CLINCHING Of NAILS IN CONTAINER CONSTRUCTION

Transcription

1 / 7 '7 CLINCHING Of NAILS IN CONTAINER CONSTRUCTION March 1948 \143Rmii) JAN No: UNITED STATES DEPARTMENT OF AGRICULTURE FOREST SERVICE LIOREST PRODUCTS LABORATORY Madison 5, Wisconsin In Cooperation with the University of Wisconsin

2 CLINCHING OF NAILS IN CONTAINER CONSTRUCTION- BY E. H. BORKENHAGEN, Engineer and H. J. KUELLING, Engineer Forest Products Laboratory2 Forest Service U. S. Department of Agriculture Summary Clinching of nails has always been advocated by the U. S. Forest Products Laboratory as an accepted method of strengthening joints of wood boxes and crates, but nobody has ascertained just how much the joint strength was improved by clinching. Recent tests made at the Forest Products Laboratory are a step toward clearing up this uncertainty. These tests show that single nails have from 45 to 464 percent more holding power when clinched than when unclinched, depending upon the state of dryness of the wood when they are driven and with drawn. Nails clinched across the grain of the wood have about 2 percent greater holding power than nails clinched with the grain. Introduction Among the many ideas advanced from time to time to improve the strength of join' of wood boxes and crates, clinching is probably the oldest and simplest. Clinch. ing has long been called for in certain phases of box and crate construction. Crate specifications require that, when the sheathing boards are fastened to the frame members and diagonals, the nails used shall be long enough to penetrate both thicknesses of material and extend beyond 1/4 inch for clinching. Box specifications require the clinching of nails when fastening cleats to the ends of nailed wood boxes and when fastening cleats to the panels of cleated plywood or cleated fiberboard boxes. Recognizing the obvious fact that individual hand clinching is laborious, some manufacturers devised methods that reduced this labor. In crate construction, a metal plate is used under the pieces being nailed together, with the nails driven at a slight angle so that the nail points, upon emergence from the wood, 1 -Published in Shipping Management, March, Maintained at Madison 5, Wis., in cooperation with the University of Wisconsin. Report No. R Agriculture-Madison

3 strike the plate and are bent into a clinch. In box construction, nailing machines are usually used in the nailing operations and most of these machines are equipped with automatic clinching bars for that purpose. As a basis for evaluating the true efficiency of clinched versus unclinched nails, the U. S. Forest Products Laboratory has conducted tests of single nails driven into and through two pieces of wood. These tests demonstrated that, under the conditions studied: (1) Clinched nails had from 56 to 161 percent more holding power than unclinched nails when driven into and pulled immediately from airseasoned lumber (moisture content about 11 percent). (2) Clinched nails had from 45 to 173 percent more holding power than unclinched nails when driven into and pulled immediately from partially dry lumber (moisture content about-22 percent). (3) Clinched nails had from 25 to 464 percent more holding power than unclinched nails when driven into partially dry lumber which was permitted. to dry to about 11 percent moisture content before the nails were pulled., (4) Nails clinched across the grain had approximately 2 percent more resistance to withdrawal than nails clinched with the grain. For practical purposes, these results showed, that the clinching of nails improves the strength of the joint practically the same amount when the nails were driven into and pulled immediately from air-seasoned or partially dry lumber. The greatest increase in joint strength occurred in partially dry wood that was subsequently kiln-dried to air-seasoned condition before the nails were pulled. This is a common enough occurrence when boxes or cpates are made of unseasoned lumber and allowed to stand around before being used. It should not be assumed, however, that such improvement in joint strength justifies the use of green lumber for box and crate construction, because it is not possible to clinch all of the nails used in the assembly of boxes and crates. In the nailing of sides, top and bottom to ends and cleats in box construction and in the nailing of ends and sides to the base or tops to the ends and sides in crate construction, it is impossible to clinch the nails. Since this assembly nailing is an important factor in determining the strength of these containers, the requirements specifying the use of: dry lumber, which give the best result, should not be waived. Since thtse tests were conducted with single nails driven through two pieces of wood, such effects of drying as warping, twisting, and splitting of lumber in a box or crate are not evaluated. The following discussion presents the basic test data upon which these general, conclusions are based. Description of Material Two sets of static withdrawal tests were conducted, the first set with Douglasfir under three different moisture conditions, and the second with three species of wood (Eastern white pine, Southern yellow pine, and Douglas-fir) in the Report No. R

