The bolt is a crucial component in modern manufacturing and has a vast range of applications in both commercial and industrial products, from airliners to gardening tools. Despite their seemingly straight forward role, bolts display a relatively complex combination of mechanical properties and design features that enable them to function properly.
A bolt’s utility is not determined simply by whether it fits into its hole or not, but rather a series of particular specifications. The type of bolt, its physical dimensions, thread density, forming material, and tightening requirements are all important characteristics for effective bolt installation. Likewise, the complementary linkage of nuts and bolts can be a significant influence on component assembly and product efficiency. Using a bolt that is too long might interfere with operations, while mounting one that lacks tensile strength may cause equipment to break down. Therefore, understanding how bolts accomplish their tasks can mean the difference between a successful project and an ineffective one. Bolt Threads
Threads are one of a bolt’s main characteristics. A thread functions under the basic concept of an inclined plane that is spiraled around the length of a fastener. This spiral requires rotational pressure to be inserted into a slot. Turning the bolt causes the threads to move the complementary slot or nut upward against the inclined plane. The greater the torque, or turning force, the greater the pressure pulling the nut forward along the threads. This in turn creates tension in the bolt and a clamping force that presses the two components together. A higher number or a denser arrangement of threads can strengthen the joint. In addition, cutting threads into the bolt after it has been heat-treated also improves thread strength. Mechanical Forces
The act of tightening a bolt engages several different types of mechanical force that help the resultant joint remain secure. The main forces generated by bolt installation include:
Clamping Force: This is a type of compression that the bolt applies to the joint, holding the two components together.
Preloading Force: Turning the bolt causes the threads to engage and stretch, which produces the preload force that keeps the threads in place.
Shear Force: The shear force is the transverse pressure working against the bolt in a perpendicular direction. It can be present alone or in combination with tension force.
Tension Force: Tension force applies to the length of the bolt, providing pressure along its vertical dimension, rather than its width.
The clamping force accomplishes the task of holding multiple parts together, and is determined by the difference between the preloading force and the tension force. When the strength of the tension force equals that of the preload force, the bolt fails and the joint breaks apart. Generally, higher preload force translates into higher clamping force, which makes for a stronger bolted joint. Ideally, precise bolt installation and tightening will create a joint that yields little or no direct shear force. More detailed equations can be found at Keeping It All Together. Manufactured for Strength
The American Society for Testing Materials (ASTM) sets most of the standards for bolt specifications in the U.S., including dimensions, forming materials, and post-fabrication finishing. Bolts are produced in a range of different lengths and can have a variety of head shapes, such as hex, oval, flat, or socket configurations. A bolt’s overall mechanical strength can be measured by the degree of strain at which it finally breaks, while its yield strength designates the level at which it bends or deforms. Although the overall, or tensile, strength is always higher than the yield strength, for a machine to run properly its bolts should have loads under the yield level. Bolts that have a wide disparity between their tensile and yield strength are considered flexible and will stretch to a relatively high degree before failing.
Bolts come in many varieties, such as T-Head or T-Slot bolts, and can be produced from a wide range of materials, but steel is the most common material used for most high-strength applications. Steel bolts are graded according to newtons per millimeter squared for tensile strength as compared to yield strength. For example, a steel bolt with a rating of 5.5 will provide roughly 500 newtons/mm2 of tensile strength and approximately 400 newtons/mm2 of yield strength. Other popular bolt materials, such as corrosion-resistant stainless steel, alloys, and composite materials can be rated in similar fashion.
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