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

Considera un martinetto a vite con filettatura quadrata che trasporta un carico. Se la vite mantiene la sua posizione anche dopo che il momento è stato rimosso, si parla di autobloccante.
Ciò accade se la direzione della forza di attrito si inverte e la forza di reazione agisce dall'altra parte della normale del filo.
In questo caso, l'angolo di attrito statico può essere maggiore o uguale all'angolo di attacco.
Per avvolgere una vite autobloccante verso il basso, è necessario applicare un momento alla vite nella direzione opposta. Applicando le equazioni di equilibrio per le forze, è possibile determinare l'entità del momento.
Se entrambi gli angoli sono uguali, la reazione agisce verticalmente, bilanciando il carico in modo tale che la vite sia sul punto di avvolgersi verso il basso.
Se l'angolo di attrito è inferiore all'angolo di attacco, la vite non è autobloccante e viene applicato un momento nella direzione opposta per arrestare l'avvolgimento verso il basso della vite.
Questo momento crea una forza orizzontale che impedisce lo scorrimento del filetto sul piano inclinato. Dal diagramma di corpo libero, l'entità di questo momento può essere determinata.

<ol>
	<li>Hibbeler, R.C. (2016). Ingegneria Meccanica: Statica. Quattordicesima edizione, New Jersey: Pearson. Pagine 432-433.</li>
	<li>Meriam, J.L., Kraige, L.G. e Bolten, J.N. (2016). Meccanica ingegneristica: statica e dinamica. Ottava edizione, Singapore: John Wiley & Sons. Pagine 355-356.</li>
	<li>Birra, F.P., Johnston, E.R., Mazurek, D.F., Cornwell, P.J. e Self, B.P. (2016). Meccanica vettoriale per ingegneri. Undicesima edizione, New York: McGraw-Hill Education. Pagine 451.</li>
</ol>

vite auto-bloccante

Un cric a vite con filettatura quadrata è un dispositivo meccanico ampiamente utilizzato per sollevare carichi pesanti o applicare una considerevole forza. Una delle caratteristiche chiave che può rendere un cric a vite più efficace e affidabile è la sua capacità di auto-bloccaggio.

Questo cric che trasporta un carico, è considerato auto-bloccante se la vite mantiene la sua posizione anche dopo che è stato rimosso il momento applicato ad essa.

<img alt="Figure 1" src="/files/ftp_upload/14320/14320_PT_Figure_1v2.svg" style="text-align: center; height: 350px;" />

Ciò si verifica quando si inverte la direzione della forza di attrito e la forza di reazione agisce sull'altro lato della normale del filetto. In questa situazione, l'angolo di attrito statico è maggiore o uguale all'angolo di avanzamento della vite. Per avvolgere una vite auto-bloccante verso il basso, deve essere applicato un momento alla vite nella direzione opposta.

<img alt="Figure 2" src="/files/ftp_upload/14320/14320_PT_Figure_2v2.svg" style="text-align: center; height: 350px;" />

Applicando le equazioni di equilibrio per le forze che agiscono sulla vite, è possibile determinare l'entità del momento necessario per avviare l'avvolgimento verso il basso.

<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;" />

Se sia l'angolo di attrito statico che l'angolo di avanzamento sono uguali, la forza di reazione agisce verticalmente, bilanciando il carico in modo che la vite sia sull'orlo dell'avvolgimento verso il basso. In questo caso, la vite si trova in uno stato di equilibrio, in cui può mantenere la sua posizione senza alcun momento aggiuntivo applicato ad essa.

Tuttavia, se l'angolo di attrito statico è più piccolo dell'angolo di avanzamento, la vite non è auto-bloccante. In questo scenario, deve essere applicato un momento nella direzione opposta per fermare l'avvolgimento verso il basso della vite. Questo momento opposto genera una forza orizzontale che impedisce lo scivolamento del filetto sul piano inclinato del cric a vite.

<img alt="Figure 3" src="/files/ftp_upload/14320/14320_PT_Figure_3v2.svg" style="text-align: center; height: 350px;" />

L'entità del momento opposto richiesto, può essere determinata analizzando il diagramma delle forze libere del cric in queste condizioni.

