What is a Screw? – Different Parts of the Screw

What is a Screw?

Screws and bolts are both a form of metal fastener that are identifiable by their externally threaded spiral. The thread is what allows them to screw into the matter they are fastening to because it will lock into internal threads that are made in the material being fastened or jam and hold tightly in softer materials.

Screws and bolts are everywhere in everyday life and industrial applications, whether you are working with wood, metal, or plastic.

The most obvious difference between a screw and a bolt is how they engage with the fastening material.

Screws are traditionally self-threading or self-tapping, meaning, when you drive a screw into a material, it makes its own threaded path, creating the internal threads that hold everything tightly together.

This gives a screw special capabilities for things like woodwork and plastic component assembly where the screw creates a grip that resists removal or being pulled out.

If you look at a screw from a physics perspective it is truly clever as it is basically an inclined plane (the thread) wound around a central core. The spiral thread acts like a wedge so when it is driven into the material it digs in further and holds tight.

In some cases, the screw threads are designed to mate with an internal thread that already exists, usually with a harder material like metal parts, but other times the screw will create a thread as it is driven in the case of softer materials.

In general, screws are designed to hold things together or to keep items in position. Almost all screws have a head on one end, which is what you grip typically with a tool that drives it i.e. a screwdriver or wrench. The head is wider than the shank which prevents the screw from falling down through the material being fastened but also provides bearing surface.

All fasteners do not follow these exact rules all the time. For instance, carriage bolts have a round head, so they cannot be driven with cleaning a wrench in the normal manner.

Set screws may have a head that is flush with the material, or even substantially less than the diameter of the thread (grub screw). There are also J-bolts which are bent into a J shape, and used as anchors in concrete, in construction.

The long cylindrical part of the screw is called the shank and runs from just under the head to the tip. It is sometimes fully threaded or partly threaded. The space from one thread to the next is called the pitch.

In basic fastener techniques a screw (or bolt) is tightened by turning it clockwise by defaults; sometimes referred to as a right-hand thread, although left-hand threads do exist but they are rare.

Left-hand screws are found where normal clockwise swing would cause the fastener to work loose; an example would be the left pedal on a bicycle which is designed to tighten in the opposite direction to avoid loosening while riding.

Parts of Screw

The three basic parts of a screw are the head, threaded shank and point.

• Screw Drive: A screw drive, also referred to as a screw head, is identified by the shape of the slot at the screw’s top. Overall, there is an impressive variety of screw drive types, and keeping multiple compatible screwdriver tips in your toolbox is always a good idea. The flat-head (commonly referred to as Schlitz), Phillips, and Pozidriv are the most common screw drives you will encounter. Each has strengths and weaknesses and it is beneficial to understand the screw drive you are using before undertaking a task.
• Screw Head: The screw head is the top portion of the screw, which is the part you will see once the screw is driven into place, and it is usually wider than the shank and below the threads that help keep the screw in place. Depending on application, screw heads may be flat, rounded (sometimes referred to as rounded), or anything in between. It is also important to note, although the screw head is the same length it can be fairly narrow or substantially wider; this will affect how the screw conforms to the surface.
• Screw Thread: This is where things get interesting: the thread. The shape and size of the thread come in a surprising variation of sizes and factors significantly in how well the screw bites into the material being affixed. The angle of the thread is the way the thread is positioned around the screw; the pitch is the distance between the thread and varies for each screw. The threads can packs closely together (fine thread) or spaced far apart (coarse thread). Each configuration is appropriate for different scenarios and depending on the materials you are working with.
• Thread Length: The next area to consider is the thread length. Not all screws have been manufactured with the intention of being threaded the same distance […]. Some screws are Fully threaded meaning they have threads down the entire length of the screw, while the screws that are partially threaded only have threads along a portion of their shank. Which is appropriated will depend on how much hold or pull you want in your application.
• Screw Shank: The shank is merely the section of smooth screw between the head and the first thread which sits just below the head. The shank is an easy part to forget about, but enhance the function of the screw particularly in partially threaded scenarios where the shank supports most of the clamping force.
• Screw Tip: Finally, dies the screw tip. The tip is the first part to anchor into the material, and may take many forms. Some screws, which might be used for specific tasks like decking, have very fine tips intended to initiate the screw embedding as seamlessly as possible. Others may be blunt or shaped for their own intended purpose.

Related Post: 30 Different Types of Screw Heads and Their Uses

What are screws made of?

Steel would be the most widely used material for screws and bolts because it is both durable and cost effective. However, most of the different projects would require each type of material.

For example stainless steel, brass and sometimes titanium would replace regular steel when there are specific properties to meet including corrosion resistance or out of concern about weight.

The final decision will depend on the application and manufacturers have a wide range of metals and alloys to meet those specific needs.

The following material can be used in the manufacturing of screws:

