A screw is a fastener type defined primarily by its ability to form a thread as it’s installed and provides “pull out resistance” from a single or multiple layered substrate.
When selecting a screw, it’s critical that the correct thread form and tip are selected to ensure the screw will perform as required and will not affect the substrate detrimentally. For example – many screws designed for use in timber will require the installer to pre-drill the substrate with an appropriate size hole to ensure that the timber does not split when the screw is installed. Other screws designed for timber may have a specially designed drill tip and thread form that negates the need to pre-drill but it’s essential that the installer verifies this prior to installation.
Screws designed for use in metal can generally be split into “self-tapping screws” and “self-drilling screws”. A self-tapping screw requires the installer to pre-drill the substrate with a defined hole size relevant to the thickness of the steel, the body diameter of the screw selected and the thickness of steel the screw is to be installed. A self-drilling screw negates the requirement to pre-drill due to its integral drill tip but is still subject to defined installation parameters.
Screws can be manufactured from many different materials including steel, brass and stainless steel and have a number of different head types, including Hexagon, round and countersunk. They can be installed using a number of different “drive” types such as slotted, pozi, torx and hex.
Screws can be used in a variety of substrates including, steel, aluminium, timber and plastic but it’s essential that the correct screw is selected based on the environment, substrate specified, and the screws required technical performance.
What is a self-drilling screw?
Self-drilling screws (often referred to as “Tek screws”) are a self-tapping screw with an integrated drill tip which allows drilling and installation of the fastener in a single action. This often makes self-drilling screws the preferred choice of installers over blind rivets or simple self-tapping screws as it eliminates the need to pre-drill the substrate.
However, installing a self-drilling screw outside the scope of its design parameters will result in reduced performance or failure, and application advice should be sought from a reputable supplier to ascertain the fasteners suitability in your intended application.
Self-drilling screws are usually manufactured from Carbon or stainless steel. Carbon steel screws can be finished in a number of different coatings which can provide limited resistance to corrosion. Stainless Steel drill screws are usually recommended for use in external or corrosive environments and are manufactured in two ways.
The entire screw including the drill tip is manufactured from a single piece of stainless steel wire. These fasteners are only suitable for drilling and thread forming in aluminium. If installed into steel, the drill tip will simply burn out.
The drill tip of the screw and lead in threads are manufactured from case hardened steel and welded onto the end of a stainless steel screw. These fasteners are suitable for drilling and thread forming in aluminium and steel substrates.
All drill screws are designed to perform and function within pre-determined design parameters with regard to their drilling capacity and effective working length and great care should be taken when specifying or selecting a drill screw. Incorrect specification of drill screws can lead to reduced performance or failure.
Why and where to use a stainless steel screw?
In most applications, stainless steel screws are considered safety critical components, and as such are relied upon to hold together structural elements or component parts that if not secured can cause risk to life or property. Because of this, it’s essential when specifying or selecting a screw that not only the functional requirements and technical performance values are considered but also the suitability of the material the screw is manufactured from in relation to the environment it is to be installed in.
It is generally accepted that Carbon steel screws are suitable only for internal, non-corrosive environments where the screw is not subjected to any conditions such as moisture or chemical vapours that will cause the fastener to corrode and ultimately fail. For environments where moisture, pollution or chemicals are present a number of different grades of stainless steel fasteners are available and should be carefully selected based on the local environmental conditions they will be used in and the required service life.
EN 12944 (Exposure categories C1 – C5 ) is a useful document in allowing specifiers to establish a defined environmental category for their project or application. Advice should then be sought from the screw supplier as to the most appropriate material suitable for the defined environment.
The three most common stainless steel grades used in the manufacture of screws are:
- (304) (A2) (1.4307 to the European standard) – Most common
- (316) (A4) (1.4401 to the European standard) – Widely specified in marine/high pollution environments
- (A5) (1.4547 to the European standard) – Specified for extreme environments – e.g. Swimming pools
The above grades of stainless steel all have similar basic chemical compositions with relatively low levels of carbon (C) 0.08% and higher levels of Chromium (Cr) 18-20 % and Nickel (Ni) 8-10% typically. When increased corrosion resistance is required due to more aggressive environments, the element Molybdenum (Mo) (Atomic number 42) is introduced in varying degrees. This is typically 2% in grade 1.4401 (A4) and 6-7% in 1.4547 (A5).
Choosing the right stainless steel fastener
With over 30 years’ experience, Mainline products can assist designers, specifiers and installers in choosing the correct stainless steel fastener for your application. Our in-depth fastener knowledge, comprehensive product range and unrivalled ability to colour match fasteners to industry leading architectural cladding panels ensure you will always have the right product on time every time.
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