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details DESIGN


Stiff Requirements Guide Casting Conversion


JITEN SHAH, PRODUCT DEVELOPMENT & ANALYSIS, NAPERVILLE, ILLINOIS CASTING PROFILE


Cast Component: Front truck cab support bracket. Casting Process: Green sand.


Material: 80-55-06 ductile iron. Weight: 35.3 lbs. Dimensions: 18 x 16 x 9 in.


Iron Works, Lethbridge, Canada, represented a consolida tion of three castings, seven welded assemblies and 16 parts into two castings, zero welded assemblies and four low-level parts. Changing the material from 65-45-12 to 80-55-06 ductile iron increased both ultimate tensile and yield strength by about 23%. With wall thicknesses of 0.25 in., the new com ponent is 7 lbs. lighter than its 42-lb. predecessor. Intricate coring and robotic grinding provided the customer the near- net-shape part without machining. T e total cost reduction was 35%.


D


aimler Trucks, Portland, Ore., wanted to consolidate the front bracket for its mid-size and large trucks into a one-size-fi ts-all design. To do so, it needed to be 10 times stiff er than the multiple- piece welded tube structure used on the Western Star platforms. T e fi nal part design delivered by casting supplier Lethbridge


Generous radii and fi llets provide smooth transitions at intersecting junctions and web corners.


• Stresses in most structural components, especially those loaded dur- ing bending, are highest on the surface. Smooth transitions at inter- secting junctions and web corners help reduce the stress concentra- tions to make the casting more durable during fatigue loading.


• In good casting design, fi llets and radii are driven by the minimum thickness of the two adjoining sections, typically 0.125 to 0.25 in.


Cross-ribs stiffen the structure.


• To eliminate tooling complexity, cross-ribs are located perpendicular to the parting plane of the pattern and corebox so they can be drawn from the mold without creating back draft, which would require a core or loose piece.


• Cross-ribs should be thinner than the base casting section so they can be fed completely by the liquid metal.


22 | METAL CASTING DESIGN & PURCHASING | Mar/Apr 2012


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