals, resulting in rupture of the flexible member. Gener- ally, 70 kPa minimum design height gauge pressure will prevent operation t
35、roubles. In some cases, half this gauge pressure will suffice, but in a few special cases up to 140 kPa may be required. 9. Basic Calculation Considerations The basic gas laws apply quite well for design calcula- tions of general characteristics in the pressure, temperature, and frequency ranges nor
36、mally used by pneumatic springs. In addition to these factors, an effective area varying with deflection must frequently be considered. This can be ac- complished by changing the size or shape of the piston. It may also be necessary to take into account the fact that fre- quently a nonproportional c
37、hange in volume occurs with changes in deflection. 10. Design Balance Factors that provide the most efficient design from a theoretical standpoint must be weighed against factors that deteriorate spring life. Thus, each operating situation must be evaluated on its overall requirements, and compromis
38、es must frequently be made. Fortunately, there is generally a variety of ways to attain the desired results, and good over- ali performance is usually attainable. 11. Spring Characteristic Features Peculiar to Constant Volume Pneumatic Springs Effective static deflection is determined by the dynamic
39、 rate at the static design position. It can be shown graphi- cally by drawing a line tangent to the dynamic load-deflec- tion curve at the static design position and extending it through the zero load line, then measuring the distance back to the static design position (See Figure. 2.1.). Natural fr
40、e- quency is directly related to the effective static deflection. Zero Deflection At Static Design Position Selected Force At Static Design Position Deflection Extension 4 Compression - Effective Static Deflection Figure 2.1-Effective Static Deflection a O LL 2 4 Rates are generally considered to va
41、ry in direct propor- tion to force. Usually the natural frequency of the system stays reasonably constant throughout the normal force range. System natural frequencies are variable and are deter- mined by spring design, spring volume, and the gas law processes involved. Figure. 2.2 shows the effect
42、of piston shape on the ef- fective area (AC) and on the dynamic spring force (F,) curves versus spring position. The lowest of the foursketches shows the effect of a large diameter flexible member combined with a small diameter piston. This variation must be done with care because of possible advers
43、e effect on the service life of a pneumatic spring assembly. b Q I Fw m Ae ! 2 Extension Compression Figure 2.2-Characteristic Variations Due to Piston Shapes and Flexible Member Size 12. Gas Law Processes A. Definitions - Units The mass of a vehicle and of its cargo is measured in kilograms (kg) an
44、d is usually called “weight.” This mass, less the unsprung mass, acts upon the suspension springs as a load, or more accurately, as a vertical downward force F (now designated force of gravity), equaling mass times acceleration of gravity and measured in newtons (N = kg x 9.806650 m/s2). With a pneu
45、matic suspension spring, this load or force is supported by a force which is developed as the product of gas gauge pressure (that is, pressure above atmospheric pres- sure) and an effective area within the flexible member of the spring. In this manual the force is measured in newtons 0, the gas pres
46、sure is measured in kilopascals pa), and the effective area is measured in square millimeters (mm). The relationship between these three values is 1 N = 1 kPa x 103mm2* *The SAE Manuals on Metai Springs use MPa (= lo6 Pa) as the SI unit for stress etc. In these other Manuals the rela- tionship betwe
47、en the three values is therefore 1 N = 1 MPa x 1 mm2. The pressure of the atmosphere at sea level is in bal- ance with a 760 mm column of mercury at 0C; it equals 101.32 kPa. The sum of the atmospheric pressure and the gauge pressure is known as absolute pressure. The funda- mental gas laws deal wit
48、h this absolute pressure. B. Vertical Supporting Force gas gauge pressure and effective area: The supporting force (F) is created as the product of F=Acx Pg Effective area (AC) can be found directly when the force and pressure are known. Then it is the result of dividing force by pressure. C. Consta
49、nt Pressure, No Gas Flow, Constant Effective Area VVVT With Constant Pressure -l = 2 or1 = 1 TI T2 v2 T, where: T = absolute temperature V = total pneumatic spring and working volume These relationships affect the pneumatic spring system when the system is at rest and only temperature changes occur. Dynamically, the only way to maintain constant pres- sure is in combination with infinite volume and thus is not generally useful. D. Constant Pressure, Constant Temperature, Gas Flow, Varying Displacement, Varying Effective Area, Varying Volume Though volume is
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