12 Structural Design Guidelines Related to Mechanical Requirements

Structural Design Guidelines:

 

1. Uniform loading criterion:

Try to avoid concentrated loads and distribute the loads on the structure as much as possible, and even distribution is ideal.

 

The strength of the structure depends on the maximum stress in the structure. It can be seen that making the structure evenly loaded can achieve the purpose of improving its strength, that is, its bearing capacity.

 

2. The shortest path criterion for force flow (analogous to the flow of water):

 

The most important feature of force flow is that the force flow preferentially follows the shorter path, more precisely the path with the greatest stiffness.

 

The path of ensuring the force flow is shorter, which can usually also improve the strength. Therefore, the closer the force flow path is to a straight line, the smaller the additional bending moment caused by the force, and the smaller the corresponding bending stress. The shape is the most ideal state of stress. The more the force line deviates from the linear shape, the greater the increase in stress.

 

The shortest path criterion for force flow, that is, the distance from the point of application of the force (the entrance of the force) to the support point of the structure (the exit of the force) is required to be as short as possible.

 

Relevant engineering application examples:

 

1) The gear on the gear shaft should be installed as close to the bearing as possible when the structure design allows it;

 

2) When the workshop is overloaded, if it is lifted as close to the track as possible, the lifting weight can be doubled;

3) For a simply supported beam bearing a uniform load, if the supports at both ends are moved inward by 0.2L, the maximum bending moment is only 20% of the former, which means that the bearing capacity of the structure is increased by 5 times.

 

3. Criteria for reducing the gap effect:

 

Notches such as holes, grooves, threads, shoulders, etc. These sudden changes in shape lead to sudden changes in force flow, and the stress rises sharply. This phenomenon is called the notch effect.

 

The sharper the change in section size, the smaller the chamfer at the top of the notch, and the stronger the notch effect.

 

The notch effect is not only related to the geometry of the notch, but also to the stress state of the component, because the root cause of the notch effect is that the force flow is forced to change its original path sharply, which leads to the Яο 哂 guide in the near road bureau due to the force flow grabbing the near road. Penalty is an increase in stress levels.

 

The characteristic of the notch effect is local. Under the static load, the plastic material is insensitive to the notch effect because it has a yield stage, and the brittle material will easily cause fracture.

 

Ways to reduce the gap effect:

 

1) Avoid shape mutation;

 

2) Reduce notch attachment Ц cut caries

 

3) Avoid sudden reduction of force flow section;

 

4) Add pre-compression internal stress;

 

5) Avoid sudden turns of force flow;

 

4. Deformation coordination criteria:

 

Stress concentration not only occurs at the notch inside a member, but also may occur in two different members. When one member and the other member are difficult to deform simultaneously at the contact point, the stress will rise sharply, and the more uncoordinated the deformation is. , the more serious the stress concentration is.

 

Reduce the stiffness of the component in the direction of force flow at the contact point, so as to reduce the obstruction to the deformation of the other component, and try to synchronize the deformation of the two components, which is the deformation coordination criterion.

 

5. Equal strength criterion:

 

The strength requirements in the component design are met by the maximum working stress in the structure being equal to or less than the required stress of the material, so that the stress outside the maximum working stress section does not reach the allowable value, and the material is not fully utilized, resulting in material damage. Waste, bulky components, and high-speed moving bulky parts consume more energy.

 

The ideal component design is that the stress is equal everywhere, and the allowable value of the material is reached at the same time, which is the equal strength criterion.

 

A large number of variable-section beams appearing in engineering are designed according to the equal-strength criterion.

 

Project example:

 

1) The horizontal arm of the radial drill;

 

2) Leaf springs for automobiles;

 

3) Step shaft;

Points to note when designing components according to the equal-strength criterion:

1) In order to facilitate manufacturing, the shape of components designed strictly according to the equal strength criterion is usually very complex, which is inconvenient to manufacture, and may not necessarily meet the structural requirements, so in practice, components with similar shapes are often manufactured;

 

2) Pay attention to the effect of secondary load, and in general design, it is negligible.

 

For example, when designing the section of a cantilever beam according to the equal-strength criterion, it is not enough to only consider the effect of the bending moment. If so, at the free end, the cross-sectional area should be zero, which obviously cannot meet the shear stress strength conditions, so it should be determined according to the shear stress strength conditions. Section height near the free end.

 

6. Additional force self-balancing criterion:

 

There are two main measures for force self-balancing: balancer and symmetrical placement.

 

7. Criteria for hollow section:

 

The bending stress or torsional stress is very small in the center of the cross-section in the gap between the two sides. In order to make full use of the material, the material should be placed in the center of the cross-section as much as possible to make it a Hollow structure, which can improve the strength and stiffness of the member, this is the hollow section criterion.

