Updated: Aug 18, 2020
Having trouble understanding what fasteners to use in your build? The google search on "which is the very best fastener for my system?" will flood you with thousands of short articles and ads without giving you any clue on how to truly select one.
Majority of the time, the root cause lies in the sheer number of variations within endless broad categories. To alleviate such a needlessly complex selection process, we at Kian Soon Mechanical Components, have decided to launch an informative series. Within this series, we break down each fastener type to help you get a better understanding on their unique traits, applications and how to choose them.
In this article, we share all that we know - about the screw. Its basic configuration, types, materials, finishing, drivers and how to go about selecting the right one for your application.
Is a screw a simple machine?
Is a screw an inclined plane?
Or is a screw a wedge?
Well the short answer is this - a screw is a mechanism that converts rotational motion/ force to a linear motion/ force. It is one of the six classical simple machines, alongside the wedge and inclined plane. Yet what is so interesting about the screw, is that geometrically, a screw can be viewed as a narrow-inclined plane wrapped around a cylinder. On the other hand, a wedge is really an inclined plane turned on its side that services to push things apart. The screw utilizes this very same concept, but instead, to bind and unbind different materials firmly, quickly, and easily.
A screw is composed of 4 crucial elements - the recess, head, thread, and point.
The Recess fits the driver (your tool) and holds the screw in place. It also provides a contra for the force of the screw unto the outer surface of the substrate you are screwing into.
From the simple slotted screw to the modern Torx, Screw Heads have been engineered to fit just about any possible scenario. Below are a few common configurations for head shapes and recesses. Combined, they dictate the Driver and security of the screw, in some cases, even ensuring that a screw maintains its straight body.
With ample rotational force is applied, it is the point of a screw that first forces an opening into your substrate and the thread that will catch on and lever its way further along.
When it comes to threads, it is important to know a few common terms:
TPI - Threads per Inch
Pitch - The incline of the thread
Internal threads - mainly on nuts.
External threads - mainly on bolts or screws.
Machine Screw Threads - used on bolts, setscrews, machine screws and designed to mate with preformed threads in nuts or tapped holes.
Spaced Threads - used on woodscrews, self-tapping screws, coach screws and Type 25 thread cutters. Designed to form its own thread, usually in a pre-drilled hole.
Some threads are "self-tapping" which means that they will cut their own way down. Other screws, like machine screws, require that your hole be pre-tapped (with a matching set of threads), or already have its matching thread cut into the hole which you are screwing into.
Below are a few common thread types.
Screw points are available in a variety of shapes, each catered to an in-depth range of applications. Screw Points are required for hole alignment, the initial entry into the material and as a means of locking against an item. Below we showcase the 5 most common point types.
Likewise, you can refer to the more in-depth article on screw configurations (Coming Soon!) if you would like to explore the more advanced makeups and customised threading & point in a screw.
Types of Screws
Now that we have a better understanding of what a Screw is made of (in its parts), we have to understand the types of screw and the variations most commonly available. Below are some of the more commonly used variations of today:
Wood Screws, in their simplest form, are a single threaded screw type. These screws generally have a flat or oval head, utilising a smaller TPI and a more aggressive pitch.
Machine Screws are used in pre-tapped holes, or pre-drilled holes for machine screws which are self-tapping. These screws will have a higher TPI and a less aggressive pitch for each thread.
Plastic Screws Coarse have sharply angled threads with round shanks to provide maximum holding strength with minimal stress.
Drywall Screws, are a twin-fast (double-helix) type screw which has a medium TPI with an aggressive pitch. They tend to be hard, brittle types of screws that are not good against corrosion.
Types of Material and Finishing
The material used depends on the use defined. Some common metal-based materials are:
Silicon bronze - used in marine environments and are highly corrosion resistant.
Stainless steel - Highly corrosion resistant material.
Zinc coated steel - Zinc is great against copper corrosion and act as a sacrificial shell. The Zinc material will rust before the metal underneath.
Brass - Soft metal and suitable for woodwork
Steel screws will generally have been treated with an oil coating to increase their shelf life, this however, will not act as an effective corrosion protection. On the other hand, Stainless Steel, Brass, and non-metallic screws have sufficient corrosion protection in normal applications.
Should your Screws be used in a more advanced application, they will need to be coated to be able to withstand corrosion. Aside from application, the coating and material will also depend heavily on the location and environment where they are designed to be used.
The most common finishes or coatings are;
Zinc Plating -This is can be applied mechanically or using electroplating (will give a bluish tint to the screw). Should the application demand an even higher corrosion resistance, a chemical passivation is applied to the zinc coating. This is chemical chromate will give the screw a yellowish colour and a harder surface.
