Screw Threads in Machinery; Explanations, Terminology and Designations

Threads are very important in machinery. In assembly, there are different kinds of threads that we use in machine element applications. So, you can find detailed information about screw threads in the topics below;

  • General terminology of screw threads
  • Different classifications of threads
  • Form types of threads
  • Designations of threads.

What is Screw Thread?

The screw thread is a geometrical shape in which the helix shape is given on the internal or external surface of cylinders. Also, we use these geometries in the mechanical assemblies. Furthermore, they are very stiff and very easy to disassemble. Screw threads are the shapes the designers mainly use for fastening applications.

Some Classifications of Threads

According to their use and functionalities, there are some classifications of threads. Check the two classifications below.

Single and Multiple Screw Threads

Multiple screw thread.

Along with the single screw threads, there are applications where multiple screw threads. So, in single threads, only one set of threads is used on a cylindrical shape. But in multiple threads, multiple sets of threads are cylindrical shapes.

They use multiple threads in mechanical assemblies where rapid motion is of prime importance. Also, the low power requirement is very important for them. For example, in valve systems, these types of threads are used.

Right-hand and Left-hand Threads

According to the assembly direction of the threads, we classify the threads as left-hand and right-hand. So, if the nut is assembled by turning clockwise rotation, this thread is called as right-hand thread. Otherwise, it is a left-hand thread.

In the general design procedures, all the threads are right-hand. Also, if a left-hand screw is used in the mechanical application, we use the ‘LH’ symbol on the technical drawing.

General Terminology about Screw Threads

Terminology of screw threads.
Image Source: Sxcco.

There are different terms for screw threads that you need to know.

External and Internal Threads

Mainly, screw threads are in two configurations like this. External threads are the type of which helical geometry at the outer surface of a cylinder. Also, the internal threads are the type of threads in which helical geometry is at the inner side of a hole.

A threaded assembly needs both internal and external threads. For example, nuts have internal thread geometries and screws have external thread geometries.

Major and Minor Diameters

In the different calculations, we use these two terms in general. The major diameter means the biggest diameter of a thread geometry. And also minor diameter is the smallest diameter.

For external threads, the major diameter is the diameter value of the thread crest. For internal threads, the major diameter is the root of the thread.

Also for internal threads, the minor diameter is the thread crest diameter. And for external threads, the minor diameter is the root of the thread.

Pitch of Screw Threads

Pitch means the distance from one point on the thread to another corresponding same next point on the screw. And also, we measure this distance parallel to the axis of the cylinder.

Pitch Diameter

This is a very important theoretical value regarding the threads. Imagine a cylinder that passes through the threads. But also, a span of the threads equal to the span of the spaces. This is the pitch diameter that is generally used in the geometrical calculations of threaded assemblies.

The Lead of Screw Thread

Lead means the total distance that one turn of the helix of the threads.

Angle of Thread

The inclination angle of the threads. We measure this angle in a plane through the axis of the screw.

Crest and Root

Threads generally have two sides. And also the crest of the thread is the joining point of the two screw threads on a cylinder. The same is the root at the bottom side of the thread.

In addition, the distance between the crest and the root gives the depth of the thread.

Screw Axis

This is the centerline axis of the screw cylinder.

Thread Series

Several threads per inch or millimeters for various diameters of threads.

General Screw Thread Forms in Engineering

There are different types of screw threads in the market and applications. According to the applications, they use different shapes and forms of threads. Here, you can find the general shapes and forms of threads.

Metric Threads


Metric threads are the ISO standard generalized type of threads. Also, the general geometry of the metric screw threads is similar to the American nation. Moreover, the crests and the roots of the threads are flat. The angle of the threads is 60° according to each other. The width of the threads is 0.125 times the pitch for metric threads.

Metric threads are generally shown by the ‘M’ symbol.

60° V Thread

This is one of the most used screw thread forms in general applications. V- threads are very useful for structural applications where rigidity is very important. The rigidity of the assembly increase with V-threads because of the high contact area between threads.

Square Threads

Square thread.

Unlike other thread types, square screw threads are not standardized. They are very good for power transmission applications. Production and disengagement are the main problems for square threads.

ACME Threads

Image Source: Wikimedia.

Instead of square threads, ACME screw thread is used generally. Because the production of ACME threads is much easier if we compare it with the square threads. Also, the disengagement of nuts from ACME threads is better.

The strength of the ACME threads is better than the square threads.

Standard worm threads are also types of ACME threads with deeper threads. They are suitable for shaft power transmission.

