 Linkage (mechanical)

This article is about assemblies of links designed to manage forces and movement. For other uses, see Linkage.
A mechanical linkage is an assembly of bodies connected together to manage forces and movement. The movement of a body, or link, is studied using geometry so the link is considered to be rigid. The connections between links are modeled as providing ideal movement, pure rotation or sliding for example, and are called joints. A linkage modeled as a network of rigid links and ideal joints is called a kinematic chain.
Linkages may be constructed from open chains, closed chains, or a combination of open and closed chains. Each link in a chain is connected by a joint to one or more other links. Thus, a kinematic chain can be modeled as a graph in which the links are vertices and the joints are paths, which is called a linkage graph.
The movement of an ideal joint is generally associated with a subgroup of the group of Euclidean displacements. The number of parameters in the subgroup is called the degrees of freedom (DOF) of the joint.
Mechanical linkages are usually designed to transform a given input force and movement into a desired output force and movement. The ratio of the output force to the input force is known as the mechanical advantage of the linkage, while the ratio of the input speed to the output speed is known as the speed ratio. The speed ratio and mechanical advantage are defined so they yield the same number in an ideal linkage.
A kinimatic chain in which one link is fixed or stationary is called a mechanism^{[1]}, and a linkage designed to be stationary is called a structure.
Contents
Uses
Perhaps the simplest linkage is the lever, which is a link that pivots around a fulcrum attached to ground, or a fixed point. As a force rotates the lever, points far from the fulcrum have a greater velocity than points near the fulcrum. Because power in to the lever equals the power out, a small force applied at a point far from the fulcrum (with greater velocity) equals a larger force applied at a point near the fulcrum (with less velocity). The amount the force is amplified is called mechanical advantage. This is the law of the lever.
Two levers connected by a rod so that a force applied to one is transmitted to the second is known as a fourbar linkage. The levers are called cranks, and the fulcrums are called pivots. The connecting rod is also called the coupler. The fourth bar in this assembly is the ground, or frame, on which the cranks are mounted.
Linkages are important components of machines and tools. Examples range from the fourbar linkage used to amplify force in a bolt cutter or to provide independent suspension in an automobile, to complex linkage systems in robotic arms and walking machines. The internal combustion engine uses a slidercrank fourbar linkage formed from it piston, connecting rod, and crankshaft to transform power from expanding burning gases into rotary power. Relatively simple linkages are often used to perform complicated tasks.
Interesting examples of linkages include the windshield wiper, the bicycle suspension, and hydraulic actuators for heavy equipment. In these examples the components in the linkage move in parallel planes and are called "planar linkages." A linkage with at least one link that moves in three dimensional space is called a "spatial linkage." The skeletons of robotic systems are examples of spatial linkages. The geometric design of these systems relies on modern computer aided design software.
The 4bar linkage is an adapted mechanical linkage used on bicycles. With a normal fullsuspension bike the back wheel moves in a very tight arc shape. This means that more power is lost when going uphill. With a bike fitted with a 4bar linkage, the wheel moves in such a large arc that it is moving almost vertically. This way the power loss is reduced by up to 30%.
History
Archimedes^{[2]} applied geometry to the study of the lever. Into the 1500s the work of Archimedes and Hero of Alexandria were the primary sources of machine theory. It was Leonardo da Vinci who brought an inventive energy to machines and mechanism.^{[3]}
In the mid1700s the steam engine was of growing importance, and James Watt realized that efficiency could be increased by using different cylinders for expansion and condensation of the steam. This drove his search for a linkage that could transform rotation of a crank into a linear slide, and resulted in his discovery of what is called Watt's linkage. This led to the study of linkages that could generate straight lines, even if only approximately; and inspired the mathematician J. J. Sylvester, who lectured on the Peaucellier linkage, which generates an exact straight line from a rotating crank.^{[4]}
The work of Sylvester inspired A. B. Kempe, who showed that linkages for addition and multiplication could be assembled into a system that traced a given algebraic curve.^{[5]} Kempe's design procedure has inspired research at the intersection of geometry and computer science.^{[6]}^{[7]}
In the late 1800s F. Reuleaux and A. B. W. Kennedy and L. Burmester formalized the analysis and synthesis of linkage systems using descriptive geometry, and P.L.Chebyshev introduced analytical techniques for the study and invention of linkages.^{[4]}
In the mid 1900's F. Freudenstein and G. N. Sandor^{[8]} used the newly developed digital computer to solve the loop equations of a linkage and determine its dimensions for a desired function, initiating the computeraided design of linkages. Within two decades these computer techniques were integral to the analysis of complex machine systems^{[9]}^{[10]} and the control of robot manipulators.^{[11]}
R.E.Kaufman^{[12]}^{[13]} combined the computer’s ability to rapidly compute the roots of polynomial equations with a graphical user interface to unite Freudenstein’s techniques with the geometrical methods of Reuleaux and Burmester and form KINSYN, an interactive computer graphics system for linkage design
The modern study of linkages includes the analysis and design of articulated systems that appear in robots, machine tools, and cable driven and tensegrity systems. These techniques are also being applied to biological systems and even the study of proteins.
