Mechanism (engineering)
Encyclopedia
A mechanism is a device designed to transform input forces and movement into a desired set of output forces and movement. Mechanisms generally consist of moving components such as gear
s and gear train
s, belt and chain drives, cam
and follower
mechanisms, and linkage
s as well as friction devices such as brakes and clutches, and structural components such as the frame, fasteners, bearings, springs, lubricants and seals, as well as a variety of specialized machine element
s such as splines, pins and keys.
The German scientist Reuleaux provides the definition "a machine is a combination of resistant bodies so arranged that by their means the mechanical forces of nature can be compelled to do work accompanied by certain determinate motion." In this context, his use of machine is generally interpreted to mean mechanism.
The combination of force and movement defines power
, and a mechanism is designed to manage power in order to achieve a desired set of forces and movement.
A mechanism is usually a piece of a larger process or mechanical system
. Sometimes an entire machine
may be referred to as a mechanism. Examples are the steering mechanism
in a car
, or the winding mechanism of a wristwatch
.
Multiple mechanisms are machines.
through the Renaissance, mechanisms were considered to be constructed from simple machines
, such as the lever
, pulley
, screw
, wheel and axle
, wedge
and inclined plane
. It was Reuleaux who focussed on bodies, called links, and the connections between these bodies called kinematic pair
s, or joints.
In order to use geometry to study the movement a mechanism, its links are modeled as rigid bodies
. This means distances between points in a link are assumed to be unchanged as the mechanism moves, that is the link does not flex. Thus, the relative movement between points in two connected links is considered to result from the kinematic pair that joins them.
Kinematic pairs, or joints, are considered to provide ideal constraints between two links, such as the constraint of a single point for pure rotation, or the constraint of a line for pure sliding, as well as pure rolling without slipping and point contact with slipping. A mechanism is modeled as an assembly of rigid links and kinematic pairs.
s. He distinguished between higher pairs which were said to have line contact between the two links and lower pairs that have area contact between the links. J. Phillips shows that there are many ways to construct pairs that do not fit this simple classification.
Lower pair: A lower pair is an ideal joint that constrains contact between a point, line or plane in the moving body to a corresponding point line or plane in the fixed body. We have the following cases:
Higher pairs: Generally, a higher pair is a constraint that requires a curve or surface in the moving body to maintain contact with a curve or surface in the fixed body. For example, the contact between a cam and its follower is a higher pair called a cam joint. Similarly, the contact between the involute curves that form the meshing teeth of two gears are cam joints.
of Greece and the South Pointing Chariot
of China. Illustrations by the renaissance scientist Georgius Agricola show gear trains with cylindrical teeth. The implementation of the involute tooth
yielded a standard gear design that provides a constant speed ratio. Some important features of gears and gear trains are:
and follower
is formed by the direct contact of two specially shaped links. The driving link is called the cam (also see cam shaft
) and the link that is driven through the direct contact of their surfaces is called the follower. The shape of the contacting surfaces of the cam
and follower
determines the movement of the mechanism.
is a collection of links connected by joints. Generally, the links are the structural elements and the joints allow movement. Perhaps the single most useful example is the planar four-bar linkage. However, there are many more special linkages:
s also known as flexure joints) that is designed to produce a geometrically well-defined motion upon application of a force.
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
s and gear train
Gear train
A gear train is formed by mounting gears on a frame so that the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, this provides a smooth transmission of rotation from one gear to the next.The transmission of...
s, belt and chain drives, cam
Cam
A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path...
and follower
Cam follower
A cam follower, also known as a track follower, is a specialized type of roller or needle bearing designed to follow cams. Cam followers come in a vast array of different configurations, however the most defining characteristic is how the cam follower mounts to its mating part; stud style cam...
mechanisms, and linkage
Linkage (mechanical)
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...
s as well as friction devices such as brakes and clutches, and structural components such as the frame, fasteners, bearings, springs, lubricants and seals, as well as a variety of specialized machine element
Machine element
Machine elements are basic mechanical parts used as the building blocks of most machines. Most are standardized to common sizes, but customs are also common for specialized applications.-Types:*Shafts**Couplings*Keys*Splines*Bearings**Roller bearings...
s such as splines, pins and keys.
The German scientist Reuleaux provides the definition "a machine is a combination of resistant bodies so arranged that by their means the mechanical forces of nature can be compelled to do work accompanied by certain determinate motion." In this context, his use of machine is generally interpreted to mean mechanism.
The combination of force and movement defines power
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...
, and a mechanism is designed to manage power in order to achieve a desired set of forces and movement.
A mechanism is usually a piece of a larger process or mechanical system
Mechanical system
A mechanical system manages power to accomplish a task that involves forces and movement. Mechanical is derived from the Latin word machina, which in turn derives from the Doric Greek μαχανά , Ionic Greek μηχανή "contrivance, machine, engine" and that from μῆχος , "means, expedient, remedy".The...
