Types of screw heads

What is the Head of a Screw?

Figure 1. The parts of a screw

The head of a screw is the most extreme part compared to the tip and represents the element of the screw where the tightening force is applied. The choice of the correct head, and therefore of the right screw, depends on its shape, the tightening force, and the tool that will be used to apply it. On the head is the recess, which also indicates the means or type of screwdriver to use to apply the element.

Focusing on the head of the screw, a first and generic classification divides the element into:

• Flat or countersunk head screws, useful for jobs where the component must disappear into the material on which it is fixed and for this reason are applied on soft materials;
• Rounded head screws, applied on supports where it is necessary for the head to protrude;
• Wing screws, used in cases where manual tightening is required, thus for quick and practical applications.

Figure 2. Some types of screw heads

Each head is characterized by a particular recess, that is what is commonly defined as the screw's drive.

Among the most common are:

• Slotted drive, the most traditional, although due to the wide slot of the element it does not guarantee optimal screwing;
• Cross or Phillips drive, invented by Thompson in the 1930s, is an element that overcomes the disadvantages of the previous one because it is self-centering;
• Pozidriv drive, similar to a cross screw, differs from the latter by having additional, much smaller and more tapered recesses. It is usually a suitable drive for wood screws;
• Allen drive, also known as "hex socket", it is an internal hexagon screw applied using a simple hex key, also called Allen key;
• Hex head screw, is an element with an external hexagon and the entire head acts as the drive;
• Torx drive, with an external and recessed hexagonal drive, it is an element that guarantees greater tightening force compared to the previous drives.

Classification of Screws

To correctly understand how screws are classified, it is first necessary to clarify the difference between a screw and a bolt. In fact, the best way to know a bolting element is to know its characteristics, applications, and the purpose for which it is produced. As described so far, the correct choice of a screw goes through its head, but not only. In addition to the latter, the size and materials in which the element will be fixed must be considered. It is therefore useful to consider:

• Compatibility between the material of the screw and that of the support;
• Load capacity, thus calculating and choosing a screw that can withstand the load to be supported;
  
  
• Type of tool for fastening, to avoid stripping or damage during installation;
• Type of thread, to be chosen based on the different types as it affects the resistance of the element and simplifies installation.

At this point, a possible categorization of screws can also be made based on their use and can be distinguished between:

• Wood screws, generally fully threaded and, in some cases, partially threaded, are usually of small diameters with flat countersunk heads, countersunk heads with a dome, and rounded heads;
• Drywall screws, generally black, are screws threaded along their entire length with countersunk or cross heads. Their particular color is due to the phosphating treatment they undergo and they can be produced as self-tapping or self-drilling;
• Metal screws, is the class to which the largest number of different products belong in shapes, lengths, diameters, and types of heads. The type of metal in which this element is produced is fundamental because it determines resistance to deformation, temperature, etc. For tightening on metal supports or elements, classic metric pitch screws, self-tapping screws, or self-drilling screws can be used.

It is also possible to distinguish between:

• Biting screw, a component that ensures tightening through a threaded hole created by its own passage;
• Through screw, tightening is ensured by a nut at the end;
• Stud screw, that is headless screws threaded at both ends.

In addition to the material on which the screw will be applied, it is essential to consider the possible heat treatment to which the screw is subjected for the intended use. In fact, it is possible to consider:

• Galvanized screws, generally silver-colored, can be subjected to different types of galvanizing and their resistance to corrosion is medium. Among the types of galvanizing, remember:
  
  1. Electrolytic galvanizing: also known as electroplating, is a coating system that takes place by immersion in a solution of zinc salts, precisely electrolytic. To obtain this coating, three main steps are performed: pre-degreasing, pickling, and degreasing, and there are different galvanizing techniques that require different processes, as described in a previous article.
  
  2. Hot-dip galvanizing: like the previous one, it is a system that takes place by immersion. The elements subjected to this treatment are first subjected to specific treatments such as pickling and then immersed in tanks containing molten zinc.
  
  
  
• Phosphated screws, usually black and therefore easily recognizable, are elements with good corrosion protection. The elements belonging to this category undergo a chemical pretreatment, precisely phosphating. Thanks to the zinc layer covering the product surface, full protection from external agents and greater durability over time are guaranteed.
  
  
  
• Nickel-plated screws, with a surface worked by chemical or electrolytic treatment, are generally precision mechanical pieces. The surface coating to which they are subjected can indeed guarantee uniformity of thickness, high hardness, and high resistance to corrosion.

Another criterion for choosing the correct element is the dimensions: selecting the right length and diameter of the component means selecting the correct resistance and therefore effectiveness. But how do you choose the right screw size?

What do the numbers on the head of the screws mean?