Mobility
The configuration of a system of rigid links connected by ideal joints is defined by a set of configuration parameters, such as the angles around a revolute joint and the slides along prismatic joints measured between adjacent links. The geometric constraints of the linkage allow calculation of all of the configuration parameters in terms of a minimum set, which are the input parameters. The number of input parameters is called the mobility, or degree of freedom, of the linkage system.
A system of n rigid bodies moving in space has 6n degrees of freedom measured relative to a fixed frame. Include this frame in the count of bodies, so that mobility is independent of the choice of the fixed frame, then we have M=6(N1), where N=n+1 is the number of moving bodies plus the fixed body.
Joints that connect bodies in this system remove degrees of freedom and reduce mobility. Specifically, hinges and sliders each impose five constraints and therefore remove five degrees of freedom. It is convenient to define the number of constraints c that a joint imposes in terms of the joint's freedom f, where c=6f. In the case of a hinge or slider, which are one degree of freedom joints, we have f=1 and therefore c=61=5.
Thus, the mobility of a linkage system formed from n moving links and j joints each with f_{i}, i=1, ..., j, degrees of freedom can be computed as,
where N includes the fixed link. This is known as KutzbachGruebler's equation
There are two important special cases: (i) a simple open chain, and (ii) a simple closed chain. A single open chain consists of n moving links connected end to end by j joints, with one end connected to a ground link. Thus, in this case N=j+1 and the mobility of the chain is
For a simple closed chain, n moving links are connected endtoend by n+1 joints such that the two ends are connected to the ground link forming a loop. In this case, we have N=j and the mobility of the chain is
An example of a simple open chain is a serial robot manipulator. These robotic systems are constructed from a series of links connected by six one degreeoffreedom revolute or prismatic joints, so the system has six degrees of freedom.
An example of a simple closed chain is the RSSR spatial fourbar linkage. The sum of the freedom of these joints is eight, so the mobility of the linkage is two, where one of the degrees of freedom is the rotation of the coupler around the line joining the two S joints.
Planar and spherical movement
It is common practice to design the linkage system so that the movement of all of the bodies are constrained to lie on parallel planes, to form what is known as a planar linkage. It is also possible to construct the linkage system so that all of the bodies move on concentric spheres, forming a spherical linkage. In both cases, the degrees of freedom of the link is now three rather than six, and the constraints imposed by joints are now c=3f.
In this case, the mobility formula is given by
and we have the special cases,
 planar or spherical simple open chain,
 planar or spherical simple closed chain,
An example of a planar simple closed chain is the planar fourbar linkage, which is a fourbar loop with four one degreeoffreedom joints and therefore has mobility M=1.
Joints
The most familiar joints for linkage systems are the revolute, or hinged, joint denoted by an R, and the prismatic, or sliding, joint denoted by a P. Most all other joints used for spatial linkages are modeled as combinations of revolute and prismatic joints. For example,
 the cylindric joint consists of an RP or PR serial chain constructed so that the axes of the revolute and prismatic joints are parallel,
 the spherical joint consists of an RRR serial chain for which each of the hinged joint axes intersect in the same point;
 the planar joint can be constructed either as a planar RRR, RPR, and PPR serial chain that has three degreesoffreedom.
Analysis and synthesis of linkages
The primary mathematical tool for the analysis of a linkage is known as the kinematics equations of the system. This is a sequence of rigid body transformation along a serial chain within the linkage that locates a floating link relative to the ground frame. Each serial chain within the linkage that connects this floating link to ground provides a set of equations that must be satisfied by the configuration parameters of the system. The result is a set of nonlinear equations that define the configuration parameters of the system for a set of values for the input parameters.