. Sometimes an entire machine
Machine (mechanical)
The mechanical properties of a machine manage power to achieve desired forces and movement. Modern machines often include computers and sensors that monitor performance and plan movement, and are called mechanical systems....
may be referred to as a mechanism. Examples are the steering mechanism
Steering
Steering is the term applied to the collection of components, linkages, etc. which will allow a vessel or vehicle to follow the desired course...
in a car
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
, or the winding mechanism of a wristwatch
Watch
A watch is a small timepiece, typically worn either on the wrist or attached on a chain and carried in a pocket, with wristwatches being the most common type of watch used today. They evolved in the 17th century from spring powered clocks, which appeared in the 15th century. The first watches were...
.
Multiple mechanisms are machines.
Types of Mechanisms
From the time of ArchimedesArchimedes
Archimedes of Syracuse was a Greek mathematician, physicist, engineer, inventor, and astronomer. Although few details of his life are known, he is regarded as one of the leading scientists in classical antiquity. Among his advances in physics are the foundations of hydrostatics, statics and an...
through the Renaissance, mechanisms were considered to be constructed from simple machines
Simple Machines
Simple Machines was a record label that operated out of Arlington, Virginia. The label was masterminded by Jenny Toomey and Kristin Thomson and, at its peak, had four paid workers- Toomey, Thomson, Pat Graham and Mickey Menard...
, such as the lever
Lever
In physics, a lever is a rigid object that is used with an appropriate fulcrum or pivot point to either multiply the mechanical force that can be applied to another object or resistance force , or multiply the distance and speed at which the opposite end of the rigid object travels.This leverage...
, pulley
Pulley
A pulley, also called a sheave or a drum, is a mechanism composed of a wheel on an axle or shaft that may have a groove between two flanges around its circumference. A rope, cable, belt, or chain usually runs over the wheel and inside the groove, if present...
, screw
Screw
A screw, or bolt, is a type of fastener characterized by a helical ridge, known as an external thread or just thread, wrapped around a cylinder. Some screw threads are designed to mate with a complementary thread, known as an internal thread, often in the form of a nut or an object that has the...
, wheel and axle
Wheel and axle
The wheel and axle is one of six simple machines developed in ancient times and is in the category of a second- or third-class lever. In its simplest form it consists of a rod attached to a wheel so that their movements are coupled when one of the parts is turned...
, wedge
Wedge
Wedge may refer to:Mathematics:* Triangular prism, a parallel triangle wedge* Wedge , a polyhedral solid defined by two triangles and three trapezoid faces* Wedge product, a mathematical term, named for the ∧ operator symbol used...
and inclined plane
Inclined plane
The inclined plane is one of the original six simple machines; as the name suggests, it is a flat surface whose endpoints are at different heights. By moving an object up an inclined plane rather than completely vertical, the amount of force required is reduced, at the expense of increasing the...
. It was Reuleaux who focussed on bodies, called links, and the connections between these bodies called kinematic pair
Kinematic pair
A kinematic pair is a connection between two bodies that imposes constraints on their relative movement. Franz Reuleaux introduced the kinematic pair as a new approach to the study of machines that provided an advance over the notion of elements consisting of simple machines.Hartenberg & Denavit...
s, or joints.
In order to use geometry to study the movement a mechanism, its links are modeled as rigid bodies
Rigid body
In physics, a rigid body is an idealization of a solid body of finite size in which deformation is neglected. In other words, the distance between any two given points of a rigid body remains constant in time regardless of external forces exerted on it...
. This means distances between points in a link are assumed to be unchanged as the mechanism moves, that is the link does not flex. Thus, the relative movement between points in two connected links is considered to result from the kinematic pair that joins them.
Kinematic pairs, or joints, are considered to provide ideal constraints between two links, such as the constraint of a single point for pure rotation, or the constraint of a line for pure sliding, as well as pure rolling without slipping and point contact with slipping. A mechanism is modeled as an assembly of rigid links and kinematic pairs.
Kinematic pairs
Reuleaux called the ideal connections between links 'kinematic pairKinematic pair
A kinematic pair is a connection between two bodies that imposes constraints on their relative movement. Franz Reuleaux introduced the kinematic pair as a new approach to the study of machines that provided an advance over the notion of elements consisting of simple machines.Hartenberg & Denavit...
s. He distinguished between higher pairs which were said to have line contact between the two links and lower pairs that have area contact between the links. J. Phillips shows that there are many ways to construct pairs that do not fit this simple classification.
