36 



Hardwood Record — Veneer & Panel Section 



August 10, 1921 



Strength of Screw Fastenings in Plywood 



If the screw fastenings in plywood construction are to be as 

 strong as the plywood itself, it is important to adapt the size of 

 screw, spacing, and margin to the particular species and thick- 

 ness of plywood used. Tests made at the Forest Products Labora- 

 tory have shown that the commonly-used plywood species may be 

 divided into the following groups, all woods in any one group re- 

 quiring the same screw fastening to develop maximum strength. 



GROUP \ 

 Low Density 

 Basswood Hemlock 



Cedar, Spanish Pine, sugar 



Cottonwood Pine, white 



Cypress, bald Poplar, yellow 



Douglas fir Redwood 



Fir, true Spruce, Sitka 



GROUP 2 

 Medium Density 

 Ash, black Hackberry 



Ash, pumpkin Magnolia 



Elm, white Mahogany 



Gum, black Maple, soft 



Gum, cotton Sycamore 



Gum, red Walnut, black 



GROUP 3 

 High Density 

 Ash, white Cherry, black 



Beech Elm, cork 



Birch Maple, hard 



The screw sizes, margin, and spacing for use with each species 

 and plywood thickness will be found in the following table. The 

 gauge is the smallest that can be used with the thickness specified 

 and not cause failure through breaking of the screw when the full 

 strength of the plywood is developed. The length of screw is the 

 shortest which will prevent the screw from pulling out before the 

 full ply\vood strength is reached. The margin is the smallest dis- 

 tance from edge of hole to edge of plywood which will insure 

 against failure by shear. The spacing is the distance from center 



to center of screw holes which gave maximum strength per linear 



inch. 



Size and Spacing of Screws for Maximum Strength in Plywood 



Tliickiit'S Scrt-w 



SpL'cifs of Gaufj^i- Itrtigth in inches Margin Spacini; 



of ply\v(jij(l (numliL'D Species receiving point_ in in 



i.lyvviMjiI in inches (»f screw White ash Spruce inches inches 



Group I 3/20 4 1/2 5/g 1/2 J/g 



3/24 5 1/2 Yi % 1/2 



3/20 6 5/g % 5/g 1/2 



3/16 7 5/g 34 5^ 5/g 



3/10 9 V, 1 34 34 



3/8 II 1 II4 V4 V4 



Group 2 3/30 5 /2 5/g 1/2 1/2 



3/24 6 5/g 3/^ 5/g 14 



3/20 7 34 ^g 5/g 5/g 



3/16 8 



3/10 10 



3/8 12 



Group 3 3/30 6 



3/24 7 



3/20 8 I I 14 



3/16 9 1!4 1/2 



3/10 II 1/2 n4 



3/8 13 1 34 2 



About equally good results were obtained with flat-headed 

 screws without washers and round-headed screws with washers. 

 Round-headed screws without washers proved an inferior means 

 of fastening. The spacing given in the table is for screws in a 

 single row, but staggering is recommended wherever possible. 



in the tests the size of frame members to which the plywood 

 might be attached was necessarily a secondary consideration, and 

 the block of wood in which the screw points were held was simply 

 made large enough to prevent failure occurring in it. Until fur- 

 ther information is obtained, designers must take particular care 

 that the frame is not split or weakened through the use of the size 

 of screw and the spacing necessary to make the fastening as strong 

 °^ '' plywood. 



Care and Use of the Hygrometer in Kiln Drying 



The correct use of the hygrometer is of vital importance in the 

 interpretation and consequent regulation of kiln conditions. Dry- 

 ing conditions altered because of a false conception of the condi- 

 tions present often result in honeycomb and other highly detri- 

 mental degrade. Several basic practices are suggested by the 

 Forest Products Laboratory for the proper handling of hygrometers. 



Calibration: Hygrometers should never be assumed as register- 

 ing the correct reading until their accuracy has been established. 

 It is not at all unusual for a thermometer to register a few degrees 

 higher or lower than the true temperature. Each thermometer 

 of a hygrometer should be checked against a standard ther- 

 mometer of known accuracy over the range of temperatures an- 

 ticipated in its use. Corrections for the several temperatures may 

 thus be determined if inaccuracy exists. Checking should be done 

 with the bulbs of the standard and the tested thermometers close 

 together and in the same medium and temperature of medium. 

 The correction factor for a thermometer at a certain reading be- 

 ing known, it is simple enough to determine the true temperature. 



Filling and Care: Hygrometer reservoirs should be filled with 

 pure w^ater only. The open-top type of reservoir is easily filled, 

 but the inverted-tube type often presents difficulties. The latter 

 type may be filled if submerged horizontally in a pail of water with 

 the water level slightly above the well opening. Other methods 

 of filling ^uich a tube are by means of a wash bottle or small bent- 



stem funnel. In any case the body should be dropped below the 

 mouth level. 



It is important that the silk or muslin covering of the wet bulb 

 be kept in good condition at all times. At least a small amount 

 of solid material is always left in the meshes after evaporation, and 

 sooner or later such a deposit impedes the transfusion of moisture. 

 It is because of this that pure or distilled water should aWays be 

 used in the reservoir and that the wick should frequently be 

 changed. 



Placing the Hygrometer: The hygrometer should be placed at 

 the exact points where information as to conditions is desired. Do 

 not place it near a door or a wall, or where it will be subjected to 

 direct radiation from the heating coils, as conditions at these 

 points are probably not representative. To obtain representative 

 conditions take an average of several readings in different parts of 

 the kiln. 



Taking the Readings: In reading the wet-bulb thermometer 

 care should be taken that there is sufficient air circulation to give 

 the maximum evaporation rate from the bulb covering. .-\t low 

 temperatures, i. e., up to 120 degrees, F., there should be an air 

 velocity of at least fifteen feet per second. At the higher tempera- 

 tures this rate is not quite so essential. -A thorough fanning of 

 the air about the wet bulb is usually required. The lowest wet- 

 bulb reading for any air condition is the one desired. 



