Structural elements and design methods of mechanical structural parts

01  Geometric elements of structural parts

The function of the mechanical structure is mainly realized by the geometric shape of the mechanical parts and the relative positional relationship between the various parts. The geometry of a part is composed of its surface. A part usually has multiple surfaces, and some of these surfaces are in direct contact with the surfaces of other parts. This part of the surface is called a functional surface. The connecting part between the functional surfaces is called the connecting surface.

The functional surface of a part is an important factor that determines the mechanical function, and the design of the functional surface is the core issue of the structural design of the part. The main geometric parameters describing the functional surface include the geometric shape, size, number of surfaces, position, order, etc. of the surface. Through the variation design of the functional surface, a variety of structural schemes for realizing the same technical function can be obtained.

 02  Links between structures

In a machine or machinery, no part exists in isolation. Therefore, in addition to studying the functions and other features of the parts themselves, the interrelationships between the parts must also be studied in the structural design.

The correlation of parts is divided into two types: direct correlation and indirect correlation. Where two parts have a direct assembly relationship, they become directly related. Correlation that does not have a direct assembly relationship becomes an indirect correlation. Indirect correlation is divided into two types: position correlation and motion correlation. Position correlation means that the two parts have requirements on the mutual position. For example, the center distance of two adjacent transmission shafts in the reducer must ensure a certain accuracy, and the two axes must be parallel to ensure the normal meshing of the gears. Motion correlation means that the motion trajectory of one part is related to another part. For example, the motion trajectory of the lathe tool post must be parallel to the centerline of the spindle. This is ensured by the parallelism of the bed guide rail and the axis of the spindle. Therefore, The position between the spindle and the guide rail is related; the tool post and the spindle are related to the movement.

Most parts have two or more directly related parts, so each part has two or more parts that are structurally related to other parts. In the structural design, the directly related parts of the two parts must be considered at the same time in order to reasonably select the heat treatment method, shape, size, precision and surface quality of the material. At the same time, it must also consider satisfying indirect related conditions, such as dimensional chain and accuracy calculations. Generally speaking, if there are more directly related parts of a part, its structure is more complicated; the more indirectly related parts of a part, the higher the precision requirement.

 03  Problems that should be paid attention to in structural design

There are many materials that can be selected in mechanical design. Different materials have different properties. Different materials correspond to different processing techniques. In structural design, appropriate materials must be selected reasonably according to functional requirements and appropriate materials must be determined according to the type of material. Processing technology, and determine the appropriate structure according to the requirements of the processing technology, only through the appropriate structure design can the selected material give full play to its advantages.

In order to select materials correctly, designers must fully understand the mechanical properties, processing performance, and cost of the selected materials. In the structural design, different design principles should be followed according to the characteristics of the selected material and the corresponding processing technology.

For example, the mechanical properties of steel under tension and compression are basically the same, so the steel beam structure is mostly symmetrical. The compressive strength of cast iron materials is much greater than the tensile strength. Therefore, the cross-sections of cast iron structures subjected to bending moments are mostly asymmetrical, so that the maximum compressive stress during load is greater than the maximum tensile stress. Figure 5.2 is a comparison of two cast iron brackets. In the steel structure design, the strength and rigidity of the structure are usually increased by increasing the cross-sectional size. However, if the wall thickness is too large in the cast structure, it is difficult to ensure the casting quality, so the cast structure is usually reinforced by stiffened plates and partitions. The stiffness and strength of the structure. Due to the poor rigidity of plastic materials, the internal stress caused by uneven cooling after molding can easily cause structural warpage. Therefore, the thickness of the ribs and the wall of the plastic structure are similar and uniform and symmetrical.

For parts that require heat treatment, the requirements for structural design are as follows: (1) The geometric shape of the part should be simple and symmetrical, and the ideal shape is spherical. (2) For parts with unequal cross-sections, the change in size and cross-section must be gentle to avoid sudden changes. If the changes in adjacent parts are too large, the large and small sections will be unevenly cooled, which will inevitably form internal stress. (3) Avoid sharp edges and sharp corners. In order to prevent the sharp edges and sharp corners from melting or overheating, a chamfer of 2 to 3 mm is generally cut on the edge of the slot or hole. (4) Avoid sections with a large difference in thickness, which are easy to deform and have a greater tendency to crack during quenching and cooling.