Strength of Screws in Wood Composites As a Function of Specific Gravity

Particleboard is widely used in furniture construction, primarily as core stock material for table and counter tops, for case sides, and for such applications as doors and drawer fronts. It is also used to a limited extent to fabricate structural members in furniture such as the front rail on certain types of sofas. Screws are most frequently used in conjunction with particleboard to attach various types of hardware such as brackets and hinges to the particleboard itself. The average ultimate withdrawal strength of screws embedded in the face surface of a sheet of particleboard may be predicted by means of the expression ( ) 2 4 / 5 2 / 1 ) 3 / ( 655 , 2 x G D L D face F × − × × = (6-22) where F = withdrawal strength from the face of particleboard, in; Gx = specific gravity based on weight when dry and volume at current moisture content; L = depth of penetration of the screw, in. For convenience a number of values of the quantity 2,655 D1/2(L D/3)5/4 have been calculated and are given in Table 6-17. It should be noted that this expression is based upon the specific gravity of the particleboard at current moisture content rather than on density which is commonly used. According to this formula withdrawal strength is proportional to the square root of the diameter of the screw. This is in contrast to the case with solid wood in which withdrawal strength is directly proportional to the diameter of the screw. A simple example serves to illustrate the use of the above equation. Example: Calculate the withdrawal strength of a number 10 sheet metal type screw that is embedded 3/4 of an inch in the face of a piece of particleboard that has a density of 50 pounds per cubic foot at a moisture content of 10 percent. In order to solve this problem we first calculate the specific gravity of the particleboard by utilizing the relationship 4 . 62 / ) 100 / 1 ( mc W G x x − × = (6-23) where Wx = the density of the board, lbs/ft3; mc = moisture content in percent. Substituting the appropriate values into this expression gives 72 . 0 4 . 62 / 45 4 . 62 / ) 100 / 10 1 ( 50 = = − × = x G Referring to Table 6-17 we see that the appropriate length-diameter factor for a number 10 screw embedded 3/4 inches in the face of particleboard is 723.4. Also, from Table 6-18 we see that Gx2, that is, 0.72 squared is 0.518. Substituting these values into equation 6-22 gives ( ) . 7 . 374 518 . 0 4 . 723 lbs face F = × =