 

Project example:

 

1) When the strength of the automobile transmission shaft is the same, the mass ratio of the solid shaft to the hollow shaft is 3. It can be seen that the use of the hollow shaft can save a lot of materials and reduce the self-mass.

 

2) Sectional shapes with the same cross-sectional area but different flexural and torsional stiffness.

 

The wall thickness of the hollow structure should not be too thin, otherwise local wrinkles will easily occur and the bearing capacity will be lost.

 

8. Closed quasi-mantle of torsion section

 

The section of the thin-walled member subjected to torsion should try to avoid being made into an open shape (open structure, low torsional rigidity.), otherwise, its torsional resistance will be greatly reduced.

 

Profile steel is basically a thin-walled open member. When it is used for torsion resistance, it should be enclosed with steel plates or used in pairs to ensure that the cross section is closed.

 

For other considerations, the open thin-walled structure is sometimes used as a torsion member, such as a truck chassis, which is composed of two channel steels connected by a transverse plate. At this time, special attention should be paid to the connection method of the transverse plate.

 

9. The best focus point criterion:

 

The location of the force point also affects the load-carrying capacity of the member.

 

(1) When the force vector does not pass through the torsion center on the cross-section of the curved beam headland, additional torque will be generated. Therefore, this situation should be avoided as much as possible. The torsion center of the symmetrical cross-section is on its symmetrical plane, and the horizontal The torsion center of the asymmetrical section steel can be found in the section steel geometric properties table. The torsion center of many sections is outside the cross section. Therefore, in order to keep these sections in a state of only bending, you must add accessories or use them in pairs. The torsion center in the cross section.

 

(2) For nodes where multiple forces act at the same time, each force vector should be converged in one place as much as possible, so as to avoid additional bending moment and reduce the stress level.

 

10. Criteria for structural flexibility under impact load:

 

Usually, the stiffer the component, the greater the stiffness and the higher the strength, but this is not always the case. Sometimes the stiffness of the structure affected by the impact load increases but leads to a decrease in its strength. This is because the impact load increases with the increase of the structural stiffness. Big.

 

Engineering application example:

 

When the grinding wheel brakes suddenly, the shaft is subjected to impact torque, which increases the length of the shaft, and the impact torque also decreases under its torsional stiffness, so the shear strength of the shaft increases instead.

 

In order to improve the impact resistance of the component, the stiffness of the system should be reduced and its flexibility should be increased, which is the structural flexibility criterion under impact load.

 

Specific measures include:

 

(1) Increase the length of the constant-section rod. such as cylinder head bolts;

 

(2) Avoid sudden changes in cross-section. For the bolt to ensure its equal section, the diameter of the polished rod part is often made equal to the inner diameter of the thread, or a hole is drilled in the screw.

 

(3) Install a buffer to absorb the energy of the shock system.

 

(4) Select materials with small elastic modulus. For example, wood structures have stronger impact resistance than steel structures.

 

11. Criteria for avoiding long compression rod instability:

 

In terms of the mechanical properties of their material, metal components are safer in compression than in tension, or at least as safe.

 

The yield point and elongation of cast iron are higher under compression than under tension, sometimes as high as 4 to 5 times. The mechanical properties of other brittle materials are similar, but metal components are not always safer in compression than in tension.

 

Because its load-bearing capacity depends not only on the material of the structure but also on its shape.

 

There are many slender pressure rods in machinery, such as the screw of the jack, the connecting rod of the internal combustion engine and the screw of the lathe, etc. These slender components cannot be designed according to the strength conditions.

 

To improve the stability of the pressure rod, the specific measures are:

 

(1) Increase the moment of inertia of the section. The buckling failure is manifested as a bending failure, and the material should be placed on the outer edge as much as possible.

 

(2) Reduce the length of the pressure rod. If the working conditions do not allow, the method of increasing the intermediate support can be adopted.

 

(3) Strengthen the restraint of the support. When the pressure rod is connected to its p-member, it should be made as rigid as possible or a tighter fit should be used.

 

(4) The optimal combination of section shape and restraint mode.

 

The pressure rod is always unstable in the longitudinal plane with small stiffness, so the stiffness in each longitudinal plane should be the same or close. When the supports at both ends of the pressure rod are column hinges, a rectangular or I-shaped section should be used.

 

(5) Reasonable selection of materials. For small and medium compliant rods in the elastic-plastic stage, high-strength steel can be used to improve their stability; for large compliant rods, high-strength steel cannot improve their stability, so economical ordinary steel should be used.

 

12. Thermal deformation free criterion:

 

Metal components have the characteristics of thermal expansion and contraction. When thermal deformation is limited, thermal stress will be generated. The purpose of reducing thermal stress cannot be achieved by increasing the cross-sectional size of the component. (installation and use temperature are different)

 

The fundamental measure to reduce thermal stress is to ensure the freedom of thermal change as much as possible.