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pp-protected|expiry=2012-07-03T19:37:53Z|small=yesAbout||the band|Pulley (band)|the village|Pulley, Shropshire|the American photographer|Gerald P. Pulley|Infobox Machine| name = Pulley| image = PulleyShip.JPG| caption = Pulleys on a ship. In this context, pulleys are usually known as block (sailing)|blocks .| classification = Simple machine | industry = Construction, transportation| application =| dimensions =| weight =| fuel_source =| powered = No| self-propelled =| wheels = 1| tracks =| legs =| aerofoils =| axles = 1| components =| invented =| inventor =| examples =A pulley is a wheel on an axle that is designed to support movement of a cable or belt along its circumference.cite book | title = Oxford English Dictionary | publisher = Oxford University Press | year = 1989 | quote = A wheel with a groove round its rim, a sheave. A wheel or drum fixed on a shaft and turned by a belt, cable, etc., Pulleys are used in a variety of ways to lift loads, apply forces, and to transmit power.
A pulley is also called a Sheave (mechanical)|sheave or drum and may have a Groove (machining)|groove between two flange s around its circumference . The drive element of a pulley system can be a rope , cable , belt (mechanical)|belt , or chain that runs over the pulley inside the groove.
Hero of Alexandria identified the pulley as one of six simple machines used to lift weights.cite book | last = Usher | first = Abbott Payson | authorlink = | coauthors = | title = A History of Mechanical Inventions | publisher = Courier Dover Publications | year = 1988 | location = USA | pages = 98 | url = http://books.google.com/books? id=xuDDqqa8FlwC& pg=PA196#v=snippet& q=wedge%20and%20screw& f=false | doi = | isbn = 0-486-25593-X Pulleys are assembled to form a block and tackle in order to provide mechanical advantage to apply large forces. Pulleys are also assembled as part of belt and chain drive s in order to transmit power from one rotating shaft to another.cite book | last = Uicker | first = John | last2 = Pennock | first2 = Gordon | last3 = Shigley | first3 = Joseph | title = Theory of Machines and Mechanisms | publisher = Oxford University Press, USA | edition = 4th | date = 2010 | isbn =978-0-19-537123-9 cite book | last = Paul | first = Burton | title = Kinematics and dynamics of planar machinery | publisher = Prentice-Hall | edition = illustrated | date = 1979 | isbn =978-0-13-516062-6
Block and tackle
A set of pulleys assembled so they rotate independently on the same axle form a block. Two blocks with a rope attached to one of the blocks and threaded through the two sets of pulleys form a block and tackle .cite web | last = Prater | first = Edward L. | title = Basic Machines | publisher = Naval Education and Training Professional Development and Technology Center, NAVEDTRA 14037 | date = 1994 | url = http://www.constructionknowledge.net/public_domain_documents/Div_1_General/Basic_Skills/Basic%20Machines%20NAVEDTRA%2014037%201994.pdf cite web | last = Bureau of Naval Personnel | title =Basic Machines and How They Work | publisher = Dover Publications | date = 1971 | url = http://www.webpal.org/SAFE/aaarecovery/5_simple_technology/basic_machines.pdf
A block and tackle is assembled so one block is attached to fixed mounting point and the other is attached to the moving load. The mechanical advantage of the block and tackle is equal to the number of parts of the rope that support the moving block.
In the diagram on the right the mechanical advantage of each of the block and tackle assembliescite book |title= http://books.google.com/books? id=Q86UKqYui-0C& printsec=frontcover& source=gbs_ge_summary_r& cad=0#v=onepage& q& f=false Handbook of Rigging: For Construction and Industrial Operations |publisher=McGraw-Hill Professional |isbn=978-0-07-149301-7 |last=MacDonald |first=Joseph A |pages=376 shown is as follows:
Gun Tackle: 2
Luff Tackle: 3
Double Tackle: 4
Gyn Tackle: 5
Threefold purchase: 6
Rope and pulley systems
A rope and pulley system, that is a block and tackle , is characterized by the use of a single continuous rope to transmit a tension force around one or more pulleys to lift or move a load---the rope may be a light line or a strong cable. This system is included in the list of simple machine s identified by Renaissance scientists.cite book | last = Avery | first = Elroy | authorlink = | coauthors = | title = Elementary physics | publisher = Sheldon and company | date = 1878 | url = http://books.google.com/books? id=QOYXAAAAIAAJ& pg=PA459& lpg=PA459& dq=Elroy+McKendree+Avery,+Elementary+Physics,+New+York+:+Sheldon+%26+Company,+1878.& source=bl& ots=JgvusHHikF& sig=a95N9DzqdzSPGoel0m3TskK_sm8& hl=en& ei=iv7jTs3LFc-asgLnvNHzBQ& sa=X& oi=book_result& ct=result& resnum=2& ved=0CCcQ6AEwAQ#v=onepage& q=wheel%20and%20axle& f=false cite book | last = Bowser | first = Edward | title = An elementary treatise on analytic mechanics: With numerous examples | publisher = D. Van Nostrand company | url= http://books.google.com/books? id=UdM3AAAAMAAJ& pg=PR9& dq=wheel+and+axle& hl=en& ei=w_xBTdD9IMzOgAf0tbDVAQ& sa=X& oi=book_result& ct=result& resnum=9& ved=0CGAQ6AEwCA#v=onepage& q=simple%20machine& f=false | edition = 5 | date = 1890 | pages = 180
If the rope and pulley system does not dissipate or store energy, then its mechanical advantage is the number of parts of the rope that act on the load. This can be shown as follows.