American National Thread Types

The only difference between the American national thread is, that the crests and roots are flat. So, these flat crests and roots are the 0.125 times pitch of the thread.

Unified Thread

Its name comes from the unification of the screw thread geometries in 1948 between the UK, US, and Canada. Also, the only difference between the unified thread and from American national thread is, that the crest may be round or flat, and the root is round.

UNEF Threads

UNEF means unified extra-fine screw thread series. In these types of threads, many threads per inch are used. the shape is the same as the American national thread. But the number of threads is more in UNEF threads.

Designers are using these types of threads in assemblies where the thread engagements are low. For example in the sheet metal assemblies, UNEF threads may be proper.

Whitworth Threads

Whitworth thread.
Image Source: Florida Center for Instructional Technology.

This is the British standard that replaced the American national screw thread. The geometrical difference of the Whitworth thread;

The angle between the threads is 55°. Depth is 0.64 times pitch and the root radius is 0.1373 times the pitch.

Knuckle Thread

Image Source: Wikimedia.

Knuckle threads are generally rolled from sheet metals. The general applications of the knuckle screw threads are the small electrical sockets and electrical bulbs. The diameter of the root and crests are 0.5 times pitch and the depth is equal to 0.5 times pitch. So, there is a sinusoidal form of threads in knuckle threads.

Buttress Threads

Buttress screw thread.
Image Source:

The difference between the buttress screw threads is that there is no angular symmetry between the threads. The top side of the threads is 7° and the bottom side is 45°. Because of this geometric feature, buttress threads are generally used in power transmission applications that are in one direction only.

We use buttress threads in car jack mechanisms.

Standard Thread Notes and Symbols

There are different types of thread noting and designation standards for metric threads and ANSI threads. With the examples below, you can understand the general noting and designations of threads below.

Metric Thread Designations

Designations of metric threads are the most basic ones. Take a look at the example below.

For example M10 X 2 – 6H/6g-LH

This is an example of metric screw thread designation. It starts with the ‘M’ which is the designation that it is the metric thread.

’10’ means the diameter of the metric thread and ‘2’ means the pitch. The diameter and the pitch are in the millimeters unit.

‘6H/6g’ is the type of fit for the internal and external screw couples.

‘LH’ is the designation that shows this metric thread is left-handed.

60° V Threads Designations

For example, we have a designation like; 1-15UNF-2A(21)

From right to left,

‘1’ is the nominal diameter of the thread in inches.

’15’ gives the number of screw threads per inch.

‘UNF’ is the letter symbol for the screw thread type.

‘2A(21)’ additional qualifying information such as gage of the thread system etc. This is the additional information that is not in the main information.

Unified Thread Designations

In general, there are different designations of internal and external unified threads. So, check the examples below.

For example, .50-15UNC-2BLH

This is an example of unified internal screw threads.

‘.50’ means the major diameter of the screw threads.

’20’ gives the threads per inch.

‘UNF’ is the information about the thread series.

‘2’ is the class of fit.

‘B’ means the internal unified thread.

‘LH’ means the left-hand thread.

Another example; .60-25UNF-3A

In this example, the only difference is the letter ‘A’ after the class of fit ‘3’. ‘A’ means, it is the external thread. All the other elements are the same with the internal unified threads.


So, as you understand that the terminology of threads at the engineering level is not simple as people think. There are different kinds of important terminology about screw threads such as depth and pict etc. Also, there are different kinds of classifications of threads such as internal and external threads and left-hand and right-hand threads.

Above all, engineers are using different kinds of thread types. Around these types, Whitworth, metric and ANSI screws. These screws have different thread shapes and geometries.

Designations of the screws and threads are very important in engineering drawings also. You can give all the required information about the type of screw thread that you use in your design. According to the standard, designations can change.

Finally, these are the general aspects of screw threads in general.

So, do not forget to leave your comments and questions below about the screw threads in mechanical design.

Your precious feedbacks are very important to us.

FAQs About Screw Threads

What is the screw thread diameter?

We measure the diameters of threads both the crests and roots of them. Root diameters are smaller than the crest diameters for external threads. And crest diameters are smaller than the root diameters for internal threads.

What is screw thread pitch? 

The Pitch of threads is the distance between the respective same points of helix geometry. Pitch is a very important thing in screw calculations.

How screw threads are made?

There are different methods in the production of screw threads. Form turning and other basic machining operations can produce screw threads.

What are the screw thread sizes?

 In general, there are different sizes of screw threads. These sizes can change according to the standard. For example for metric threads, there are diameter sizes ranging from 1 mm to 60 mm.


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