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Short communicationInnovative design of the lifting mechanisms for forklift trucksJian-Yi Wang, Jing-Shan Zhao, Fu-Lei Chu, Zhi-Jing FengDepartment of Precision Instruments and Mechanology, Tsinghua University, Beijing 100084, PR Chinaa r t i c l ei n f oa b s t r a c tArticle history:Received 17 June 2010Received in revised form 3 August 2010Accepted 4 August 2010Available online 1 September 2010Forklift truck is one of the most important tools in logistics. However, the general mast systemof a forklift truck not only restrains the drivers vision, but also increases the whole weight of atruck and decreases the fuel economy. Therefore, this paper focuses on the innovative design ofa new lifting mechanism for forklift truck. Firstly, a spatial multi-link lift-guidance mechanismis proposed. And then, under the constraints of this mechanism, the mobility of the fork andfork frame is investigated in theory. Lastly, a new lifting mechanism based on it is presentedand computer simulation is used to demonstrate the feasibility of motion. This multi-link liftingmechanism takes advantage of flexible cable drive and rigid body guidance, which not onlyprovides the operator with a wider field of vision but also reduces the equilibrate weight of avehicle and therefore improves the fuel economy. 2010 Elsevier Ltd. All rights reserved.Keywords:Forklift truckSpatial multi-link mechanismRigid body guidanceFlexible driving method1. IntroductionForklift trucks are usually used at railway stations, warehouses, ports and factories for loading, unloading and conveying. Ageneral weight-balanced forklift truck consists of a chassis and a work device which can be tilted and lifted vertically.However, the general forklifts have the following major disadvantages. First, the mast system composed of several largecomponents will badly affect the drivers field of vision because it locates in front of the driver. Many accidents involving collisionsbetween pedestrians and trucks are due to inherently bad visibilities of the forklift trucks 1,2. In addition, the mast system playsan important role in supporting the loads and guiding the fork frame to lift vertically. So its strength and stiffness must be highenough and its self-weight has to be increased. The weight of the rear equilibrator is increased as a result, which surely improvesthe unnecessary waste of energy.Expanding the drivers vision has always been an important research project. Some kinds of trucks have improved visibility viachanging the forms of the mast 2,3, while others utilize the visual guidance methods of mobile camera-space manipulation 4 orcomputer vision guidance 5,6. These trucks have better visibility for drivers but they are not fundamental solutions to overcomethe shortages of the general mast structures, and the problem of high self weight has not been cracked completely either.In order to improve the driving comfort and safety for the drivers and to reduce energy consumption, this paper focuses on theinnovative design of the lifting mechanism for forklift trucks. A spatial multi-link lift-guidance mechanism is proposed, and then anew kind of lifting mechanism based on it is presented. This mechanism not only expands the vision field of the driver but alsoreduces the whole weight of the vehicle.2. Proposition of a lift-guidance mechanismAs is well known, a planar RRR-open-chain linkage that consists of three revolute pairs and two links generates planar motions.Suppose that two such planar RRR-linkages are placed in two planes with a certain nonzero subtended angle. The ends of the twoRRR-open-chain linkages are connected with a same rigid body through revolute pairs shown in Fig. 1. Link 1 is the base and rigidMechanism and Machine Theory 45 (2010) 18921896 Corresponding author. Fax: +86 10 62788308.E-mail address: jingshanzhao (J.