Screw (simple machine)

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A screw is a mechanism that converts rotational motion into linear motion, and torque (rotation force) into linear force. It is one of six classic simple machines. The most common form consists of a cylindrical shaft with a helical or back groove called yarn around the outside. Screw through the hole in the object or other media, with the yarn on the inside of the hole that integrates with the screw thread. When the screw shaft is rotated relative to the stationary thread, the screw moves along its axis relative to the surrounding medium; for example turning a wooden screw forced it into the wood. In the screw mechanism, either the screw shaft can rotate through the threaded hole in the stationary object, or the threaded collar as the nut can rotate around the stationary screw shaft. Geometrically, the screw can be seen as a narrow sloping plane that is wrapped around a cylinder.

Like other simple machines, screws can strengthen strength; a small rotational force (torque) on the shaft can exert a large axial force on the load. The smaller the pitch (the distance between screw threads), the greater the mechanical advantage (the ratio of output to input power). Screws are widely used in threaded fasteners to store objects together, and in devices such as screw tops for containers, vises, screw jacks and screw presses.

Another mechanism that uses the same principle, also called a screw, does not have to have a shaft or thread. For example, a bottle opener is a helical shaped rod with a sharp point, and the Archimedes screw is a water pump that uses a rotating helical chamber to move water up the hill. The general principle of all screws is that the spinning helix can cause linear motion.

Video Screw (simple machine)

Histori

The screws are one of the last simple machines created. The screws appeared in ancient Greece and Egypt, and in the first century BC were used in the form of screw presses and Archimedes screws, but when it was found unknown. The Greek philosopher Archytas of Tarrentum (428 - 347 BC) was said by the Greeks to create a screw. Although the Greek philosopher Archimedes was credited by the Greeks by creating a water pump screw Archimedes around 234 BC. records show it was first used in ancient Egypt. Archimedes first studied screws as a machine, so he was sometimes considered the inventor of a screw. The Greek philosopher defines a screw as one of the simplest machines and can calculate its mechanical superiority (ideal). For example, the Heron of Alexandria (52 AD) lists the screws as one of five mechanisms that can "organize the load in motion", defines it as a cylindrical sloping plane, and describes its fabrication and use, including describing a tap to cut the screw thread.

Because they had to be painstakingly cut by hand, the screws were only used as connectors in some machines in the ancient world. Screw fasteners only began to be used in the 15th century in hours, after which the screw-cutter lathes were developed. The screws are also apparently applied to drilling and moving materials (other than water) around this time, when auger drawings and exercises begin to appear in European paintings. The complete dynamic theory of simple machines, including screws, was done by the Italian scientist Galileo Galilei in 1600 at Le Meccaniche ("On Mechanics").

Maps Screw (simple machine)

Lead and pitch

The smoothness or roughness of screw threads is determined by two closely related quantities:

• The steering is defined as the axial distance (parallel to the screw axis) of the screw running in a full rotation (360 Â °) of the shaft. Leading determines the mechanical superiority of the screw; the smaller the tin, the higher the mechanical advantage.
• The pitch is defined as the axial distance between the adjacent thread crests.

In most screws, called single start screw, which has one helical string wrapped around it, the lead and pitch are the same. They are just different in the " lots of start " screws, which have several interlocked threads. In this screw, the lead is equal to the pitch multiplied by the number of starts . Multi-starting screws are used when large linear motions for a given rotation are desired, for example on a screw cap on a bottle, and a ball point pen.

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Handedness

The helix of screw threads can rotate in two possible directions, known as handedness . Most screw threads are oriented so that when viewed from above, the screw rod moves away from the viewer (screw tightened) when rotated clockwise. This is known as the right-hand thread (RH ), because it follows the right hand rule rule: when the fingers of the right hand curve around the stem in the direction of the round, the thumb will point in the direction shaft movement. A thread oriented in the opposite direction is known as left-handed ( LH ).

