Structural design and optimization of airport multi-purpose lifting platform

Jian-shu GAO，Xiao-ning FENG，Xiang-sen MENG，Pu-ning ZHANG

（1 Civil Aviation University of China，School of Airport，Tianjin 300300，Chian）

（2Civil Aviation University of China，School of Aviation Engineering，Tianjin 300300，China）

（3 Civil Aviation University of China，School of Electronic Information and Automation，Tianjin 300300，China）

Abstract：In view of the huge domestic air cargo market and the improvement of domestic equipment support capability for aviation emergency rescue，this paper proposes a lifting platform that serves airports and can realize multiple purposes.On the basis of structural design，the lifting part is taken as the main research object for optimal design，and the scissor lifting structure driven symmetrically is taken as its lifting mechanism.The parametric model of the scissor lifting frame is established by using virtual prototype technology.The optimization target is determined as the minimum／maximum thrust of the oil cylinder，and the optimization object is the position of the main hinge point.After optimization，the maximum thrust of the oil cylinder is reduced by 28.4%，and the fluctuation range of the radial velocity of the oil cylinder is obviously reduced compared with that before optimization.Using the finite element analysis software ANSYS，the strength of the main stressed components at the most dangerous moment is checked.The analysis shows that the components are reasonable in structure，safe and reliable，which can provide theoretical support for the production of real objects.

Key words：Lifting platform，Hydraulic fork lifting deck，ADAMS，ANSYS，Optimization

1 Introduction

Air cargo is an important indicator of economic vitality and trade.By the end of 2019，there are only 13 domestic cargo carriers operating，with a total of 174 all-cargo planes.All-cargo planes account for a small proportion of China’s air traffic，and according to the analysis of global air cargo market data in the International Airport Association（ACI）［1］，as shown in Fig.1，China’s per capita air cargo volume is far from that of developed countries such as Europe and the United States，which also shows that China’s air cargo market has great potential［2］.In 2019，China ranked second in the world in terms of cargo scale，with 7.526 million tons of air cargo and mail traffic，up 1.9%year on year（the development data of China’s air cargo market from 2010 to 2019［3-4］are shown in Fig.2.

Fig.1 Comparison of air cargo volume per capita between China and some developed countries in Euro-American

Fig.2 Data of China’s air cargo and mail traffic from 2010 to 2019

Air cargo has always been an important force in national emergency rescue.After the outbreak of the novel coronavirus epidemic，with the increasing number of confirmed cases，the distribution of emergency medical supplies has become an important part of winning the epidemic prevention battle.Many airlines have taken the initiative to undertake the transportation of epidemic prevention supplies.According to the statistics of the Civil Aviation Administration，as of March 23，the civil aviation system had guaranteed 30，333 flights（including charter flights）involving epidemic prevention and control，transporting 4，149，519 items of prevention and control materials，totaling 38，654.68 tons.With a huge air cargo market and undertaking very important emergency rescue tasks，the research and development of supporting equipment for air cargo are also particularly important.One of the functions of the lifting platform designed in this topic is a special airport safeguard equipment that can load and unload large and medium-sized cargo between the ground and the cargo hold of the aircraft.

Another important function of the lifting platform is to lift the damaged aircraft.When the aircraft is damaged or unable to move，it may be necessary to lift the aircraft to a certain height for maintenance or other operations，which can take a long time for large equipment to enter the site，and the labor and material costs are relatively high.The lifting platform can realize fast approach and fast lifting，so that the aircraft can leave the flight area in time or quickly carry out maintenance inspection，thus realizing the safe operation of the airport and the aircraft.Fig.3 is a composition diagram of the main types of civil aviation accidents in 2018［3-4］.Most accidents may cause damage to the landing gear of the aircraft.At this time，the damaged aircraft needs to be lifted and the lifting platform can give full play to its role.To sum up，designing a multi-purpose lifting platform has great practical significance.

