Misplaced Pages

#791208

45-502: The term "jack post" is one of many describing the same system. Other names include any assortment of "adjustable", "steel", "jack" and "shoring" with "post", "column" and "prop", depending on their precise construction - the "jack" refers to examples with a jack screw to adjust the precise length of the post. They are also known by any number of trade names, notably "Acrow prop". Jack posts are widely standardized. Jack posts are telescopic tubular steel props consisting of two primary parts,

90-638: A ( F A ⋅ e A ⊥ ) − b ( F B ⋅ e B ⊥ ) = a F A − b F B , {\displaystyle F_{\theta }=\mathbf {F} _{A}\cdot {\frac {\partial \mathbf {v} _{A}}{\partial {\dot {\theta }}}}-\mathbf {F} _{B}\cdot {\frac {\partial \mathbf {v} _{B}}{\partial {\dot {\theta }}}}=a(\mathbf {F} _{A}\cdot \mathbf {e} _{A}^{\perp })-b(\mathbf {F} _{B}\cdot \mathbf {e} _{B}^{\perp })=aF_{A}-bF_{B},} where F A and F B are components of

135-424: A square thread or buttress thread is used, because it has the lowest friction and wear. In technical applications, such as actuators , an Acme thread is used, although it has higher friction, because it is easy to manufacture, wear can be compensated for, it is stronger than a comparably sized square thread and it makes for smoother engagement. The ball screw is a more advanced type of leadscrew that uses

180-692: A British Standard BS4074. A similar lightweight European prop design exists, these have a maximum capacity of only 5 kilonewtons (1,100 lb f ). Props are made in a range of five BS standard sizes: The safe load for each prop depends upon the size and extension of the prop. Size 1, 2 and 3 props can take about 34 kN unextended, dropping progressively to less than half of that when fully extended. Size 0 props can take at least 23 kN, whereas size 4 props at most 20 kN dropping to 7 kN at their maximum extension. Props must be loaded concentrically, i.e. vertically for simple weights, to within 25mm. Jack screw A jackscrew , or screw jack ,

225-504: A foot pedal was used for the earliest horizontal frame loom . In Mesopotamia (modern Iraq) c.  3000 BC , the shadouf , a crane-like device that uses a lever mechanism, was invented. In ancient Egypt , workmen used the lever to move and uplift obelisks weighing more than 100 tons. This is evident from the recesses in the large blocks and the handling bosses which could not be used for any purpose other than for levers. The earliest remaining writings regarding levers date from

270-480: A greater output force, which is said to provide leverage , which is mechanical advantage gained in the system, equal to the ratio of the output force to the input force. As such, the lever is a mechanical advantage device , trading off force against movement. The word "lever" entered English around AD 1300 from Old French : levier . This sprang from the stem of the verb lever , meaning "to raise". The verb, in turn, goes back to Latin : levare , itself from

315-431: A hinge, or pivot, called a fulcrum. The ideal lever does not dissipate or store energy, which means there is no friction in the hinge or bending in the beam. In this case, the power into the lever equals the power out, and the ratio of output to input force is given by the ratio of the distances from the fulcrum to the points of application of these forces. This is known as the law of the lever . The mechanical advantage of

360-446: A jackscrew on a Yakovlev Yak-42 airliner due to design flaws resulted in the crash of Aeroflot Flight 8641 in 1982. The failure of a jackscrew on a McDonnell Douglas MD-80 due to deficient maintenance brought down Alaska Airlines Flight 261 in 2000. A MRAP armoured vehicle being transported aboard National Airlines Flight 102 in 2013, a Boeing 747-400BCF freighter, broke loose immediately after takeoff and smashed through

405-446: A lever can be determined by considering the balance of moments or torque , T , about the fulcrum. If the distance traveled is greater, then the output force is lessened. T 1 = F 1 a , T 2 = F 2 b {\displaystyle {\begin{aligned}T_{1}&=F_{1}a,\quad \\T_{2}&=F_{2}b\!\end{aligned}}} where F 1