By general convention, the right hand is the default handling for screw threads. Therefore, most spare parts and fasteners have a right hand thread. One explanation for why the right-handed thread becomes standard is that for right-handed people, tightening the right-handed screw with a screwdriver is easier than tightening the left-handed screw, as it uses a stronger supinator muscle from the arm than the weaker pronator's muscle. Since most people are not left-handed, the right hand thread becomes standard on threaded fasteners. Left hand screw screw is used on some machines and in this application:

• Where a rotation of the shaft will cause the conventional right hand hole to loosen rather than tighten due to an unsteady recession. Examples include:
  • Left hand pedal on bike.
  • The left screw holds a circular saw blade or grinding wheel bench.
• On some devices that have threads at both ends, such as turnbuckle and segments of removable pipes. These sections have one right hand and one left-handed thread, so rotate pieces tighten or loosen both threads at the same time.
• In some gas supply connections to prevent dangerous misconnections. For example in gas welding, the flammable gas supply line is attached to the left hand thread, so it will not inadvertently switch to the oxygen supply, which uses the right hand thread.
• To be useless to the public (therefore preventing theft), left-handed lightbulbs are used in some train and subway stations.
• The lid of the coffin is said to have been traditionally held with left-handed screws.

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Screw screw

Various shapes (profiles) of yarn are used in screws used for different purposes. The threaded screws are standardized so that parts manufactured by different manufacturers will mate properly.

Threaded corner

The thread angle is the included angle, measured on the part parallel to the axis, between the two sides of the bearing of the yarn. The angle between the axial and normal load forces to the bearing surface is approximately equal to the half-angle of the thread, so the yarn angle has a great effect on the friction and screw efficiency, as well as the wear and strength levels. The larger the screw angle, the greater the angle between the load vector and the normal surface, the greater the normal force between the threads required to support the given load. Therefore, increasing the angle of the yarn increases the friction and wear of the screw.

The outer edge of the outer bearing face, when acted upon by the load force, also apply the radial force (outward) to the nut, causing tensile stress. This radial style explodes increases with increasing yarn angle. If the tensile strength of the nut material is insufficient, excessive burden on the nut with a large screw angle can break the nut.

The yarn angle also affects the strength of the yarn; thread with a large angle has a wide root compared to its size and is stronger.

Thread type

In threaded fasteners, a large amount of friction is acceptable and usually desirable, to prevent fasteners from opening the lid. So the yarn used in the fastener usually has a large 60 Â ° yarn angle:

• (a) V extension - This is used if additional friction is required to ensure the screw remains immobilized, such as the setcrew screw and adjustment, and where the connection should be a tight liquid as in the pipe connection threaded.
• (b) National America - This has been replaced by an almost identical Unified Yarn Standard. It has the same 60 Â ° thread angle as the V thread but is stronger because of its flat roots. Used in bolts, nuts, and various kinds of fastener.
• (c) Whitworth or English Standard - A very similar British standard is replaced by the Integrated Yarn Standard.

In machine relations such as screws or jackscrew, on the contrary, friction must be minimized. Therefore, a thread with a smaller angle is used:

• (d) Square flow - This is the strongest and lowest friction thread, with an angle of 0 Â °, and does not apply the blasting force to the nut. However difficult to fabricate, requires a single point cutting tool because of the need to weaken the tip. These are used in high load applications such as jackscrews and screw leads but most have been replaced by Acme threads. A modified square thread with a small 5 Â ° thread angle is sometimes used instead, which is less expensive to make.
• (e) Acme thread - With the thread angle 30 Â ° it has a friction higher than the square thread, but is easier to make and can be used with a separate nut to adjust wear. It is widely used in vises, C-clamps, valves, scissor jacks and lead screws in machines such as lathes.
(f) Buttress thread - This is used in high load applications where load forces are applied in only one direction, such as a screw jack. With an angle of 0 Â ° from the bearing surface is as efficient as a square thread but is stronger and easier to fabricate.
• (g) Knuckle thread - Similar to a square thread where the corners have been rounded to protect it from damage, it also provides higher friction. In low-power applications it can be produced cheaply from sheet stock by rolling. These are used in light bulbs and sockets.

