Fig. 4. Remove unnecessary metal for a lighter casting with less strain in the radial veins. This also remedies shrinkage defects.
Fig. 3. If possible, design uniform sections. This will reduce the weight of casting, lower the manufacturing cost and fi x the shrink- age problem.
4 5
PROPORTION Inner Walls Correctly
Inner sections of castings, resulting from complex
cores, cool slower than outer sections, causing varia- tions in strength properties. Reduce inner sections to 0.9 times the thickness of the outer wall to limit the variation (Fig. 4). The inside diameter of cylinders and bushings should exceed the wall thickness of the cast- ing. A casting with an extremely thin core is neither safe nor cost effective, and it is better to cast the sec- tion as a solid. Sometimes, a slight change in design can eliminate a core, make the design simpler and yield significant cost reductions.
KNOW Your Junctions
Consider the stress concentration of X, T, L, K, L-T
Table 2. Property Comparisons for Gray Iron Classes Property
Brinell Hardness Tensile Strength
Modulus of Elasticity Tensile Poisson’s Ratio Compression Poisson’s Ratio
Compression-to-Tensile Strength Ratio
and X-T junctions. Simplify the complex junctions such as X, V, Y and X-T junctions to T or L junctions. To reduce stress concentration, use large fillet radii or concave corner pieces at the junction of cast sections. Use radii equal to section thickness in order to balance stress concentration and design and casting consider- ation. The number of fillet radii in one pattern should be the minimum possible (preferably only one).
6 207
AVOID Bosses, Lugs and Pads
Bosses and pads increase metal thickness, create hot spots and cause open grain or draws. Instead of using bosses to support bolts, use techniques such as milling, countersinking or a continuous rib. If the features have to be used in a design, blend them into the casting by tapering or flattening the fillets. When a design has several lugs and bosses on one surface, the elements should be joined to facilitate machining. Bosses should be uniform in thickness to simplify machining.
Class 25 (as-cast) Class 30 (as-cast) Class 30 (annealed) Class 35 (as-cast) Class 40 (as-cast) 187
109 212 235
29.9 ksi (206 MPa) 33.7 ksi (232 MPa) 20.6 ksi (142 MPa) 34.8 ksi (240 MPa) 41.9 ksi (289 MPa) 16.6 Msi (114 GPa) 17.0 Msi (117 GPa) 14.5 Msi (100 GPa) 18.0 Msi (124 GPa) 18.2 Msi (126 GPa) 0.29 0.27 3.68
0.19 0.28 3.84
36 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2011
0.21 0.26 4.05
0.22 0.28 3.63
0.24 0.23 3.71
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