Oficios Técnicos

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MECÁNICA : Máquinas básicas

Durante muchos años los artefactos mecánicos básicos que entran en la composición o formación de máquinas se conocieron como potencias mecánicas. Dado que estos artefactos mecánicos se consideran en un sentido más estático que dinámico (es decir, la consideración de fuerzas  opuestas en equilibrio en lugar de tender a producir movimiento), es más correcto referirse a las mismas como máquinas básicas. Estrictamente hablando, el término potencia es un término dinámico en relación con el régimen de tiempo haciendo trabajo. Cuando los elementos de una máquina están en equilibrio, ningún trabajo se realiza. Por lo tanto, es incorrecto referirse a las máquinas básicas como potencias mecánicas.

Se debe entender que la acción de todas las máquinas básicas depende del principio de trabajo, que es: La fuerza aplicada, multiplicada por la distancia a través de la cual se mueve, es igual a la resistencia a superar, multiplicada por la distancia a través de la cual es superada.

A continuación se presentan la máquinas básicas:

- Palanca
- Rueda y eje
- Polea
- Plano inclinado
- Tornillo
- Cuña

Estas máquinas pueden reducirse aún más a tres clases de máquinas, como sigue:

- Un cuerpo sólido girando sobre un eje
- Una cuerda flexible
- Una superficie inclinada dura y lisa

El principio de los momentos es importante en el estudio de las máquinas básicas. Este importante principio puede enunciarse de la siguiente manera: "Cuando dos o más fuerzas actúan sobre un cuerpo rígido y tienden a girar en un eje, existe equilibrio si la suma de los momentos de las fuerzas que tienden a girar el cuerpo en una dirección es igual a la suma de los momentos de las fuerzas que tienden a girar el cuerpo en la dirección opuesta alrededor del mismo eje.”

Palanca

Una palanca es una barra de metal, madera, u otra sustancia que se utiliza para ejercer una presión o para sostener un peso en un punto de su longitud al recibir una fuerza en un segundo punto, y es libre de dar vuelta en un tercer punto o punto fijo llamado fulcro. Su aplicación se basa en el Principio de Momentos. La siguiente regla general se puede aplicar a todas las clases de palancas.

Regla: La fuerza P, multiplicada por su distancia desde el fulcro F, es igual a la carga W,  multiplicada por su distancia desde el fulcro.

Por lo tanto, la fórmula de cálculo que implica las tres clases de palancas es:

F ×  distancia  = W x distancia

Como se muestra en la figura 1, hay tres clases de palancas.

Figura 1. Las tres clases de palancas.

Problema

¿Qué fuerza P se requiere en un punto a 3 pies de distancia del  punto de apoyo F para equilibrar un peso de 112 libras aplicado en un punto a 6 pulgadas del punto de apoyo?

Solución

Las distancias o longitudes de las palancas son 3 pies y 6 pulgadas, respectivamente. Dado que las distancias deben ser de la misma denominación, los 3 pies deben reducirse a pulgadas (3 × 12), o sea 36 pulgadas. Entonces, aplicando la regla:

Rueda y eje

Figura 2.  El principio del aparejo diferencial, dando manivela en sentido horario para levantar el peso (a la izquierda), y la operación de elevación completa (a la derecha).

Una comparación de la rueda y el eje con la palanca de primer orden (véase la figura 1) indica que son similares en principio. La misma  fórmula puede ser utilizada para los cálculos que implican la rueda y el eje. Figura 2 muestra la rueda y el eje chino. Esta es una modificación de la rueda y el eje, y se puede utilizar para obtener apalancamiento extremo. Este es también el principio básico del aparejo diferencial. A medida que la manivela se gira hacia la derecha, el cable se enrolla en el tambor B y se desenrolla del tambor A. Debido a que el tambor B es más grande en diámetro, la longitud del cable entre los dos tambores es gradualmente absorbida, levantando la carga. Por lo tanto, si la diferencia en los diámetros de los dos tambores es pequeña, se obtiene un apalancamiento extremo, lo que permite que pesos pesados puedan ser levantados con poco esfuerzo. Además, la carga permanece suspendida en cualquier punto, porque la diferencia en los diámetros de los dos tambores es demasiado pequeña para superar la fricción de las partes.

