Steam-bending properties of southern pine

Abstract

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 rating where 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 end made with face of stock--averaged 3.5^o) bent to a radius of 12 times thickness, 75 percent of the 1-inch specimens and 80 percent of the 1/2-inch specimens and bending radius of 12 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^o 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.

Citation

Lemoine, Truett J.; Koch, Peter. 1971. Steam-bending properties of southern pine. Forest products Journal, Vol. 21(4): 34-42