Freudenstein introduced a method to use these equations for the design of a planar fourbar linkage to achieve a specified relation between the input parameters and the configuration of the linkage. Another approach to planar fourbar linkage design was introduced by L. Burmester, and is called Burmester theory.
Planar one degreeoffreedom linkages
The mobility formula provides a way to determine the number of links and joints in a planar linkage that yields a one degreeoffreedom linkage. If we require the mobility of a planar linkage to be M=1 and f_{i}=1, the result is
or
This formula shows that the linkage must have an even number of links, so we have
 N=2, j=1: this is a twobar linkage known as the lever;
 N=4, j=4: this is the fourbar linkage;
 N=6, j=7: this is a sixbar linkage. A sixbar linkage two links that have three joints, called ternary links, and there are two topologies of this linkage depending how these links are connected. In the Watt topology, the two ternary are connected by a joint. In the Stephenson topology the two ternary links are connected by binary links;^{[14]}
 N=8, j=10: the eightbar linkage has 16 different topologies;
 N=10, j=13: the 10bar linkage has 230 different topologies,
 N=12, j=16: the 12bar has 6856 topologies.
See Sunkari and Schmidt^{[15]} for the number of 14 and 16bar topologies, as well as the number of linkages that have two, three and four degreesoffreedom.
The planar fourbar linkage is probably the simplest and most common linkage. It is a one degreeoffreedom system that transforms an input crank rotation or slider displacement into and output rotation or slide.
Examples of fourbar linkages are:
 the crankrocker, in which the input crank fully rotates and the output link rocks back and forth;
 the sliderdrank, in which the input crank rotates and the output slide moves back and forth;
 draglink mechanisms, in which the input crank fully rotates and drags the output crank in a fully rotational movement.
Other interesting linkages
 Pantograph (fourbar, two DOF)
 Five bar linkages often have meshing gears for two of the links, creating a one DOF linkage. They can provide greater power transmission with more design flexibility than fourbar linkages.
 Klann linkage is a sixbar linkage that forms the leg of a walking mechanism;
 Toggle mechanisms are fourbar linkages that are dimensioned so that they can fold and lock. The linkage is dimensioned so that the linkage reaches a toggle position just before it folds. The high mechanical advantage allows the input crank to deform the linkage just enough push the it beyond the toggle position. This locks the input in place. Toggle mechanisms are used as clamps.
Straight line mechanisms
 James Watt's parallel motion and Watt's linkage
 Peaucellier–Lipkin linkage, the first planar linkage to create a straight line output from rotary input; eightbar, one DOF.
 A Scott Russell linkage, which converts linear motion, to (almost) linear motion in a line perpendicular to the input.
 Chebyshev linkage, which provides nearly straight motion of a point with a fourbar linkage.
 Hoekens linkage, which provides nearly straight motion of a point with a fourbar linkage.
 Sarrus linkage, which provides motion of one surface in a direction normal to another.
Biological linkages
Linkage systems are widely distributed in animals. The most thorough overview of the different types of linkages in animals has been provided by M. Muller,^{[16]} who also designed a new classification system, which is especially well suited for biological systems. A wellknown example is the cruciate ligaments of the knee.
An important difference between biological and engineering linkages is that revolving bars are rare in biology and that usually only a small range of the theoretically possible is possible due to additional mechanical constraints (especially the necessity to deliver blood).^{[17]} Biological linkages frequently are compliant. Often one or more bars are formed by ligaments, and often the linkages are threedimensional. Coupled linkage systems are known, as well as five, six, and even sevenbar linkages.^{[16]} Fourbar linkages are by far the most common though.
Linkages can be found in joints, such as the knee of tetrapods, the hock of sheep, and the cranial mechanism of birds and reptiles. The latter is responsible for the upward motion of the upper bill in many birds.
Linkage mechanisms are especially frequent and manifold in the head of bony fishes, such as wrasses, which have evolved many specialized feeding mechanisms. Especially advanced are the linkage mechanisms of jaw protrusion. For suction feeding a system of linked fourbar linkages is responsible for the coordinated opening of the mouth and 3D expansion of the buccal cavity. Other linkages are responsible for protrusion of the premaxilla.
