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A Comprehensive Guide to Understanding Different Types of Threads

Table of Contents

Threads represent a fundamental aspect of mechanical design and assembly. A thread can be defined as a helical ridge or groove that wraps around a cylindrical surface. These helical structures serve various purposes, including fastening, joining, or transmitting motion between components. Threads are ubiquitous in industries such as automotive, aerospace, construction, and textiles. They play a crucial role in ensuring the integrity, functionality, and reliability of assembled components. Different types of threads are tailored to specific applications, offering unique characteristics suited for various environments and requirements.

Types of Threads

Basic Concepts of Thread Design

Thread design involves careful consideration of several key parameters to ensure optimal performance and functionality. Understanding these parameters is essential for engineers and designers involved in the manufacturing process.

Major Thread Design Parameters

Pitch-The pitch refers to the distance between adjacent thread crests measured parallel to the thread axis. It determines the axial movement per revolution and influences the mechanical advantage and load-carrying capacity of the thread.

Thread Angle- The thread angle is the angle between the flanks of the thread. For standard V-shaped threads, this angle is typically 60 degrees for metric threads and 55 degrees for unified threads. The thread angle affects the self-locking ability, strength, and ease of engagement during assembly.

Major Diameter- The major diameter is the largest diameter of the threaded portion of a screw or nut. It is crucial for determining the fit between mating components and plays a significant role in load distribution and thread strength.

Differences Between Thread Profiles

Thread profiles describe the cross-sectional shape of a thread. Different profiles are used for various applications due to their unique properties.

ISO Metric Thread: This is the most widely used thread profile globally, characterized by a 60-degree angle. It’s known for its simplicity and versatility.

Unified Thread Standard (UTS): Predominantly used in the United States and Canada, UTS threads also have a 60-degree angle but differ in thread pitch and diameter standards compared to metric threads.

Acme Threads: These have a trapezoidal profile, making them more suitable for heavy loads and power transmission applications. Their design allows for easier manufacture and tolerates dirt and damage better.

British Standard Whitworth (BSW):  Featuring a 55-degree thread angle with rounded peaks and valleys, BSW threads are known for their strength and resistance to loosening.

Thread Direction

The direction of a thread can be right-hand or left-hand. The significance lies in the direction in which a threaded component must be turned to tighten or loosen.

Right-Hand Threads: These are the most common. Tightening occurs when the thread is turned clockwise. Most screws, bolts, and nuts use right-hand threads due to convention and the natural tendency for right-handed tightening.

Left-Hand Threads: Less common, these are used in applications where the direction of rotation or forces would cause a right-hand thread to loosen. For example, left-hand threads are found in the left side of bicycles’ pedals to prevent them from becoming unscrewed while pedalling.

Different Types Of Screw Threads

There are many types of threads that can be classified according to different characteristics.Threads are divided into connection threads and transmission threads according to their working properties. The equivalent friction angle of the thread for connection is large, which is conducive to achieving reliable connection; the equivalent friction angle of the thread for transmission is small, which is conducive to improving the efficiency of transmission.

Connection threads

  1. Ordinary thread

Tooth type is equilateral triangle, tooth angle a = 60 °, internal and external threads screwed together to leave a radial clearance. The root of the external thread is allowed to have a larger rounded angle to reduce stress concentration. The same nominal diameter is divided into coarse and fine threads according to the pitch size. Fine thread type and coarse thread similar, but the pitch is small, small angle of rise, self-trivial sex is better, high strength, because of the fine teeth are not wear-resistant, easy to slip buckle.

General connection with coarse threads, fine threads are commonly used in small parts, thin-walled fittings or shock, vibration and variable load connection, but also as a fine-tuning mechanism of the adjustment thread.

Ordinary thread
  1. Non-thread sealing pipe threads

The tooth type is an isosceles triangle, tooth angle a = 55 °. The top of the teeth has a large rounded corner, and there is no radial clearance after the internal and external threads are screwed together. Pipe threads for the British fine threads, size code for the inner threads of the pipe large diameter.

Applicable to pipe fittings, plugs, valves and other accessories.

  1. Pipe thread with thread sealing

The tooth type is an isosceles triangle, tooth angle a = 55 °. The top of the tooth has a large rounded corner, the thread is distributed in the taper of 1:16 on the wall of the cone. It includes conical internal thread and conical external thread, and cylindrical internal thread and cylindrical external thread two forms of connection. After the threads are screwed together, the deformation of the threads themselves can ensure the tightness of the coupling, without any packing, and the sealing is simple. Suitable for pipes, pipe fittings, plugs, valves and other threaded accessories.

4.Metric tapered thread

Tooth angle a = 60 °, the top of the thread for the flat top, thread distribution in the taper of 1:16 conical pipe wall. Used for gas or liquid piping systems rely on thread sealing coupling thread.

pipe threads

Transmission threads

  1. Rectangular thread

The tooth angle is square, tooth type its transmission efficiency than another thread angle a = 0 ° high, but the root strength is weak, spiral vice wear, the gap is difficult to repair and compensate, and transmission accuracy is low. To facilitate the milling and grinding process, it can be made into an I0 angle.


  1. Trapezoidal thread

The tooth type is an isosceles trapezoid, tooth angle a = 30 °. The internal and external threads are not easy to loosen with the taper surface. Compared with the rectangular thread, the transmission efficiency is low, but the process is good, the root strength is high, and the alignment is good. Such as with a split nut. It can also adjust the clearance. The trapezoidal thread is the most commonly used transmission thread.

  1. Serrated thread

The tooth type is not isosceles trapezoid, the working surface of the tooth side angle of 3 °, non-working surface of the tooth angle of 30. external thread root has a larger rounded angle to reduce stress concentration. Commonly used in screw presses.

How to distinguish between threads

-Observe the pitch and tooth shape

Fine threads have a smaller pitch and a more pointed tip angle, while coarse threads have a larger pitch and a more rounded tip angle. In addition, standard threads have a uniform pitch and tooth shape, while speciality threads may be different.


-Distinguish between metric, imperial and American threads

Metric threads are usually marked with an “M” plus a nominal diameter, such as M16 or M12, and have a tip angle of 60 degrees. Imperial threads have a tip angle of 55 degrees, and their dimensions are expressed in inches, with the pitch expressed as the number of threads in 1 inch, such as 8-32, 4-20. American threads include NPT, PT and G, which belong to different standards.


-Observe the thread type and direction

Threads are available in thread types such as straight threads, slanted threads and curved threads and directions such as left-hand and right-hand.


-Distinguish between internal and external threads

The top of the tooth (large diameter) and the thread termination line of external threads are indicated by thick solid lines, and the bottom of the tooth (small diameter) is indicated by thin solid lines. The bottom, top and thread termination lines of internal threads are usually dashed.

Types Of Threads


EASIAHOME has 17 years of experience machining bearings and fasteners, and we specialize in providing you with a one-stop shop and customized threading solutions to meet the needs of your project. We understand that choosing the right thread type and machining method for different products is critical.

Depending on the load, sealing, and durability characteristics of your product, we will help you choose the optimal thread type, such as trapezoidal, square, or V-thread, and offer a wide range of machining methods to choose from, including traditional cutting, rolling, and highly efficient CNC machining techniques.

Through our extensive experience and dedicated team of professionals, we will ensure that you are assured of the best possible performance, quality and reliability of your products. No matter what your project needs are, EASIAHOME will be dedicated to providing you with a satisfactory solution.

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