工程机械用三级液压缸的设计与仿真
参考文献
[1] 雷天觉.液压工程手册[M].北京：机械工业出版社，1990.23
[2] 成大先.机械设计手册：下册[M].北京：机械工业出版社，1993.43 [3] 何存兴.液压元件[M].北京：机械工业出版社，1982.70
[4] 黎启柏.液压元件手册[M].北京：机械工业出版社，1992.124-159 [5] 刘会国.双作用伸缩式液压缸[J].液压与气动[J]，2003（09）.62
[6] 吴兴奎.一种高可靠的新结构多级套筒液压缸[J].液压与气动[J]，20039（03）.3 [7] 邓飘,邱义,张宝生.液压缸行程检测技术研究现状[J].液压与气动，2008.15-34 [8] 徐灏．机械设计手册（第四、五卷）[M]．北京：机械工业出版社，1991.9 [9] 江洪，江帆．Solid works 机械设计实例解析[M]．机械工业出版社2006.1 [10] 刘鸿文.材料力学（第四版）[M].北京：高等教育出版社，2004.54
[11] 张仁杰．液压缸的设计制造与维修[M]．北京：机械工业出版社，1989.6 [12] 陈奎生．液压与气压传动传动[M]．武汉理工大学出版社，2001.8 [13] 许福玲．液压与气压传动[M]．北京：机械工业出版社，1997.9
[14] 朱辉．画法几何及工程制图[M]．上海：上海科学技术出版社，2007.294-320 [15] 翼宏．液压气压传动与控制[M]．华中科技大学出版社，2009,9.78-95
[16] S M O．Extrusion and Other Processes ． Conveyors for Bulk Materials 1992 [17] Mennesmann Rexroth．Hydraulic Components．Mennesmann Rexroth Gmbh [18] Mennesmann Rexroth．Proportional．Highresponse and Servo-Valves．Electrolic Components and Systems.mennesmann Rexroth Gmbh．RE29003/03．4
[19] Eugenio de01iveira Simonetto , Denis Borenstein . A decision support
system forthe operational planning of solid waste collection [J] ,Waste Management，2006.6
[20] Stem .Jeffrey. Southgate Douglas and Strasma. John. Improving garbage
collection in Latin America’s slums： some lessons from Machala Ecuador. Resource [J]，Conservation and Recycling .1997,7.21
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附录
外文翻译（原文） DESIGN OF BIAXIAL TENSILE TESTING MACHINES S.A.Kurkin, V.F.LukWyanov, and M.N.Krumbol’DT
Welded sheet structures and shells experience biaxial tension when loaded with an internal pressure.Under these conditions there is often an increased sensitivity of the sheet metal to the presence of stress raisers or to a local change of he mechanical properties of the metal related with the manufacturing process, which can lead to a marked decrease of the strength of the structure.
Therefore, when selecting the material and manufacturing process of crucial sheet structures the data obtained in the usual uneasily tests of specimens are insufficient. This necessitates testing large full-scale assemblies and mock-ups of articles maximally reflecting the real operating conditions of the base metal and welded joints. Such tests are of interest for checking the strength of structure at the final stage of its manufacture, but they are quite expensive and as a rule convey insufficient information on the causes of premature failure.
In this connection we should conduct tests which would most fully reflect the real working conditions of structure and in which laboratory methods of investigation would be applicable.
With reference to sheet structures we should take into account in such tests:
a) The state of stress (biaxial with equal or unequal components of the principal stresses);
b) Character of loading (static or repeated static with different cycles); c) Effect of the ambient medium; d) Effect of operating temperature.
The equipment for the tests should be simple in design and provide a high output of investigations.
In this article we will present the experience of designing machines for testing metal and welded joints in a state of biaxial stress. An analysis conducted in [1, 2] showed that the working conditions of metal and welded joints in sheet structure are reproduced most fully in testing by the hydrostatic buckling method. In this case the specimen is secured about the contour and loaded by a hydraulic pressure.
The stressed state arising in the metal depends on the shape of the specimen and die(Fig.1).Thus, in the case of hydrostatic pressure loading of a plane specimen supported about the contour of the round hold of the die, biaxial bending occurs and a considerable part of the outer convex surface of the specimen experiences uniform tension with equal componentsσ1=σ2(Fig.la).If the plane specimen is reliably fixed about the contour of the hole of the die, biaxial tension is superposed on biaxial bending. In the case of loading, not a plane specimen, but a spherical segment(Fig.lb) with a sufficiently large ratio of the diameter of the hole of the die to the thickness of the specimen, the bending component is small, and we can consider that the central part of the specimen experiences biaxial tension withσ2/σ1=1.
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工程机械用三级液压缸的设计与仿真
Fig.1 loading schemes for producing biaxial tension
Biaxial tension with unequal components within the ratio σ2/σ1 = 1.0-0.75 can be produced by buckling the specimens according to the scheme given in Fig.lb with the use of dies having elliptic holes. A further decrease of the ratio σ2/σ1(0.7-0.3)is achieved by means of the scheme shown in Fig.1c, where a specimen in the form of a cylindrical panel restrained by a flange part between the cylindrical die and cylindrical punch is loaded by hydrostatic pressure [3]. For sheet structures working under pressure two types of loading are characteristic: single (static) and low cycle (repeated static).For the first it is expedient to use schemes a and b (see Fig.1). Testing according to scheme a under a static load can be done both on specimens in the form of plane sheets and in the form of reshaped spherical segments [4, 5]. Sheets are preferable, since their manufacture is less timeconsuming.Specimens in the form of spherical segments permit reducing the influence of the edge effect from securing the specimen about the contour; however, the preparation of such specimens requires plastic deformation, and this can lead to a change of the mechanical properties of the material which is not always rectifiable even by subsequent heat treatment.
