Name: square-thread screw mechanics and friction
Uploaded: 2023-09-22T11:49:29.000Z
Duration: 1 min 17 s
Channel: JoVE

La maggior parte delle viti sono caratterizzate da una cresta elicoidale e sono comunemente usate come elementi di fissaggio; le viti a filettatura quadrata sono spesso impiegate nelle macchine per trasmettere potenza o movimento tra le parti.
Considera un martinetto con una vite a filettatura quadrata che trasporta un carico. Quando viene applicata una forza all'impugnatura, la vite gira e il carico viene sollevato.
La vite può essere considerata un albero cilindrico con una cresta quadrata inclinata avvolta su di esso.
La distanza tra le creste di due filettature adiacenti è il passo e la distanza che la vite avanza in un giro è nota come passo.
Quando il filo viene srotolato di un giro, assomiglia a una rampa inclinata con un angolo. L'angolo di inclinazione della filettatura è chiamato angolo di attacco ed è espresso in termini di raggio medio della filettatura e passo.
Poiché la forza di attrito tra le superfici è indipendente dall'area di contatto, la vite è rappresentata come un blocco sopra la filettatura del martinetto.
L'angolo tra la forza di reazione e la normale alla filettatura è l'angolo di attrito.

<ol>
	<li>Hibbeler, R.C. (2016). Ingegneria Meccanica: Statica. Quattordicesima edizione, New Jersey: Pearson. Pagine 432.</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 354-355.</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 450-451.</li>
</ol>

forze di attrito sui viti

Le viti sono caratterizzate da una cresta elicoidale nota come filettatura avvolta intorno ad un albero cilindrico. Sono comunemente utilizzate come fissaggi per tenere insieme oggetti o per trasmettere potenza e movimento nelle macchine. Un tipo di vite particolarmente utile per la trasmissione di potenza è la vite a filetto quadro.

Un cricchetto con una vite a filetto quadro è un dispositivo meccanico utilizzato per sollevare carichi pesanti applicando una forza alla sua maniglia. Quando viene applicata la forza, la vite si gira, sollevando il carico. La vite può essere considerata un albero cilindrico con una cresta quadrata inclinata o una filettatura avvolta intorno ad essa. Questa filettatura ha caratteristiche specifiche che ne definiscono la funzionalità.

La distanza tra due filettature adiacenti su una vite è chiamata passo. Questa misura è fondamentale, poiché determina quanto le filettature sono vicine. Un'altra caratteristica essenziale di una vite è il suo avanzamento, che si riferisce alla distanza che la vite compie in una rotazione completa.

Quando il filetto di una vite viene srotolato da una rotazione, assomiglia ad una rampa inclinata con un angolo specifico. Questo angolo di inclinazione, noto come angolo di avanzamento, può essere espresso in termini di raggio medio del filetto (la distanza media dal centro della vite alla linea centrale del filetto) e dell'avanzamento. L'angolo di avanzamento è un parametro cruciale, poiché influisce sull'efficienza della vite e sulla quantità di forza necessaria per girarla.

Per comprendere la meccanica di una vite a filetto quadro, è utile considerare le forze di attrito che agiscono sulla vite. Poiché la forza di attrito tra due superfici è indipendente dell'area di contatto, la vite può essere rappresentata come un blocco sopra il filetto del cricchetto. L'angolo tra la forza di reazione (la forza esercitata dal filetto sul blocco) e la normale al filetto (la direzione perpendicolare alla superficie del filetto) è chiamato angolo d'attrito. Questo angolo è determinato dai materiali a contatto e dalla lubrificazione utilizzata tra di loro.

L'attrito tra la vite e il carico che trasporta svolge un ruolo cruciale nell'efficienza e nella funzionalità della vite a filetto quadro. Le forze di attrito devono essere superate per girare la vite e sollevare il carico. Tuttavia, queste forze forniscono anche stabilità, impedendo alla vite di girare e al carico di cadere anche quando la forza viene rimossa dalla maniglia. Questa caratteristica di bloccaggio automatico è particolarmente utile in applicazioni in cui la vite deve tenere il carico in posizione senza una forza di input continua.

A causa della loro elevata efficienza e delle capacità di bloccaggio automatico, le viti a filetto quadro sono spesso impiegate nelle macchine per trasmettere potenza o movimento tra parti. Esempi di tali applicazioni includono cricchetti, morsetti, morse e diverse presse. Queste viti sono progettate per resistere a carichi pesanti e fornire un movimento regolare e controllato.

Forze di attrito sui viti

Le viti sono caratterizzate da una cresta elicoidale nota come filettatura avvolta attorno a un albero cilindrico. Sono comunemente usati come elementi di fissaggio per tenere oggetti ...

Education

Didattica

JoVE Core

Mechanical Engineering

Ingegneria Meccanica

Unit 1

Unità 1

Friction

Attrito

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

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 one full revolution, and the lead angle is the angle of that advance when the thread is unwrapped into a ramp.

The lead angle depends on the mean thread radius, which is the average distance from the center of the screw to the centerline of the thread, and on the lead itself. This angle matters because it affects screw efficiency and the force needed to turn the screw. To study a square-threaded screw, the frictional forces acting on it must also be considered.

