Abstract
Wood softened by the action of moisture and heat or by other agencies can be upset or shortened very considerably, but can be stretched or lengthened but little. Hence, since the concave and convex faces of a bent piece differ in length, it is necessary in bending to prevent stretch and to cause the entire difference in length to take place as upset, this upset being a maximum at the concave face and decreasing to zero near the convex face. This can be accomplished by supplying a metal strap with securely attached end fittings through which the compressive force, or end pressure, can be applied. The end pressure must be distributed over the ends of the stock so that localized crushing does not occur, and the rotation of the end fitting must be restrained to obviate release of the end pressure. The common arrangement of parts of wood-bending apparatus does not fulfill these requirements. The requirements can be attained, however, by utilizing the principle of the reversed lever.
The distance between end fittings cannot be made exactly the same for a number of straps and cannot be kept constant, neither can pieces of wood be cut exactly to the required length. Hence, means for adjusting the distance between end fittings is necessary. Pieces whose width is considerably less than their depth must be restrained to prevent sidewise bending and the formation of lateral crooks and buckling.
This paper, which is based on studies at the Forest Products Laboratory, deals only with the requirements of bending apparatus and manipulation, emphasizes their importance, and points out that they must be correct before the large breakage losses incurred in the bending of wood can be reduced materially.
