Machine Design

Design Considerations

Several structural design considerations should be taken into account for economical and efficient welding. Many of these apply to other joining methods, and all apply to both subassemblies and the complete structure.

Recognize and analyze the design problem: Designs must perform well under expected and worst-case conditions. The designer should consider this before sitting down at the drawing board or CAD terminal. Considerations include: Is it more economical to build an irregular shape from welded pieces or to cut it from a plate, with the accompanying waste? Can bending replace a welded joint? Are preformed sections available? How, when, and how much should the structure be welded? Can weight be reduced cost-effectively by using welded joints? Will fewer parts offer equal or better performance?

Determine load conditions: Structures will be subject to tension, compression, torsion, and bending. These loads must be calculated under service conditions. Locations of critical loads must be determined and the structure designed to handle the loads efficiently. Careful designers will locate joints away from high-stress areas when possible.

Consider producibility:The most elegant design is useless if it cannot be made efficiently. Welders cannot always fabricate what designers think up. Designers should spend time in the shop and consult foremen or manufacturing engineers during design to become familiar with the challenges of translating drawings into products.

Optimize layout: When drawing the preliminary design, engineers should plan layout to reduce waste when the pieces are cut from plate. Preformed beams, channels, and tubes also may reduce costs without sacrificing quality.

Anticipate plate preparation: Many designers assume that metals are homogeneous, but real-world metal does not have equal properties in all directions. Therefore, the type of plates used should be considered.

Many properties of rolled plates are directional, with the most desirable properties in the direction of rolling. Strength and ductility often are low through the thickness because nonmetallic impurities in many plates weaken them in that direction. Thus, designers should avoid loading rolled plates through their thickness.

One way to get around through-thickness problems is called buttering. Joint surfaces are gouged or ground out and refilled with weld metal, so welds are made against weld metal instead of base metal. The properties of weld metal do not vary so much with direction because the weld metal was never rolled.

Designers must also plan for residual stresses in joints from weld shrinkage. These stresses may cause lamellar tearing in the base metal, especially in thick plate weldments.

In addition to inherent metal properties, designers must consider how plates must be prepared for welding. Parts must be thoroughly cleaned before they are welded, and some joints require machined bevels or grooves.

Consider using standard sections and forms: Preformed sections and forms should be used whenever possible. Specifying standard sections for welding is usually cheaper than welding many individual parts. In particular, specifying bent components is preferable to making welded corners.

Select weld-joint design: There are five basic types of joints: butt joints, corner joints, T-joints, lap joints, and edge joints. In addition, the American Welding Society recognizes about 80 different types of welding and joining processes. Each process has its own characteristics and capabilities, so joint design must be suitable for the desired welding process. In addition, the joint design will affect access to the weld.

Restrain size and number of welds: Welds should match, not exceed, the strength of the base metal for full joint efficiency. Overwelding is unnecessary, increases costs, and reduces strength.

Welding sometimes induces distortions and residual stresses in structures. It is best to specify the minimum amount of welding needed. To check for overwelding, determine joint stresses versus stress in the adjoining members.

When full-penetration joints through the thickness of the material are used, weld-metal strength must equal or exceed that of the base metals. Fillet welds must be appropriately sized to provide full strength. To determine adequate fillet size or depth of penetration, calculate stresses and joint loads, as well as the safety factor required. Designers often specify much bigger fillet welds than codes demand even with a safety factor. This adds to cost and distortion.

Use subassemblies: Whenever possible, large assemblies should be made in smaller sections before final welding. Subassemblies are easier to transport, position, and access for welding. They also permit some distortion control at intermediate stages of fabrication.

TAGS: Fasteners
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