450-415: A lever is the ratio of output force to input force. M A = F 2 F 1 = a b . {\displaystyle MA={\frac {F_{2}}{F_{1}}}={\frac {a}{b}}.\!} This relationship shows that the mechanical advantage can be computed from ratio of the distances from the fulcrum to where the input and output forces are applied to the lever, assuming

495-481: A recirculating-ball nut to minimize friction and prolong the life of the screw threads. The thread profile of such screws is approximately semicircular (commonly a "gothic arch" profile) to properly mate with the bearing balls . The disadvantage to this type of screw is that it is not self-locking. Ball screws are prevalent in powered leadscrew actuators. Jackscrews are also used extensively in aircraft systems to raise and lower horizontal stabilizers . The failure of

SECTION 10

#1732791848792

540-423: A rigid bar connected to a ground frame by a hinged joint called a fulcrum. The lever is operated by applying an input force F A at a point A located by the coordinate vector r A on the bar. The lever then exerts an output force F B at the point B located by r B . The rotation of the lever about the fulcrum P is defined by the rotation angle θ in radians. Let the coordinate vector of

585-437: A screw jack, the ratio of the force the jack exerts on the load to the input force on the lever ignoring friction is where The screw jack consists of two simple machines in series; the long operating handle serves as a lever whose output force turns the screw. So the mechanical advantage is increased by a longer handle as well as a finer screw thread. However, most screw jacks have large amounts of friction which increase

630-430: A stationary support frame with a wide base resting on the ground. A rotating collar on the head of the screw has holes into which the handle, a metal bar, fits. When the handle is turned clockwise, the screw moves further out of the base, lifting the load resting on the load table. In order to support large load forces, the screw is usually formed with Acme threads . An advantage of jackscrews over some other types of jack

675-450: A strong 'needle' or 'strongboy' is placed through the hole. A pair of props are then used, one under each end. Existing windows or doorways may also be supported directly, or via needles. As the plates on the end of the posts are typically small, they offer little sideways support. If there is any sideways force, props should be strutted or 'laced' with scaffolding poles. Metal props, their size, strength and construction, are now described by

720-415: A weightless lever and no losses due to friction, flexibility or wear. This remains true even though the "horizontal" distance (perpendicular to the pull of gravity) of both a and b change (diminish) as the lever changes to any position away from the horizontal. Levers are classified by the relative positions of the fulcrum, effort and resistance (or load). It is common to call the input force "effort" and

765-409: Is a type of jack that is operated by turning a leadscrew . It is commonly used to lift moderate and heavy weights, such as vehicles; to raise and lower the horizontal stabilizers of aircraft; and as adjustable supports for heavy loads, such as the foundations of houses. A screw jack consists of a heavy-duty vertical screw with a load table mounted on its top, which screws into a threaded hole in

810-428: Is given by: M A = F B F A = a b . {\displaystyle MA={\frac {F_{B}}{F_{A}}}={\frac {a}{b}}.} This is the law of the lever , which was proven by Archimedes using geometric reasoning. It shows that if the distance a from the fulcrum to where the input force is applied (point A ) is greater than the distance b from fulcrum to where

855-405: Is important, such as for horizontal stabilizers on aircraft. The large area of sliding contact between the screw threads means jackscrews have high friction and low efficiency as power transmission linkages, around 30%–50%. So they are not often used for continuous transmission of high power, but more often in intermittent positioning applications. In heavy-duty applications, such as screw jacks,

900-429: Is not limited by the limitations on screw jacks and can be far greater. After World War II, improvements to the grinding of hydraulic rams and the use of O ring seals reduced the price of low-cost hydraulic jacks and they became widespread for use with domestic cars. Screw jacks still remain for minimal-cost applications, such as the little-used tyre-changing jacks supplied with cars, or where their self-locking property

945-426: Is that they are self-locking , which means when the rotational force on the screw is removed, it will remain motionless where it was left and will not rotate backwards, regardless of how much load it is supporting. This makes them inherently safer than hydraulic jacks , for example, which will move backwards under load if the force on the hydraulic actuator is accidentally released. The ideal mechanical advantage of

