Strumento per il calcolo delle sollecitazioni di viti e bulloni | Analisi dei giunti strutturali
The difference between Screw and Bolts explained.
A screw is usually installed into a tapped hole, unless it is a self-tapping screw which creates its own thread. Screws don’t need nuts, as they become secure by being tightened into the hole with a screwdriver or driver bit which fits into the drive recess.
Bolts are designed to be installed with a suitable nut. The hole for a bolt is not tapped as the bolt is pushed through and is fixed and tightened using a nut at the back of the material being fastened. So a bolt will be longer than the width of the material it is being used on, as it needs to protrude through to the other side to screw into the nut.
Screw and Bolt Stress check tool under preload and combined load.
Given the axial preload (you can calculate it with our free preload calculator here), it is possible to perform a stress check of your bolt/screw connection under external load. For the bolt/screw stress check tool to work, you only need the following:
- External loads (axial and shear loads)
- Yield strength of the material of the bolt/screw. You can find a datasheet of Yield Strength for the most common materials qui.
- Nominal diameter of resistant minimum section of the bolt/screw
- Shear factor of the section (1.33 for circular sections).
The tools work with both Metric/International Units and Imperial Units. If a safety factor < 1.5 is revealed, the joint connection may be unsafe. In case the output on the compressive force is negative, the joint connection is likely to open under the external axial load (in general because external axial load > preload force).
Von Mises Criterion Explained for Bolt and Screw fasteners.
Von Mises stress criterion is an equivalent stress value based on distortion energy to decide if a ductile material will fail (yield or fracture) under a given loading condition. The Von Mises failure theory indicates that a material will fail if the Von Mises stress or effective stress of that material under load is equal or greater than the yield limit of the same material under a simple uniaxial tension test. Usually the preload force is set at 75% yield stress of the joint material.
Why Von Mises stress is so unsed in industry.
The Von Mises theory predicts ductile yielding with more accuracy as compared to the maximum shear stress theory. It is more real and less conservative than maximum shear stress theory and hence, product cost reduces. During the mechanical design of elements, it is the duty of every engineer to keep the Von Mises Stress (σv) value below the yield strength (σy) of that material to make the design safe. For this reason the theory finds wide application in Finite Element Analysis (FEM).
La general mathematical formula of the Von Mises Stress is:
The above mathematical expression can be expressed also in terms of principal stress. For more info, please refer to this link.