Linkages are also present as locking mechanisms, such as in the knee of the horse, which enables the animal to sleep standing, without active muscle contraction. In pivot feeding, a fourbar linkage at first locks the head in a ventrally bent position by the alignment of two bars. The release of the locking mechanism jets the head up and moves the mouth toward the prey within 510 ms.
See also
 Engineering mechanics
 Cam
 Kinematic pairs
 Kinematic coupling
 Kinematics
 Overconstrained mechanism
 Machine
 Threepoint hitch
 Lever
 Parallel motion
 Whippletree, a multibar linkage to evenly distribute force.
 Burmester theory of the design of linkages to reach multiple specified poses
 Kinematic models in Mathcad  http://communities.ptc.com/groups/kinematicmodelsinmathcad
References
 ^ OED
 ^ T. Koetsier, “From Kinematically Generated Curves to Instantaneous Invariants: Episodes in the History of Instantaneous Planar Kinematics,” Mechanism and Machine Theory, 21(6):489 498, 1986
 ^ A. P. Usher, 1929, A History of Mechanical Inventions, Harvard University Press, (reprinted by Dover Publications 1968)
 ^ ^{a} ^{b} F. C. Moon, “History of the Dynamics of Machines and Mechanisms from Leonardo to Timoshenko,” International Symposium on History of Machines and Mechanisms, (H. S. Yan and M. Ceccarelli, eds.), 2009. doi: 10.1007/97814020948591
 ^ A. B. Kempe, “On a general method of describing plane curves of the nth degree by linkwork,” Proceedings of the London Mathematical Society, VII:213216, 1876
 ^ D. Jordan and M. Steiner, “Configuration Spaces of Mechanical Linkages,” Discrete and Computational Geometry, 22:297315, 1999
 ^ R. Connelly and E. D. Demaine, “Geometry and Topology of Polygonal Linkages,” Chapter 9, Handbook of discrete and computational geometry, (J. E. Goodman and J. O’Rourke, eds.), CRC Press, 2004
 ^ F. Freudenstein and G. N. Sandor, “Synthesis of Path Generating Mechanisms by Means of a Programmed Digital Computer,” ASME Journal of Engineering for Industry, 81:159168, 1959
 ^ P. N. Sheth and J. J. Uicker, “IMP (Integrated Mechanisms Program), A ComputerAided Design Analysis system for Mechanisms and Linkages,” ASME Journal of Engineering for Industry, 94:454464, 1972
 ^ C. H. Suh and C. W. Radcliffe, Kinematics and Mechanism Design, John Wiley, pp:458, 1978
 ^ R. P. Paul, Robot Manipulators: Mathematics, Programming and Control, MIT Press, 1981
 ^ R. E. Kaufman and W. G. Maurer, ”Interactive Linkage Synthesis on a Small Computer”, ACM National Conference, Aug.35, 1971
 ^ A. J. Rubel and R. E. Kaufman, 1977, “KINSYN III: A New HumanEngineered System for Interactive Computeraided Design of Planar Linkages,” ASME Transactions, Journal of Engineering for Industry, May
 ^ L. W. Tsai, Mechanism design: enumeration of kinematic structures according to function, CRC Press, 2000
 ^ R. P. Sunkari and L. C. Schmidt, "Structural synthesis of planar kinematic chains by adapting a Mckaytype algorithm," Mechanism and Machine Theory 41 (2006) 1021–1030
 ^ ^{a} ^{b} Muller, M. (1996). "A novel classification of planar fourbar linkages and its application to the mechanical analysis of animal systems". Phil. Trans. R. Soc. Lond. B 351: 689–720. doi:10.1098/rstb.1996.0065.
 ^ Dawkins, Richard (November 24, 1996). "Why don't animals have wheels?". Sunday Times. Archived from the original on February 21, 2007. http://web.archive.org/web/20070221073440/www.simonyi.ox.ac.uk/dawkins/WorldOfDawkinsarchive/Dawkins/Work/Articles/19961124wheels.shtml. Retrieved 20081029.
Further reading
 Bryant, John; Sangwin, Chris (2008). How round is your circle? : where engineering and mathematics meet. Princeton: Princeton University Press. pp. 306. ISBN 9780691131184. — Connections between mathematical and realworld mechanical models, historical development of precision machining, some practical advice on fabricating physical models, with ample illustrations and photographs
 Erdman, Arthur G.; Sandor, George N. (1984). Mechanism Design: Analysis and Synthesis. PrenticeHall. ISBN 0135723965.