Fig.3 Composition chart of main types of civil aviation accident signs in 2018

2 Structural design of airport multi-purpose lifting platform

The lifting platform designed for this time is mainly used for lifting aircraft and air cargo，which requires heavy load and sufficient reliability.Therefore，the lifting part of the multi-purpose lifting platform at the airport adopts a symmetrically driven heavy-load scissor lifting platform.The lifting part consists of a platform support，a hydraulic oil cylinder，an inner swing rod and an outer swing rod，a sliding block，a platform support plate and hinged pin shaft parts.A diamond-shaped connecting block is designed on the middle shaft of the inner swing rod and is hinged with the push rods of the four hydraulic cylinders respectively，so that the lifting force can be effectively amplified；the lifting platform moves horizontally along with the inner swing rod and the outer swing rod and the hydraulic cylinders；in order to improve the bearing capacity and the reliability of the lifting platform，the four hydraulic cylinders are symmetrically installed；and due to the adoption of the reasonable layout form，the lifting mechanism structure of the multi-hydraulic cylinders is still compact.In order to facilitate cooperation with other airport special vehicles and increase their practicability，the starting height of the lifting platform is designed to be as low as possible，so the driving device is not designed，and other power vehicles are used to drive and walk.When the lifting platform needs to move，the walking hydraulic oil cylinder contracts，and the walking wheel contacts the ground to realize its movement；When the lifting platform works，the traveling hydraulic oil cylinder extendsout，the traveling wheel retracts，and the supporting bottom plate contacts the ground.At this time，the platform contacts the ground in a large plane，which is suitable for working under various ground conditions and has good stability.The three-dimensional drawing software CREO has three characteristics：interoperability，ease of use and openness.This design uses this drawing software to draw the three-dimensional model.The three-dimensional model of the airport multi-purpose lifting platform is shown in Fig.4.

Fig.4 Three-dimensional model of airport multi-purpose lifting platform

Fig.5（a）is a schematic diagram of lifting an aircraft and cargo.When the working object is an aircraft，the working process is as follows：① the lifting platform moves to a suitable position below the fuselage，the walking wheel is retracted，and the support plate contacts the ground；② the lifting hydraulic cylinder acts，the lifting platform stops at the position contacting the fuselage；the push rod hydraulic cylinder acts，pushing the side support plate to rotate to the position contacting the aircraft fuselage to stop；③the lifting hydraulic cylinder extends out again，lifting the aircraft to a suitable height，and inserting a safety pin.When the working object is cargo，as shown in Fig.5（b），the working process is as follows：① the lifting platform moves directly below the warehouse door，the walking wheel is retracted，and the support plate contacts the ground；② the cargo are moved to the lifting platform through the roller；③ the lifting hydraulic cylinder moves，the lifting platform is lifted to the docking height with the warehouse，and the goods are removed.

Fig.5 Schematic diagram of lifting platform

According to the fuselage dimensions of most aircraft and the height of warehouse doors，as well as the current standards［5］for loading special vehicles with other goods，the main technical requirements of the lifting platform are determined as shown in Table 1.

Table 1 Main technical requirements for lifting platform

3 Mechanism dynamics analysis

The shear fork lifting mechanism is modeled and analyzed，and the dynamic analysis diagram of the mechanism is shown in Fig.6.

Fig.6 Dynamic analysis diagram

In the figure，point A，C-the hinge point between the swing rod and the slide block，point B，D-the junction point between the swing rod and the lifting platform and the supporting bottom plate；point H，K-the hinge point between the push rod of the hydraulic cylinder and the swing rod；point M，N-hinge point of hydraulic cylinder barrel and swing rod；Point E-the hinge point of the inner and outer swing rods，and is the midpoint of the swing rod；ψ-Angle between hydraulic cylinder and horizontal plane；θ-angle between swing rod and horizontal plane，where：

From the geometric relationship in the figure，we can get：

The whole lifting mechanism is regarded as a balanced object，and each hinge constraint is regarded as an ideal constraint.Due to the rotation friction of each hinge and the rolling friction of the roller have little influence on the lifting force of the hydraulic cylinder during the operation of the mechanism，it is ignored.Load weight P，total hydraulic cylinder piston thrust F.According to the principle of virtual displacement［6］，the sum of the virtual work done by all the active forces acting on the particle system in any virtual displacement is equal to 0，namely：

The formula（5）and the formula（6）are brought into the formula（4）to obtain：

From equation（7），it can be known that the thrust F of the hydraulic cylinder is related to the load weight P，the hinge center length L at both ends of the swing rod，the hinge positionsλ，ηat both ends of the hydraulic cylinder and the included angleθbetween the swing rod and the horizontal plane.At the beginning of the design，the lifting height and geometric parameters of the lifting platform have been determined according to the design requirements，thus determining the length of the swing rod.Therefore，under the condition that the load weight P is constant，the thrust F of the hydraulic cylinder is related toλ，η，θ.Under the condition of bearing the same size，if the maximum thrust of the hydraulic cylinder can be minimized in the lifting process，the working oil pressure of the hydraulic cylinder can be idealized，thus effectively avoiding the impact on the hydraulic cylinder，reducing the failure rate，improving the lifting performance and the service life of the original parts，and reducing the cost of hydraulic devices.In order to achieve this goal，the hinge point position of the hydraulic cylinder needs to be optimized.