The numbers engraved on the head of the screw represent its metric size, described with the letter M and a number indicating the value of the nominal external diameter in millimeters.

For example, the writing M4 will describe a nominal diameter of 4 mm and a coarse metric thread. Moreover, in most cases, the indication of the material is also reported. The indication of the type of material, which we find on the head, provides information on the material's resistance. For example, the numbers 8.8 on the head of the element will indicate that the screw material in question is steel with an 8.8 resistance. The presence of these indications provides the necessary information to calculate the yield point.

At this point, we can proceed with the distinction of the types of screws and bolts based on the conformation of the head. Below we list some of the best-known ones.

Hex Head Screws

Among the vast number of fastening elements on the market, hex head screws are among the most used products. The reason lies precisely in the particular shape of the head: the hexagonal shape allows for a more secure connection between the element and the support and, therefore, finds wide application in mechanical assemblies, infrastructural and construction types, etc. To ensure optimal fastening, it is advisable to choose the right tool that has the same dimensions as the screw recess: for example, for some types of products with hex heads, it is recommended to use a torque wrench.

Cylindrical Head Screws

Cylindrical head screws are a particular category of elements, created specifically to be used with nuts. The head is equipped with a recess that fits into the nut and facilitates the use of a wrench to tighten or loosen the screw. There are some particular cylindrical head screws, for example, those with DIN 912 or UNI 5931 metric pitch, which also have indications about the resistance class. Specifically, screws of this type provided in class 12.9, have a particular engraving or knurling right around the head.

The DIN 912 is the most well-known cylindrical head screw and is often referred to. In reality, the market offers different types of elements with cylindrical heads such as, for example, the cylindrical head screw with a slot or the cylindrical head screw with a capped slot, etc.

Often used for fastening sheet metal or in furniture construction, they can be easily confused with rounded head screws due to their rounded shape and final installation (both remain on the surface of the element to which they are applied). It should be noted that cylindrical head screws have their particular conformation and to measure them, the diameter, length, and pitch are considered:

• The diameter is the widest part of the screw head;
• The length is measured from under the head to the tip;
• The pitch is the distance between the threads, generally coarse or fine.

TCEI Screws: Explanation and Sizes

TCEI is the abbreviation of the more extended term Cylindrical Head Hex Socket Screw. TCEI screws are very particular: these elements have a cylindrical-shaped head with a central cavity, which can be hexagonal or torx. Generally used as a bolt element, they are used in fastenings that will be closed at one end with a nut. Most of these elements are threaded only on part of the shank to allow the union of two parts or to create a fastening point on a surface. These are components widely used in the mechanical field because they guarantee good resistance to traction and stress. This resistance can be further increased using washers.

Given their conformation, TCEI screws are available on the market in high-strength steel (8.8, 10.9, 12.9) raw or galvanized and stainless steel. As for the dimensions of a TCEI screw, they are variable. These elements can be supplied from a diameter of M3 up to important sizes like M56.

Countersunk Head Screws

The term countersunk head screw indicates a type of fastening element used when a smooth and flat surface is needed. In fact, the main characteristic of this screw is its flat head which, once inserted, does not protrude from the support on which it is fixed.

This product finds wide application in various sectors such as woodworking, metalworking to join parts or metal sheets while maintaining a flat surface, in plastic material processing, and in the automotive and aerospace sectors.

One of the most well-known and used products is the countersunk head screw with DIN 7991 hex socket: it is one of the most commercial elements with this type of head and allows for the assembly of various elements even if the space for tightening is not sufficient.

Rounded Head Screws

As the name itself suggests, rounded head screws are characterized by a head shape similar to a spherical cap. Therefore, it is a head without edges, in some cases also called a button head.

Rounded head screws are used for mechanical assemblies, wooden artifacts, etc. The most well-known and commercial product is ISO 7380 but, even in this case, there are other various types such as, for example, the rounded head screw with a slot, rounded wide combi head screw, or wide combi knurled head screw, etc.

Headless Screws

Headless screws, also known as set screws, are particular fastening elements that do not have the traditional head. These components are in the form of a cylindrical shank that can be fully threaded or threaded at both ends. For some of these screws, it is possible to find a hex socket or a screwdriver slot on the upper part to allow installation. Available in various sizes, pitches, and lengths, these elements are used in industrial and mechanical applications such as machinery assembly, electronic equipment construction, and the installation of fixtures. Among the most used products are:

• Headless screws with slot and flat tip;
• Headless screws with slot and conical tip;
• Hexagon socket set screws with flat tip;
• Hexagon socket set with conical tip;
• Hexagon socket set with cylindrical tip;
• Hexagon socket set with cup tip;
• Partially threaded headless screws;
• Etc.

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