| Aerial work platform is a special vehicle which carries operators and tools to the aerial appoint place for work. It makes a greatly development in recent years. Aerial work platform has four types according to the booms, they are uprightness action type; articulated booms type(A-type); telescopic booms type(T-type) and complex booms type(C-type). When traveling, T-type aerial work platform's booms are retracted, and the booms are extend when it works. The extension of booms increases the platform's work height. Compare with A-type platform, T-type platform offers higher efficiency, easier control and more smooth action. C-type platform has articulated booms and telescopic booms at the same time, its work function is best, but its structure is most complex. This thesis discusses the method to design a C-type platform which working height is 25 meters, it place emphasis on the analysis and research for the platform's metal Structure, include booms and subframe. In this article, two methods are used to study the metal structure. One is Analytics method, the other is the finite element method. As a result, the article will find the proper method, which could optimize the structure and improve the mechanical capability of the platform. The aerial work platform studied in the article provides 25m maximum work height. Its booms are comprised of 3-section full power synchronized telescoping booms of box construction and 1-section articulating boom of box construction. One end of telescoping boom is connected to the turret by articulate joint. Turret are fixed on the subframe though slewing bearing, it can rotate 360o continuous in either direction driven by the planetary gear swing. The subframe is rigidly mounted on the frame of chassis, together with four hydraulically operated outriggers. When working, outriggers extend, support on the earth, absorb all stresses acting on the platform. The booms and turret movements are controlled by a multiple hydraulic valves, it serves to control the direction of motion and provide infinite control of the speed of motion regardless of their load. Several actions may be operated simultaneously and independently of each other. The platform's safety devices include: emergency pump; automatic range limited device; outriggers monitors. (1)research of the telescoping booms The telescoping booms are 3-section full power synchronized, and they are box construction. The synchronization system consists of a double-acting hydraulic cylinder, an extension chain and a retraction chain. Each end of 3 sections mounted slide pad, which are used to bear the stresses and lower friction. It is required that the platform could work with maximum load in the basket at all range. So the most dangerous status of the telescoping booms is one of the following status:①Maximum load in the basket, the boom extend horizontally to the Max. Working outreach.②Maximum load in the basket, the booms extend completely and it is at the Max. working outreach. In this article, the stress and strain at the two work conditions of the telescoping booms are calculated and analysed. The permissible values of stresses comply with "Aerial work platform--safety requirements(GB9645-88): σs—yield strength; S—safety factor; f1-stress concentration factor; f2-fluctuant load factor According to the results of calculate, the condition that maximum load in the basket, the booms extend completely and it is at the Max. working outreach is the worst case. At this case, the second boom is most [ ]12sσ= S ?σf?fdangerous. When the telescoping booms extend completely, the booms will occur elastic deformation. The deformation would influence work parameters and security of the platform. In the calculation, the following preconditions are based on: 1,When deforming, section of the booms are still plane; 2,The deformation is micro-deformations, and the flexibility curves are continuous and smooth. 3,The deformations on raise-lower plane and on the slewing plane are independent of each other. In the calculation, the deformation δa which due to the effects of force perpendicular to the booms and the deformation δb which due to the effects of force parallel to the booms are calculated separately, then composed. Because transverse force and axial force affect each other, the change of deformation due to the effects of axial force is not linearity with the change of force. So the elastic deformationsδa andδb can not be composed use superposition theorem, it can be calculated using integral method. )FMxEIF LcL(FF(MxEIL cFEIL c)FExCdxYExdxYdx ?=+????+?1163211232 1312 )FYδδMxEIF LcLc(FExdxYLab?=+=+??1163 21 2 The finite element analysis and research for the telescoping booms adopted the software—ANSYS. In the analysis and research, the telescoping booms regard as incorporation, avoiding the extra complex situation in the calculation of ventral plate when elastic edge constrained condition of cover plate is taken into account. Considering the mass of booms, the booms modeling is processedbased on its true work position(elevation θ) ensuring the mass center accurate. Choose solid45 element of the ANSYS cell library。Each box section of booms are divided by free and mapped model, Slide pads are divided by sweep model, ensure the shape of divided element is tetrahedron. The grid sizes are manual setting. For the masses of the booms, after input density of the booms'material and acceleration of gravity into ANSYS fore treatment module, program will accumulate element load factor into overall load automatically according to element type and real constant. The rated load act on the end of telescoping booms as concentrated load. According to the finite element analysis and research for the telescoping booms, the max. deformations occur at the end of telescoping boom. The large stress of the boom occur on the base plate that near the action point of lower slide pad and the area near the articulated point of the elevation cylinder and the boom. The max. stresses occur at the base plate of the second boom contact with the action point of left lower slide pad. The result of strength and rigidity analysis according to the finite element method is more accurate and reliable than that according to traditionary analytic method, and can get the stress distribution in local area, such as the area where the boom contact with slide pad or the position where the elevation cylinder articulate to the boom, and these areas are usually the most danger. (2)Analysis and research for the subframe Subframe main body of the 25 meters aerial work platform is sheet metal close box-form structure, they are weld at the position where connecting turret, and slewing bottom ring is weld up the reinforcing plate. The horizontal-vertical outriggers are established on the subframe. When working, outriggers extend, support on the earth, absorb all stresses actingon the platform. The calculations about stress and strain of subframe are made in the most unfavorable stability in the permissive range. The calculations are also according to the statically Indeterminate structure theorem and considering subframe torsion rigidity and outriggers bend rigidity are not infinity. The aerial work platform requires each outriggers support force is not less than zero. The accurate model of subframe is shell. The finite element method is suited for the calculations. The finite element method could analyse and research for the stress and strain of subframe particularly and completely. The rear part of the subframe include two rear outriggers are Choose for anslysing. Subframe and outriggers are regard as sheet metal structure; the sheet metals are modeled using sizes of its mid plate. Slewing bottom ring model as solid. When dividing grids, the sheet metals disperse on element shell63. Object divide on 8 node hexahedron element solid 45. When subframe model as one body, the moving outriggers are fixed with the dead outriggers. The large stresses occur around the weld line between the slewing bottom ring and cover plate. Then anslyse outriggers separate from the subframe, the maximum stresses area occur in the moving outriggers where supported on the dead outriggers base plate. (3)basket leveling system The 25 metres aerial work platform use the parallelogram mechanist to level the basket for the articulated boom. The hydraulic servo cylinder automatic leveling system is used for the telescoping booms. The parallelogram mechanist is fitted with draw rod. The draw rod and the articulated boom act as the two parallel side of the parallelogram, two points selected on the turret which angle is fixed to earth act as one side, and two points selected on the basket act as another side, setting the side on the basket parallel with the side on the turret, all the sides are articulated. When the articulated boom rotate, the parallelogram mechanistensure angle between the basket and earth fixed. The hydraulic servo cylinder automatic leveling system is composed of the lower cylinder articulated between the turret and the telescoping boom and the upper cylinder articulated between the basket and the base and hydraulic system connect the two cylinders. The moving process of upper cylinder is opposite to the lower cylinder's, and the upper cylinder would extend or retract the approximate same distance when the lower cylinder retract or extend. The lower cylinder, the turret and the telescoping boom form a trigon. The upper cylinder, the basket and the base are also form a trigon. It is realized in the structure design that the corresponding angles of the two cylinders will change equally on the opposite direction when the cylinders move the same distance on the opposite direction. Then the system realized basket leveling. The 25 metres aerial work platform has trial-manufactured. According to the test results, the analyse and research of the telescoping boom and the subframe are right, the basket leveling system satisfy the require.
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