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Maximize Design of Ribs and Brackets

Ribs have two functions: to increase stiffness and to reduce weight. If they are too shallow or too widely spaced, they can be ineffective. The thickness of ribs should approximate 80% of the adjoining thickness and should be rounded at the edge. In other words, thin ribs should be avoided when joined to a heavy section or they may lead to high stresses and cracking. The design preference is for the ribs to be deeper than they are thick; the ribs should solidify before the casting section they adjoin. In addition, the space between ribs should prevent any localized accumulation of metal.

In general, ribs in compression offer a greater safety factor than ribs in tension. However, castings having thin ribs or webs in compression may require design changes to provide necessary stiffening and avoid buckling.

Avoid cross ribs or ribbing on both sides of a casting. Cross ribbing creates hot spots and makes feeding difficult. Instead, design cross-coupled ribs in a staggered double "T" form. Avoid complex ribbing, which complicates molding, hinders uniform solidification and creates hot spots. Consequently, ribs are only preferable when the casting wall cannot be made strong or stiff enough on its own.

Ribs meeting at acute angles also may cause molding difficulties, increase costs and aggravate the risk of casting defects. "Honeycombing" often will provide increased strength and stiffness without creating hot spots (Fig. 7).

Brackets carrying offset loads introduce bending moments—localized and in the body of the casting. As a remedy, taper "L"-shaped brackets and make the length of contact with the main casting as ample as possible. As another option, brackets may frequently be cast separately and then attached, simplifying molding.

A ribbed bracket will offer a stiffness advantage, but avoid heat concentration by providing cored openings in webs and ribs. Such openings should be as large as possible, and consistent with strength and stiffness. Avoid rectangular-shaped cored holes in ribs or webs; use oval-shaped holes with the longest dimension in the direction of the stresses.

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10 Rules for Engineered Quality

Visualize the Casting in the Mold
Design for Soundness
Avoid Sharp Angles and Corners
Minimize the Number of Sections
Employ Uniform Sections
Correctly Proportion Inner Walls
Fillet All Sharp Angles
Avoid Abrupt Section Changes
Maximize Design of Ribs and Brackets
Avoid Using Bosses, Lugs, and Pads

This article was adopted from a section of Casting World Magazine. Reprinted with permission.