Galvanised - This is another form of Zinc plating, where the screws or fixings are dipped into molten zinc, then spun to ensure an even coating across the whole surface. It is more effective than an electroplated finish, but it is rougher so cannot be used on all but the roughest of screws and more typically found on nails.
A galvanised finish is often wax coated to make the fixing easier to use/insert. As this coating is relatively thick, any clearance holes and nuts will have to be tapped oversize to accommodate the additional excess.
Nickel plating - Nickel electroplating is often used to provide corrosion or wear resistance in a metal fastener but can be used to build up a heavily worn, or undersized parts for salvage. The nickel plating provides high resistance to corrosion and is especially beneficial if there is likely to be wear or friction in the product as nickel does not flake or chip like other decorative finishes.
It is also ideal in these situations as it provides an even coating on any fixing, even in recesses or holes. Engineering applications for this plating can be found in several industries but are most commonly used in the marine, gas and oil industries.
There are also a range of decorative, secondary coating, as follows;
Brass Electro Base - applied by electroplating, often directly used to finish polished iron or as an intermediate layer for subsequent nickel plating
Black Japanned Screws - The screws are covered with a black enamel which can be used in black fittings and will provide corrosion resistance.
Chrome Screws - Chrome electroplated copper screws that provide a very hard and highly shinny surface, able to within stand high levels of condensation.
A driver is a tool used to turn a screw into a substrate. Finding a screw compositable to your Driver, or vice versa, is essential to, the form and fit of the screw, and the manual requirements to drive the screw into your substrate. There are four types of drivers for screws.
Manual Screwdriver - Most commonly see and come in many different sizes, formats, and lengths.
Torque Screwdriver - Torqued drivers have a clutch in the driver which can be set to a specific, required torque. The clutch will engage when to ensure that you don't over-tighten the screw. These are available in manual, electric, and pneumatic types. Note that although no single tool covers the entire range, low-, mid-, and high-torque ranges are available.
Ratchet Screwdriver - For tight spots or screws that you'd like to do by hand, these will save you a significant amount of effort. They have a ratcheting mechanism that permits the tool to apply turning force only in one direction, while moving freely in the opposite. So it can be set to drive or remove screws, without having to move the bit in and out of the screw head at the end of each turn.
Mechanical Screwdriver - Mechanized screw drivers come in many variations, from the hefty, heavy-duty types to the light and easy variations all with the same goal of reducing manual labour in fitting of the screw.
So now you have a basic understanding of the common variations available, how do you choose the Right screw for the Right job?
With what you have learnt as above, identity the Screw Type and Finishing which best fits your application. Once you have selected, you need to identify the length and gauge required. The general rule of thumb is that the screw should enter at least half the thickness of the bottom material – For example 3/4″ into a 2 x 4.
The screw’s diameter, come in gauges 2 through 16. The average application will call for an 8-gauge screw. If working with very thick or heavy material, consider 12 to 14-guage screws.
You will also need to consider the application of the screw through its lateral and withdrawal loads as the total length of the thread that is inserted into the substrate, is dependent on these forces.
If the forces will apply lateral pressure, also known as sheer pressure, consider using a thicker screw. On the other hand, if the forces will apply a withdrawal pressure, be sure that ample screw threads have at least 1 to 1,1/2 inches of thread into the receiving substrate. One method is to use multiple short screws.
Now, the standard screw size are vast and slightly confusing. Refer to our Metric Imperial Screw Conversion Chart (Coming Soon!) to make sure you have the right size screw for your job.
There are many types of screws, heads, drivers, and materials. The sheer variety can be over-whelming in scope, and we hoped this article has been useful in navigating this landscape. Should you have any queries you might need help on, feel free to drop us an email at email@example.com Our team experts hold over 50 years of experience combined and will be able to advise you on the most intricate of setups.
But wait, what about Bolts? While closely related, a bolt is not a "screw" in the traditional sense. The common differentiation is that a bolt passes through the materials it is binding together and has a fastener (nut) on the other end. Read up on our next series on Bolts – Coming soon!
For more information, publication and marketing related queries, please contact: Kai Xuan, Marketing Manager for Kian Soon Mechanical Components firstname.lastname@example.org
About Kian Soon Mechanical Components
Kian Soon Mechanical Components was founded in 1977 and is one of the leading distributors of C parts in South-East Asia. Aside from their head office in Singapore, the company also has subsidiaries in Malaysia, Indonesia and Thailand. The company was acquired by Bufab in 2017.
In 2019, we officially announced the completion of our ISO 7 Cleanroom under sister company PureSys.
Through PureSys, Bufab seeks to bring added value to current and future customers, starting from the life science, medical and pharmaceutical industries.