Screw | Types, Parts, Manufacturing, Material Selection, Applications

Table of Contents

Screws are fasteners with helical grooves on their shank. These grooves can be either throughout the shank or only in the beginning. These grooves provide necessary friction and traction to avoid pull-out. They are generally made from a metal.

A screw is sometimes also known as cap screw or tap bolt.

Note: Those who don’t know ‘What is a fastener?’. A fastener is a hardware device which mechanically affixes or joins two or more parts together.

Note: Those who don’t know ‘shank’. It is discussed later in this article.

Screws are male part and the joining bodies are female part (unlike Bolt). We will discuss differences between screw and bolt later in this article.

On the basis of threading mechanism there are basically two types of screws.

Self-threading screws
Non-self-threading screws

When we move self-threading screws clockwise (with some pressure) against the joining material, it makes its way into the material (mainly because of its sharp tip). They are generally suitable when the joining parts are made of either wood or plastic.

In many cases self-threading screws can also be inserted into joining parts with the help of a hammer, for example in case of wood and plastic.

In case of non-self-threading screws, we have to first make suitable helical grooves in the joining material and then insert screw carefully. They are generally used when the joining parts are made of metal.

Screws can also be categorized in various other types, we will discuss them later in this article.

Difference between bolt and screw

Bolt and Screw

However, there is no clear-cut differentiation (by definition) between screws and bolts but there are some basic differences (by observation).

Bolt requires nut to give it grip against the joining parts. However, it is not the case with screw, it takes help of helical grooves of joining parts to have a grip. In simple language we can say that screw do not requires a nut.
Screws can have a tapered shank but bolts can’t because they have to take a nut.
Many say that they are not the devices which are different, but how they are used.
Screws are smaller in size when compared to bolts.
Screws have a large variety of head whereas bolts have either hexagonal or square head.

Parts of screw

Body of a screw can mainly be divided into four parts.

Head (a): It is top most part of a screw. A screw can be tighten or loosen from the head of the screw. It has provisions for accommodating screwdriver or wrench.
Not threaded shank (b): This is the part of shank which do not have threads. This part could be absent in some screws.
Threaded shank (c): This is the part of screw having threads on it. It is the place from where screw gets its grip on joining bodies. It has helical grooves in it.
Tip (d): It is the bottom most part of the screw. It helps screw in penetrating through the joining bodies. It could be missing in some screws where threads are already present in joining bodies. It is essential part of self-threading screws, as mentioned earlier.

Screw material

Following material can be used in the manufacturing of screws.

Titanium: Screws made of Titanium are hard, strong, light and corrosion resistant. When alloyed with other materials it can increase strength and durability.
Stainless steel: Screws made from stainless steel are chemical and corrosion resistant. They have an appealing surface finish. They can not be hardened like carbon steel.
Hardened steel: Screws made from hardened steel are hard but brittle in nature.
Steel: Screws made from steel are strong but they are vulnerable to corrosion.
Plastic: Screws made from plastic are inexpensive and corrosion resistant. They are used for light loads. They are commonly used near water, such as water pools.
Molybdenum: Screws made from Molybdenum have very high melting point and are exceptionally strong.
Copper alloy: Screws made from copper alloy have good load capacity and are wear resistant. They are suitable for use near magnets.
Brass: Screws made of Brass are strong, conductive and corrosion resistant. They have low magnetic permeability.
Aluminium: Screws made of Aluminium are light and easy to manufacture. They are thermally and electrically conductive. They are resistant to corrosion.
Super alloys: Screws made of super alloys show good mechanical strength, surface stability, corrosion resistance and resistance to creep at higher temperatures. Some common super alloys include Hastelloy®, Inconel®, Incoloy®, and Monel®.

Apart from above material, sometimes some finishing material is also used in screws. It can provide durability and corrosion resistance to the screw. Here are some finishing materials used in screws.

Zinc: Its coating acts as a sacrificial anode, protecting the underlying metal. It is applied as fine white dust.
Chrome: Its coating gives a bright, reflective finish. It is decorative and very durable. It is applied by electroplating.
Black oxide: Its coating is mostly used for aesthetic purpose. It does not enlarge the dimensions of the screw. It is a processed black rust.

Different shapes of screw heads

(a) Pan head: It is circular in shape but flat at top. It has relatively high outer edge.

(b) Dome or button head: It is circular in shape and hemi spherical at top. It has relatively high outer edge compared to round head.

(d) Truss or Mushroom head: It is circular in shape and hemi spherical at top. It has relatively low outer edge compared to round head.

(e) Flat or countersunk head: It is conical in shape but has a flat head.

(f) Oval head: It is conical in shape but has a spherical head.

Types of screws

Following are some main types of screws which are generally used in our day to day life.

Wood screws
Wing screws
Thumb screws
Shoulder screws
Sheet metal screws
Set screws
Self-Drilling screws
Particleboard screw
Mirror screws
Machine screws
Masonry screw
Masonry screw
Jack screw
Jack screw
Dry wall screw
Drive screw
Dowel screw
Deck screw
Cap screw

Manufacturing of screws

There are three major steps in the manufacturing a screw.

Heading
Thread rolling
Coating

Screws are normally made from wire. The wire is then cut to the proper length for the type of screw being made. Heading produces the head of the screw. The shape of the die in the machine dictates the features to be pressed into the screw head for example a round head screw uses a round die.

The threads are generally produced via thread rolling. However, some are machined.

Finally a coating, such as electroplating with zinc or black oxide, is applied to prevent corrosion.

Applications of screw

Screw can be used in following conditions

When the parts that are fastened, are thick enough to accommodate threaded hole.
There is no place to accommodate the nut.
When the parts that are fastened, have sufficient strength to accommodate durable threads.
These are used when the parts are seldom dismantled.

Relative advantages and disadvantages of screws and bolts

Screws are cheaper compared to bolts.
Bolts are good for frequent dismantling and reassembling, unlike screws.
Bolts carry load on a larger shank area when compared to screw.

Image sources:

Bolt and screw: By © 2010 by Tomasz Sienicki [user: tsca, mail: tomasz.sienicki at gmail.com] – Photograph by Tomasz Sienicki (Own work), CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=8944465

Parts of screw: By Rfc1394, Waterced – https://commons.wikimedia.org/wiki/File:Wood_screw_big.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=65221519

Bolt and Nut: By Pearson Scott Foresman – Archives of Pearson Scott Foresman, donated to the Wikimedia Foundation. This file has been extracted from another file: Bolt (PSF).jpg, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2555706

Types of screw heads: CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2452317