Basic Knowledge of Screws and Bolts


Screws and bolts that connect parts together. Despite their simple structure, they come in a wide variety of types, features, and applications.
Additionally, depending on the material, there are differences in durability and maintenance methods, making them extremely complex components.
Therefore, on this page, we have compiled information about screws and bolts, which are parts that significantly affect the quality of your products.
We have also included a list of specialized terminology related to the industry, so please see in detail.


What is a screw?

A screw is a cylindrical metal component with spiral-shaped grooves. Its basic function is to act as a male thread, while a female thread in the form of a nut or a component is used in conjunction with the male thread to secure parts together.
Screws come in various types, depending on factors such as the shape of the thread, diameter, length, and pitch, and they are utilized for a wide range of applications.
Commonly, they are used in machinery, automobiles, buildings, household appliances, and various other products.


What is a bolt?

A bolt is a cylindrical metal component with threaded grooves that is used in conjunction with a nut to secure two parts together.
Bolts come in different types based on factors such as the shape of the threads, diameter, and length, and they are used for various applications.
There are different head shapes available, including hexagonal, flanged, and flat heads, each suitable for different fastening methods.
Bolts are also classified based on their materials, with common options being carbon steel, stainless steel, and titanium alloy.
It is important to select the appropriate type of bolt based on the intended application and environmental factors.


Basics of screws

When selecting screws, there are various factors to consider, such as “screw type,” “head shape,” “drive type,” “material,” “surface treatment,” and “size”.
It is important to choose these elements accurately, taking into account the intended application and safety aspects. We have summarized the selection process briefly below.

selection 1

Selection of screw threads

There are various standards (which may vary by country) for screw specifications. Among the common metric screws in the JIS (Japanese Industrial Standards) specification, there are metric coarse threads, metric fine threads, metric trapezoidal threads, pipe parallel threads, pipe taper threads, as well as special screws with unique features produced by various manufacturers. The selection of screw threads differs based on the mating material, intended application, and required strength.

selection 2

Selection of head shape

Screws not only consist of the threaded portion but also have different head shapes that can enhance their functionality based on the intended application. Head shapes include flat heads, pan heads, binding heads, truss heads, round heads, countersunk heads, hexagonal heads,…

selection 3

Selection of drive type

The choice of drive type for the driving portion of the screw (such as cross recess, slotted, …) also affects the torque management during tightening. Common drive types include cross recess, slotted, Phillips, hex socket, tamper-resistant, and others.

selection 4

Selection of material

Choosing the right material is also one of the most important factors. Simply referring to it as “iron” encompasses a range of materials with varying strengths, including basic iron materials, high-strength iron for specific applications, and even ultra-high-strength iron. Additionally, materials can be further categorized based on heat treatment and other factors that affect strength. Material selection is not solely based on strength but also takes into account corrosion resistance. For example, materials like stainless steel 304, 316, or even titanium are chosen to enhance corrosion resistance, making material selection a crucial factor based on the intended application.

selection 5

Selection of surface treatment

Surface treatment is an important factor that encompasses various aspects. It ranges from anti-corrosion treatments aimed at preventing corrosion to decorative plating or achieving a matte finish for aesthetic components. Surface treatment also includes color coding for identification purposes.

selection 6


The selection of size significantly affects the strength of the product. While choosing a larger size may increase strength, it also adds weight to the product. Therefore, selecting the appropriate size to meet the required strength without unnecessarily increasing product weight becomes a crucial decision-making factor in the final product.


Common types of screws

Below are some common types of screws:

  • Flathead screw
  • Roundhead small screw
  • Panhead small screw
  • Truss head small screw
  • Binding head small screw
  • Tapping screw
  • Hexagonal tapping screw
  • Self-drilling screw
  • Wood screw
  • Set screw
  • Washer assembly screw
  • Interior screw (universal screw)
  • Coarse thread


Common types of bolts

Here are some common types of bolts:

  • Washer Assembly Hex Bolt
  • Hex Bolt
  • Fully Threaded Hex Bolt
  • Hex Socket Bolt
  • Anchor Bolt (Foundation Bolt)
  • U-Bolt
  • Eye Bolt
  • Knob Bolt
  • Long Thread Bolt
  • Hanger Bolt


Standards and sizes of screws

Screws and bolts have various standards such as JIS (Japanese Industrial Standards), ISO (International Organization for Standardization), and DIN (Deutsches Institut für Normung) standards. These standards define the diameter, length, thread shape, and head shape of screws and bolts.


