Threaded Screws: Basic Concepts

Threaded screws are some of the most innovative tools invented. If you know your way around them, you can make a lot of equipment to work properly. 

Although there are plenty of types of threads, the principles are the same which allow screws to become stronger and support heavy loads. 

With Rotary thread, we’ve innovated on a tool which works well with the mechanics and design of different thread types, regardless of the standard it follows.

It’s important to understand some basic concepts about screw threads, though, and ultimately, how Rotary thread helps keep screws working properly via maintenance and repair.

What is a Screw thread?

Threads are the ridges that traverse the surface of a cylinder. These ridges are uniform and follow the shape of a helix. 

Threads can be inner or male, which means it is on the surface of a screw. If the threads are inside a nut or a hole, it is called an outer thread or female. Male and female threads join together to lock the screw to the joint.

Threaded Fasteners

Threaded fasteners are anything with inner or outer threads. Any rod, shaft, nut, bolt or screw 

which are threaded are considered threaded fasteners.

Some of the types of threaded fasteners are nuts, bolts, screws, thread inserts, and studs. 

Threaded screws are some of the most used fasteners and come in different standards. These screws are manufactured and considered either in inch, metric, or pipe. 

The threads on the fasteners are used mainly for two purposes:

• To fasten two objects, as in to fasten an automobile wheel
• To connect two objects, as in a hose to its cap

Basic Thread Concepts

Screw threads allow for proper installation of and screws. By providing the friction between the mating inner and outer threads, it achieves two things:

Here we use screw and bolt interchangeably, as similar forces apply to the joint and its components.

• Allow the bolt or screw to withstand vibration
• Support the shear load (sliding force between objects)

The pre-tension of the bolt, or the load coming from the torque, makes the bolt more able to withstand the load applied to the joint. 

In the image above, the two objects in blue and in red are compressing against one another. The bolt in between them counteracts these forces by acting like a spring, providing a tensile stress to the joint. 

Yield stress is the force on the bolt where the bolt will assume plasticity and will no longer be able to return to its original form. Threads that are in good condition prevent the bolt or screw from reaching this point. This is why maintaining threads is important, it will reduce stress on the joint and on the equipment.

Thread Pitch and Diameters

Pitch and diameter are the essential measurements of a thread. Proper measurements are made to check which screw threads work with each other. Otherwise, if the measurements do not match and its is forced, the threads might get stripped. 

Pitch is the measured distance between the ‘crests’ of a thread. The crests are the peaks of the threads. The greater the pitch, the greater the torque needed applied to the screw to achieve a certain tension. 

The pitch is measured using a thread pitch gauge for accuracy. 

Diameter can be understood as pitch diameter, major diameter and minor diameter. Of those three, pitch diameter is most important to know about for maintenance. It would tell you if two threads can be mated together. A diameter is measured using a caliper.

Parallel and Tapered Threads

Threads are made either parallel or tapered. Tapered threads become narrower as you reach the tip. Parallel threads, meanwhile, retain the diameter constantly on both ends. Parallel threads are also called straight threads.

To know if you have a tapered or parallel thread, you should use a caliper to measure the diameter of the first, fourth, and last crests. If all the measurements are the same, then you have a parallel thread. If the measurements are not the same, then you have tapered threads. 

Metric, Inch, and Pipe Threads

Metric threading directly measures the pitch in millimeters. An example of it would be 10mm x 1mm.

Whereas metric threads are measured by the distance per thread or the thread pitch, inch threads are measured by the number of threads at a given distance. The standard used is the number of threads per inch (TPI).  

The standards which use TPI are SAE, English, Standard, and Imperial. An example of an inch standard would be 9/16″ x 20 TPI. 

Pipe thread measurements are NPT (National Pipe Thread) and ISO 7/1 or British Standard Pipe. You may refer to a chart here for the nominal sizes. To be able to use the chart, you need to measure the inner diameter of the female thread and the outer diameter of the male thread.

Though these standards vary, Rotary thread does not distinguish any kind of thread when repairing. It can fix any metric, pipe, or inch. 

Coarse and Fine Threads

Coarse and fine threads have different applications. Coarse threads are often used in construction because they can withstand more rigorous conditions. Coarse threads are also less prone to getting stripped and cross-threaded because of its wider thread.Coarse threads are often used for assembly and disassembly. 

Fine threads, on the other hand, are used more often on engines and equipment in need of stability, because fine threads can resist vibration well with its high tensile strength. 

When in need of maintenance, both can be repaired by Rotary thread.

Rotary Thread

Rotary thread can be used on any metric, inch or pipe thread. It wouldn’t matter the number of threads per inch, or the distance between the pitches. 

Most of all, there is no need to measure the pitch or the diameter. You won’t need to use a caliper to measure diameter or thread gauge for the pitch.

It gives you the option of repairing even if the threads are galled, dented, corroded, or damaged. This saves you money and time. Overall, knowing the basics of how threaded screws work gives you ample advantage in repairing them.