All three schemes shown in Fig.1 can be used in tests under low-cyclic loading conditions.However, preference should be given to biaxial bending according to scheme a, since the latter permits testing greater thickness. To increase the zone with uniform distribution of stresses, the contour of the specimen can be hinged to the die. Tests by low-cyclic loading according to schemes b and c(see Fig. 1) are conducted only on reshaped specimens in the form of a spherical segment or cylindrical panel. It is necessary to take into account that at the place of attachment of such specimens considerable bending stresses, exceeding the stresses in the center of the specimen, occur.
The ambient medium has an especially strong effect on the results of long-term and repeated static tests. Of great interest is the life of materials in the case of repeated static loading in corrosive environments.
From the standpoint of the effect of the environment, the latter can be used as the working fluid acting on the specimen under pressure. In this case any of the present test schemes is used.
To prevent corrosion of the main systems and parts of the machine, the corrosive fluid is placed under the specimen and is separated from the main working fluid by a partitioning piston equipped with seals. The same role can be played by an elastic membrane located in the cavity under the specimen.
The main shortcoming of this scheme is the impossibility of observing the process of
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fracture.Therefore,in those cases where the action of the corrosive medium without pressure is of interest, the corrosive medium should be placed over the specimen, which permits its easy replacement as it is contaminated by the corrosion products and also observation of the course of fracture visually and by means of still and motion-picture photography.
For protection against corrosion the upper plate can be coated with a layer of epoxy resin or lacquer, or a circular rubber molding preventing the spread of the corrosive medium beyond the limits of the part of the specimen being tested can be glued on the specimen. In the case of corrosion tests with the medium above the specimen the loading scheme shown in Fig.la is recommended.
The temperature is an important facto determining the test results. The data of investigations[6] showed that negative temperatures down to -196°can be obtained in static loading( by local cooling of the specimen with liquid nitrogen or its vapors).In the case of long-time tests it is expedient to cool the entire machine by placing it in a cooling chamber.
Heating the specimen to temperatures 200-250°can be done by electric heaters placed above the specimen. For better heat exchange there should be a layer of mineral oil above the specimen which is intensely agitated during testing.
The initial data for designing the machine are the mechanical properties of the sheet metal, range of thicknesses to be tested, and the loading conditions.
The 10-year experience of the design and operation of such machines indicates that in the case of a single static load the maximum pressure in the hydraulic system should not be above 600-700 gauge atm and in the case of repeated static loading it should not exceed 150-200 gauge atm.
The main parameter determining the dimensions and construction of the machine is the size of the hole of the die 2r, which depends on the thickness t of the specimen.
We will consider the selection of the value of the ratio r/t for the different test schemes. For scheme a (see Fig.1) an increase of the ratio r/t is accompanied by an increase of membrane tresses in comparison with the stresses in the specimen from bending, which is undesirable, since membrane stresses affect considerably the rate of development of fracture and can hamper an analysis of the test results. In addition, the pressure under the specimen required for testing increases with a decrease of the ratio r/t.
Taking into account the considerations expressed above, we recommend selecting the ratio between the radius r of the die and the thickness t of the specimen in conformity with the inequality
where σy is the yield point of the material; E is the modulus of elasticity; P is the maximum pressure under the specimen.
In static tests of specimens restrained about the contour (Fig.1, scheme b) the ratio r/t is determined in the following way.
The experimental data showed that the bending component of strain in the central part of the specimen is characterized by the expression
where Ψb is the logarithmic necking deformation in fracture by uneasily tension.
It is desirable that by the time of fracture the value of ebend does not exceed 0.03, for which fulfillment of the inequality
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工程机械用三级液压缸的设计与仿真
is required.
Moreover, a considerable pressure P is necessary for fracture of the specimen in the case of small values of the ratio r/t. Using the studies of Tomlenov [7], Sandier and Khodulin [8], we can recommend a dependence of the ratio r/t on strength, plasticity of the material, and pressure P for fracture of the specimen:
where Sk is the true resistance to breaking.
From relationships (3) and (4) we must take the larger r/t.Taking into account that during the test the radius of curvature of the specimen can decrease not only as a consequence of deformation of the metal but also due to slippage of the flange part in the fastening, we must increase the ratio obtained by 10-15%.
To prevent the flange from being pulled into the die, the width of the flange part of the specimen should be not less than 0.25-0.3 of the diameter of the die.
In testing specimens in the form of a reshaped spherical segment the radius of the segment R should be selected from the inequality
which lessens the influence of the edge effect [8].
For testing by scheme c (see Fig.1) we use a specimen in the form of a cylindrical panel. The cylindrical panel should form an arc in the cross section with a central angle not less than 120-160°.The length of the panel (without the flange part) should be not less than 2.5-3 radii of its cross section. To increase, the working zone of the specimen, the die should be made in the form of an oval whose major axis coincides with the generatrix of the panel.
Fig.2 Designs of machines for biaxial tension testing
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