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Machine (mechanical)

Diesel engine, friction clutch and gear transmission of anautomobile.

From Wikipedia, the free encyclopedia

Machines employ power to achievedesired forces and movement (motion).A machine has a power source andactuators that generate forces andmovement, and a systemof mechanisms that shape the actuatorinput to achieve a specific application ofoutput forces and movement. Modernmachines often include computers andsensors that monitor performance and planmovement, and are called mechanicalsystems.

The meaning of the word "machine" istraced by the Oxford English Dictionary[1] toan independently functioning structure and by Merriam­Webster Dictionary[2] to something that hasbeen constructed. This includes human design into the meaning of machine.

The adjective "mechanical" refers to skill in the practical application of an art or science, as well asrelating to or caused by movement, physical forces, properties or agents such as is dealt withbymechanics.[1] Similarly Merriam­Webster Dictionary[3] defines "mechanical" as relating to machineryor tools.

Power flow through a machine provides a way to understand the performance of devices ranging fromlevers and gear trains to automobiles and robotic systems. The German mechanician FranzReuleaux[4] wrote "a machine is a combination of resistant bodies so arranged that by their means themechanical forces of nature can be compelled to do work accompanied by certain determinate motion."Notice that forces and motion combine to define power.

)

More recently, Uicker et al.[5] state that a machine is "a device for applying power or changing itsdirection." And McCarthy and Soh[6] describe a machine as a system that "generally consists of a powersource and a mechanism for the controlled use of this power."

Contents [hide]

1 Simple machines2 Power sources3 Mechanisms

3.1 Gears and gear trains

3.2 Cam and follower mechanisms3.3 Linkages3.4 Flexure mechanisms

4 Structural components

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Flint hand axefound in Winchester

5 Mechanics5.1 Dynamics of machines5.2 Kinematics of machines

6 Kinematic chains6.1 Planar mechanisms6.2 Spherical mechanisms6.3 Spatial mechanisms6.4 Kinematic diagram

7 Machine design8 Machine elements9 See also

10 References11 Further reading

Simple machines [edit]

The idea that a machine can be decomposed into simple movable elements led Archimedes to definethe lever, pulley and screw as simple machines. By the time of the Renaissance this list increased toinclude the wheel and axle, wedge and inclined plane. The modern approach to characterizingmachines focusses on the components that allow movement, known as joints.

Wedge (hand axe):Perhaps the first example of a device designed to managepower is the hand axe, also see biface andOlorgesailie. A hand axe is made bychipping stone, generally flint, to form a bifacial edge, or wedge. A wedge is asimple machine that transforms lateral force and movement of the tool into atransverse splitting force and movement of the workpiece. The available poweris limited by the effort of the person using the tool, but because power is theproduct of force and movement, the wedge amplifies the force by reducing themovement. This amplification, or mechanical advantage is the ratio of the inputspeed to output speed. For a wedge this is given by 1/tanα, where α is the tipangle. The faces of a wedge are modeled as straight lines to form a slidingor prismatic joint.

Lever: The lever is another important and simple device for managing power. This is a body that pivotson a fulcrum. Because the velocity of a point farther from the pivot is greater than the velocity of a pointnear the pivot, forces applied far from the pivot are amplified near the pivot by the associated decreasein speed. If a is the distance from the pivot to the point where the input force is applied and b is thedistance to the point where the output force is applied, then a/b is the mechanical advantage of thelever. The fulcrum of a lever is modeled as a hinged or revolute joint.

Wheel: The wheel is clearly an important early machine, such as the chariot. A wheel uses the law ofthe lever to reduce the force needed to overcome friction when pulling a load. To see this notice thatthe friction associated with pulling a load on the ground is approximately the same as the friction in asimple bearing that supports the load on the axle of a wheel. However, the wheel forms a lever thatmagnifies the pulling force so that it overcomes the frictional resistance in the bearing.

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Eary Ganz Electric GeneratorinZwevegem, West Flanders, Belgium

Power sources [edit]

Natural forces such as wind and water powered largermechanical systems.Waterwheels appeared around the worldaround 300 BC to use flowing water to generate rotary motion,which was applied to milling grain, and powering lumber,machining and textile operations. Modern water turbines usewater flowing through adam to drive an electric generator.Early windmillscaptured wind power to generate rotary motionfor milling operations. Modern wind turbines also drives agenerator. This electricity in turn is used to drivemotors formingthe actuators of mechanical systems.

