Amarish Take a look at Shigley's Mechanical Engineering Design, Section 8-4. There are a lot of details left out of your original post that can affect the size and placement (and therefore quantity) of fasteners used for this application. In addition to those noted above - is the fastener held by a nut/washer combination on the side opposite the head? Or is it threaded into the (relatively soft) lower plastic component? Does the washer (if present) span a larger diameter than the nut (if present) or the head of the fastener? If so, by how much? Remember, you're trying to spread out the load created by tensioning the fastener, so in general, a larger surface contact is better.
As noted above, the angle of incidence when the components make contact with the "ground" makes a difference in the ability to maintain a proper joint. Note that the mass of the dropped components also impacts the ability to constrain the joint - a "massier" object will produce more force on impact, resulting in more stress on the joint. What type of fastener are you considering? Does the "screw" have to be at or below the surface of the component? Does that mean you need a countersink - thereby increasing the flange thickness?
If you're interested - fastening two steel plates of 200x150x50 mm to prevent separation (or maintain gasket tightness) would mean using 0.375 inch (10 mm) diameter bolts at no more than 6 inch (150 mm) spacing. This would typically result in one of two cases: 1) if a bolt is located at the corner, then a quantity of 6 is required (three each on each long side) or 2) a total of 8 would be used (roughly 5 inch = 125 mm spacing), with some (equal) gap between the actual corner and the first fastener along a given side.
Using smaller diameter fasteners would mean using a proportionately larger amount in each case.
Converting energy to motion for more than half a century