Polea

Los dos tipos de son fijos y móviles. Ninguna ventaja mecánica se obtiene de la polea fija. Su uso es importante en realizar un trabajo que sea adecuado (elevar agua de un pozo, por ejemplo).

La polea móvil, al distribuir de su peso en porciones separadas, está dotada de ventajas mecánicas proporcionales al número de puntos de apoyo. Como se ilustra en la figura 3, la relación entre la fuerza aplicada y la carga levantada es cambiada por las diversas combinaciones de polea básicas. Por supuesto, una gama aún mayor se pueden obtener por medio de poleas adicionales, pero hay un límite práctico a que esto sea mecánicamente conveniente. La norma siguiente establece la  relación entre la fuerza y la carga.

Figura 3. Relación entre la fuerza aplicada y la carga elevada en diversas combinaciones básicas de poleas.

Regla: La carga W que puede ser levantada por una combinación de poleas es igual a la fuerza F veces el número de cables que soportan el bloque inferior o móvil.

Plano Inclinado

Si un camino en pendiente o inclinado es sustituido por una línea ascendente directa de ascenso, un peso dado puede ser levantado por un peso más pequeño. Por lo tanto, el plano inclinado es una máquina básica, debido a una fuerza menor puede ser aplicada para levantar una carga (ver Figura 4).

Regla: A medida que la fuerza P es aplicada a la carga W, la misma relación se cumple para la altura H a la longitud del plano inclinado L.

Así, el cálculo es:

fuerza: carga = altura: longitud del plano

Problema

¿Qué fuerza P se requiere para levantar una carga de 10 libras si la altura es 2 pies y el plano es de 12 pies de largo?

Figura 4. Plano inclinado. Un menor peso (P) puede ser utilizado para levantar una carga o el peso (W), porque la carga es parcialmente soportada por el plano inclinado.

Solución

Sustituyendo en la ecuación anterior:

Tornillo

Este tipo de máquina básica es meramente un plano inclinado envuelto alrededor de un cilindro. El tornillo se utiliza para ejercer una presión severa a través de un espacio pequeño. Dado que se lo somete a una alta pérdida de fricción, el tornillo generalmente ejerce una pequeña cantidad de potencia en sí mismo, pero una gran cantidad de potencia se puede derivar cuando se combina con la palanca o la rueda.

Regla: La fuerza aplicada P es a la carga W, como el paso de rosca es a la longitud del hilo de rosca por vuelta.

Así, el cálculo es:

fuerza aplicada P : carga W = paso : longitud de rosca por vuelta

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2002 JEEP WRANGLER (TJ) Bestop HighRock 4x4 High Access Front Winch Bumper in Matte Black

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Store: 4 Wheel Drive Hardware

