When the screw rod is matched with the main shaft of the reduction box with a longer cylindrical surface, the screw rod can be used as a cantilever beam with one end fixed. The force state of the screw during the extrusion process can be simplified as shown in the figure:
1. The force of the screw
1) Self-weight G;
2) Torque M required to overcome material resistance;
3) Axial force P produced by material pressure.
The screw is generally scrapped due to long-term wear and the gap between the screw and the barrel is too large and cannot be extruded normally. However, there are also examples of damage caused by improper design or operation caused by working stress exceeding the strength limit. Therefore, the screw should also meet certain strength requirements.
4) The dangerous section of the screw is generally at the smallest thread root diameter in the feeding section.
According to material mechanics, for plastic materials, the composite stress is calculated by the third strength theory, and its strength conditions are:
2. Strength calculation
3. Other situations
1) In the case of a floating connection between the tail of the screw and the main shaft of the reduction box, because the screw is floating in the barrel, the bending stress caused by the weight of the screw is equal to zero, so the calculation is only based on the compression stress and shear stress of the screw.
2) The bending stress caused by the weight of the screw is very small (the screw is full of materials), even in the former case, it can be omitted, so the two methods are actually the same.
3) There are also methods to estimate the screw diameter by pure torque.