Strength of 1/4-14 Sheet Metal Screws in 12g SS

[schulkfl]

I have a question concerning the use of 1/4-14 sheet metal screws in 12 gage stainless steel sheet metal.

We currently have a unit (weight=600lbs) that is lifted straight up with eyebolts. The eyebolts tranfer the 600# tensile load through qty.8 1/4-14 stainless steel self tapper screws evenly placed along the two sides of the unit(4 along each side).

While I understand that using sheet metal screws for structural purposes is bad practice in general, I am having trouble finding any mechanical properties of these screws to see if they will support the load.

Could someone help with where I might find this information and what are the appropriate calculations?

Thanks,

Fred S

 

[trainguy]

shulkfl,

Are the screws in shear or in tension?

tg

 

[Screwman]

It's not the strength of the screws that is so questionable (although they are pretty questionalbe from a mechanical standpoint too), it's the unknown strength of the one "thread" that is formed into the sheet metal. Slight changes in hole size and even the clearance hole size can reduce the pull-out by 50%. There is no way that I would allow any unit that weighs 600# to be supported overhead using sheet metal screws. Once you had one part fail you would end up with a zipper failure on the rest and down it would all come.
Even if the parts are loaded in shear, they will go into bearing and then they will start to cock, which will put the nut threads into tensile and then you will get a failure.

 

[schulkfl]

The screws are in tension.

Screwman- I agree with your statement. Is there however, a way to quantitavely prove this? I have no data to support that hunch. I am looking for both the tensile strength of a 1/4-14 and the thread strength in 12g stainless. Besides actually testing this on an Instron, I'm not sure where/how I could find what I'm looking for...

 

[CoryPad]

What you are asking is not documented because it is poor practice and not to be promoted.

We have no idea what kind of screw you have. If you purchase them to established standards, then we can use some quantitative information.

For example, a 1/4-14 screw can be obtained according to ISO 1479 Hexagon head tapping screws. These screws have a thread form according to ISO 1478 Tapping screws thread, and mechanical properties according to ISO 2702 Heat-treated steel tapping screws – Mechanical properties. However, you stated stainless steel, so we can try and use the values of ISO 3506-1 Mechanical properties of corrosion resistant stainless-steel fasteners. With these standards, we can do some simple calculations. The minor diameter is a minimum of 4.70 mm, for a screw core sectional area of 17.3 mm[sup]2[/sup]. The material has a minimum tensile strength of ~ 500 MPa (assuming a low strength austenitic alloy). This produces a minimum screw tensile strength of 8.67 kN. The screw also can shear through the threads, so finding the sectional area of the thread form requires some geometry. Assuming the hole size is ~ screw minor diameter, it can be shown that the sectional area of a single thread is 9.75 mm[sup]2[/sup]. The shear strength is ~ 0.6 [·] 500 MPa, or 300 MPa. This provides a minimum screw stripping strength of 2.92 kN. The object has a mass of 272 kg (600 lb), which produces a force of 2.67 kN.

However, you still need to consider the sheet metal, which is the weak link. It will bend and the hole will enlarge and the screw will disengage. This is not easily calculated. It is easily measured. This is what you should do.


Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Screwman]

Cory,
Good response.
Best idea is make em and break em.
SMS threads are highly variable in results, so make sure you use a large enough sample size to ensure that you have statistcally valid results.

 

[schulkfl]

Thank you all for your help- If I can manage to get some testing done, 0I will post the appropriate data,

Thanks,
Fred S

 

[trainguy]

Tell me, shulkfl,

Why don't you use weld nuts, or riv-nuts in your application?

These have to be at least 5 times better than your sheet metal screw option.

Any thoughts?

tg