Brand: Bestop

2010 JEEP WRANGLER (JK) Fab Fours Lifestyle Winch Bumper with Pre-runner Grille Guard
 

2010 JEEP WRANGLER (JK) Fab Fours Lifestyle Winch Bumper with Pre-runner Grille Guard

$1,709.99

Lifestyle Winch Bumper with Pre-Runner Grille Guard Fits 2007 to 2016 JK Wrangler, Rubicon and Unlimited;2 and 4-door models;2 stage black powder coat - Epoxy primer with a black Semi Gloss Semi Textured top coat;3/16" construction throughout;Tapered sides and bottom slides off obstacles;Light cut-outs and Hella 90MM fog And 60MM turn indicator lights are included with the bumper;Winch ready JEEP WRANGLER (JK) 70TH ANNIVERSARY ISLANDER MOUNTAIN RUBICON SAHARA SPORT UNLIMITED 70TH ANNIVERSARY UNLIMITED ISLANDER UNLIMITED MOUNTAIN UNLIMITED RUBICON UNLIMITED SAHARA UNLIMITED SPORT UNLIMITED X X 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 3.6L ERB (220) Pentastar 3.8L (3778cc) GAS with Light Mounts Included with D-Ring Mount Included with Winch Mount Included with Grille Guard Included Fab Fours Fab Fours Lifestyle Winch Front Bumper with Pre-runner Grille Guard (Black) - JK07-B1850-1 JK07-B1850-1 Front Bumpers [Read more]

Store: 4 Wheel Drive Hardware

Brand: Fab Fours

2010 JEEP WRANGLER (JK) Fab Fours Lifestyle Winch Bumper with Pre-runner Grille Guard
 

2010 JEEP WRANGLER (JK) Fab Fours Lifestyle Winch Bumper with Pre-runner Grille Guard

$1,669.99

Lifestyle Winch Bumper with Pre-Runner Grille Guard Fits 2007 to 2016 JK Wrangler, Rubicon and Unlimited;2 and 4-door models;Bare steel;3/16" construction throughout;Tapered sides and bottom slides off obstacles;Light cut-outs and Hella 90MM fog And 60MM turn indicator lights are included with the bumper;Winch ready JEEP WRANGLER (JK) 70TH ANNIVERSARY ISLANDER MOUNTAIN RUBICON SAHARA SPORT UNLIMITED 70TH ANNIVERSARY UNLIMITED ISLANDER UNLIMITED MOUNTAIN UNLIMITED RUBICON UNLIMITED SAHARA UNLIMITED SPORT UNLIMITED X X 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 3.6L ERB (220) Pentastar 3.8L (3778cc) GAS with Light Mounts Included with D-Ring Mount Included with Winch Mount Included with Grille Guard Included Fab Fours Fab Fours Lifestyle Winch Front Bumper with Pre-runner Grille Guard (Bare) - JK07-B1850-B JK07-B1850-B Front Bumpers [Read more]

Store: 4 Wheel Drive Hardware

Brand: Fab Fours

2010 JEEP WRANGLER (JK) Fab Fours Lifestyle Winch Bumper without Grille Guard
 

2010 JEEP WRANGLER (JK) Fab Fours Lifestyle Winch Bumper without Grille Guard

$1,549.99

Lifestyle Winch Bumper without Grille Guard Fits 2007 to 2016 JK Wrangler, Rubicon and Unlimited;2 and 4-door models;2 Stage matte black powder coat;Provides more tire clearance;Winch ready (Accepts up to 10,000 lb. winches or smaller); 3/4 D-ring mounts;1/4 boxed mounting system with adjustability to allow a custom fit;Fab Fours signature injection molded light housings;Fab Fours 90mm fog and 60mm turn indicator lights included FREE with the bumper;3/16 Steel construction;100% Made in the USA JEEP WRANGLER (JK) 70TH ANNIVERSARY ISLANDER MOUNTAIN RUBICON SAHARA SPORT UNLIMITED 70TH ANNIVERSARY UNLIMITED ISLANDER UNLIMITED MOUNTAIN UNLIMITED RUBICON UNLIMITED SAHARA UNLIMITED SPORT UNLIMITED X X 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 3.6L ERB (220) Pentastar 3.8L (3778cc) GAS with Light Mounts Included with D-Ring Mount Included with Winch Mount Included Fab Fours Fab Fours Lifestyle Winch Front Bumper without Grille Guard (Black) - JK07-B1851-1 JK07-B1851-1 Front Bumpers [Read more]