-S. Zhao).0094-114X/$ see front matter 2010 Elsevier Ltd. All rights reserved.doi:10.1016/j.mechmachtheory.2010.08.002Contents lists available at ScienceDirectMechanism and Machine Theoryjournal homepage: /locate/mechmtbody 4 is connected with the base through two parallel RRR-kinematic chains, ABC and DEF. It is easy to find that, these twokinematicchainsarebothplanarRRR-kinematiclinkages.Thereforethetrackofrigidbody 4must beparalleltoboth ofplane andplane . That is, rigid body 4 can only make a free translation parallel to the intersection line, mn, of the two planes. As a result, thetrack of rigid body 4 is a straight line.Slider-crank mechanism is a common mechanism that applies RRR-open-chain structure. It is widely used and examples of itsapplications are easily found in gasoline and diesel engines 7. However, in the applications of slider-crank mechanism, deadpoints possibly lead to actuating failure. In order to improve the stability and avoid the dead points, this paper replaces the RRR-kinematic chains with RPR-kinematic chains, which are composed of two revolute pairs and one prismatic pair and can also onlygenerate planar motions without dead points.Suppose the two vertical planes that the support blocks and their RPR-kinematic chains located are symmetrical withrespect to the longitudinal symmetrical plane of the vehicle. As is shown in Fig. 2, the subtended angle between the twoplanes is denoted by (0bb180). Two RPR-kinematic chains are placed in the two planes, respectively. Every RPR-kinematic chain includes two revolute pairs and one prismatic pair, a connecting rod and a slideway. The connecting rod andthe slideway are connected through a prismatic pair. The fork frame has two extruded revolute pairs connected with theends of the two RPR-kinematic chains. As is discussed above, in such a spatial multi-link structure, the track of the forkframe is in a straight line.In addition, one can improve the strength and stiffness by adding another similar kinematic chain in each vertical plane, inwhich the support blocks located, to connect the fork frame and the truck chassis. Meanwhile, the upper and lower kinematicchains are connected by constraint rods. As the track of the upper and lower kinematic chains are parallel, the constraint rodsprovide redundant restraints and improve the stiffness and stability of the lift-guidance mechanism and the load-carryingcapacity 8.Now the kinematic principles of the spatial multi-link lift-guidance mechanism can be analyzed in theory. In order toinvestigate the degree of freedom (DoF) of the fork frame, one can establish a Cartesian coordinate system, where xoy-plane is theplane determined by the axes of revolute pairs A and D, the origin of coordinate system is the intersection point of axes of A and D,x-axis is the axis of revolute pair A. The coordinate system is shown in Fig. 3.Assume that the subtended angle of the axes of revolute pair D and revolute pair A is denoted by (0obb180o), the distancefrom revolute pairs A and D to the origin of coordinate system are equal and denoted by a. The coordinates of A and D area00 andacosasin 0, respectively. In addition, the coordinates of C and F can be expressed asayCzC andxFyFzF, individually.It is not difficult to find that, the axes of revolute pairs A and C are parallel, and so are the axes of revolute pairs D and F.s1=100Tdenotes the direction vector of the axes of revolute pairs A and C, s2=cos sin 0Tdenotes the unitdirection vector of the axes of revolute pairs D and F, s3=0yCzCTdenotes the direction vector of prismatic pair B ands4=xFbcos yFbsin zFTdenotes the direction vector of prismatic pair E.Fig. 1. Spatial multi-link structure.1893J.-Y. Wang et al. / Mechanism and Machine Theory 45 (2010) 18921896According to the screw theory 9, the terminal constraint screw matrix of a kinematic chain can be obtained by solving thereciprocal screw equation$TE$= 01where $ is a kinematic screw matrix and $is the terminal constraint screw.The kinematic screw matrix of a kinematic chain, ABC, can be expressed as$ABC= $A$B$C? =1000000000yCzC1000zCyC2435T2Therefore, in accordance with Eq. (1), one can obtain the terminal constraint screw matrix composed of a set of base screws ofthe terminal constraint screw system.$ABC=1000000000100000012435T3Fig. 2. A new lift-guidance mechanism. 1fork; 2fork frame; 3extruded revolute pair; 4connecting rod; 5constraint rod; 6slideway; 7support block.Fig. 3. Cartesian coordinate system.1894J.