<img alt="Equation 4" src="/files/ftp_upload/14320/14320_PT_Equation_4v2.svg" style="text-align: center; height: 13px;" />

Questa informazione è cruciale per progettare e operare efficacemente un cric a vite, poiché consente agli ingegneri di selezionare i parametri appropriati per l'applicazione specifica.

Vite auto-bloccante

Un martinetto a filettatura quadrata è un dispositivo meccanico ampiamente utilizzato per il sollevamento di carichi pesanti o l'applicazione di una forza considerevole. Una delle caratteristiche chiave che può ...

Education

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JoVE Core

Mechanical Engineering

Ingegneria Meccanica

Unit 1

Unità 1

Friction

Attrito

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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

Static Friction at the Point of Slip

Static friction is the force that keeps two surfaces from starting to slide past each other. It acts when there is a tendency for motion, but not yet actual slipping. This force helps explain everyday actions like walking on the ground and driving a car. A towing example shows how static friction changes as force is applied. A truck pulls a car with a rope, and the car stays still at first. The car is in static equilibrium, which means the forces on it are balanced. Gravity and the normal...

kinetic friction

Sliding Friction Between Moving Surfaces

Kinetic friction is the sliding friction that acts when two surfaces move against each other. It opposes relative motion and acts parallel to the contact surface, in the opposite direction of travel. In the truck and car example, the car first stays still because static friction holds it in place. When the truck’s force becomes greater than the limiting static frictional force, the friction at the contact surface drops to a lower value. The car then begins to slide, and its speed increases. At...

characteristics of dry friction

Static and Kinetic Dry Friction

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types of friction problems

Dry Friction in Crate, Ladder, and Frame Problems

Dry friction problems often focus on crates, ladders, and frames that are close to slipping. Dry friction is the force that resists relative motion, or the tendency for motion, between two surfaces in contact. In many physics and engineering problems, this force helps keep an object still or defines the point where motion is about to begin. One common case has no clear impending motion. A force may be applied to a crate, but the crate stays at rest. The friction force between the floor and the...

friction: problem solving

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wedges

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frictional forces on screws

Square-Thread Screw Mechanics and Friction

Square-threaded screws use a helical thread to turn force into lifting motion. They are common fasteners, but they are also useful for transmitting power and motion in machines. A jack with a square-threaded screw lifts heavy loads when force is applied at the handle and the screw turns. A screw can be viewed as a cylindrical shaft with an inclined square ridge wrapped around it. The distance between two neighboring threads is called the pitch. The lead is the distance the screw advances in...

upward impending motion

Screw Jack Force and Friction Analysis

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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,...

screw: problem solving

Screw Torque and Self-Locking

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frictional forces on flat belts

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flat belts: problem solving

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pivot bearings

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Pivot bearings support axial loads on a rotating shaft. A bearing helps two parts move relative to each other while reducing friction and wear. In this design, the pivot is placed at the end of the shaft to carry the axial thrust. The pivot surface can be flat, conical, or truncated, depending on the application. In a flat pivot bearing, the axial load is spread over the bearing area. The pressure is uniform and can be found by dividing the load by the area. To study the forces more closely,...

collar bearings

Friction Torque in Collar Bearings

Collar bearings support axial loads on rotating shafts, and their friction helps determine the torque needed to start motion. They are used in many machines and may have a single collar or multiple collars, depending on the application and load needs. For a single collar bearing under an axial load, the contact area is the ring-shaped region between the external radius and the internal radius of the collar. If the bearing is assumed to support the load evenly, the uniform normal pressure is...

journal bearings

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Journal bearings support rotating shafts and axles while helping keep them stable. They are used in machines such as engines, turbines, and pumps. Their job is to reduce friction, wear, and vibration. A journal bearing works by keeping a thin film of lubricant between the bearing surface and the rotating shaft. This film limits direct contact and lowers frictional force. The video uses a rope winch with dry or only partly lubricated journal bearings to show the idea. As the spool turns...

bearings: problem solving

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rolling resistance

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rolling resistance: problem solving

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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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mechanical engineering

Screw Jack Self-Locking Behavior

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