This article combines actual experience of structural optimization design of precision castings. It analyzes influence of six different types of "ribs" on advantages and disadvantages of product structure, and refines "rib" arrangement method suitable for precision casting structure, which has certain guiding significance for optimization design of precision casting products.

 

In recent years, product designers have successfully realized secondary lightweight transformation of some precision castings using various process techniques and have achieved certain results; however, due to lack of systematic summary of design techniques and uneven design level of designers, there are often certain regrets in product structure.
In common force-bearing castings. Ribs are main channel for force transmission, and more than 70% of load is borne by ribs. Arrangement of ribs has become key to lightweight design of castings and determines strength of parts; industrial beauty of castings depends on their appearance, and ribs are main component of casting appearance. Arrangement of ribs has also become a standard for measuring whether it reflects industrial aesthetics. Therefore, layout of ribs directly affects strength and beauty of parts, determines competitiveness of products.
How to make ribs of castings stand out, be both rigid and flexible? How to use the most limited ribs to carry maximum load? This is a topic that casting structural designers will always study.

Traditional ribs are mainly straight ribs, which are generally not recommended; "S" type, "U" type, cross type, trapezoidal, and annular ribs are widely used in lightweight design of castings.

So-called straight ribs are defined as ribs that are mainly connected by straight lines, have uniform wall thickness, are rigid and blunt, similar to triangular ribs "△" and rectangular ribs "□".
This type of rib has simple three-dimensional modeling and convenient two-dimensional drawing processing. It is preferred type for many domestic technical engineers, as shown in Figures 1 and 2. However, due to following disadvantages, it is generally not recommended for lightweight design of castings.
Disadvantage 1: Transition of ribs is unnatural, which leads to obstruction of expansion and contraction of casting, and stress concentration during pouring, heat treatment and correction process is prone to cracks;
Disadvantage 2: Material distribution is too uniform, force flow transmission is not considered, which violates principle of lightweight lean design;
Disadvantage 3: Structure is rigid and lacks industrial aesthetic elements.

 

Shape of "S" ribs is similar to a large "S". According to different structures of parts, composition form is slightly different. It is generally divided into two categories: one is composed of three large arcs; the other is composed of large arcs at both ends and straight sections in the middle gate. General layout of this type of ribs is:

a) Used for strengthening connection between single plates or single columns, as shown in Figure 3, a compressor bracket and flip bracket. Ribs on both sides are similar to a large "S", which is an evolution and upgrade of near-triangular ribs, with a smooth and natural transition. It is recommended that triangular reinforcement ribs be arranged in this way!

 

Figure 3: "S" ribs
b) Used for large plane strengthening and segmentation, as shown in Figure 4, a commercial vehicle front axle leaf spring bracket. Bottom ribs are "S" shaped, dividing large plane, strengthening bottom surface and preventing casting defects such as bulging and depression on large plane. This solution can be considered for layout of ribs on large plane!

 

Figure 4: Plane "S" ribs

"U" ribs are similar to "U" shaped, usually composed of three arcs, middle arc is a large arc (diameter is generally more than 3 times that of two ends), which is evolution and upgrade of "rectangular ribs" and is usually used for connecting two columns or two plates. As shown in Figure 5, it is a passenger car compressor bracket, which is used to connect two cylinders; as shown in Figure 6, it is a commercial vehicle flip bracket. It is used to connect two plates.

 

Cross ribs refer to ribs that cross each other. Common types include "cross", "T", "L", etc. This type of ribs is usually used for web or large plane reinforcement. Its layout should systematically consider transmission of force flow, treatment of casting heat nodes, and appearance of appearance.
a) Cross-shaped type, as shown in Figure 7, is a commercial vehicle oil cylinder upper bracket.

 

Figure 7: Cross-shaped type
A area is a traditional cross-shaped rib, and intersection of ribs forms a heat node, which will cause serious shrinkage and shrinkage; B area avoids direct intersection of ribs in the form of process holes, while ensuring strength of part, heat node is limited to reduce internal quality of part.
b) T-shaped cross-shaped type, as shown in Figure 8, is a commercial vehicle double front axle leaf spring bracket

 

Figure 8: T-shaped cross-shaped type
Four ribs in the A, B, C, and D areas intersect to form a "T" shape. If simply connected (as shown in upper left picture), four heat nodes will appear at intersection, which is prone to shrinkage; in order to reduce heat node and eliminate casting defects, intersection is often connected by a circular arc transition to make wall thickness uniform.
c) "L" cross rib
According to structure of part, this type of rib is often transformed into an "S" rib after being processed with large arcs at both ends, which will not be repeated here.

Trapezoidal ribs refer to ribs with a shape similar to a trapezoid. Their function is to remove excess material and eliminate casting heat nodes. They are usually set at intersection of parts corners. As shown in Figure 9, a passenger car leaf spring bracket has a large heat node at intersection of rib and plate, which is prone to shrinkage. By opening process holes to form trapezoidal ribs, material is minimized while ensuring strength of parts, and heat node is effectively eliminated.

 

Figure 9: Trapezoidal ribs

Annular ribs refer to ribs with a structure similar to that of an annular shape. They are mainly used for flanging reinforcement of process holes and process grooves, see Figures 10 and 11.

 

Arrangement of lightweight "ribs" breaks traditional design model and provides a reference for lean design of precision castings. Through more than a hundred case practices, following conclusions are drawn:
(1) Arrangement of "ribs" should take into account distribution of force flow. In force flow transmission path, its cross-section is variable and non-uniform thickness;
(2) Arrangement of "ribs" should follow principle of uniform wall thickness to minimize thermal nodes, avoid shrinkage cavities and shrinkage;
(3) Arrangement of "ribs" should give priority to "arc-shaped" ribs to improve product yield, avoid generation of heat treatment and correction cracks;
(4) Arrangement of "ribs" should have both industrial aesthetics, prominent ribs, a good balance of rigidity and flexibility.