How the type of thread can completely change the characteristics of fasteners
When the average person looks at a box of bolts or screws, they don’t think much past the basic size of the screw, or, if it will be exposed, maybe the color of the metal. But in the components industry, there is so much more thought that goes into the threads.
Male vs. Female
In today’s world, gender plays a big role in everything that we do. In this industry, it turns out, that’s the case, too. Every set of matched threads between nuts and bolts, have a gender. A screw typically has “male” threads, while the matching nut has a set of female threads. Together, they guide the bolt down into the nut, hence the male and female reference.
A thread can twist in either a clockwise or counterclockwise direction in relation to the viewer. This threading property is known as handedness. Most threads are turned in a clockwise direction – hence the old phrase “righty-tighty; lefty-loosey.” This familiar type of thread is known as a right-handed (RH) thread, because it follows the right hand grip rule. Meanwhile, threads that follow the opposite rule and tighten by a counterclockwise direction are left-handed (LH).
While a large majority of the nuts and bolts that CFI makes do follow the standard right-handed thread, the applications where we do make LH nuts are in applications where the nuts are deliberately chosen to trip up, or cause pause in the user. For example, applications where there could be dangerous misconnections such as gas welding connections, LH nuts are chosen, so the user must think twice before tightening the bolts.
On a bolt or screw, the cross-sectional shape of the thread is called the threadform. There are several standard forms – square, triangular and trapezoidal. Triangular threadforms are also called V-threads because they form the shape of the letter V. These threadforms typically have a thread angle of 60 degrees.
A screw or bolt’s lead is the distance along a screw’s axis that is covered by a 360-degree turn of the screw. A closely related term is called the pitch. While related, pitch is not the same as a screw’s lead. Pitch is the distance from the crest of one thread to the next. Most standard bolts and screws are given a single-start threadform, so the Lead and Pitch are deliberately made one and the same.
We’ve just scratched the surface of threading today, so if you want to learn more, contact one of CFI’s engineers at 847-818-0333.