Consider the set of pulleys that form the moving block and the parts of the rope that support this block. If there are p of these parts of the rope supporting the load W, then a force balance on the moving block shows that the tension in each of the parts of the rope must be W/p. This means the input force on the rope is T = W/p. Thus, the block and tackle reduces the input force by the factor p.
How it works
The simplest theory of operation for a pulley system assumes that the pulleys and lines are weightless, and that there is no energy loss due to friction. It is also assumed that the lines do not stretch.
In equilibrium, the forces on the moving block must sum to zero. In addition the tension in the rope must be the same for each of its parts. This means that the two parts of the rope supporting the moving block must each support one-half the load.
These are different types of pulley systems:
Fixed: A fixed pulley has an axle mounted in bearings attached to a supporting structure. A fixed pulley changes the direction of the force on a rope or belt that moves along its circumference. Mechanical advantage is gained by combining a fixed pulley with a movable pulley or another fixed pulley of a different diameter.
Movable: A movable pulley has an axle in a movable block. A single movable pulley is supported by two parts of the same rope and has a mechanical advantage of two.
Compound: A combination of fixed and a movable pulleys forms a block and tackle . A block and tackle can have several pulleys mounted on the fixed and moving axles, further increasing the mechanical advantage.
The mechanical advantage of the gun tackle can be increased by interchanging the fixed and moving blocks so the rope is attached to the moving block and the rope is pulled in the direction of the lifted load. In this case the block and tackle is said to be "rove to advantage."cite web | title = seamanship reference | publisher = sccheadquarters.com | url = http://www.sccheadquarters.com/UserData/root/Files/Training/Specialisations/Seamanship/Chapter%205%20-%20General%20Rigging.pdf Diagram 3 shows that now three rope parts support the load W which means the tension in the rope is W/3 . Thus, the mechanical advantage is three.
By adding a pulley to the fixed block of a gun tackle the direction of the pulling force is reversed though the mechanical advantage remains the same, Diagram 3a. This is an example of the Luff tackle.
Belt and pulley systems
A belt and pulley system is characterized by two or more pulleys in common to a belt. This allows for power (physics)|mechanical power , torque , and speed to be transmitted across axles. If the pulleys are of differing diameters, a mechanical advantage is realized.
A belt drive is analogous to that of a chain drive , however a belt sheave may be smooth (devoid of discrete interlocking members as would be found on a chain sprocket, spur gear, or timing belt) so that the mechanical advantage is approximately given by the ratio of the pitch diameter of the sheaves only, not fixed exactly by the ratio of teeth as with gears and sprockets.
In the case of a drum-style pulley, without a groove or flanges, the pulley often is slightly convex to keep the flat belt centered. It is sometimes referred to as a crowned pulley. Though once widely used in factory line shaft s, this type of pulley is still found driving the rotating brush in upright vacuum cleaner s. Agricultural tractors built up to the early 1950s generally had a Belt Pulley Magazine|belt pulley . It had limited use as the tractor and equipment being powered needed to be stationary. It has thus been replaced by other mechanisms, such as power take-off and hydraulic machinery|hydraulics .
See also
Block and tackle
Differential pulley
Hoist (device)|Hoist
Portsmouth Block Mills
Reel
V-belt
References
Wiktionarycommons category|Pulleys reflistSimple machines Category:Mechanics Category:Simple machines Category:Mechanical power transmission