-Y. Wang et al. / Mechanism and Machine Theory 45 (2010) 18921896Similarly, the terminal constraint screw matrix of kinematic chain DEF is obtained:$DEF=cossin0000000sincos00000012435T4Consequently, the terminal constraint screw matrix on CF can be expressed as$CF=$ABC$DEF?5According to Eq. (1), so long as 0bb180, the kinematic screw matrix of the frame CF can be solely obtained$CF=000001T6It proves that the fork frame has only one DoF along z-direction, i.e., the track of the fork frame is restricted to a straight line.Under the constraints of the lift-guidance mechanism, the fork frame can be lifted up and down perpendicular to the ground.3. Implementation of a new lifting mechanismThis section will present a new lifting mechanism for forklift truck based on the lift-guidance mechanism proposed above andsimulate its motion in a computer.As is discussed above, the lift-guidance mechanism restrains the track of the fork frame in a straight line. In order to lift goodswhilenotaffectingthedriversfield ofvision,onecanusewindlassandsoftsteelcablesto lifttheforkandforkframe.The structureis shown in Fig. 4(a). One can set a windlass at the rear of the forklift truck and places some pulleys and supporting rods on the topof the cab. One end of each cable is connected with the fork frame and the other end is fixed to the windlass. When the windlassworks, the cables will lift or drop the fork frame.In order to verify whether this kind of forklift truck can achieve the desired movements, especially insure the fork frame to beraised vertically, Pro/engineer software is utilized to simulate its motion. Fig. 4(a), (b) and (c) show different positions when themechanism lifts the fork and fork frame up. From the simulation one finds that the track of the fork and fork frame is a straight lineperpendicular to the ground. In addition, without the mast system, the driver has a better vision forwards and backwards.This kind of lifting mechanism consists of the flexible cable drive and rigid body guidance with better structural performances.Cable-driven manipulators have been widely investigated in applications for their unique advantages such as low inertia, lightweight and so on 10. It is not difficult to find that the weight of goods is supported mostly by the cables. Therefore, therequirement for strength and stiffness of the lift-guidance mechanism is lower, and the weight of it can be decreased as a result.Meanwhile, compared with components of the general forklift trucks, such as cylinder, chains, chain wheels and the mast system,this kind of lifting mechanism utilizes windlass, cables and several connecting rods. Their weight is lower and the gravity center ofthem moves backwards. Consequently, the weight of rear equilibrator is greatly decreased and the whole weight of the truckbecomes much lower. So the proposed forklift truck reduces energy consumption and improves the fuel economy of the vehicle.Fig. 4. Implementation of the new lifting mechanism and computer simulations. 1fork frame; 2front pulley; 3front supporting rods; 4back supporting rods;5cables; 6back pulley.1895J.-Y. Wang et al. / Mechanism and Machine Theory 45 (2010) 189218964. ConclusionsThis paper proposes a new kind of lifting mechanism, which is based on a spatial multi-link lift-guidance mechanism, forforklift trucks. Theoretical analysis and computer simulations are used to verify the single DoF of the fork frame under theconstraints of the mechanism. The lifting mechanism consists of flexible cable drive and rigid body guidance, and therefore shouldseparate the lifting from the guidance which decreases the self weight of a mast system. Compared with the general forklift trucks,the driver has a wider field of vision without the mast system and hence both the reliability of a truck and the driving comfort areenhanced obviously. In addition, because of lower requirement of strength and stiffness for every link of the new liftingmechanism, the whole weight of a truck is greatly decreased. Consequently, it improves the fuel economy of the vehicle.AcknowledgementsThis research was supported by the National Natural Science Foundation of China under Grant 50805083 and a Foundation forthe Author of National Excellent Doctoral Dissertation of China under Grant 200741. The authors gratefully acknowledge thesesupport agencies.References1 T. Horberry, T.J. Larsson, I. Johnston, J. Lambert, Forklift safety, traffic engineering and intelligent transport systems: a case study, Applied Ergonomics 35(2004) 575581.2 G. Rechnitzer, T.J. Larsson, Forklift Trucks and Severe Injuries: Priorities for Prevention, , 1992 Available from: b/muarc/reports/muarc030c.pdfN, cited March 22, 2010.3 R.L. Dunnhow, How to buy a fo