The word engine derives from "ingenuity" and originally referredto contrivances that may or may not be physical devices. See Merriam­Webster's definition of engine .A steam engine uses heat to boil water contained in a pressure vessel; the expanding steam drives apiston or a turbine. This principle can be seen in the aeolipile of Hero of Alexandria. This is calledan external combustion engine.

An automobile engine is called an internal combustion engine because it burns fuel(anexothermic chemical reaction) inside a cylinder and uses the expanding gases to drive a piston. A jetengine uses a turbine to compress air which is burned with fuel so that it expands through a nozzle toprovide thrust to an aircraft, and so is also an "internal combustion engine."[7]

The heat from coal and natural gas combustion in aboiler generates steam that drives a steamturbineto rotate an electric generator. A nuclear power plant uses heat from a nuclear reactor togenerate steam and electric power. This power is distributed through a network of transmission lines forindustrial and individual use. Electric motors use either AC or DC electric current to generate rotationalmovement. Electric servomotors are the actuators for mechanical systems ranging fromroboticsystems to modern aircraft. Hydraulic andpneumatic systems use electrically driven pumps to drivewater or air respectively into cylinders to power linear movement.

Mechanisms [edit]

A machine consists of an actuator input, a system of mechanisms that generate the output forces andmovement, and an interface to the user. Electric motors, hydraulic and pneumatic actuators provide theinput forces and movement. This input is shaped by mechanisms consisting of gears and gear trains,belt and chain drives, cam and follower mechanisms, and linkages as well as friction devices such asbrakes and clutches. Structural components consist of the frame, fasteners, bearings, springs,lubricants and seals, as well as a variety of specialized machine elements such as splines, pins andkeys.[4][5] The user interface ranges from switches and buttons to programmable logic controllers andincludes the covers that provide texture, color and styling.

Gears and gear trains [edit]

The transmission of rotation between contacting toothed wheels can be traced back to the Antikytheramechanism of Greece and the south­pointing chariot of China. Illustrations by the renaissancescientist Georgius Agricola show gear trains with cylindrical teeth. The implementation of the involute

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The Antikythera mechanism (mainfragment)

Schematic of the actuator and four­bar linkage thatposition an aircraft landing gear.

tooth yielded a standard gear design that provides aconstant speed ratio. Some important features of gearsand gear trains are:

The ratio of the pitch circles of mating gears definesthe speed ratio and the mechanical advantage of the gearset.A planetary gear train provides high gear reduction in acompact package.

It is possible to design gear teeth for gears that are non­circular, yet still transmit torque smoothly.The speed ratios of chain and belt drives are computed inthe same way as gear ratios. Seebicycle gearing.

Cam and follower mechanisms [edit]

A cam and follower is formed by the direct contact of two specially shaped links. The driving link iscalled the cam (also see cam shaft) and the link that is driven through the direct contact of theirsurfaces is called the follower. The shape of the contacting surfaces of the cam and followerdeterminesthe movement of the mechanism.

Linkages [edit]

A linkage is a collection of links connected by joints.Generally, the links are the structural elements andthe joints allow movement. Perhaps the single mostuseful example is the planar four­bar linkage.However, there are many more special linkages:

Watt's linkage is a four­bar linkage thatgenerates an approximate straight line. It wascritical to the operation of his design for thesteam engine. This linkage also appears invehicle suspensions to prevent side­to­sidemovement of the body relative to the wheels.Also see the article Parallel motion.

The success of Watt's linkage lead to the designof similar approximate straight­line linkages,such as Hoeken's linkage and Chebyshev'slinkage.The Peaucellier linkage generates a truestraight­line output from a rotary input.

The Sarrus linkage is a spatial linkage that generates straight­line movement from a rotary input.Select this link for an animation of theSarrus linkageThe Klann linkage and the Jansen linkage are recent inventions that provide interesting walkingmovements. They are respectively a six­bar and an eight­bar linkage.

