Screw Pull-Out Force in Polypropylene - Calculation

[506818]

I'm designing a system housed in an enclosure (Peli case style) that will hold 1 psi (0.0069 N/mm^2) pressure with a top panel held in by a number of screws.

The top panel has a surface area of 83607mm^2, therefore, a total force of = 0.0069*83607 = 577N. At present, I have 10 screws holding the part in place (57.7N per screw).

The case is made from polypropylene with M4x0.7 stainless steel screws used to hold the top panel in place. There are mounting holes on the case that I am planning on drilling larger and tapping (5 mm of thread used by screw) to fit the M4 screws. However, I need to ensure that the screws hold the top panel in place without stripping the plastic thread on the case. I'm not worried by the screw, more about the case.

I've found this calculation for internal threads shear force area (imperial-based):

https://www.engineersedge.com/thread_strength/thread_bolt_stress.htm

Associated calculator:

https://www.engineersedge.com/calculators/internal-thread-shear-area.htm

The values I have come up (metric):
n = 25.4/0.7 = 36.29
Le = 5 mm
Dsmin = 3.838 mm
Enmax = 3.523 mm
As = 0.07102 in^2 = 45.819 mm^2 (not sure if calculator is correct on the website as I get 30.71 mm^2)

Above thread dimensions calculated using:

http://www.amesweb.info/Screws/IsoMetricScrewThread.aspx

Then with F = shear strength * shear area
(shear for polypropylene = 0.5* tensile strength = 0.5*31 = 15.5 N/mm^2, data from here:

http://www.sdplastics.com/pdf/pp.pdf)

Therefore, the max force each screw can theoretically handle is:
F = 15.5*45.819 = 710 N

This seems to indicate that I should be fine, but it's all theoretical and would like some idea if I'm on the right path. Am I?

 

[506818]

Just want to say thank you for all your input - this is great discussion! Should've mentioned from the start (didn't want to deviate from the direct problem), but here is the case:

https://www.nanuk.com/nanuk-918.php?lang=lang-en

I think you've all convinced me that more screws is the way to go should I proceed along this path! I can have up to 24 screws, but this adds considerable cost to the product (sealing screws don't come cheap).

This is not a Peli case and definitely made from PP (have the datasheet and it's imprinted on the case).

The image is deceptive - there is some sealing around the screw, but it's very thin and relies on the gasket to form around the screw once it's been 'punched' through. Here is the actual case and panel/gasket that you can buy:

https://www.nanuk.com/docs/918_NANUK_Waterproof_Panel_Kit_Lower.pdf

Note it is a series of self-tapping screws on closed-cell neoprene.

The case shouldn't be opened up in the field - all access can be done using the features on the cover plate.

https://www.youtube.com/watch?v=JxJ05TdjQEY[/URL]]

I follow now, although this wouldn't be the end of the world if the case bowed and pressure was released - better than the cover plate flying off! Obviously want to avoid this scenario altogether.

Can you explain more about your design? Did you add pressure-equalisation features in the end?

The aim here is to have as close to zero condensation in the box, and the temperature will go down below 0 so the internal pipes and metalwork will accumulate condensation at low temperatures very quickly. Therefore, the PRVs on Pelican cases are essentially free-breathers and I'll get condensation in no time even if I cram in bags of desiccant.

 

[3DDave]

One method is to use a bellows that allows volume changes without exchanging outside air. This drastically cuts the differential pressure as it is limited by the elasticity of the bellows. Add in a pressure limiting valve so that the bellows is not over-extended. There are micro-porous materials that can exclude liquid water ingestion and can be used in conjunction with desiccants.

 

[IRstuff]

I still don't have a feel for your application, but prevention of internal moisture is tricky, particularly if there is moisture at the seal interface and if the enclosure experiences temperature/pressure changes. We've had non-hermetic enclosures that would get sun heated during the day and after sunset they would suck in external condensation, essentially a slow, solar-powered water pump. On the US east coast, we could get inches of water inside the enclosure over some number days/weeks.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[506818]

Great suggestion! Tbf, I have some internal volume to play with - this could be something that's retrofitted to the assembly if it becomes an issue. Tube + piston + spring won't be too difficult or expensive to create.

Tell me about it... What you describe about the accumulation of water is exactly what I can see happening.

Primary requirements:
- Flow water into a temperature control chamber in the box (from a water bath that can vary from -2°C to +40°C)
- Reduce condensation (0 is preferable)
- Box can't explode
- Aesthetically desirable and able to carry around (hence the Peli-style case)

 

[3DDave]

The bellows could just be a plastic bag. It may take a bit of tuning to get it right, but it seems like for a stationary box the sensitivity to setup will be really low. Just make sure it has a drain to the outside so it doesn't fill with water.

 

[506818]

Just curious, is there anything off-the-shelf that could be used for this? Guessing you would need to pre-set the internal bellows pressure (could just be ambient).

 

[3DDave]

There's no need to set the bellows pressure. It is open to the outside and any pressure difference is transferred. It's like breathing into a plastic bag. The bag only sees pressure as a resistance to deformation - the ideal case would be no pressure.

More important is setting the bellows default volume, which could be controlled by changing the box internal gas volume at final assembly. It needs to be such that the bellows can move freely at the minimum and maximum internal gas volume and not be stretched tightly or fully compressed.

 

[506818]

OK - so I was thinking a little outside the box. How about something that wasn't exposed to the outside, but sat inside the box? So as the internal pressure increased, the bellows inside would shrink due to the walls of the box being stiffer than the walls of the bellows?

With the pressure relief valve I have atm, it should be fine to set an external bellows so that it doesn't expand/compress beyond its capacity.