Bolts are for the assembly of two unthreaded components, with the aid of a nut. Screws in contrast are used with components, at least one of which contains its own internal thread, which even may be formed by the installation of the screw itself. Many threaded fasteners can be described as either screws or bolts, depending on how they are used.
Bolts are often used to make a bolted joint. This is a combination of the nut applying an axial clamping force and also the shank of the bolt acting as a dowel, pinning the joint against sideways shear forces. For this reason, many bolts have a plain unthreaded shank as this makes for a better, stronger dowel. The presence of the unthreaded shank has often been given as characteristic of bolts vs. screws, but this is incidental to its use, rather than defining.
Bolted joints are one of the most common elements in construction and machine design. They consist of fasteners that capture and join other parts, and are secured with the mating of screw threads.
There are two main types of bolted joint designs: tension joints and shear joints.
In the tension joint, the bolt and clamped components of the joint are designed to transfer the external tension load through the joint by way of the clamped components through the design of a proper balance of joint and bolt stiffness.
The shear joint transfers the applied load in shear on the bolt shank and relies on the shear strength of the bolt. Tension loads on such a joint are only incidental. A preload is still applied but is not as critical as in the case where loads are transmitted through the joint in tension.
The complexity of the simple nut and bolt is frequently underestimated. A fully tightened bolt does not perform like a loose bolt. A fully tightened bolted joint can sustain millions of load cycles without problems, a joint consisting of untightened bolts will frequently fail within a few cycles. The reason for this is the way a bolted joint carries an external load – a fully tightened bolt sustains only a small proportion of any externally applied load.
Thread engagement is the length or number of threads that are engaged between the screw and the female threads. Screws are designed so that the bolt shank fails before the threads, but for this to hold true, a minimum thread engagement must be used.
The following equation defines this minimum thread engagement:
Where Le is the thread engagement length, At is the tensile stress area, D is the major diameter of the screw, and p is the pitch. This equation only holds true if the screw and female thread materials are the same.
The tensile stress area At is calculated by the following formula:
Where π: 3,14, D: the screw major diameter (mm), and p: screw thread pitch (mm).
As always, you can try using fxSolver, with the minimum thread engagement equation, as long as the tensile stress area formula in our database.
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