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Using

• Since the self-locking property (see below) the screw is widely used in threaded fasteners to hold objects or materials together: wood screws, metal sheet screws, studs, and bolts and nuts.
• The self-locking property is also the key to the use of screws in a variety of other applications, such as bottle caps, top screw caps, threaded pipe connections, clamps, C clip, and screw jack.
• The screw is also used as a liaison in the machine for power transfer, in the worm gear, lead screw, ball screw, and roller screws. Because of its low efficiency, screw connections are rarely used to carry high power, but are more commonly used in low power, intermittent use such as positioning actuators.
• Rotating spiral or chambered screw blades are used to remove material in Archimedes screw, auger earth drill, and screw conveyors.
• The micrometer uses precision calibration screws to measure lengths with high accuracy.

The screw blades, despite sharing the name screws , work on physical principles that are very different from the screw type above, and the information in this article does not apply to it.

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Moved distance

Jarak linier ${\ displaystyle d \,}  ÂÂ$ batang sekrup bergerak ketika diputar melalui sudut ${\ displaystyle \ alpha \,}  ÂÂ$ derajat adalah:

${\ displaystyle d = l {\ frac {\ alpha} {360 ^ {\ circ}}} \,}  ÂÂ$

di mana ${\ displaystyle l \,}  ÂÂ$ adalah ujung sekrup.

The distance ratio of a simple machine is defined as the ratio of the applied force spacing moving to the moving load distance. For the screw it is the ratio of the circular distance d _at the point at the edge of the shaft moves to a linear distance d _out the moving axis. If r is the radius of the axis, in a loop a point on the moving screw loop is distance of 2? r , while the axis moves linearly with a distance of l . So the distance ratio

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Mechanical advantage without friction

The mechanical gain MA of the screws is defined as the axial output force ratio F _out applied by the shaft at the load to the rotational force < at applied to the edges of the shaft to change it. For no friction screws (also called screws ideal ), from energy conservation, the work done on the screw by the input power that rotates is the same as the work done by < i> screw on load style:

${\ displaystyle W_ {in} = W_ {out} \,}$

Pekerjaan sama dengan gaya yang dikalikan dengan jarak yang dilakukannya, sehingga pekerjaan yang dilakukan dalam satu putaran sekrup lengkap adalah ${\ displaystyle W_ {in} = 2 \ pi rF_ {in} \,}  ÂÂ$ dan pekerjaan yang dilakukan pada beban adalah ${\ displaystyle W_ {out} = lF_ {keluar} \,}  ÂÂ$ . Jadi ideal keuntungan mekanik dari sekrup sama dengan rasio jarak :

It can be seen that the screw mechanical advantage depends on the instructions, ${\ displaystyle l \,}$ . The smaller the distance between the thread, the greater the mechanical advantage, and the greater the force the screw can exert the given power. However, the most actual screws have a large amount of friction and mechanical advantages less than that given by the above equation.

Torque shape

Gaya rotasi yang diterapkan pada sekrup sebenarnya adalah torsi ${\ displaystyle T_ {in} = F_ {in} r \,}  ÂÂ$ . Karena ini, kekuatan input yang diperlukan untuk memutar sekrup tergantung pada seberapa jauh dari poros itu diterapkan; semakin jauh dari poros, semakin sedikit kekuatan yang diperlukan untuk mengubahnya. Gaya pada sekrup biasanya tidak diterapkan pada pelek seperti yang diasumsikan di atas. Ini sering diterapkan oleh beberapa bentuk tuas; misalnya baut diputar oleh kunci pas yang pegangannya berfungsi sebagai pengungkit. Keuntungan mekanis dalam hal ini dapat dihitung dengan menggunakan panjang lengan tuas untuk r dalam persamaan di atas. Faktor asing ini r dapat dihapus dari persamaan di atas dengan menulisnya dalam hal torsi:

${\ displaystyle {\ frac {F_ {out}} {T_ {in}}} = {\ frac {2 \ pi} {l}} \,}  ÂÂ$

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Keuntungan dan efisiensi mekanik yang sebenarnya

Due to the large area of ​​contact between the moving thread and the stationary, the screw usually has a large friction energy loss. Well-lubricated jack screws also have an efficiency of 15% - 20%, the rest of the work applied to turn them into friction. When friction is included, the mechanical advantage is no longer equal to the distance ratio but also depends on the efficiency of the screw. From energy conservation, the work of W _in is done on the screw by the input power which rotates it equal to the amount of work performed moving the load W _out , and the work is dissipated as heat by friction W _fric in screw

$Â\; Â{\displaystyle Â\; Â\; Â\; Â\; Â\; Â\; Â}$ _{   ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂﯯ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯                         me      Â ·                           =           ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂﯯ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯                 Â             u     ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂï <½                                    ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂﯯ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯                 Â       Â               me      ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂï <½                                        {\ displaystyle W_ {in} = W_ {out} W_ {fric} \,}  Â}

Efisiensi ? adalah bilangan tak berdimensi antara 0 dan 1 yang didefinisikan sebagai rasio kerja output terhadap pekerjaan input

${\ displaystyle \ eta = W_ {out}/W_ {in} \,}  ÂÂ$

${\ displaystyle W_ {out} = \ eta W_ {in} \,}  ÂÂ$

atau dalam hal torsi

${\ displaystyle {\ frac {F_ {out}} {T_ {in}}} = {\ frac {2 \ pi \ eta} {l}} \ qquad \, }  ÂÂ$

So the actual mechanical advantage of the screw is reduced from what would be the ideal screw and friction by efficiency ${\ displaystyle \ eta \,}$ . Due to its low efficiency, screw powered machines are not often used as connectors for transferring large amounts of power but are more often used in intermittent positions.

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The self-locking property

A large friction force causes most screws in practical use to be "self-locking", also called "non-reciprocal " or " non-overhauled > ". This means that applying torque to the shaft will cause it to spin, but no amount of axial load force on the shaft will cause it to reverse direction, even if the torque used is zero. This is in contrast to some other simple "reciprocal" or " non locking machines, which means that if the load force is large enough, they will move backwards or" > reshuffle ". Thus, the machine can be used in both directions. For example, in a lever, if the force at the end of the load is too large it will move backwards, doing the work on the applied force. Most screws are designed to lock themselves, and in the absence of torque on the shaft will remain in any remaining position. However, some screw mechanisms with considerable pitch and good lubrication are not self locked and will be repaired, and very little, like a push drill, use a screw in the sense of "backward", apply axial force to the shaft to turn the screw.

This self-locking property is one of the reasons for the use of very large screws in threaded fasteners such as wood screws, sheet metal screws, buttons and bolts. Tightening the fasteners by turning them puts the compression force on the material or part that is tightened together, but no amount of force from the parts will cause the screw to open fast. This property is also the basis for the use of screws in the upper container lid of screws, vises, C-clamps, and screw jacks. A heavy object can be raised by rotating the jack shaft, but when the shaft is released, it will remain at whatever height is raised.

Sekrup akan self-locking jika dan hanya jika efisiensinya ${\ displaystyle \ eta \,}  ÂÂ$ di bawah 50%.

${\ displaystyle \ eta = {\ frac {F_ {out}/F_ ​​{in}} {d_ {in}/d_ {out}}} = {\ frac {F_ {out}} {F_ {in}}} {\ frac {l} {2 \ pi r}} & lt; 0,50 \,}  ÂÂ$

Whether the self-locking screw ultimately depends on the pitch angle and the coefficient of friction from the yarn; very lubricated yarn, low friction with a large enough pitch can "remodel".

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References

Source of the article : Wikipedia