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Flexure mechanisms [edit]

A flexure mechanism consists of a series of rigid bodies connected by compliant elements (also knownas flexure joints) that is designed to produce a geometrically well­defined motion upon application of aforce.

Structural components [edit]

A number of machine elements provide important structural functions such as the frame, bearings,splines, spring and seals.

The recognition that the frame of a mechanism is an important machine element changed thename three­bar linkage into four­bar linkage. Frames are generally assembledfrom truss orbeam elements.

Bearings are components designed to manage the interface between moving elements and are thesource of friction in machines. In general, bearings are designed for pure rotation orstraight linemovement.Splines and keys are two ways to reliably mount an axle to a wheel, pulley or gear so that torquecan be transferred through the connection.Springs provides forces that can either hold components of a machine in place or acts asasuspension to support part of a machine.

Seals are used between mating parts of a machine to ensure fluids, such as water, hot gases, orlubricant do not leak between the mating surfaces.Fasteners such as screws, bolts, spring clips, and rivets are critical to the assembly of componentsof a machine. Fasteners are generally considered to be removable. In contrast, joining methods,such as welding,soldering, crimping and the application ofadhesives, usually require cutting theparts to disassemble the components

Mechanics [edit]

Usher[8] reports that Hero of Alexandria's treatise on Mechanics focussed on the study of lifting heavyweights. Today mechanics refers to the mathematical analysis of the forces and movement of amechanical system, and consists of the study of the kinematics and dynamics of these systems.

Dynamics of machines [edit]

The dynamic analysis of machines begins with a rigid­body model to determine reactions at thebearings, at which point the elasticity effects are included. The rigid­body dynamics studies themovement of systems of interconnected bodies under the action of external forces. The assumptionthat the bodies are rigid, which means that they do not deform under the action of applied forces,simplifies the analysis by reducing the parameters that describe the configuration of the system to thetranslation and rotation of reference frames attached to each body.[9][10]

The dynamics of a rigid body system is defined by its equations of motion, which are derived usingeither Newtons laws of motion or Lagrangian mechanics. The solution of these equations of motiondefines how the configuration of the system of rigid bodies changes as a function of time. Theformulation and solution of rigid body dynamics is an important tool in the computer simulation

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Illustration of a four­bar linkagefrom Kinematics of Machinery, 1876

ofmechanical systems.

Kinematics of machines [edit]

The dynamic analysis of a machine requires the determination of the movement, or kinematics, of itscomponent parts, known as kinematic analysis. The assumption that the system is an assembly of rigidcomponents allows rotational and translational movement to be modeled mathematically asEuclidean,or rigid, transformations. This allows the position, velocity and acceleration of all points in a componentto be determined from these properties for a reference point, and the angular position, angular velocityand angular acceleration of the component.

Kinematic chains [edit]

The classification ofsimple machines to provide a strategy for thedesign of new machines was developed by Franz Reuleaux, whocollected and studied over 800 elementary machines. Herecognized that the classical simple machines can be separatedinto the lever, pulley and wheel and axle that are formed by a bodyrotating about a hinge, and the inclined plane, wedge and screwthat are similarly a block sliding on a flat surface.[11]

Simple machines are elementary examples ofkinematicchains or linkages that are used to modelmechanicalsystems ranging from the steam engine to robot manipulators. Thebearings that form the fulcrum of a lever and that allow the wheeland axle and pulleys to rotate are examples of a kinematicpair called a hinged joint. Similarly, the flat surface of an inclinedplane and wedge are examples of thekinematic pair called a slidingjoint. The screw is usually identified as its own kinematic pair called a helical joint.

This realization shows that it is the joints, or the connections that provide movement, that are theprimary elements of a machine. Starting with four types of joints, the rotary joint, sliding joint, cam jointand gear joint, and related connections such as cables and belts, it is possible to understand a machineas an assembly of solid parts that connect these joints called a mechanism .[5]

Two levers, or cranks, are combined into a planarfour­bar linkage by attaching a link that connects theoutput of one crank to the input of another. Additional links can be attached to form a six­bar linkage orin series to form a robot.[5]

Planar mechanisms [edit]

While all mechanisms in a mechanical system are three­dimensional, they can be analyzed usingplanegeometry, if the movement of the individual components are constrained so all point trajectories areparallel or in a series connection to a plane. In this case the system is called a planar mechanism. Thekinematic analysis of planar mechanisms uses the subset of SE(3) consisting of planar rotations andtranslations, denote SE(2).

