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Short communication Innovative design of the lifting mechanisms for forklift trucks Jian Yi Wang Jing Shan Zhao Fu Lei Chu Zhi Jing Feng Department of Precision Instruments and Mechanology Tsinghua University Beijing 100084 PR China a r t i c l ei n f oa b s t r a c t Article history Received 17 June 2010 Received in revised form 3 August 2010 Accepted 4 August 2010 Available online 1 September 2010 Forklift truck is one of the most important tools in logistics However the general mast system of a forklift truck not only restrains the driver s vision but also increases the whole weight of a truck and decreases the fuel economy Therefore this paper focuses on the innovative design of a new lifting mechanism for forklift truck Firstly a spatial multi link lift guidance mechanism is proposed And then under the constraints of this mechanism the mobility of the fork and fork frame is investigated in theory Lastly a new lifting mechanism based on it is presented and computer simulation is used to demonstrate the feasibility of motion This multi link lifting mechanism takes advantage of fl exible cable drive and rigid body guidance which not only provides the operator with a wider fi eld of vision but also reduces the equilibrate weight of a vehicle and therefore improves the fuel economy 2010 Elsevier Ltd All rights reserved Keywords Forklift truck Spatial multi link mechanism Rigid body guidance Flexible driving method 1 Introduction Forklift trucks are usually used at railway stations warehouses ports and factories for loading unloading and conveying A general 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 large components will badly affect the driver s fi eld of vision because it locates in front of the driver Many accidents involving collisions between pedestrians and trucks are due to inherently bad visibilities of the forklift trucks 1 2 In addition the mast system plays an important role in supporting the loads and guiding the fork frame to lift vertically So its strength and stiffness must be high enough and its self weight has to be increased The weight of the rear equilibrator is increased as a result which surely improves the unnecessary waste of energy Expanding the driver s vision has always been an important research project Some kinds of trucks have improved visibility via changing the forms of the mast 2 3 while others utilize the visual guidance methods of mobile camera space manipulation 4 or computer vision guidance 5 6 These trucks have better visibility for drivers but they are not fundamental solutions to overcome the 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 the innovative design of the lifting mechanism for forklift trucks A spatial multi link lift guidance mechanism is proposed and then a new kind of lifting mechanism based on it is presented This mechanism not only expands the vision fi eld of the driver but also reduces the whole weight of the vehicle 2 Proposition of a lift guidance mechanism As 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 two RRR open chain linkages are connected with a same rigid body through revolute pairs shown in Fig 1 Link 1 is the base and rigid Mechanism and Machine Theory 45 2010 1892 1896 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 08 002 Contents lists available at ScienceDirect Mechanism and Machine Theory journal homepage body 4 is connected with the base through two parallel RRR kinematic chains ABC and DEF It is easy to fi nd that these two kinematicchainsarebothplanarRRR kinematiclinkages Thereforethetrackofrigidbody 4must beparalleltoboth ofplane and plane That is rigid body 4 can only make a free translation parallel to the intersection line mn of the two planes As a result the track 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 its applications are easily found in gasoline and diesel engines 7 However in the applications of slider crank mechanism dead points 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 only generate planar motions without dead points Suppose the two vertical planes that the support blocks and their RPR kinematic chains located are symmetrical with respect to the longitudinal symmetrical plane of the vehicle As is shown in Fig 2 the subtended angle between the two planes is denoted by 0 b b180 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 and the slideway are connected through a prismatic pair The fork frame has two extruded revolute pairs connected with the ends of the two RPR kinematic chains As is discussed above in such a spatial multi link structure the track of the fork frame is in a straight line In addition one can improve the strength and stiffness by adding another similar kinematic chain in each vertical plane in which the support blocks located to connect the fork frame and the truck chassis Meanwhile the upper and lower kinematic chains are connected by constraint rods As the track of the upper and lower kinematic chains are parallel the constraint rods provide redundant restraints and improve the stiffness and stability of the lift