4 air-seasoned condition. The tests were made on wood with moisture content at time of test as follows: Average moisture content at time of test (Percent) Kiln-dried Douglas-fir 1 Partially dry Douglas-fir 22 Partially dry :Douglas-fir nailed at 22 percent and kiln-dried for 8 days 11 Air-seasoned Eastern-white pine, Southern yellow pine, and Douglas-fir 11. The type of test specimen used for these'tests is shown in figure 1. Each test specimen contained three nails of one size and kind driven through a 3/4- inch thick face plate (block A of Fig. 1) and into either_another 3/4-inch or 1-5/8-inch block (biock. 114jf Fig. 1), the thickness of the second block depending upon the length of the nail. For the tests of unclinched nails, the nails were driven through both blocks of wood with the points left protruding in the instances where the length of the nail exceeded the total thickness of the two block8:: For clinched nails, the nails were driven through both blocks of wood and clinched individually with a hammer, some across the grain of the wood and some With the.grain. All nails were driven perpendicular to the surface of the block and spaced as shown in figure 1. The sizes and types of nails tested are listed in table 1. Fifteen nails of each size, type, and clinching condition were tested in Douglas-fir at each of the three moisture conditions, and in each of the three species, Eastern white pine, Southern yellow pine, and Douglas-fir, in the air-seasoned condition. Method of Testing In the first set of tests, using only Douglas-fir wood blocks, nails were driven into kiln-dried wood and pulled immediately; driven into partially dried wood and pulled immediately; and driven into partially dried wood which was kiln-dried to 11 percent before the nails were pulled. In the second set of tests, the nails were driven into and pulled immediately from Eastern white pine, Southern yellow pine, and Douglas-fir, all in the air-seasoned condition. Immediately before testing, the top block of wood in each specimen (block A, Fig. 1) was split off around the nails to make it possible to pull each nail individually. The nails were pulled at a speed of.182 inch per minute by means of astecial gripping device used in connection with a universal testing machine (Fig 2). Load was. applied until the nail pulled out. Maximum load, which usually occurred when the nail pulled only a fraction of an inch, is recorded in tables 2 and 3. Report No. R1777

5 Effect of Moisture Condition on Strength of Joint The present tests,are in accord with previous studies made at the Forest Products Laboratory, which show that unclinched nails lose much of their resistance to withdrawal when driven into partially dried wood which is subsequently dried further before the nails are pulled. These tests showed that the average withdrawal load of the five sizes of unclinched nails, driven into partially dried wood which was then kiln-dried before they were.119d.was about 59 percent lower than that obtained from those driven into and pulled immediately from kiln-dried wood; and 57 percent Tower than obtained fro those driven into and pulled immediately from partially dried wood. Then the nails were clinched, however, there was no significant difference between the withdrawal loads under the three moisture Conditions (table,4). Comparisons of Clinched and UnClincheOlails of the Same Size The importance of clinching is further demonstrated by comparisons of the withdrawal loads required to pull the same sizes of nails, 4inc4ed and unclinched. The results in table. 2 show that, for nails driven into dry lumber and pulle once, the sixpenny clinched pright_box. nails required 42 percent greater withdrawal loads than the unclinched sixpenny cement-coated, sinker nails; when driven into partially dried wood and pulled at once the clinched bright box nails required 11 percent greater withdrawal loads; and when driven into partially dried wood which to 11 percent before pulling the nails, the clinched bright box nails required 25 percent greater withdrawal loads. The sixpenny clinched common nails required 75 percent, 175 percent, and 35 percent greater withdrawal loads, respectively, than the sixpenny cement-coated nails under the above conditions of moisture content. In these comparisons, some cognizance should be given to the fact that the bright box and common nails have a greater-shank - diameter than the sinker nail; hence, some increase in the withdrawal loads would be expected. The comparison has a practical significance, however, inasmuch as box and crate specifications require cement-coated nails where no clinch is made, but bright nails are'permitted where they are clinched. Therefore, in practice, a sixpenny bright box or common nail might be used where clinching is specified, whereas a sixpenicementcoated sinker might be used where clinching is not specified. The clinched tenpenny common nails required 16 Percent, 45 percent, and. 464 percent greater withdrawal loads, respectively, than the unclinched tenpenny nails under the three moisture conditions, Clinching of Naii.- P With and across the Grain of the Wood" The tests of the three kinds of nails which were clinched"with or across the grain showed that clinching of the nails across the grain resulted in withdrawal loads approximately 2 percent greater than clinching with the grain. The results are tabulated in table 5. Report No. R