Storage methods for screws

Screws are affected by their storage methods, which can impact their quality and lifespan. When storing screws, it is important to keep them in a dry location to prevent moisture and corrosion.
The use of anti-corrosion tools such as rust-preventive paper, rust-preventive vinyl, and silica gel is crucial. For long-term storage, applying rust-preventive oil can help prevent rust and corrosion. Each type of screw may require specific storage methods, so proper management is essential.


Are screws in need of maintenance?

Screws require regular maintenance to function properly.
Especially when exposed to the external environment for a long period, it is important to clean them regularly to prevent rust and corrosion. Additionally, if wear or damage occurs, replacement is necessary.


Common problems with screws

  • Stripped threads (loss of screw functionality)
  • Shaft breakage
  • Rust
  • Poor seating or loosening
  • Deformation of the head groove

※You can check the table by scrolling left and right.

Troubleshooting Classification of Causes
Mistakes in screw type selection and hole design Mistakes in driver selection Mistakes in screw material selection Poor tightening torque Mistakes in insufficient safety margin calculation Lack of electrical knowledge
Screw threads damaged
Screw shafts broken
Diagonal fastening
Poor seating or loosening
Grooves on the head damaged

*Cause analysis of screw troubles (quoted from: Yoshiaki Kunii, Learning from Screws and Springs! Mechanical Elements for Designers, Nikkan Kogyo Shimbunsha, 2017, p.48)


Mechanisms and solutions for screw-related issues

Improper selection of screws can lead to problems such as stripped threads, screw breakage, rust, poor connect or loosening of screws, and damage to the drive portion. It is important to make careful selections to avoid these potential issues.

・Stripped threads: refers to the damage to the screw threads caused by frequent tightening and loosening, especially when the initial tightening is done with high torque using tools. When the screw threads are damaged, it reduces the friction between head screw and nut, which can lead to loosening or inability to tighten properly, resulting in potential issues. The countermeasures include performing careful tightening during screw installation and cleaning, such as removing debris from the male and female threads.

・Thread breakage: Thread breakage can occur due to factors such as insufficient strength, fatigue degradation, or loosening caused by fatigue reduction. This problem occurs when there are loads on the components being fastened, as well as vibrations or repeated load fluctuations. It is important to consider the specific application and the occurrence of vibrations in order to select the appropriate strength of screws.

・Rust formation: Rust is a common problem that can occur due to factors such as insufficient thickness of surface treatments (plating, painting,…), environments with extreme oxidation tendencies near coastlines, or accelerated degradation caused by contact with different factors. It is important to implement preventive measures such as adequate surface treatment and proper storage methods to minimize the occurrence of rust.

・Poor connect or loosening of screws: This is an eternal challenge in the mating of head screw and nut. Loosening can occur due to over-tightening during installation or as a result of vibrations. To prevent loosening, various products with anti-loosening features are available, such as nuts with built-in features in head screw or nut (such as nylon nuts, U-nuts, hard-lock nuts), or washers (such as Nord-Lock washers). Selecting products with anti-loosening functions, available in Japan and other countries, is one approach to addressing this issue.

・Drive portion deformation: This is a common problem that frequently occurs with screws. Through repeated tightening, the cross recess for a Phillips screwdriver may become worn, or excessive force may cause the hex socket to become stripped. These are typical issues that arise. To prevent this, it is important to select appropriate and non-worn-out tools, and when using tools for tightening, it is recommended to initially hand-tighten before applying excessive force. In the event that the drive portion is damaged, there are also various removal tools available. Please feel free to inquire for more information