Because friction does not depend on contact area, the screw can be modeled as a block moving over the thread of the jack. The angle between the reaction force from the thread and the normal to the thread surface is called the angle of friction. It depends on the materials in contact and on any lubrication used between them.

Friction between the screw and the load must be overcome to raise the load. At the same time, that same friction helps keep the screw from turning back when the handle force is removed. This self-locking feature is useful when the load must stay in place without continuous force.

Square-threaded screws are valued for high efficiency and self-locking behavior. They are used in jacks, clamps, vises, and presses. These machines rely on the screw to hold heavy loads and produce smooth, controlled motion.

Square-Thread Screw Mechanics and Friction

square-thread-screw-mechanics-and-friction

dry friction

Dry Friction in Crates and Everyday Motion

Dry friction appears when two solid surfaces touch and try to move past each other. Students can see it when they walk, slide a box across a table, or rub their hands together. Even though it is common, the forces behind it are not easy to see. A simple crate on a rough horizontal floor helps explain the idea. When an external force pushes the crate sideways, two main forces act on it. The normal force acts perpendicular to the surface and helps balance the crate’s weight, while the frictional...

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

Dry friction is the resistance between two solid surfaces that move against each other without lubrication or any fluid. A common example is a gardener pushing a wheelbarrow across the ground. The wheel and the ground create dry friction that slows the motion. Before the wheelbarrow starts to move, static friction acts at the contact surface. Static friction is the force that opposes motion before slipping begins, and it helps keep the wheelbarrow stable. It is present only when there is no...

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

Friction on a Banked Road

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wedges

Wedges and Self-Locking Stability

A wedge is a simple machine that can lift, support, and split heavy objects. It can also change the height of structural or mechanical parts. Because it amplifies force, a wedge makes it easier to move or hold large loads with less effort. One example is lifting a heavy slab. In that case, the wedge turns the applied force into a much larger force that acts almost perpendicular to the original push. That force change helps raise the load without requiring a large input force. To analyze wedge...

frictional forces on screws

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

A square-threaded screw jack lifts heavy loads by turning handle force into a twisting moment. Its upward motion depends on overcoming static friction between the screw threads and the jack. To study this motion, the thread can be treated as if it were a block on the groove of the jack after the thread is completely unraveled. A free-body diagram helps show the forces acting on this simplified system. The force at the handle creates a horizontal force that pushes the movable thread up the...

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,...

screw: problem solving

Screw Torque and Self-Locking

Screw torque and self-locking are analyzed by looking at a threaded screw shaft and a plate gear. The shaft has a mean radius and a lead. The plate gear also has a mean radius. The coefficient of static friction between the screw and the gear is part of the setup. The first step is to find the static friction angle, φ. This angle is defined by the coefficient of static friction. Next, the lead angle is calculated from the lead and the mean radius of the shaft. It gives the screw geometry...

frictional forces on flat belts

Flat Belt Tension and Friction

Flat belt tension and friction control how power moves from one pulley to another. Flat belts are used in many industrial systems to transmit power between pulleys. As the belt wraps around the pulleys, the tension changes because of friction between the belt and the pulley surface. When a pulley turns counterclockwise, the belt side moving away from the pulley has the higher tension, called T2. The side moving toward the pulley has the lower tension, called T1. To estimate these tensions, the...

flat belts: problem solving

Flat Belt Forces and Pulley Moment

Flat belts transmit power from one pulley to another, and the forces on the belt help determine the moment on a pulley. In this example, a belt wraps around pulleys A and B, with radii of 30 cm and 10 cm. The belt meets the pulleys at a 20 degree angle from the horizontal, which sets up the force balance in the system. As pulley B rotates clockwise, it drives pulley A. That motion creates tension T2 on one side of the belt and tension T1 on the other side. The belt-to-surface contact angle, β,...

pivot bearings

Axial Load Support in Pivot Bearings

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

Friction Moment in Journal Bearings

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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Double-collar bearing torque calculations show how friction and axial force affect bearing performance. Engineers use these calculations to test whether the bearing can handle the forces and moments it experiences during operation. This helps them choose and design bearings for specific applications and setups. For a double-collar bearing with set dimensions and an axial force applied, the maximum frictional moment can be found with a standard expression. The equation is rearranged, and the...

rolling resistance

Tire Deformation and Rolling Resistance

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rolling resistance: problem solving

Lawn Roller Force Calculation

Rolling resistance is the force that makes a rolling object harder to move across a surface. It is also called rolling friction. It comes from deformation in the object and the surface that touch each other. Internal friction, hysteresis, and other energy losses in the materials also play a role. Because of this force, extra energy is needed to keep a wheel, tire, or ball moving. The video applies this idea to a lawn roller. The roller has a mass of 100 kg, a radius of 25 cm, and a coefficient...

key terms and concepts

Learn how square-threaded screws lift heavy loads, use pitch and lead, and stay self-locking through friction in jacks, clamps, vises, and presses.

Square-Thread Screw Mechanics and Friction | JoVE K12

JoVE Core Mechanical Engineering offers concise, easy-to-understand animated videos covering key topics such as equilibrium, forces, truss analysis, and structural mechanics. Explore their real-world applications, and witness these concepts in action through scientist-led experiments.

mechanical engineering

Square-Thread Screw Mechanics and Friction

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