4 Simulation and optimization of shear lifting mechanism

4.1 Parametric modeling of scissors lifting mechanism

Parameterized modeling based on ADAMS is to use the parameterized function provided by ADAMSto set the position parameters of each component in the lifting mechanism as changeable variables［7］.Compared with the traditional direct import of ADAMS from three-dimensional drawing software，the advantage lies in：in order to modify the size of a component or change the existing design scheme，the model is not reconstructed，but the method of directly modifying the values of design variables can save a lot of time and energy.Optimal design is to determine the degree of influence of design variables on the performance of the model and the value of design variables under the premise of meeting the work requirements，the system performance index of the model can reach the optimal［8］，so parametric modeling must be carried out on the model before optimal design.

The position parameters of each hinge point and the geometric dimensions of parts are obtained through measuring the built three-dimensional model，and the initial position coordinates of each hinge point of the scissor lift mechanism are obtained by taking the center of mass of the support bottom plate as the coordinate origin O（0，0）.Since the change of coordinate values in the Z-axis direction only changes the lateral stability of the lift platform and has no influence onthe mechanical characteristics of the parts，it is not analyzed.Meanwhile，two pairs of X and Y coordinate values in the four hydraulic cylinders are the same，so only the position coordinates of the two hydraulic cylinders are represented，as shown in Table 2.

Table 2 Articulation point description and initial position coordinates of scissors lifting mechanism

In the whole mechanism，the mass of the hydraulic cylinder is quite different from other components，which has little influence on the lifting force，so its mass attribute is ignored.In order to get closer to the actual working conditions，the mass of the inner and outer swing rods and the lifting platform are defined，and the sliding friction of the slide block is considered.In addition，each hinge point is regarded as a rigid original，and the friction of each hinge point is ignored［9］.The length of the cylinder rod and cylinder barrel of the hydraulic cylinder and the angle of the corresponding MARKER point are parametrically defined，so that the initial position of the hydraulic cylinder will change along with the change of the counting variables.The connection relationship between each component is shown in Table 3.

After determining the relevant and the connection relationship between the various components，the parametric model of the lifting mechanism of the airport multifunctional moving platform is obtained，as shown in Fig.7.

Table 3 Link relation between components

Fig.7 Parametric model of scissors lifting mechanism

4.2 Simulation and optimization of shear lifting mechanism

Define the material properties and mass of the main components and start the gravity acceleration module.The operation process of the scissor lift mechanism is completed by four symmetrical hydraulic cylinders in cooperation.When the lifting platform reaches the specified operation height，the hydraulic cylinders are locked.The initial setting time for completing the whole lifting process is 30 s.The lifting function of the hydraulic cylinders is defined as：

Through the simulation of the preliminarily designed virtual model of the lifting mechanism of the airport multifunctional lifting platform，the lifting force output curve of the hydraulic cylinder running for 30s under 16 tons of load is obtained，as shown in Fig.8.In the figure，the maximum lifting force of the hydraulic cylinder appears at the initial stage of lifting，and the maximum lifting force is 6.219×105N.The curve is smooth and there is no obvious fluctuation，whichshows that the structural design is reasonable.

Fig.8 Oil cylinder thrust curve at the beginning of design

The optimization analysis defines variables according to the parameterized size of the mechanical structure through design research，experimental research and optimization calculation，establishes an objective function according to the design requirements of the mechanical structure，analyzes the degree of influence of variables on the performance indexes of the prototype，thus obtaining which parameters are more important to the performance indexes of the prototype，and determines the design variable value that enables the system performance indexes to reach the optimum［10］.

In order to facilitate the optimization analysis，the names of the main hinge points are marked in the structural diagram of the scissor lift mechanism in Fig.9.as shown in the figure，O1，O2，O3and O4are the main layout parameters of the scissor lift mechanism.The structure of the inner and outer swing rods is directly affected by their coordinate values.Since the change of the coordinate values in the Z-axis direction will only change the lateral stability of the lifting platform and have no influence on the thrust of the hydraulic cylinder，it is not taken as a design variable.Therefore，the optimized design variables are finally determined as O1（x，y），O2（x，y），O3（x，y），O4（x，y）.According to the previous simulation analysis of the lifting mechanism model，it is found that the lifting force is relatively large in the initial stage.In order to reduce the working pressure of the hydraulic system and realize the economic selection of the oil cylinder，the maximum thrust of the oil cylinder is taken as the optimization target.Firstly，the target is optimized with a single variable DV_2_Y（the coordinate value of the hinge point y between the hydraulic cylinder and the inner swing rod）.The variable changes within the range of（0，100），within which the lifting platform can work normally，the change of hydraulic cylinder lifting force is shown in the figure when DV_2_Y takes different values，when DV_2_Y takes different values，the change of hydraulic cylinder lifting force is shown in Fig.10.When DV_2_Y is 100，the maximum lift force of the hydraulic cylinder is the smallest，which is 3.736×105N，which is 39.9%lower than that before optimization.

