[Guide] For motor products, when the shaft has different size sections, the stress will be concentrated in the abrupt change of the cross-sectional area, which is the weak link of the entire shaft. In order to improve this phenomenon, round corners are used for transition at the sudden change of the cross-sectional area of the shaft to improve the overall comprehensive ability of the shaft.

The stress problem in elastic mechanics refers to the phenomenon of locally increased stress in an object, which generally occurs in places where the shape of the object changes drastically, such as gaps, holes, grooves, and rigid constraints. Stress concentration can cause fatigue cracks in objects, and static load fracture of parts made of brittle materials. At the stress concentration point, the large value of the stress, that is, the peak stress, is related to the geometry and loading method of the object. The locally increased stress decays rapidly with increasing distance from the peak stress point. Since the peak stress often exceeds the yield limit, resulting in stress redistribution, the actual peak stress is often lower than the theoretical peak stress calculated by elastic mechanics.

For motor products, when the shaft has different size sections, the stress will be concentrated at the abrupt change of the cross-sectional area, which is the weak link of the entire shaft. In order to improve this phenomenon, round corners are used for transition at the sudden change of the cross-sectional area of the shaft to improve the overall comprehensive ability of the shaft.

The working capacity of shaft parts (step shafts, crankshafts, etc.) that are subjected to alternating bending and torsional stress is usually determined by the ability of the shaft to resist fatigue damage caused by alternating stress. Practice has proved that fatigue failure often occurs in parts where stress is concentrated when parts are working, that is, at the transition groove angle of shaft parts. Therefore, various measures are often adopted in the structural design of the shaft to reduce the stress concentration and ensure the fatigue strength of the shaft. The main measure to reduce the stress concentration on the shaft is to increase the radius of the transition arc at the corner of the groove. In general design, the radius of the transition arc is not less than 0.05 times the diameter of the shaft.

1. Motor knowledge expansion 1 Measures taken to reduce and avoid stress concentration

In order to avoid the damage of materials or components due to stress concentration, the following measures are mainly adopted in engineering:

●Surface strengthening. Shot peening, rolling, nitriding and other treatments on the surface of the material can improve the fatigue strength of the surface of the material;

● Avoid sharp corners. That is, change the corners to excessive rounded corners, and appropriately increase the radius of the transition arc, the effect is better;

●Improve the shape of parts. The shape of the radius of curvature gradually changing is conducive to reducing the stress concentration factor. The ideal method is to use a streamline or double curvature, which is more convenient for engineering applications;

●Local reinforcement of the hole edge: the use of a reinforcement ring or a local thickening at the hole edge can reduce the stress concentration factor, and the degree of decrease is related to the shape and size of the hole, the shape and size of the reinforcement ring and the load form;

●Appropriately select the location and direction of the opening: the location of the opening should avoid the high stress area as far as possible, and the increase of the stress concentration factor due to the mutual influence between the holes should be avoided. For the elliptical hole, the long axis should be parallel to the direction of the external force , Which can reduce the peak stress;

●Increase the stress in the low stress area, reduce the thickness of the part in the low stress area, or increase the gap or round hole in the low stress area, so that the transition of the stress from the low stress area to the high stress area tends to be smooth;

●Using residual stress: When the peak stress exceeds the yield limit, unloading will produce residual stress, and reasonable use of residual stress can also reduce the stress concentration factor.

2. Stress mitigation measures adopted in the design process

At the abrupt change of size, if the fillet transition is not adopted, the stress will increase infinitely in the finite element analysis. In fact, in structural design, in order to reduce the stress concentration factor, there are some basic design rules, briefly described as follows:

Modify shape

●Rounded corners: Sharp corners are strictly prohibited in components. It is known from theoretical analysis that when the radius of curvature of the rounded corners approaches zero, its stress concentration coefficient tends to infinity. Replacing sharp corners with rounded corners can effectively relieve stress concentration.

● Streamlined. For the tension rod or compression rod with variable cross-section, if streamlined transition is adopted, the stress of the component can be made uniform, thereby avoiding stress concentration.

●Oval hole. On the premise of ensuring the normal operation of the component, turning the round hole into an elliptical hole can often improve the strength of the component.

3. Stress control method during drilling

●The stress concentration factor should be selected in the place with low stress. For example, in the punching position, do not place the hole on a section with a large bending moment; do not place the hole near the edge.

● It is necessary to consider whether there will be interference with the parent body, resulting in increased stress. If the position of the hole is particularly close to the edge, the problem of stress concentration will increase.

●When making long holes, choose the long axis of the hole to be consistent with the long side of the base, and the stress will be smaller.

●If the hole is added near the processed hole, the stress coefficient will decrease.

● Digging a certain thickness of material on the upper and lower sides of the hole will reduce the rigidity of this part to achieve the purpose of easing stress concentration. In these figures, the stress concentration factor is small in the latter case.

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