 Hartenberg, R.S. & J. Denavit (1964) Kinematic synthesis of linkages, New York: McGrawHill — Online link from Cornell University.
 Kidwell, Peggy Aldrich; Amy AckerbergHastings, David Lindsay Roberts (2008). Tools of American mathematics teaching, 18002000. Baltimore: Johns Hopkins University Press. pp. 233–242. ISBN 9780801888144. — "Linkages: a peculiar fascination" (Chapter 14) is a discussion of mechanical linkage usage in American mathematical education, includes extensive references
 How to Draw a Straight Line — Historical discussion of linkage design from Cornell University
External links
 Kinematic Models for Design Digital Library (KMODDL) — Major web resource for kinematics. Movies and photos of hundreds of working mechanicalsystems models in the Reuleaux Collection of Mechanisms and Machines at Cornell University, plus 5 other major collections. Includes an ebook library of dozens of classic texts on mechanical design and engineering. Includes CAD models and stereolithographic files for selected mechanisms.
 How round is your circle? — Supplementary online information to support the book, How round is your circle? : where engineering and mathematics meet, by John Bryant and Chris Sangwin. Includes sections about Watt's and other linkages, many photographs and videos of linkages, errata and elaborations to the published book
 Digital Mechanism and Gear Library (DMGLib) (in German: Digitale Mechanismen und Getriebebibliothek) — Online library about linkages and cams (mostly in German)
 Linkage calculations
 Java animated linkages
 GIFformat animated linkages
 Introductory linkage lecture
 Virtual Mechanisms Animated by Java
 SAM: design, analysis and optimization of (linkage) mechanisms
Categories: Machines
 Mechanisms
 Linkages
Wikimedia Foundation. 2010.
Look at other dictionaries:
Linkage — generally means the manner or style of being united , and can refer to:*Genetic linkage *Linkage (mechanical) *Linkage (policy) *Linkage (linguistics) *Linkage (software) *Linkage (BitTorrent client)*Linkage is also a commonly used phrase in most … Wikipedia
Mechanical system — This article is about systems that manage mechanical movement. For other uses, see Machine (disambiguation). A mechanical system manages power to accomplish a task that involves forces and movement. Mechanical is derived from the Latin word… … Wikipedia
linkage — 1874, from LINK (Cf. link) (v.) + AGE (Cf. age). To understand the principle of Peaucellier s link work, it is convenient to consider previously certain properties of a linkage, (to coin a new and useful word of general application), consisting… … Etymology dictionary
linkage — /ling kij/, n. 1. the act of linking; state or manner of being linked. 2. a system of links. 3. Genetics. an association between two or more genes on a chromosome that tends to cause the characteristics determined by these genes to be inherited… … Universalium
mechanical brakes — Service brakes that are actuated by a mechanical linkage (rather than hydraulic lines) connecting the brakes to the brake pedal. The parking brake on most vehicles is mechanical … Dictionary of automotive terms
linkage — noun a) A mechanical device that connects things. A linkage in my cars transmission is broken so I cant shift out of first gear. b) A connection or relation between things or ideas. Syn: association, bond, connection, link, r … Wiktionary
mechanical system — noun a system of elements that interact on mechanical principles • Hypernyms: ↑system • Hyponyms: ↑fuel injection, ↑fuel injection system, ↑linkage, ↑lubricating system, ↑force feed lubricating system, ↑ … Useful english dictionary
Cognate linkage — In Kinematics, cognate linkages are linkages that ensure the same input output relationship or coupler curve geometry, while being dimensionally dissimilar. In case of four bar linkage coupler cognates, the Roberts–Chebychev theorem states that… … Wikipedia
Four bar linkage — A four bar linkage or simply a 4 bar or four bar is the simplest movable linkage. It consists of 4 rigid bodies (called bars or links), each attached to two others by single joints or pivots to form a closed loop. Four bars are simple mechanisms… … Wikipedia
Watt's linkage — (also known as the parallel linkage) is a type of mechanical linkage invented by James Watt (19 January 1736 ndash; 25 August 1819) to constrain the movement of a steam engine piston in a straight line. The idea of its genesis using links is… … Wikipedia