Lower pair: A lower pair is an ideal joint that constrains contact between a point, line or plane in the moving body to a corresponding point line or plane in the fixed body. We have the following cases:
- A revolute pair, or hinged joint, requires a line in the moving body to remain co-linear with a line in the fixed body, and a plane perpendicular to this line in the moving body maintain contact with a similar perpendicular plane in the fixed body. This imposes five constraints on the relative movement of the links, which therefore has one degree of freedom.
- A prismatic joint, or slider, requires that a line in the moving body remain co-linear with a line in the fixed body, and a plane parallel to this line in the moving body maintain contact with a similar parallel plan in the fixed body. This imposes five constraints on the relative movement of the links, which therefore has one degree of freedom.
- A cylindrical joint requires that a line in the moving body remain co-linear with a line in the fixed body. It is a combination of a revolute joint and a sliding joint. This joint has two degrees of freedom.
- A spherical joint, or ball joint, requires that a point in the moving body maintain contact with a point in the fixed body. This joint has three degrees of freedom.
- A planar joint requires that a plane in the moving body maintain contact with a plane in fixed body. This joint has three degrees of freedom.
Higher pairs: Generally, a higher pair is a constraint that requires a curve or surface in the moving body to maintain contact with a curve or surface in the fixed body. For example, the contact between a cam and its follower is a higher pair called a cam joint. Similarly, the contact between the involute curves that form the meshing teeth of two gears are cam joints.
Gears and gear trains
The transmission of rotation between contacting toothed wheels can be traced back to the Antikythera mechanismAntikythera mechanism
The Antikythera mechanism is an ancient mechanical computer designed to calculate astronomical positions. It was recovered in 1900–1901 from the Antikythera wreck. Its significance and complexity were not understood until decades later. Its time of construction is now estimated between 150 and 100...
of Greece and the South Pointing Chariot
South Pointing Chariot
The south-pointing chariot was an ancient Chinese two-wheeled vehicle that carried a movable pointer to indicate the south, no matter how the chariot turned. Usually, the pointer took the form of a doll or figure with an outstretched arm...
of China. Illustrations by the renaissance scientist Georgius Agricola show gear trains with cylindrical teeth. The implementation of the involute tooth
Involute gear
The involute gear profile is the most commonly used system for gearing today. In an involute gear, the profiles of the teeth are involutes of a circle. In involute gear design contact between a pair of gear teeth occurs...
yielded a standard gear design that provides a constant speed ratio. Some important features of gears and gear trains are:
- The ratio of the pitch circles of mating gears defines the speed ratioGear ratioThe gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...
and the mechanical advantageMechanical advantageMechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force...
of the gear set. - A planetary gear trainEpicyclic gearingEpicyclic gearing or planetary gearing is a gear system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear. Typically, the planet gears are mounted on a movable arm or carrier which itself may rotate relative to the sun gear...
provides high gear reduction in a compact package. - It is possible to design gear teeth for gears that are non-circularNon-circular gearA non-circular gear is a special gear design with special characteristics and purpose. While a regular gear is optimized to transmit torque to another engaged member with minimum noise and wear and with maximum efficiency, a non-circular gear's main objective might be ratio variations, axle...
, yet still transmit torque smoothly. - The speed ratios of chainChain driveChain drive is a way of transmitting mechanical power from one place to another. It is often used to convey power to the wheels of a vehicle, particularly bicycles and motorcycles...
and belt drivesBelt (mechanical)A belt is a loop of flexible material used to link two or more rotating shafts mechanically. Belts may be used as a source of motion, to transmit power efficiently, or to track relative movement. Belts are looped over pulleys. In a two pulley system, the belt can either drive the pulleys in the...
are computed in the same way as gear ratios. See bicycle gearingBicycle gearingA bicycle gear or gear ratio refers to the rate at which the rider's legs turn compared to the rate at which the wheels turn. Bicycle gearing refers to how the gear ratio is set or changed. On some bicycles, there is only one gear so the ratio is fixed. Most modern bicycles have multiple gears,...
.
Cam and follower mechanisms
A camCam
A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path...
and follower
Cam follower
A cam follower, also known as a track follower, is a specialized type of roller or needle bearing designed to follow cams. Cam followers come in a vast array of different configurations, however the most defining characteristic is how the cam follower mounts to its mating part; stud style cam...
is formed by the direct contact of two specially shaped links. The driving link is called the cam (also see cam shaft
Camshaft
A camshaft is a shaft to which a cam is fastened or of which a cam forms an integral part.-History:An early cam was built into Hellenistic water-driven automata from the 3rd century BC. The camshaft was later described in Iraq by Al-Jazari in 1206. He employed it as part of his automata,...
) and the link that is driven through the direct contact of their surfaces is called the follower. The shape of the contacting surfaces of the cam
Cam
A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path...
and follower
Cam follower
A cam follower, also known as a track follower, is a specialized type of roller or needle bearing designed to follow cams. Cam followers come in a vast array of different configurations, however the most defining characteristic is how the cam follower mounts to its mating part; stud style cam...
determines the movement of the mechanism.
