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# Calculation to decide the screw quantity between 2 plastic parts in assembly

## Calculation to decide the screw quantity between 2 plastic parts in assembly

(OP)
Hi,

I need a calculation method on how to decide the no. of screws to assemble the 2 plastic parts firmly.

E.g.: Part 1 with size of 200 x 150 x 50mm and part 2 size of 200 x 150 x 50mm, here how many screws needed to hold these 2 blocks firmly. Either by 4 or 6 or 8 screws with equidistance space.

E.g.: Personal Air Cooler Product

Thanks,
Amarish

How much firmly?

### RE: Calculation to decide the screw quantity between 2 plastic parts in assembly

(OP)
Air cooler product dropped from a height of 1meter (1000mm), the individual parts in the assembly should not open up and there should not be any gap.
I know there are so many ways apart from screw mounting, but i need how to evaluate the product i.e. no. of screws will secure my product.

### RE: Calculation to decide the screw quantity between 2 plastic parts in assembly

200mm x 150mm ... so something like a piece of paper (8.5" x 11") folding in 1/2.

4 seems light, particularly if at corners ... maybe more reasonable is on the long sides, something like 150mm apart

6 seems good,
8 seems too many.

what do other people (making similar things today) do ?

how critical is the requirement ? sounds sort of "made up", what would happen if these two pieces separated when dropped ? (ie would ebola virus escape ? ie super critical)
what is the price difference ? how critical is the price difference ??

maybe make some prototypes and test ?

as important as the number of screws is the design of the flange and the preload of the fasteners.
would does disassembly affect your design ? are these fasteners permanent or required to be disassembled quickly (and often) ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

### RE: Calculation to decide the screw quantity between 2 plastic parts in assembly

further to rb1957 : if it drops which is the part of the air-cooler touching first the ground (in practice where is the CG/center of gravity) ? in case the two platic parts in flat position or by the edge/corner ?

### RE: Calculation to decide the screw quantity between 2 plastic parts in assembly

and how rigid/resilient the surface, and ultimately "african or european ?" (this is a joke, taken from Monty Python, but how undefined properties can have a profound effect on the solution.)

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

### RE: Calculation to decide the screw quantity between 2 plastic parts in assembly

If there are ribs or other reinforcement that stiffen these pieces that will dictate the best locations for fasteners.
It will also dictate how large they can be.
It isn't so much the fastener strength as it is how much the load can be spread out.
6 sounds fair if you can have large heads and there are good stiffeners.
Otherwise you might need 10 to get good uniform retention.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

### RE: Calculation to decide the screw quantity between 2 plastic parts in assembly

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

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