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

Considere un gato de tornillo de rosca cuadrada que transporta una carga. Si el tornillo conserva su posición incluso después de que se retira el momento, se denomina autobloqueo.
Esto sucede si la dirección de la fuerza de fricción se invierte y la fuerza de reacción actúa en el otro lado de la normal de la rosca.
Aquí, el ángulo de fricción estática puede ser mayor o igual al ángulo de avance.
Para enrollar un tornillo autoblocante hacia abajo, se debe aplicar un momento al tornillo en la dirección opuesta. Aplicando las ecuaciones de equilibrio para las fuerzas, se puede determinar la magnitud del momento.
Si ambos ángulos son iguales, la reacción actúa verticalmente, equilibrando la carga de tal manera que el tornillo está a punto de enrollarse hacia abajo.
Si el ángulo de fricción es menor que el ángulo de avance, el tornillo no es autoblocante y se aplica un momento en la dirección opuesta para detener el bobinado descendente del tornillo.
Este momento crea una fuerza horizontal que impide el deslizamiento de la rosca en el plano inclinado. A partir del diagrama de cuerpo libre, se puede determinar la magnitud de este momento.

<ol>
	<li>Hibbeler, R.C. (2016). Ingeniería Mecánica: Estática. Decimocuarta edición, Nueva Jersey: Pearson. Págs. 432-433.</li>
	<li>Meriam, J.L., Kraige, L.G. y Bolten, J.N. (2016). Ingeniería Mecánica: Estática y Dinámica. Octava edición, Singapur: John Wiley & Sons. Págs. 355-356.</li>
	<li>Beer, F.P., Johnston, E.R., Mazurek, D.F., Cornwell, P.J. y Self, B.P. (2016). Mecánica vectorial para ingenieros. Undécima Edición, Nueva York: McGraw-Hill Education. Págs. 451.</li>
</ol>

tornillo auto-trabante

Un gato mecánico con rosca cuadrada es un dispositivo ampliamente utilizado para levantar cargas pesadas o aplicar una fuerza considerable. Una de las características clave que puede hacer que un gato de tornillo sea más efectivo y confiable es su capacidad auto-trabante. Se considera que un gato de tornillo con rosca cuadrada que lleva una carga es auto-trabante si el tornillo mantiene su posición incluso después de que se retira el momento aplicado sobre él. <img alt="Figure 1" src="/files/ftp_upload/14320/14320_PT_Figure_1v2.svg" style="text-align: center; height: 350px;"> Esto ocurre cuando la dirección de la fuerza de fricción se invierte y la fuerza de reacción actúa en el otro lado de la normal de la rosca. En esta situación, el ángulo de fricción estática es mayor o igual al ángulo de avance del tornillo. Para enrollar un tornillo auto-trabante hacia abajo, se debe aplicar un momento en la dirección opuesta al tornillo. <img alt="Figure 2" src="/files/ftp_upload/14320/14320_PT_Figure_2v2.svg" style="text-align: center; height: 350px;"> Aplicando las ecuaciones de equilibrio para las fuerzas que actúan sobre el tornillo, se puede determinar la magnitud del momento necesario para iniciar el enrollamiento hacia abajo. <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;"> Si tanto el ángulo de fricción estática como el ángulo de avance son iguales, la fuerza de reacción actúa verticalmente, equilibrando la carga de manera que el tornillo esté al borde de enrollarse hacia abajo. En este caso, el tornillo está en un estado de equilibrio, donde puede mantener su posición sin que se le aplique ningún momento adicional. Sin embargo, si el ángulo de fricción estática es menor que el ángulo de avance, el tornillo no es auto-trabante. En este escenario, se debe aplicar un momento en la dirección opuesta para detener el enrollamiento hacia abajo del tornillo. Este momento opuesto genera una fuerza horizontal que impide el deslizamiento de la rosca sobre el plano inclinado del gato de tornillo. <img alt="Figure 3" src="/files/ftp_upload/14320/14320_PT_Figure_3v2.svg" style="text-align: center; height: 350px;"> La magnitud del momento opuesto requerido se puede determinar analizando el diagrama de cuerpo libre del gato de tornillo bajo estas condiciones. <img alt="Equation 4" src="/files/ftp_upload/14320/14320_PT_Equation_4v2.svg" style="text-align: center; height: 13px;"> Esta información es crucial para diseñar y operar un gato de tornillo de manera efectiva, ya que permite a los ingenieros seleccionar los parámetros apropiados para la aplicación específica.

Tornillo auto-trabante

Un gato de tornillo de rosca cuadrada es un dispositivo mecánico ampliamente utilizado para levantar cargas pesadas o aplicar una fuerza considerable. Una de las características clave que puede ...

Education

Educación

JoVE Core

Mechanical Engineering

Ingeniería Mecánica

Unit 1

Unidad 1

Friction

Fricción

KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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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frictional forces on screws

Square-Thread Screw Mechanics and Friction

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upward impending motion

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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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