Name: screw jack self-locking behavior
Uploaded: 2023-09-22T11:49:29.000Z
Duration: 1 min 16 s
Channel: JoVE

Weźmy pod uwagę podnośnik śrubowy z gwintem kwadratowym przenoszący ładunek. Jeśli śruba zachowuje swoją pozycję nawet po wykręceniu momentu, nazywa się to samohamownym.
Dzieje się tak, gdy kierunek siły tarcia odwraca się, a siła reakcji działa po drugiej stronie normalnej gwintu.
W tym przypadku kąt tarcia statycznego może być większy lub równy kątowi wyprzedzenia.
Aby nawinąć samozabezpieczającą w dół, należy przyłożyć moment do w przeciwnym kierunku. Stosując równania równowagi dla sił, można określić wielkość momentu.
Jeśli oba kąty są równe, reakcja działa pionowo, równoważąc obciążenie tak, że śruba jest na skraju nawijania w dół.
Jeśli kąt tarcia jest mniejszy niż kąt przystawienia, śruba nie jest samoblokująca i przykładany jest moment w przeciwnym kierunku, aby zatrzymać uzwojenie w dół.
Ten moment wytwarza siłę poziomą, która zapobiega przesuwaniu się gwintu po nachylonej płaszczyźnie. Na podstawie diagramu swobodnego ciała można określić wielkość tego momentu.

<ol>
	<li>Hibbeler, R.C. (2016). Mechanika inżynierska: Statyka. Czternasta edycja, New Jersey: Pearson. Strony 432-433.</li>
	<li>Meriam, J.L., Kraige, L.G. i Bolten, J.N. (2016). Mechanika inżynierska: statyka i dynamika. Ósma edycja, Singapur: John Wiley & Sons. Strony 355-356.</li>
	<li>Beer, F.P., Johnston, E.R., Mazurek, D.F., Cornwell, P.J. i Self, B.P. (2016). Mechanika wektorowa dla inżynierów. Jedenasta edycja, Nowy Jork: McGraw-Hill Education. Strony 451.</li>
</ol>

Śruby samoblokujące

Śruba samoblokująca to urządzenie mechaniczne powszechnie stosowanym do podnoszenia ciężkich ładunków lub wywierania znacznej siły. Jedną z kluczowych cech, które sprawiają, że śruba staje się bardziej skuteczna i niezawodna, jest jej zdolność do samoblokowania.

Śruby stosowane do podnoszenia ładunków uważane są za samoblokujące, jeśli śruba utrzymuje swoją pozycję nawet po usunięciu przyłożonego do niej momentu.

<img alt="Figure 1" src="/files/ftp_upload/14320/14320_PT_Figure_1v2.svg" style="text-align: center; height: 350px;" />

Dzieje się to, gdy kierunek siły tarcia zmienia się, a siła reakcji działa po drugiej stronie prostopadłej do gwintu. W tej sytuacji kąt tarcia statycznego jest większy lub równy kątowi nachylenia gwintu. Aby owinąć samoblokującą śrubę w dół, na śrubę musi być przyłożony moment w przeciwnym kierunku.

<img alt="Figure 2" src="/files/ftp_upload/14320/14320_PT_Figure_2v2.svg" style="text-align: center; height: 350px;" />

Przy zastosowaniu równań równowagi dla sił działających na śrubę można określić wartość momentu wymaganego do rozpoczęcia nawijania w dół.

<img alt="Equation 1" src="/files/ftp_upload/14320/14320_PT_Equation_1v2.svg" style="text-align: center; height: 29px;" />

<img alt="Equation 2" src="/files/ftp_upload/14320/14320_PT_Equation_2v2.svg" style="text-align: center; height: 13px;" />

<img alt="Equation 3" src="/files/ftp_upload/14320/14320_PT_Equation_3v2.svg" style="text-align: center; height: 13px;" />

Jeśli kąt tarcia statycznego jest równy kątowi skoku, siła reakcji działa pionowo, równoważąc ładunek, a śruba znajduje się na granicy obrotu w dół. W tym przypadku śruba jest w stanie równowagi, utrzymując swoją pozycję bez potrzeby dodatkowego momentu obrotowego.

Jednak jeśli kąt tarcia statycznego jest mniejszy niż kąt skoku, śruba nie jest samoblokująca. W takim przypadku, aby zatrzymać nawijanie w dół śruby, musi być przyłożony moment w przeciwnym kierunku. Ten przeciwny moment generuje siłę poziomą, która zapobiega ślizganiu się gwintu na nachylonej płaszczyźnie śrubowej.

<img alt="Figure 3" src="/files/ftp_upload/14320/14320_PT_Figure_3v2.svg" style="text-align: center; height: 350px;" />

Wielkość wymaganego przeciwnego momentu można określić, analizując wykres swobodnego ciała śruby w tych warunkach.

<img alt="Equation 4" src="/files/ftp_upload/14320/14320_PT_Equation_4v2.svg" style="text-align: center; height: 13px;" />

Te informacje są istotne dla skutecznego projektowania i obsługi śrub samoblokujących, ponieważ pozwalają inżynierom wybrać odpowiednie parametry dla konkretnej aplikacji.

Podnośnik śrubowy z gwintem kwadratowym jest urządzeniem mechanicznym szeroko stosowanym do podnoszenia ciężkich ładunków lub przykładania znacznej siły. Jedną z kluczowych funkcji, które mogą ...

Education

JoVE Core

Mechanical Engineering

Inżynieria mechaniczna

Unit 1

Jednostka 1

Friction

Tarcie

KEY TERMS AND CONCEPTS

Kluczowe terminy i pojęcia

Self-Locking Screw

screw jack self-locking behavior

A square-threaded screw jack can hold a heavy load because of its self-locking behavior. This feature makes the device more reliable when it is used to lift large weights or apply a strong force. If the screw keeps its position after the turning moment is removed, the jack is self-locking.

Self-locking happens because friction changes the way the forces act on the thread. The friction force reverses direction, and the reaction force moves to the other side of the thread’s normal. In this case, the static friction angle is larger than or equal to the lead angle of the screw.

To wind a self-locking screw downward, an opposite moment must be applied. The required moment can be found by using the equilibrium equations for the forces on the screw. If the static friction angle and the lead angle are equal, the reaction force acts vertically and the screw is on the verge of winding down.

When the static friction angle is smaller than the lead angle, the screw is not self-locking. Then an opposite moment is needed to stop the screw from winding downward. That moment creates a horizontal force that prevents the thread from sliding on the screw jack’s inclined plane. Engineers can determine the size of this opposing moment from a free-body diagram, which helps them choose the right screw jack design for the job.

Screw Jack Self-Locking Behavior

dry friction

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static friction

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frictional forces on screws

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self-locking screw

A square-threaded screw jack can hold a heavy load because of its self-locking behavior. This feature makes the device more reliable when it is used to lift large weights or apply a strong force. If the screw keeps its position after the turning moment is removed, the jack is self-locking. Self-locking happens because friction changes the way the forces act on the thread. The friction force reverses direction, and the reaction force moves to the other side of the thread’s normal. In this case,...

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key terms and concepts

Learn how a square-threaded screw jack stays in place, when it is self-locking, and how friction and lead angle affect the moment needed to move it.

Screw Jack Self-Locking Behavior | JoVE K12

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Screw Jack Self-Locking Behavior

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