Threads: How Many Types Exist

How many types of threads exist? How are they recognized?

For a good understanding of the different coupling possibilities between screw and nut, the starting point is the knowledge of the types of threads and their designation. Each thread is distinguished by the different elements that characterize it. Specifically, these are:

• Length, measured in relation to the axis of the screw or nut;

• Nominal diameter of the thread, in reference to the characteristic diameters of the threads such as the external diameter (De), the core diameter (Dn), and the mean diameter (Dm) obtained by adding the first two;

• Height, obtained by calculating the difference between De and Dn;

• Thread pitch, a factor measured with a tool called a thread gauge;

• Direction of the helix, whether the threading is clockwise or anticlockwise and therefore whether it is a right-handed or a left-handed thread;

• Helix angle, which describes the geometric shape of the thread;

• Crest or Vertex, the upper surface that joins the two sides;

• Root or Bottom, the lower surface that joins the two sides;

• Flanks, the lateral surface located between crest and root.

Below is an explanatory image of the characteristics just listed.

Based on the aforementioned characteristics, the standard and most common thread shapes can be distinguished into two main types:

• Triangular threads
• Non-triangular threads.

The triangular thread is usually used as a connection. The angle between the threads is 60° while the crest can be flat or rounded.
Among the most used triangular thread systems is the ISO metric thread, the classic screw identified by an M with an ideal profile equivalent to an equilateral triangle, and the Gas thread, a coupling that guarantees sealing in the passage of fluids and is indeed used in gas pipelines.

As for non-triangular threads, the most common forms are:

• Square thread
• Trapezoidal thread

Unlike the triangular thread, these two systems are not primarily used as a connection but for power transmission.
For example, trapezoidal threads can undergo numerous modifications without sacrificing the efficiency of the element.
The angle between the threads can vary from 10° to 15°, thus reducing the space between the teeth.

Types of Threads and Their Designation

The different types of threads are distinguished not only by their specifics but also by their designation.
The way a thread is indicated and, therefore, its designation, is distinguished by some elements:

• Lateral symbol, which identifies the thread;
  
  
• Nominal diameter (corresponding to the external diameter of the screw, measured in inches or millimeters, or to a conventional diameter in Gas threads);
  
  
• Pitch (measured in millimeters when it differs from the nominal diameter);
  
  
• Number of starts (expressed when it is greater than one, it refers to the number of parallel and independent threads present on the surface of the screw);
  
  
• Direction of the helix (if left or LH, Left Hand).

For example, the designation "M 16 x 1.5 left" indicates: ISO metric thread with a triangular profile, nominal diameter of 16 mm, left helix, and fine pitch.
But, in detail, what is meant by fine pitch and coarse pitch? What is the difference between these two nomenclatures?

Coarse and Fine Thread Pitches: Summary Table

Among the values indicated in the designation, there are precisely the thread pitches.
In essence, the thread pitch is the distance between two consecutive threads of the same helix, thus between the grooves of the screw, and is found in the ISO metric thread.
The ISO metric thread is the standard thread recognized by the International Organization for Standardization and has a triangular profile and an angle, of profile, of 60°.
The threads of screws produced according to this metric are identified with an M.
This letter may be followed by a value, expressed in mm, when the thread is a fine pitch. Indeed, there are two types of pitches: coarse and fine.

The coarse pitch thread is the one with the standard nominal diameter that ensures sealing against unscrewing.
To this series corresponds a single pitch value and the adjective "coarse" indicates that such pitch is the one with the higher value for that diameter.
The fine pitch thread is used for special applications such as, for example, when a higher anti-unscrewing capacity is required, or a greater fluid sealing capacity, or when a very precise screwing adjustment is needed.
Fine pitch threads can be found in several sizes for the same diameter.

Below is a summary table of diameters and corresponding thread pitches.

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