Fig.9 Schematic diagram of lifting structure

In the same optimization mode，the other seven design variables are optimized one by one，wherein the design variables corresponding to the（x，y）coordinates of O3need to meet the constraint conditions corresponding to equation 9，and the value range of the design variables corresponding to the（x，y）coordinates of O1is obtained symmetrically about the coordinate origin by equation 9

Finally，the optimal values of all design variables are obtained，and the optimal results of all hinge point parameters are shown in Table 4.

Table 4 Design variable optimization results

After optimizing all design variables，the thrust output curve of the optimized hydraulic cylinder is shown in Fig.10，comparing to the two thrust curves before and after optimization，it can be seen that the maximum thrust of the hydraulic cylinder is reduced from 6.219×105N to 4.450×105N，a decrease of 28.4%，since the optimization is based on the preliminary design，the optimization effect belongs to the normal optimization range，the slope of the optimized thrust output curve is lower than that before optimization，and the curve trend is gentle.Figs.11 and 12 are radial velocity fluctuation curves of hydraulic cylinders before and after optimization.the velocity fluctuation amplitude in the initial stage of lifting after optimization is significantly reduced compared with that before optimization，which is of great significance for improving the stability of the lifting platform and prolonging the service life of the hydraulic system［11］.Fig.13 is the velocity curves of lifting platform before and after optimization.After optimization，the slope of the velocity curve is slightly reduced，and the lifting stability is improved，thus ensuring the safety of damaged aircraft and cargo.Fig.14 shows the displacement curve of the optimized lifting platform.The optimized lifting platform stroke is 2 820 mm，which meets the design requirements of 700-3 500 mm.

Fig.10 Optimization of hydraulic cylinder thrust curves before and after

Fig.11 Optimizing radial velocity fluctuation curve of front hydraulic cylinder

Fig.12 Radial velocity fluctuation curve of optimized

Fig.13 Optimization of lifting platform velocity curves before and after

Fig.14 Displacement curve of lift platform after optimization

5 Finite element analysis

The simulation analysis of ADAMS shows that the maximum thrust of the hydraulic cylinder appears in the initial lifting stage，at which time the inner swing rod bears the largest load and generates the largest deformation.Therefore，the initial lifting stage is taken as the most dangerous moment，and the optimized inner swing rod is analyzed by finite element method.The maximum stress value of each hinge point at that moment is obtained through ADAMS，and the stress and strain are checked in ANSYS.In order to facilitate analysis and speed up grid division and simulation，minor details are ignored and accessory components are omitted before analyzing the inner swing rod.

The optimized finite element model is established by the ANSYS.Q345 steel is selected as the material of the inner swing rod，and its allowable stress is［σ］＝σs／n＝257.5 MPa.The stress and strain nephogram of the optimized inner swing rod are obtained.As shown in Figs.15 and 16，the location where the maximum stress is near the hinge hole at the fixed end，and the maximum stress is 232.89 MPa，which is less than the allowable stress of the material.The component is reasonable in structure，safe and reliable.As shown in Fig.17，the stress profile of the intermediate connecting rod shows that the maximum stress generated by the intermediate connecting rod is 3.003 9 MPa，which is far less than the allowable stress.this result also proves that the layout mode of fixing the stressed diamond block on the inner side of the swing rod can effectively transmit the force to the swing rod，which is different from the traditional mode of transmitting the force through the intermediate connecting rod and effectively avoids the damage of the intermediate connecting rod due to excessive torque.

Fig.15 Equivalent stress nephogram of inner swing rod

Fig.16 Equivalent strain nephogram of inner swing rod

Fig.17 Stress profile of intermediate connecting rod

6 Conclusion

Based on the comprehensive consideration of domestic air cargo market and emergency rescue，this paper designs a multi-purpose lifting platform that serves the airport.The scissor lifting mechanism driven symmetrically can realize stable lifting of large load.CREOis used to build a three-dimensional model of the lifting platform.According to the overall structural design of the lifting platform，the virtual prototype technology is used to determine the optimization target as the minimum／maximum lift force of the hydraulic cylinder.The position of the main hinge point of the scissor lift mechanism is taken as the optimization object for optimization analysis.After optimization，the maximum thrust force of the hydraulic cylinder is reduced by 28.4%，and the fluctuation range of the radial velocity of the hydraulic cylinder is significantly reduced compared with that before optimization.Using the finite element analysis software ANSYS，the stress distribution of the swing rod is analyzed and judged，and the strength of the main stressed components at the most dangerous moment is checked.The analysis shows that the components are reasonable in structure，safe and reliable.The analysis results can guide the design of scissor lift mechanism［12］and provide theoretical support for the production of real objects.