Store: 4 Wheel Drive Hardware

Brand: Fab Fours

2010 JEEP WRANGLER (JK) Fab Fours Lifestyle Winch Bumper without Grille Guard
 

2010 JEEP WRANGLER (JK) Fab Fours Lifestyle Winch Bumper without Grille Guard

$1,519.99

Lifestyle Winch Bumper without Grille Guard Fits 2007 to 2016 JK Wrangler, Rubicon and Unlimited;2 and 4-door models;Bare steel;3/16" construction throughout;Tapered sides and bottom slides off obstacles;Light cut-outs and Hella 90MM fog And 60MM turn indicator lights are included with the bumper;Winch ready JEEP WRANGLER (JK) 70TH ANNIVERSARY ISLANDER MOUNTAIN RUBICON SAHARA SPORT UNLIMITED 70TH ANNIVERSARY UNLIMITED ISLANDER UNLIMITED MOUNTAIN UNLIMITED RUBICON UNLIMITED SAHARA UNLIMITED SPORT UNLIMITED X X 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 3.6L ERB (220) Pentastar 3.8L (3778cc) GAS with Light Mounts Included with D-Ring Mount Included with Winch Mount Included Fab Fours Fab Fours Lifestyle Winch Front Bumper without Grille Guard (Bare) - JK07-B1851-B JK07-B1851-B Front Bumpers [Read more]

Store: 4 Wheel Drive Hardware

Brand: Fab Fours

2010 JEEP WRANGLER (JK) Fab Fours Full Metal Jacket Stubby Winch Bumper without Grill Guard
 

2010 JEEP WRANGLER (JK) Fab Fours Full Metal Jacket Stubby Winch Bumper without Grill Guard

$849.99

Full Metal Jacket Stubby Winch Bumper without Grill Guard Fits 2007 to 2016 JK Wrangler, Rubicon and Unlimited;2 and 4-door models;2 stage black powder coat;Epoxy primer with a black semi-gloss textured top coat;3/16" construction throughout;3/4 D-ring mounts;Tapered sides and bottom slides off obstacles;Light cut-outs for Rigid Dually lights;Winch ready JEEP WRANGLER (JK) 70TH ANNIVERSARY ISLANDER MOUNTAIN RUBICON SAHARA SPORT UNLIMITED 70TH ANNIVERSARY UNLIMITED ISLANDER UNLIMITED MOUNTAIN UNLIMITED RUBICON UNLIMITED SAHARA UNLIMITED SPORT UNLIMITED X X 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 3.6L ERB (220) Pentastar 3.8L (3778cc) GAS with Light Mounts Included with D-Ring Mount Included with Winch Mount Included Fab Fours Fab Fours Full Metal Jacket Stubby Winch Front Bumper without Grill Guard (Black) - JK07-B1855-1 JK07-B1855-1 Front Bumpers [Read more]

Store: 4 Wheel Drive Hardware

Brand: Fab Fours

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Donde "paso" es la distancia entre elementos similares dispuestos en un modelo o entre dos puntos de una pieza mecánica como, por ejemplo, la distancia entre las cimas de dos ranuras consecutivas de un disco, o entre dos filetes consecutivos de un tornillo.

Problema

Si el paso o distancia entre filetes es de 1/4 pulgada y una fuerza P de 100 libras es aplicada, que carga de peso W se puede mover por el tornillo si la longitud de la rosca por vuelta del tornillo es de 10 pulgadas?

Solución

Sustituyendo en la ecuación anterior:

Cuña

La cuña es prácticamente un par de planos inclinados que se colocan espalda a espalda o en contacto a lo largo de sus bases (ver figura 5).

Figura 5. Aplicación de una cuña (P) para levantar una carga pesada o peso (W).

Regla:  La fuerza aplicada P es la carga W, como el espesor de la cuña es a su longitud.

Así, el cálculo es:

fuerza aplicada P : carga W = espesor de la cuña: longitud de la cuña

Problema

¿ Qué fuerza aplicada  P se requiere para elevar una carga W de 2000 libras, utilizando una cuña que tiene 4 pulgadas de espesor y 20 pulgadas de longitud?

Solución

Sustituyendo en la ecuación:

 

 

 

 

 

 

 

 
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