Screws¶

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

Screw and hex nut.¶

Afrank99 CC BY-SA 3.0 via Wikimedia Commons.

Parts of a screw¶

The denomination of the different parts of the screw is as follows.

Stem: Cylindrical part of the screw where the grooves of the thread are carved.
Neck: Uncut part of the screw shank.
Thread: It is the inclined plane wound helically around the stem.
Screw head: It is the extreme part of the screw, used to turn the thread. It is usually square or hexagonal in large screws.
Fillet: It is the protruding part of the groove of the thread.
Passed: It is the distance between two consecutive crests of the thread.
Nut: It is a mechanical part with a threaded hole that is attached to the screw. The nut usually has a square or hexagonal shape to facilitate its turning using torque wrenches.

Partes de un tornillo y tuerca hexagonal.

Parts of a bolt and hex nut.¶

Screw Applications¶

One of the applications of screws is to make removable joints.

Another great application for screws is to build mechanisms that move slowly, precisely, and with great force. For example, the mechanism of a mechanical jack to lift cars is based on a screw that moves a pair of scissors.

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

Mechanical jack to lift cars, with a screw that moves the mechanism.¶

Interiot Public Domain via Wikimedia Commons.

Bolt calculation¶

The parameters of a screw are its pitch or distance between threads, the number of turns it turns and the linear feed it achieves when turning. The formula that relates these variables is the following.

$Feed = Turn \cdot Step$

Being

Lead = distance the screw travels

Turn = number of turns the screw turns

Pitch = distance between two screw threads

Both the Advance and the Pace must be expressed in the same units of distance.

Exercise chair¶

A workshop chair is raised by means of a screw with a thread pitch equal to 4 millimeters per turn. If we want to raise the chair 6 centimeters, how many turns will it be necessary to give the screw?

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

$Lead=6cm=60\:mm$

$Pitch = 4 \: mm / turn$

Next we write the formula and substitute the known quantities.

$Feed = Turn \cdot Step$

$60 \: mm = Turn \cdot 4 \: mm / turn$

Finally, we clear the unknown to find the result.

$Turn = \cfrac{60}{4} = 15 \: turns$

Exercise vise¶

A vise opens a distance of 12 centimeters after turning the crank a total of 24 turns. What is the pitch of the screw?

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

$Lead=12cm=120\:mm$

$Turn = 24 \: turns$

Next we write the formula and substitute the known quantities.

$Feed = Turn \cdot Step$

$120 \: mm = 24 \: turns \cdot Pitch$

Finally, we clear the unknown to find the result.

$Pitch = \cfrac{120}{24} = 5 \: mm / turn$

Microscope screw exercise¶

A microscope has a screw to raise and lower the stage and be able to correctly focus the object to be viewed. If the pitch of the screw is 0.5 millimeters and we make a turn of 16 turns, how much will the plate advance?

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

$Step = 0.5 \: mm / turn$

$Turn = 16 \: turns$

Next we write the formula and substitute the known quantities.

$Feed = Turn \cdot Step$

$Feed = 16 \: turns \cdot 0.5 \: mm / turn$

To finish, it is not necessary to clear and we can directly calculate the result.

$Feed=8\:mm$