Knowing the right screw size is important for ensuring that you use the correct fastener in your project. However, many people are confused about how screw sizes are determined.
Screws with greater outer diameter exhibit better failure force characteristics on 3-point bending tests and torsional tests than smaller screws. Additionally, there is a good correlation between the displaced diameter and pull-out strength.
Thread Pitch
Thread pitch is the distance between adjacent thread crests. It affects how much torque is needed to tighten a screw and has implications for overall thread strength, wear and efficiency.
Unlike a perfectly sharp V-thread, which would have a depth of thread (height from root to crest) equal to the pitch value, most screw threads are “truncated,” meaning that they are not entirely sharp at the crest or root. This is done to allow for a degree of tolerance.
The major diameter and thread pitch are often confused with each other. Generally, the major diameter is the largest diameter of the screw and is noted as such in inches, for example 1/2-13 refers to a coarse thread. The thread pitch is the distance between two consecutive thread crests and is typically measured with a pitch gauge or go/no-go gage. The pitch gauge measures the distance from the center of one thread to the center of the next and is expressed as threads per inch, for example, 20 threads per inch means that a screw has a fine thread pitch.
Thread Diameter
The diameter of a screw is defined as the distance over which all of the threads pass, or the axial distance that one thread travels in a 360deg turn. It is also called the major diameter. The minor diameter is the distance over which only a portion of the thread passes, or the thread pitch.
Both of these factors determine the functional diameter of a screw and its strength, and they can be used to distinguish between different types of screws and bolts. For example, a coarse thread has a larger diameter than a fine thread. This difference is made possible by the fact that the threads in a coarse thread are closer together than they are in a fine thread, which makes it easier for them to dig into the surface of the material.
A screw or bolt’s diameter and thread pitch are usually indicated by a number followed by a capital letter, such as M (for metric) or UTS. It is sometimes also helpful to know if the screw or bolt has left-hand threads, which are designated by an LH symbol.
Thread Depth
The distance from the center of a thread’s flank to the base of its flank is its depth. The deeper the thread, the more stress it can withstand and the stronger it is.
In an ideal world, the straight sides of a thread’s flanks would coincide at the point called the pitch diameter (PD). However, this condition is rarely met due to tolerance ranges and other factors.
Because of this, some amount of truncation — either at the crest or the root of the sharp V-shaped form — has to occur in order for a threaded connection to work. This truncation results in a final thread depth that can be measured as a fraction of the thread’s pitch value. If the truncation is moderate and there are sufficient positive root-crest clearances, then the threads of a screw and nut will come into intimate contact at the PD line. Otherwise, the threads may shear and loosen.
Material
Screws are made from a variety of materials and can be produced in several ways, such as cutting, grinding or rolling. These methods influence the thread’s shape and strength. The thickness of the screw shank is also important. Screws with a thicker shank are more resistant to breakage and may be used in structural applications.
Screw threads can be characterized by their major diameter, which is usually indicated as the first number in a screw’s gauge size, which ranges from #0 to #12 (or 1/2 inch). The screw’s gauge is determined by counting how many thread peaks there are on one inch of the screw’s length.
The next number in a screw’s gauge is its thread pitch, which indicates the number of thread peaks on a complete revolution of the screw. The last number is the screw’s overall length, which can be measured from the end of the shank to its head for hex, pan and button screws or from the tip of the threads for countersunk or trim heads.8 screw diameter