guidance mechanism and the load carrying capacity 8 Now the kinematic principles of the spatial multi link lift guidance mechanism can be analyzed in theory In order to investigate the degree of freedom DoF of the fork frame one can establish a Cartesian coordinate system where xoy plane is the plane 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 0ob b180o the distance from revolute pairs A and D to the origin of coordinate system are equal and denoted by a The coordinates of A and D are a00 andacos asin 0 respectively In addition the coordinates of C and F can be expressed asayCzC and xFyFzF individually It is not diffi cult to fi nd that the axes of revolute pairs A and C are parallel and so are the axes of revolute pairs D and F s1 100 Tdenotes the direction vector of the axes of revolute pairs A and C s2 cos sin 0 Tdenotes the unit direction vector of the axes of revolute pairs D and F s3 0yCzC Tdenotes the direction vector of prismatic pair B and s4 xF bcos yF bsin zF Tdenotes the direction vector of prismatic pair E Fig 1 Spatial multi link structure 1893J Y Wang et al Mechanism and Machine Theory 45 2010 1892 1896 According to the screw theory 9 the terminal constraint screw matrix of a kinematic chain can be obtained by solving the reciprocal screw equation TE 0 1 where 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 100000 0000yCzC 1000zC yC 2 4 3 5 T 2 Therefore in accordance with Eq 1 one can obtain the terminal constraint screw matrix composed of a set of base screws of the terminal constraint screw system ABC 100000 000010 000001 2 4 3 5 T 3 Fig 2 A new lift guidance mechanism 1 fork 2 fork frame 3 extruded revolute pair 4 connecting rod 5 constraint rod 6 slideway 7 support block Fig 3 Cartesian coordinate system 1894J Y Wang et al Mechanism and Machine Theory 45 2010 1892 1896 Similarly the terminal constraint screw matrix of kinematic chain DEF is obtained DEF cos sin 0000 000sin cos 0 000001 2 4 3 5 T 4 Consequently the terminal constraint screw matrix on CF can be expressed as CF ABC DEF 5 According to Eq 1 so long as 0 b b180 the kinematic screw matrix of the frame CF can be solely obtained CF 000001 T 6 It 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 mechanism This section will present a new lifting mechanism for forklift truck based on the lift guidance mechanism proposed above and simulate 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 goods whilenotaffectingthedriver sfi eld ofvision onecanusewindlassandsoftsteelcablesto lifttheforkandforkframe The structure is 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 top of the cab One end of each cable is connected with the fork frame and the other end is fi xed to the windlass When the windlass works 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 be raised vertically Pro engineer software is utilized to simulate its motion Fig 4 a b and c show different positions when the mechanism lifts the fork and fork frame up From the simulation one fi nds that the track of the fork and fork frame is a straight line perpendicular 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 fl exible 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 light weight and so on 10 It is not diffi cult to fi nd that the weight of goods is supported mostly by the cables Therefore the requirement 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 of them moves backwards Consequently the weight of rear equilibrator is greatly decreased and the whole weight of the truck becomes 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 1 fork frame 2 front pulley 3 front supporting rods 4 back supporting rods 5 cables 6 back pulley 1895J Y Wang et al Mechanism and Machine Theory 45 2010 1892 1896 4 Conclusions This paper proposes a new kind of lifting mechanism which is based on a spatial multi link lift guidance mechanism for forklift trucks Theoretical analysis and computer simulations are used to verify the single DoF of the fork frame under the constraints of the mechanism The lifting mechanism consists of fl exible cable drive and rigid body guidance and therefore should separate 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 fi eld of vision without the mast system and hence both the reliability of a truck and the driving comfort are enhanced obviously In addition because of lower requirement of strength and stiffness for every link of the new lifting mechanism the whole weight of a truck is greatly decreased Consequently it improves the fuel economy of the vehicle Acknowledgements This research was supported by the National Natural Science Foundation of China under Grant 50805083 and a Foundation for the Author of National Excellent Doctoral Dissertation of China under Grant 200741 The authors gratefully acknowledge these support agencies References 1 T Horberry T J Larsson I Johnston J Lambert Forklift safety traffi c engineering and intelligent transport systems a case study Applied Ergonomics 35 2004 575 581 2 G Rechnitzer T J Larsson Forklift Trucks and Severe Injuries Priorities for Prevention 1992 Available from b reports muarc030