Linkages
A linkageLinkage (mechanical)
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...
is a collection of links connected by joints. Generally, the links are the structural elements and the joints allow movement. Perhaps the single most useful example is the planar four-bar linkage. However, there are many more special linkages:
- Watt's linkageWatt's linkageWatt's linkage is a type of mechanical linkage invented by James Watt in which the central moving point of the linkage is constrained to travel on an approximation to a straight line...
is a four-bar linkage that generates an approximate straight line. It was critical to the operation of his design for the steam engine. This linkage also appears in vehicle suspensions to prevent side-to-side movement of the body relative to the wheels. Also see the article Parallel motionParallel motionThe parallel motion is a mechanical linkage invented by the Scottish engineer James Watt in 1784 for his double-acting steam engine.In previous engines built by Newcomen and Watt, the piston pulled one end of the walking beam downwards during the power stroke using a chain, and the weight of the...
. - The success of Watt's linkage lead to the design of similar approximate straight-line linkages, such as Hoeken's linkage and Chebyshev's linkageChebyshev linkageThe Hoekens linkage is a four-bar mechanism that converts rotational motion to approximate straight-line motion. The Hoekens linkage is a cognate linkage of the Chebyshev linkage.-See also:*Straight line mechanism*Peaucellier–Lipkin linkage...
. - The Peaucellier linkagePeaucellier-Lipkin linkageThe Peaucellier–Lipkin linkage , invented in 1864, was the first planar linkage capable of transforming rotary motion into perfect straight-line motion, and vice versa...
generates a true straight-line output from a rotary input. - The Sarrus linkageSarrus linkageThe Sarrus linkage, invented in 1853 by Pierre Frédéric Sarrus, is a mechanical linkage to convert a limited circular motion to a linear motion without reference guideways. The linkage uses two perpendicular hinged rectangular plates positioned parallel over each other...
is a spatial linkage that generates straight-line movement from a rotary input. - The Klann linkageKlann LinkageThe Klann linkage is a planar mechanism designed to simulate the gait of legged animal and function as a wheel replacement. The linkage consists of the frame, a crank, two grounded rockers, and two couplers all connected by pivot joints....
and the Jansen linkageTheo JansenTheo Jansen is a Dutch artist and kinetic sculptor. He builds large works which resemble skeletons of animals, that are able to walk using wind power on the beaches of the Netherlands...
are recent inventions that provide interesting walking movements. They are respectively a six-barSix-bar linkageA six-bar linkage is a one degree-of-freedom mechanism that is constructed from six links and seven joints. An example is the Klann linkage used to drive the legs of a walking machine....
and an eight-bar linkage.
Flexure mechanisms
A flexure mechanism consisted of a series of rigid bodies connected by compliant elements (flexure bearingFlexure bearing
A flexure bearing is a bearing which allows motion by bending a load element.A typical flexure bearing is just one part, joining two other parts. For example, a hinge may be made by attaching a long strip of a flexible element to a door and to the door frame...
s also known as flexure joints) that is designed to produce a geometrically well-defined motion upon application of a force.
See also
- Outline of machines
- Linkage (mechanical)Linkage (mechanical)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...
- Four-bar linkage
- Machine (mechanical)Machine (mechanical)The mechanical properties of a machine manage power to achieve desired forces and movement. Modern machines often include computers and sensors that monitor performance and plan movement, and are called mechanical systems....
- Mechanical systemMechanical systemA mechanical system manages power to accomplish a task that involves forces and movement. Mechanical is derived from the Latin word machina, which in turn derives from the Doric Greek μαχανά , Ionic Greek μηχανή "contrivance, machine, engine" and that from μῆχος , "means, expedient, remedy".The...
- Mechanical watchMechanical watchA mechanical watch is a watch that uses a mechanical mechanism to measure the passage of time, as opposed to modern quartz watches which function electronically. It is driven by a spring which must be wound periodically...
- Virtual workVirtual workVirtual work arises in the application of the principle of least action to the study of forces and movement of a mechanical system. Historically, virtual work and the associated calculus of variations were formulated to analyze systems of rigid bodies, but they have also been developed for the...
- GearGearA gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
- Gear trainGear trainA gear train is formed by mounting gears on a frame so that the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, this provides a smooth transmission of rotation from one gear to the next.The transmission of...
- Gear ratioGear ratioThe gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...
External links
- Balanced hinge-lever mechanism
- 507 Mechanical Movements a 1908 publication by Henry T. Brown
- Machines and Mechanisms Wiki
- Kinematic Models for Design Digital Library (KMODDL) collections of movies and photos of hundreds of mechanism models.