CNC Knowledge: Screw force analysis

There are many ways to estimate the allowable torque of a screw, and the shape is not a regular cylinder, so the values ​​vary. Finite element analysis can handle such complex shape constraint calculation problems.

The screw material is 38CrMoAl, with a tensile strength of 1000 MPa and a yield strength of 835 MPa.

The screw diameter is 20mm and the screw grooves are 4.5 and 3.5mm respectively for comparison.

According to the calculated screw torque load, the values ​​are 100 NM and 150 NM respectively.

The torque acts on the screw and the maximum stress must be at the minimum diameter of the screw. Therefore, an accurate analysis is performed on the screw root and rough calculations performed at other locations do not affect the accuracy of the results.

As shown in the figure, the mesh at the root of the screw is refined to ensure the calculation accuracy.

1) Screw groove depth 4.5mm, torque load 100NM

At present, the root stress is 782 MPa, which is close to the yield. The safety factor is approximately 1.

2) The screw groove depth is 4.5mm, the torque load is 100NM, and the screw edge is beveled.

The front end surface of the spiral edge changes from a straight surface to an oblique side surface, with an angle of 15°. At this point, the stress at the base of the screw decreases slightly.

3) The screw groove depth is 3.5mm, the torque load is 150NM, and the screw edge is beveled.

Increasing the diameter of the screw root can effectively reduce stress and improve the load capacity of the screw.

4) The screw groove depth is 3.5mm, the torque load is 100NM, the screw edge is beveled, and the safety factor is 1.5.

It’s safe at the moment, but the load capacity is only 100NM and the screw cannot work normally most of the time.

Summary: Considering the screw power and load capacity, the screw groove depth is 3.5mm, the load capacity is 150NM, and the safety factor is about 1. Or 3.2mm threaded to provide more security redundancy.