Threaded Rod 101: Dimensions
Welcome back to our Threaded 101 series! Be sure to read our previous articles on finishes, materials, and tensile strength to catch up. This article provides an overview of dimensions associated with threaded rods.
With so many different sizes and measurements, threaded rod dimensions can be complicated. The important dimensions for threaded products are diameter, thread pitch, and length (Figure 1). This is also the order typically used to specify a product- diameter, thread pitch, length.
Figure 1: Threaded Rod Dimensions
There are several different diameter measurements associated with threaded rods. First is the major diameter, which is the largest diameter of the screw thread, measured from thread crest to thread crest. Next is the minor diameter, which is the smallest diameter of the screw thread, measured from thread root to thread root. Minor diameter is sometimes referred to as root diameter. Lastly, pitch diameter is the simple effective diameter of screw thread, approximately halfway between the major and minor diameters. All diameters are measured in either inches or millimeters.
When specifying a threaded rod, the nominal major diameter should be used. This can be expressed as a fraction, decimal, or machine screw number.
Thread Count and Pitch
Thread count is measured in threads per inch (TPI). Typically, Inch-Standard fasteners use thread count while thread pitch is used for Metric fasteners. Thread pitch is the distance from a point on the screw thread to a corresponding point on the next thread measured parallel to the axis, typically measuring the distance between thread crests. Thread pitch is often measured in millimeters. TPI and thread pitch are determined by the thread series. The most common thread series are Unified Coarse (UNC), Unified Fine (UNF), Acme, 8-pitch (UN8), Metric coarse, and Metric fine.
When specifying a threaded rod, the TPI or thread pitch comes after the major diameter. Inch-Standard fasteners use TPI while Metric fasteners use thread pitch. A dash (-) commonly separates the TPI from the major diameter specified. An “x” commonly separates the thread pitch from the major diameter for Metric threads. Specifying the thread series is not uncommon, but it is not necessary since the TPI or thread pitch will indicate that information.
Class of Fit
Class of fit is a designation used to specify the amount of tolerance/allowance between mating threads. Essentially, it is a designation of how the internal and external threads fit together. For Unified thread series there are six main classes, three for external threads and three for internal threads. These are designated by the numbers 1, 2, and 3 followed by either “A” or “B”. The “A” designation represents external threads while the “B” designation represents internal threads. Class 1A fits allow for the largest tolerances and are used in many commercial applications for easy assembly and disassembly. Class 2A fits are tighter than 1A fits and are used for some commercial and aerospace applications. Class 3A fits are the tightest and allow for the smallest tolerances. These are used in precision applications where a close fit is required.
For Metric thread series, there are many more classes. These have two sets of alphanumeric designations, one for the pitch diameter and one for the major diameter. They are usually shown in the following format: 6g6g. When the two tolerances are the same, the class of fit can be simplified to one alphanumeric pairing (in this case: 6g). The numbers represent the tolerance grade while the letters represent the tolerance position. Smaller numbers indicate a tighter fit while smaller letters (closer to the beginning of the alphabet) indicate more allowance. External threads are designated by lower-case letters. Internal threads are designated by upper-case letters. The most common class of fit for Metric thread series is 6g for external threads and 6H for internal threads.
When specifying a threaded rod, the class of fit comes after the TPI/thread pitch. A dash (-) commonly separates the TPI/thread pitch from the class of fit.
For the length, it is important to identify if the overall length of the product is being measured, or from first thread to first thread (see Figure 2 below). Overall length refers to the entire length of the product, from end to end. First thread to first thread refers to the measurement from the first complete thread on one end to the first complete thread on the other end. This measurement does not take into account the partial threads and chamfers on the ends of the rod.
Figure 2: Length: Overall vs 1st Thread to 1st Thread
When specifying a threaded rod, the length comes after the class of fit. The two are commonly separated by an “x” (read as “by”). Length should be in units of inches for Inch-Standard fasteners and millimeters for Metric fasteners.
Another area of measurement for double-ended and single-ended studs is the thread length, which refers to the threading on the end of the product. Thread lengths can be equal or unequal on the threaded product. For example, the product can be a 20” overall length double-ended stud, with a 2-inch thread on one end, and a 6-inch thread on the other end.
To review, when specifying a threaded rod, the nominal major diameter comes first. This is followed by a dash (or “x”) and the TPI/thread pitch along with the thread series. Next is a dash and the class of fit. The length is the last necessary dimension, being separated from the class of fit by an “x”.
Example of how to specify dimensions for fully threaded rods:
¼” – 20 UNC – 1A x 6”
M12 x 1.75 – 6g x 200mm
Now that you have a basic understanding of the dimensions associated with threaded rods, you can be confident that your order will fulfill the requirements of your application.