SECTION 20

#1732791848792

990-481: Is the generalized coordinate that defines the configuration of the lever, and the generalized force associated with this coordinate is given by F θ = F A ⋅ ∂ v A ∂ θ ˙ − F B ⋅ ∂ v B ∂ θ ˙ =

1035-445: Is the input force to the lever and F 2 is the output force. The distances a and b are the perpendicular distances between the forces and the fulcrum. Since the moments of torque must be balanced, T 1 = T 2 {\displaystyle T_{1}=T_{2}\!} . So, F 1 a = F 2 b {\displaystyle F_{1}a=F_{2}b\!} . The mechanical advantage of

1080-422: The eardrum to the oval window of the cochlea . The lever is a movable bar that pivots on a fulcrum attached to a fixed point. The lever operates by applying forces at different distances from the fulcrum, or a pivot. As the lever rotates around the fulcrum, points further from this pivot move faster than points closer to the pivot. Therefore, a force applied to a point further from the pivot must be less than

1125-467: The 3rd century BC and were provided, by common belief, by the Greek mathematician Archimedes , who famously stated "Give me a lever long enough and a fulcrum on which to place it, and I shall move the world." Autumn Stanley argues that the digging stick can be considered the first lever, which would position prehistoric women as the inventors of lever technology. A lever is a beam connected to ground by

1170-451: The 3rd class lever. A compound lever comprises several levers acting in series: the resistance from one lever in a system of levers acts as effort for the next, and thus the applied force is transferred from one lever to the next. Examples of compound levers include scales, nail clippers and piano keys. The malleus , incus and stapes are small bones in the middle ear , connected as compound levers, that transfer sound waves from

1215-578: The adjective levis , meaning "light" (as in "not heavy"). The word's primary origin is the Proto-Indo-European stem legwh- , meaning "light", "easy" or "nimble", among other things. The PIE stem also gave rise to the English word "light". The earliest evidence of the lever mechanism dates back to the ancient Near East c.  5000 BC , when it was first used in a simple balance scale . In ancient Egypt c.  4400 BC ,

1260-421: The force located at a point closer in, because power is the product of force and velocity. If a and b are distances from the fulcrum to points A and B and the force F A applied to A is the input and the force F B applied at B is the output, the ratio of the velocities of points A and B is given by a/b , so we have the ratio of the output force to the input force, or mechanical advantage,

1305-400: The forces that are perpendicular to the radial segments PA and PB . The principle of virtual work states that at equilibrium the generalized force is zero, that is F θ = a F A − b F B = 0. {\displaystyle F_{\theta }=aF_{A}-bF_{B}=0.\,\!} Thus, the ratio of the output force F B to

1350-542: The fulcrum to the input point A and to the output point B , respectively. Now introduce the unit vectors e A and e B from the fulcrum to the point A and B , so r A − r P = a e A , r B − r P = b e B . {\displaystyle \mathbf {r} _{A}-\mathbf {r} _{P}=a\mathbf {e} _{A},\quad \mathbf {r} _{B}-\mathbf {r} _{P}=b\mathbf {e} _{B}.} The velocity of

1395-426: The input force F A is obtained as M A = F B F A = a b , {\displaystyle MA={\frac {F_{B}}{F_{A}}}={\frac {a}{b}},} which is the mechanical advantage of the lever. This equation shows that if the distance a from the fulcrum to the point A where the input force is applied is greater than the distance b from fulcrum to

Jack post - Misplaced Pages Continue

1440-400: The input force necessary, so the actual mechanical advantage is often only 30% to 50% of this figure. Screw jacks are limited in their lifting capacity. Increasing load increases friction within the screw threads. A fine pitch thread, which would increase the advantage of the screw, also reduces the speed of which the jack can operate. Using a longer operating lever soon reaches the point where

1485-451: The lever will simply bend at its inner end. Screw jacks have now largely been replaced by hydraulic jacks . This was encouraged in 1858 when jacks by the Tangye company to Bramah 's hydraulic press concept were applied to the successful launching of Brunel 's SS  Great Eastern , after two failed attempts by other means. The maximum mechanical advantage possible for a hydraulic jack

