Screws

A screw is a simple machine formed by an inclined plane that is wound around a cylindrical axis.

Tornillo y tuerca hexagonal

Screw and hexagonal nut.

Afrank99, CC BY-SA 3.0, vía Wikimedia Commons.

Parts of a screw

The different parts of a screw have the following names.

Shank
Cylindrical part of the screw where the thread grooves are cut.
Neck
Part of the screw shank that is not threaded.
Thread
Inclined plane wound helically around the shank.
Screw head
End part of the screw used to rotate it. In large screws it is usually square or hexagonal in shape.
Thread ridge
Protruding part of the thread groove.
Pitch
Distance between two consecutive crests of the thread.
Nut
Mechanical part with a threaded hole that fits onto the screw. It usually has a square or hexagonal shape to allow tightening with wrenches.
Partes de un tornillo y tuerca hexagonal.

Parts of a screw and a hexagonal nut.

Applications of screws

Removable joints

One of the applications of screws is to make removable joints, that is, joints that can be easily assembled and disassembled.

Por ejemplo, la carcasa de un ordenador está unida con tornillos.

Mechanisms that move with precision

Screws allow very precise movements.

Por ejemplo, el tornillo de un grifo giratorio permite abrir o cerrar el paso de agua con mucha precisión. Otro ejemplo son las sillas a tornillo, que se pueden subir o bajar poco a poco dando vueltas al asiento.

Mechanisms to move with force

Otra aplicación importante de los tornillos es la construcción de mecanismos que avanzan con mucha fuerza.

Por ejemplo, el mecanismo de un gato mecánico está basado en un tornillo que mueve un sistema de tijeras con suficiente fuerza como para levantar un automóvil.

Gato mecánico para levantar automóviles, con un tornillo que mueve el mecanismo.

Gato mecánico para levantar automóviles, con un tornillo que mueve el mecanismo.

Interiot, Public Domain, vía Wikimedia Commons.

Screw calculations

The main parameters of a screw are its pitch, the number of turns, and the linear advance obtained when turning it.

La relación entre estas magnitudes se expresa mediante la siguiente fórmula:

Advance = Turns \cdot Pitch

Where:

Advance = linear distance travelled by the screw, in millimetres.

Turns = number of turns made by the screw.

Pitch = distance the screw advances per turn.

Both Advance and Pitch must be expressed in the same units of length, usually millimetres.

Chair exercise

A workshop chair is raised using a screw with a thread pitch of 4 millimeters per turn. If we want to raise the chair by 6 centimeters, how many turns must be made on the screw?

To solve the problem, we first write down the available data, converting all distances to the same unit.

Advance = 6 cm = 60 \: mm

Pitch = 4 \: mm / turn

Next, we write the formula and substitute the known quantities:

Advance = Turns \cdot Pitch

60 \: mm = Turns \cdot 4 \: mm / turn

Finally, we solve for the unknown to find the result:

Turns = \cfrac{60}{4} = 15 \: turns

Bench vise screw exercise

A bench vise opens a distance of 12 centimeters after turning the handle a total of 40 turns. What is the pitch of the screw?

To solve the problem, we first write down the available data, converting all distances to the same unit.

Advance = 12 cm = 120 \: mm

Turns = 40 \: turns

Next, we write the formula and substitute the known quantities.

Advance = Turns \cdot Pitch

120 \: mm = 40 \: turns \cdot Pitch

Finally, we solve for the unknown to find the result.

Pitch = \cfrac{120}{40} = 3 \: mm / turn

Microscope screw exercise

A microscope uses a screw to raise and lower the stage in order to focus correctly on the object being observed. If the screw pitch is 0.5 millimeters and we make 16 turns, how much will the stage move?

To solve the problem, we first write down the available data, converting all distances to the same unit.

Pitch = 0.5 \: mm / turn

Turns = 16 \: turns

Next, we write the formula and substitute the known quantities.

Advance = Turns \cdot Pitch

Advance = 16 \: turns \cdot 0.5 \: mm / turn

Finally, there is no need to solve for an unknown, and we can calculate the result directly.

Advance = 8 \: mm

Exercises

  1. What is a screw?

  2. Name the nine different parts shown in the following image:

    Ejercicio de tornillos.

  3. What is the pitch of a screw?

  4. What three applications do screws have? Write one example for each application.

  5. Why do you think screws allow linear motion to be produced with great precision?

  6. A press uses a screw to close its jaw. The screw has a pitch of 6 mm per revolution. If we want to close the press by 18 cm, how many turns must we give to the screw?

  7. A mechanical lift raises a platform 2 meters using a screw with a pitch of 10 mm per revolution. How many turns of the screw are required to raise the platform?

  8. The screw that adjusts the height of a sewing machine needle has a pitch of 1 mm. If we want to lower the needle by 5 mm, how many turns must be made?

  9. A screw in a laboratory press allows an opening of 40 mm after 25 turns. What is the pitch of the screw?

  10. A screw in a hydraulic press raises the piston 0.75 m with 150 turns. What is the pitch of the screw?

  11. A bench vise jaw moves 15 centimeters after turning the handle 60 times. What is the pitch of the screw?

  12. A carpenter's clamp is adjusted using a screw with a pitch of 5 mm per revolution. If we turn the handle 12 times, how far will the jaw move?

  13. A laboratory chair has a screw with a pitch of 2 mm per revolution. If the screw is turned 25 times, how much does the chair rise?

  14. A table saw has a support adjustment screw with a pitch of 1.5 mm per revolution. If we turn the handle 40 times, how far will the support move?