3-18 DESIGN DETAILS 



For laminates reinforced with various types of reinforcement, the table giving the 

 lowest strength should be used. These tables give fastener type, size, recommended 

 minimum penetration; below this amount strengths are erratic and unreliable; strengths 

 for this minimum penetration, and maximum strength and penetration. Straight line 

 interpolation may be used between these values. Penetrations greater than the given 

 maximum do not increase the strength. 



Fasteners given are standard steel machine screws, and self-tapping screws of two 

 types, thread forming and thread cutting. Drilling and tapping of holes for machine screws 

 should be in accordance with normal practice. The drilling of pilot holes for the self- 

 tapping screws was investigated and it was found that, in general, the smallest hole which 

 permits the screw to be driven without excessive difficulty gives the best results. The use 

 of either larger or smaller holes results in decreased strengths. 



The use of the tables is illustrated by Design Example 3-1. It must be noted that the 

 strength values given are for load at failure and a reasonable factor of safety must there- 

 fore be applied as shown in the example. 



DESIGN EXAMPLE 3-1 - USE OF TABLES FOR DETERMINING 

 THREADED FASTENERS 



A pad eye is to be fastened to a 1/2 inch thick mat laminate, and the pad eye 

 holds a line with a breaking strength of 1500 pounds applied normal to the laminate. 

 The pad eye is held to the laminate by 4 fasteners, which cannot penetrate the 

 laminate. The problem is to determine the type and size of fastener to use. 



If equal load distribution is assumed, each fastener must support 1/4 of the 

 load, or 3 75 pounds. When the breaking strength of the load applying part is 

 used a relatively small factor of safety may be used; assume 2 for this problem. 

 The choice of factor of safety is discussed in Chapter 6. Each fastener must 

 therefore be designed to support an ultimate load of 2 x 375 or 750 pounds. 



As a first trial, take a number 4-40 thread cutting self-tapping screw, at 

 a 3/8 inch maximum penetration from Table 3-4. 



Axial holding force for minimum 2/16 inch penetration = 80 pounds 



Axial holding force for maximum 7/16 inch penetration = 900 pounds 



Axial holding force for 3/8 inch = 6/16 inch penetration = 



80 + 4/5(900-80) = 736 pounds 



Since 736 pounds is less than the 750 pound ultimate load, this fastener is 

 not satisfactory. 



By repetition of the above calculation for different fasteners, the 6-32 self- 

 tapping, thread cutting screw is found to be satisfactory. If the maximum allowable 

 depth of penetration is increased to 7/16 inch the 4-40 fastener will be satisfactory. 



Attachments to sandwich construction, decks or bulkheads, require special treatment, 

 since lightweight core materials cannot retain threaded fasteners and may be crushed when 