• Titanium: Titanium screws are a clear winner because they are tough, lightweight, and resistant to rust. When titanium is combined with other metals, it will obtain much more strength and durability. This is why titanium is a good choice when all these factors can be an issue.
• Stainless Steel: Stainless steel screws are also resistant to chemicals and rust, which is why they maintain good aesthetics and endure. However, unlike carbon steel, you can only harden stainless steel so much, so there are limitations on heavy-duty uses.
• Hardened Steel: If you are thinking of screws that are well-defined by strength, you will probably think of hardened steel screws. These max out on hardness, but can be a bit brittle, so in applications that are extreme vibration or shifting, risks exist.
• Steel: Steel screws are strong, but they will rust, so they are not really suitable for wet or outdoor applications unless they are coated in sealer, paint, or other protective barriers.
• Plastic: When price or corrosion resistance is favored over strength, plastic screws will suffice. Plastic screws are low duty screws and they are great around water (e.g., pool equipment and plumbing) because they will not rust.
• Molybdenum: Molybdenum screws are unique because they the most heat resistant, and are incredibly strong screws for specialized industrial use.
• Copper Alloy: Copper alloy screws can withstand a load without a lot of wear, and are advantageous when working near magnets, given they do not impact prior magnetism as much.
• Brass: When discussing brass screws several things surface: strength, good electrical conductivity, and rust resistance. They have low magnetic permeability, and are often used in electronics and around sensitive magnets.
• Aluminum: Aluminum screws are obviously lightweight and great for DIY; and they are good conductors of heat and electricity. Aluminum screws are naturally corrosive resistant, contributing to their usefulness in applications.
• Superalloys: This is probably what most people are researching when they speak of performance, and keeping an eye on high-temperature capabilities–superalloy screws would be a top choice since they can withstand heavy stress, apart from being rust resistant, and do not lose their shape easily.

In addition to the above materials, sometimes some finishing materials are used in screws. It provides durability and corrosion resistance on the screw. discussed next are some finishing materials used in screws.

• Zinc: Zinc coatings are a popular choice because they act as a sacrificial anode—meaning they corrode first, which protects the metal underneath from rusting. The coating is usually just a fine white powder on the surface of the metal that creates a barrier to moisture and other oxidizing agents.
• Chrome: Chrome coatings are unique in that they are shiney and durable. Not only do chrome coatings look good but they “hold up” well over time. Chrome is generally electroplated for a reflective surface that is commonly found in decorative hardware or auto parts.
• Black Oxide: Black oxide coatings are most often selected for aesthetics, producing a dark, matte finish that does not alter the size of the screw at all. The nice part about this process is that it creates a layer of black rust, which is nice subtle appearance, with some protection, particularly indoors.

What about coatings?

Coatings applied to screws are important because they enhance the screws’ fundamental properties, allowing them to perform satisfactorily in a wider range of situations. Coatings come in a broad variety of types which allow manufacturers to apply fasteners made specifically for the environment and use.

Coating materials are numerous, as examples exist using copper, ceramic, and galvanized. The advantages of coatings can vary. Depending on the situation, coatings can be used to give strength to a screw, to protect against corrosion, and/or give a pleasing appearance.

An example of a coatings use are galvanized steel screws. These types of screws provide a much better level of protection against corrosion than plain steel each time they are used. Another example is when a fastener must be visible and have a better appearance than standard steel. A copper-plated screw will provide a more pleasing option than standard steel.

The manufacturing process of screws

The production of a screw typically involves three main stages:

• Heading,
• Thread rolling, and
• Coating.

The whole process starts with wire, which is the initial starting material. This wire is first cut into segments of the desired length and the length is based on the size of the intended screw. The next step is heading which forms the head of the screw.

This is the process of putting the wire segment in a die; the die will ultimately determine the features of the head. For example, a round-head screw requires a round die; other head styles will use other die shapes.

After a head is put on the screw, you will have a screw blank. Then threads are put onto the screw. Most often, threads are created using a process called thread rolling.

Thread rolling is a process where the screw blank is squeezed or pressed between dies and the threads become imprinted. While that is mostly the case, threads may also be cut using alternative machining methods, depending on the design of the screw and any design criteria.

The last process is coating. There is usually a surface treatment like zinc electroplating or black oxide to protect the screw from corrosion and because it enhances the performance of the screw.

The coating improves the overall quality and durability of the screw and may also enhance its look to also save time by virtue of making it more suitable for some environments.

Related Post: 35 Different Types of Screws and Their Uses

Uses of Screw

Screws are important in threaded fasteners for many applications. Screws frequently attach items together which applies to both common items as well as specialized tools. For example, we find screws in screw-top lids on containers, as well as vises, screw jacks, screw presses, and more.

Some typical situations involving screws include:

1. Threaded holes are ideal for fastening objects together when they have enough thickness to support threaded fastening. In any situation where enough thickness is not present, the threaded fastening will fail faster and typically result in a weaker fastening.
2. Bolts are often better than screws for fastening items you are going to take apart and put back together often. Screws will eventually lose grip after a number of assembly/disassembly pairs and will often require a replacement after a large number of cycles.
3. Both screws and bolts have their pros and cons. Bolts are usually stronger and are typically better suited for heavy-duty applications, while screws are better fitted for moderately weighted applications or connecting items in tight areas.
4. It will be important to be sure the thickness of the material you want to connect will provide enough strength to maintain a threaded connection and help to prevent screw stripping or failure in the threaded connection.
5. Screws are often acceptable fasteners when you can unfasten the joint every once in a while. Screws work well in configurations that require assembling the entire configuration, but only require unfastening infrequently in a much heavier duty manner and interval.
6. Sometimes there is not enough room for a nut on the other end of a bolt or through hole. Then using a screw or a screw connection is almost your only option.
7. When considering price, screws are often less expensive than bolts so you can use screws for connections, which is great for panels and budgets.
8. Having a seating face of a bolt helps to carry load better than a screw in many ways, just because they have one bearing surface. It is common for bolts to be the preferred option under high stress types of work or structural work.

Advantages and Disadvantages of screw and bolt

• Generally speaking, screws are often cheaper than bolts.
• Bolts are a much better way to go with things that you are taking apart and putting together frequently (which screw are not).
• Another difference is that bolts are intended to deal with the load over a wider shank area than screws.

FAQs

References:

1. What is Screw?: https://en.wikipedia.org/wiki/Screw