1530-406: The main part of the post, and the jack screw or other adjustable fitting on one or both ends. Both ends are normally fitted with flat metal plates on the end, providing additional support area. A recent improvement to Acrow props was to shape this baseplate with notches, allowing pallet loads of horizontal props to be stacked neatly, rather than randomly piled. Most jack posts are split in two near

1575-447: The middle, with the upper end carrying the jack designed to slide within the lower portion. Gross adjustment for length is first made by pulling a pin and sliding the two sections within each other until they almost fill the gap, inserting the pin to lock them, then using the screw to close any remaining gap. Other designs used two threaded pipes instead of sliding sections, ratechetting or clamping sections, or other similar concepts to lock

1620-402: The one illustrated, the jack screws also help align the connectors and overcome the large frictional forces involved in inserting or removing the connector. When unscrewed, they allow the connector halves to be taken apart. Jackscrews in electrical connectors may have ordinary screw heads or extended heads designed as thumbscrews . The idea of incorporating jack screws into electrical connectors

1665-409: The output force "load" or "resistance". This allows the identification of three classes of levers by the relative locations of the fulcrum, the resistance and the effort: These cases are described by the mnemonic fre 123 where the f fulcrum is between r and e for the 1st class lever, the r resistance is between f and e for the 2nd class lever, and the e effort is between f and r for

1710-399: The output force is applied (point B ), then the lever amplifies the input force. On the other hand, if the distance a from the fulcrum to the input force is less than the distance b from the fulcrum to the output force, then the lever reduces the input force. The use of velocity in the static analysis of a lever is an application of the principle of virtual work . A lever is modeled as

1755-416: The point P that defines the fulcrum be r P , and introduce the lengths a = | r A − r P | , b = | r B − r P | , {\displaystyle a=|\mathbf {r} _{A}-\mathbf {r} _{P}|,\quad b=|\mathbf {r} _{B}-\mathbf {r} _{P}|,} which are the distances from

1800-562: The points A and B are obtained as v A = θ ˙ a e A ⊥ , v B = θ ˙ b e B ⊥ , {\displaystyle \mathbf {v} _{A}={\dot {\theta }}a\mathbf {e} _{A}^{\perp },\quad \mathbf {v} _{B}={\dot {\theta }}b\mathbf {e} _{B}^{\perp },} where e A and e B are unit vectors perpendicular to e A and e B , respectively. The angle θ

1845-480: The rear bulkhead.  The cockpit voice and flight data recorders were knocked off line, hydraulic lines were severed and most critically, the horizontal stabilizer actuator’s jackscrew was destroyed, rendering the aircraft uncontrollable. A machinist's jack is a miniature screw jack used to support protruding parts of a workpiece or to balance clamping forces on that workpiece during machining operations. Aside from their size, these frequently look no different from

Jack post - Misplaced Pages Continue

1890-469: The screw jacks used to lift buildings off their foundations. Machinist's jacks can be as simple as a threaded spacer with a bolt in it to serve as a jackscrew. The term jackscrew is also used for the captive screws that draw the two parts of some electrical connectors together and hold them mated. These are commonly encountered on D-subminiature connectors, where they serve primarily to prevent accidental disconnection. On larger connectors such as

1935-402: The system at a specific length. Jack posts are mostly used for shoring : temporary supports during building repair or alteration work, rather than scaffolding : access platforms for workers. A typical use is to support an existing horizontal beam while its original masonry supports are removed or repaired. When masonry itself is to be supported, holes are first knocked through the brickwork and

1980-478: The wrong connector from being connected to the wrong socket. Lever A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge , or fulcrum . A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is divided into three types . It is one of the six simple machines identified by Renaissance scientists. A lever amplifies an input force to provide

2025-475: Was not considered novel in the late 1950s and early 1960s. Some patents from that era show pairs of jackscrews on opposite sides of a multi-pin connector. Another shows a single central jackscrew. These patents mention the phrase "jack screw" incidentally, without asserting a claim to the idea. Jack screws may have either male or female threads, and on some connectors, the genders of the screws as well as various alignment pins may be mixed in order to prevent

#791208