Читать книгу The Handyman's Book of Tools, Materials, and Processes Employed in Woodworking - Paul N. Hasluck - Страница 166

The holding powers of nails, of course, vary with the shape of the nails and the kind of wood into which they are driven. The following statements have been made as the result of tests carried out at Sibley College, Cornell University:—Cut nails are superior to wire nails in all positions, and if pointed they would be 30 per cent. more efficient in direct tension. The advantage of a wire nail is the sharp point; without a point it would have but one-half its ordinary holding power. The surface of the nail should be slightly rough, but not barbed; barbing decreases the efficiency of cut nails by about 32 per cent. Nails should be wedge-shaped in both directions when there are no special dangers of splitting. The length of nails to be used in tension should be about three times the thickness of the thinnest piece nailed. Nails usually hold about 50 per cent. more when driven perpendicularly to the grain than when parallel to it. When subjected to shock, nails have been found to hold less than one-twelfth the dead load they will stand when weight is applied gradually. Professor Soule has proved that, under all ordinary conditions, cut nails hold better than wire nails. Holding effect increases with the length of a nail, but not in proportions expressible by any simple formula. When a cut nail is properly pointed, its hold is increased by 33 per cent. Nails driven into certain woods, Californian redwood for example, will take a better hold the longer they are left in; the reverse happens with other woods, Douglas pine for instance. Deals, when nailed to a block, show about equal holding powers for equal areas of nail, whether a few thick nails or a greater number of thin nails are employed; the advantage is slightly in favour of the thick nails. Haupt, in his “Military Bridges,” gives a table showing the holding power of wrought iron nails weighing 77 to the lb., and having a length of about 3 in. The nails were driven through a 1-in. board into a block from which it was dragged in a direction perpendicular to length of nails. Taking a pine plank nailed to a pine block, with eight nails to the square foot, the average breaking weight per nail was found to be 380 lb.; in oak the power was 415 lb.; with twelve nails per square foot the holding power was 542·5 lb.; and with six nails in pine 463·5 lb. The highest result was obtained for twelve nails per square foot in pine, the breaking weight being 612 lb. per nail. The average strength decreased with the increase of surface. In Bevan’s experiments the force in pounds required to extract “threepenny” brads from dry Christiania deal, at right angles to grain of wood, was found to be 58 lb.; the force required to draw a wrought-iron “sixpenny” nail was 187 lb., the length forced into the wood being 1 in. The relative adhesion, when the nail is driven transversely and longitudinally, is in deal about 2 to 1. To extract a common “sixpenny” nail from a depth of 1 in., in dry beech, across grain, required 167 lb.; in dry Christiania deal, across grain, 187 lb., and with grain 87 lb.; in elm the required force was 327 lb. across grain, and 257 lb. with grain; and in oak 507 lb. across grain. From Lieutenant Fraser’s experiments, it would appear that the holding power of spike nails in fir is 460 lb. to 730 lb. per inch in length; while the adhesive power of “wood screws” 2 in. long and 22·1 diameter, at exterior of threads, 12 to the inch, driven into 1/2-in. board, was 790 lb. in hard wood, and about half that amount in soft wood.