Steam-Bending Properties of Southern Pine

Southern pine wood can be successfully steam-bent if the bending jig incorporates a flexible metal bending strap together with a mechanism to apply a uniform end-compression load during the bending operation. With clear, 1/2- and 1-inch thick southern pine at 17-percent moisture content, highest bending ratings, were obtained with fast-grown, vertical-grain, low-density wood free of cross grain. With this optimum stock (cross grain–as measured by the maximum angle that resin canals on either edge made with face of stock–averaged 3.5?) bent to a radius of 12 times thickness, 75 percent of the 1-inch specimens and 80 percent of the 1/2-inch specimens had bending ratings of 8 or above on a scale that ran from 0 (worst) to 10 (best). With optimum stock bent to a radius of 12 times thickness plus 3 inches, 85 percent of the half-inch and 68 percent of the 1-inch specimens rated 9 or 10. To obtain 75-percent yield, of near perfect pieces, a bending radius of about 16 times thickness was required. A steaming time of 20 minutes per inch of thickness was adequate. Placement of pith side of specimen–toward or away from concavity–had no significant effect on the bending ratings. When restraint was removed from specimens previously bent 180? over a semicircular form and dried under restraint, immediate increase in diameter averaged 0.5 inch. If then stored unrestrained in a water-saturated atmosphere, they further increased substantially in diameter (average of 2.5-inch increase after 4 weeks’ exposure). If stored unrestrained in a dry atmosphere for 4 weeks, however, specimens contracted slightly (0.2 inch). Best stability was obtained with vertical-grain wood steamed 20 minutes (compared to 10 minutes) per inch of thickness. one-half-inch stock bent to a 6-inch radius was more stable than if bent to a 9-inch radius; it was also more stable than 1-inch stock bent to a 15-inch radius.

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