Bolt Boot Camp: 6 Things to Know About Threaded Fasteners

Rookie Year: The Engineering Intern’s Toolbox
A Series of Technical Blogs for Engineering Interns & Entry-Level Engineers

While warmer temps are upon us and school is out for the summer, it’s time for many to begin that most interesting season of the year… summer internships. Across the nation, engineering interns are getting their first taste of professional life and hopefully learning some basics which will serve them for years. However, some of the principles they are expected to know on day one are rarely covered in class. To help our young, budding engineers, here’s one basic lesson for their toolbox… literally the nuts and bolts.

How Do Threaded Fasteners Work?

Threaded fasteners all contain a spiraling ramp on a cylindrical shaft. This ramp is a simple mechanism for converting rotational motion into linear motion. More importantly, it converts a torque (rotational force) into a linear force. The ramp is what is referred to as a thread. When the threads are on the outside of a cylindrical shaft it is referred to as an external or male thread while a cylindrical hole with a thread is called an internal or female thread. Rotational motion is converted into linear motion by interacting (mating) an internal thread with an external thread.

How are Fasteners Meant to be Used?

Threaded fasteners are almost always intended to clamp (fasten) two or more pieces of material together. Unless specially designed to do so, threaded fasteners are not intended to locate pieces of material relative to each other or prevent pieces of material from sliding. Threaded fasteners are strongest in tension (being pulled apart) not in shear (slide apart). As a result, they prevent parts from sliding relative to each other by their clamp force not from the body of the fastener acting like a pin.

What is the Difference Between a Bolt and Screw?

This is a very confusing principle for new engineers because some bolts are screws and others are not. For example, a carriage bolt is often used as a screw and a socket head cap screw (SHCS) is often used as a bolt. The simple answer is a bolt uses a nut while a screw does not. Screws thread directly into one of the pieces of material to be clamped. In this case, the material within one of the items to be clamped acts as the internal female thread. A bolt on the other hand passes through all of the pieces of material to be clamped and does not interact with any of them. Instead, a nut or other pre-made female thread is applied to the end of the bolt and the material is clamped between the nut and head of the bolt.

What is Meant by a Fine or Coarse Thread?

Thread pitch deals with how many threads are in a given length of a fastener per its nominal diameter. For example, a fastener could have large and widely spaced threads. This is referred to as having a coarse pitch. A fastener with the same nominal diameter that has more frequent threads per unit of length has what is known as a fine pitch.Coarse threads are bigger and have more material in them. However, this means that they have a smaller non threaded cylinder section compared to fine pitch threads. Therefore, coarse pitch threads have stronger threads and weaker bodies. Fine pitch threads are the opposite which have weaker threads and stronger bodies.

When Should I Use a Fine vs. Coarse Pitch?

Most standard fasteners have a common coarse and fine pitch thread size. Coarse pitch threads are ideal when threading into a base material which is equal to or stronger then the fastener. As stated earlier the extra material in the coarse pitch threads means that the threads are stronger then fine pitch. This means they are better at being installed and removed frequently without damaging the threads. Therefore, it is better to use coarse pitch threads in soft materials when the fastener will be repetitively removed and reinstalled.Fine pitch threads give you more threads per length and therefore more load to be distributed into the base material. This is ideal if you are working with thin materials or when the base materials are softer than the fastener. Also, because the fastener’s shank is larger, the fine pitch threads have higher tensile (pulling) strength. The trouble with fine threads are that the threads themselves are much weaker. Repeatedly taking the fastener in and out will damage the threads. The other issue is that it is easy to “cross thread” the nut when installing on a fine threaded bolt.

How Deep Should I Thread a Fastener?

How deep a fastener needs to be threaded is directly related to its diameter. The bigger a fastener is, the deeper it will need to be in order to be strong. Many studies have found that there is a simple rule of thumb for standard pitch fasteners. The depth of a fastener should be at least two times its nominal diameter. For example a 1/4in (0.25in) fastener should be threaded at least 1/2in (0.50in) deep. Any deeper than this and the threads don’t really do much except add assembly time.Thread pitch also plays a part in how deep a fastener should be. For example, fine pitch threads distribute more loads per unit of length. Therefore they do not need to be threaded as deep. Most fine pitch threads obtain most of their strength by one diameter of engagement.

*Tip. When threading into thin material, use fine pitch threads when two diameters are not possible.

Written By: Sean Derrick – Product Development Engineer |
Sean has a B.S. in Engineering Design and a M.S. in Manufacturing Engineering from Western Michigan University. He has over 7 years of product development experience in a variety of industries ranging from medical, automotive, furniture, consumer products, and defense. Sean enjoys hiking, mountaineering, skiing, movies, and is pursuing a Ph.D in his free time.