6 Table 1.--Sizes and types of nails tested Size of : Type of nail Tested under nail : following conditions Fivepenny : Cement-coated sinker : Not clinched Sixpenny : Cement-coated sinker : Not clinched Sixpenny : Bright common : Clinched across grain Sixpenny : Bright common : Clinched with grain Sixpenny : Bright box : Clinched across grain Sixpenny : Bright box : Clinched with grain Eightpenny Ninepenny : Cement-coated sinker : : Cement-coated sinker : Not clinched Not clinched Tenpenny : Bright common Tenpenny : Bright common Tenpenny : Bright common Not clinched : Clinched across grain : Clinched with grain Report No: 81777

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9 Table 4.--Average withdrawal loads, showing effect of moisture conditions on clinched and unclenched nails in Douglas-fir Test condition : Average : Relation to : withdrawal :"driven-dry, : load : pulled-dry" : load Pounds : Percent Unclinched nails (average of 5d, 6d, 8d, 9d, and 1d nails) 1 Driven dry--pulled dry 179 : 1 Driven partially dry--. : pulled partially dry 171 : 95 Driven partially dry-- : : pulled dry 74 : 41 Clinched nails (average of 6d and 1d nails clinched : : both ways). : Driven dry pulled dry 27 : 1 Driven partially dry-- : : pulled partially dry 278 : 13 Driven partially dry-- : : pulled dry : 11 : : : : Report No. R1777

10 Table --Average withdrawal loads of nails clinched with the grain and across the grain Type of nail and test condition t Average withdrawal : Relation of load : cross-grain to with-grain : Clinched : Clinched : clinch : with : across : grain : grain : Pounds : Pounds : Percent Tests made in Douglas-fir under various moisture conditions Sixpenny bright box nails. : Driven dry--pulled dry 149 : 155 : 14 Driven partially dry--pulled partially 137 : dry 181 :: 132 Driven partially dry--pulled dry 145 : 183 : 126 AVERAGE 121 : Sixpenny common nail : : : Driven dry--pulled dry 174 : 199 : 114 Driven partially dry--pulled partially dry : 179 : 254 : 142 Driven partially dry--pulled dry 187 : 235 : 126 AVERAGE 127 Tenpenny common nail : : : Driven dry--pulled dry : 51 : Driven partially dry--pulled partially dry : 396 : 524 : 132 Driven partially dry--pulled dry 437 : 589 : 135 AVERAGE 128 Tests made in three species - all air-seasoned Sixpenny bright box nail Eastern white pine 12 : 143 : 119 Southern yellow pine 144 : 182 : 126 Douglas-fir 26 : 217 : 15 AVERAGE 117 Sixpenny common nail : : : Eastern white pine : 165 : 131 Southern yellow pine 185 : 215 : 116 Douglas-fir 25 : 298 : 119 AVERAGE 122 Tenpenny common nail : :. Eastern white pine 293 : Southern yellow pine 357 : 425 : 119 Douglas-fir AVERAGE 122 Report No

11 ZM 7:,5733. P Figure -Test specimen for making direct withdrawn]. tests of nails. Report No, Rl777

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13 ZM F Figure 2.--Special gripping device used on the universal testing machine for making direct nail withdrawal tests. Report No

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