Fastener Threads

Fastener threads are essential components of mechanical engineering and play a crucial role in securing objects and structures.

Understanding the basics of fastener threads is essential to ensure they function as intended and can bear the loads and forces placed upon them.

However, with so many threaded fasteners to select from – from external threads and internal threads to fine threads and coarse threads – it can be difficult to know where to begin. Read on to discover more.

What are fastener threads?

Threaded fasteners are those with helical grooves or ridges, such as military bolts and sheet metal screws.

These threads are designed to mate with corresponding threads in a tapped hole or a nut, creating a secure and reliable joint between two or more objects.

Threads are typically formed by cutting, rolling, or molding the surface of a fastener, and they come in a variety of shapes and sizes to meet different application requirements.

Screw threads must be precisely formed to ensure that they engage fully and uniformly with the mating threads, providing adequate clamping force and resistance to loosening under load.

Fastener threads are a critical component of many mechanical assemblies, and their design and performance can have a significant impact on the overall reliability and safety of the assembly.

Different types of fastener threads

There are several different types of threads used in fasteners, from external threads and internal threads to male threads and female threads. Each has its own specific characteristics and applications.

Here are some of the most common types of threads:

Unified Thread Standard (UTS)

The UTS is the most common type of thread used in North America. It is a standard thread design that has a 60-degree thread angle and comes in several different thread classes, depending on the application requirements.

Metric Thread Standard

The metric thread standard is used in most of the world outside of North America. It has a 60-degree thread angle and comes in several different classes, similar to the UTS.

Acme Thread

The Acme thread is a trapezoidal thread design that is commonly used in power transmission applications, such as lead screws and jackscrews. It has a 29-degree thread angle and a large thread depth, making it ideal for high-load applications.

Buttress Thread

The Buttress thread is a modified trapezoidal thread that is used in applications that require high axial load capacity. It has a 45-degree thread angle on one side and a 5-degree angle on the other, making it ideal for applications where the load is primarily in one direction.

Square Thread

The Square thread is a high-efficiency thread design that is commonly used in power transmission applications, such as lead screws and jackscrews. It has a square-shaped profile that provides a large contact area between the mating threads, resulting in a high load-carrying capacity.

Knuckle Thread

The Knuckle thread is a thread design that has a large radius at the root of the thread, which helps to distribute stress more evenly across the threads. This makes it ideal for applications that require high fatigue resistance, such as aerospace and custom engineering for military projects.

Thread dimensions, specifications, and the importance of proper thread engagement

Thread dimensions and specifications are critical to ensuring proper engagement between mating threads and achieving the desired level of clamping force and resistance to loosening.

Here are some of the key aspects of thread dimensions and specifications:

1. Thread Pitch – The thread pitch is the distance between adjacent threads and is typically expressed in millimeters or threads per inch (TPI). Threaded fasteners with a higher thread pitch have threads that are closer together, resulting in greater clamping force and resistance to loosening.
2. Thread Diameter – The thread diameter is the outer diameter of the threaded portion of a fastener and is typically expressed in millimeters or inches. It is essential to match the thread diameter of the fastener with the mating hole or nut to ensure proper engagement and prevent thread stripping or cross-threading.
3. Thread Length – The thread length is the portion of the fastener that is threaded and is typically expressed in millimeters or inches. It is essential to ensure that the thread length is sufficient to provide proper engagement between the mating threads, without exceeding the depth of the threaded hole.
4. Thread Class – The thread class refers to the tolerance of the threads and is typically expressed as a number or letter. A higher thread class means the threads are more precisely formed, resulting in greater clamping force and resistance to loosening.

All you need to know about thread forming screws, nuts and bolts

All things considered, fastener threads are a critical component of mechanical assemblies, and proper selection of thread type, dimensions, and specifications is essential to achieving the desired level of clamping force and resistance to loosening.

With a variety of thread types and standards available, it is important to consider the specific application requirements when selecting a thread design.