The group SE(2) is three­dimensional, which means that every position of a body in the plane is definedby three parameters. The parameters are often the x and y coordinates of the origin of a coordinate

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frame in M measured from the origin of a coordinate frame in F, and the angle measured from the x­axis in F to the x­axis in M. This is often described saying a body in the plane has threedegrees­of­freedom.

The pure rotation of a hinge and the linear translation of a slider can be identified with subgroups ofSE(2), and define the two joints one degree­of­freedom joints of planar mechanisms. The cam jointformed by two surfaces in sliding and rotating contact is a two degree­of­freedom joint.

Select this link to see Theo Jansen's Strandbeest walking machine with legs constructed from planareight­bar linkages

Spherical mechanisms [edit]

It is possible to construct a mechanism such that the point trajectories in all components lie in concentricspherical shells around a fixed point. An example is the gimbaled gyroscope. These devices arecalled spherical mechanisms.[6] Spherical mechanisms are constructed by connecting links with hingedjoints such that the axes of each hinge passes through the same point. This point becomes center ofthe concentric spherical shells. The movement of these mechanisms is characterized by the groupSO(3) of rotations in three­dimensional space. Other examples of spherical mechanisms arethe automotive differential and the robotic wrist.

Select this link for an animation of a Spherical deployable mechanism .

The rotation group SO(3) is three­dimensional. An example of the three parameters that specify aspatial rotation are the roll, pitch and yaw anglesused to define the orientation of an aircraft.

Spatial mechanisms [edit]

A mechanism in which a body moves through a general spatial movement is called a spatialmechanism. An example is the RSSR linkage, which can be viewed as a four­bar linkage in which thehinged joints of the coupler link are replaced by rod ends, also called spherical joints or ball joints. Therod ends allow the input and output cranks of the RSSR linkage to be misaligned to the point that theylie in different planes, which causes the coupler link to move in a general spatial movement. Robotarms, Stewart platforms, and humanoid robotic systems are also examples of spatial mechanisms.

Select this link for an animation of Bennett's linkage , which is a spatial mechanism constructed fromfour hinged joints.

The group SE(3) is six­dimensional, which means the position of a body in space is defined by sixparameters. Three of the parameters define the origin of the moving reference frame relative to thefixed frame. Three other parameters define the orientation of the moving frame relative to the fixedframe.

Kinematic diagram [edit]

A kinematic diagram reduces the machine components to a skeleton diagram that emphasizes thejoints and reduces the links to simple geometric elements. This diagram can also be formulated asagraph by representing the links of the mechanism as vertices and the joints as edges of the graph.This version of the kinematic diagram has proven effective in enumerating kinematic structures in theprocess of machine design.[12]

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A CNC metalworking lathe.

An important consideration in this design process is the degree of freedom of the system of links andjoints, which is be determined using theChebychev–Grübler–Kutzbach criterion.

Machine design [edit]

Machine designrefers to the procedures and techniques usedto address the three phases of a machine's lifecycle:

1. invention, which involves the identification of a need,development of requirements, concept generation,prototype development, manufacturing, and verificationtesting;

2. performance engineering involves enhancingmanufacturing efficiency, reducing service andmaintenance demands, adding features and improvingeffectiveness, and validation testing;

3. recycle is the decommissioning and disposal phase and includes recovery and reuse ofmaterials and components.

Machine elements [edit]

The elementary mechanical components of a machine are termed machine elements. These elementsconsist of three basic types (i) structural components such as frame members, bearings, axles,splines, fasteners, seals, and lubricants, (ii)mechanisms that control movement in various ways suchas gear trains, belt or chain drives, linkages,cam and follower systems, including brakes andclutches,and (iii) control components such as buttons, switches, indicators, sensors, actuators and computercontrollers.[13] While generally not considered to be a machine element, the shape, texture and color ofcovers are an important part of a machine that provide a styling and operational interface between themechanical components of a machine and its users.