Screw cap axial force and pressure

[JimmyWeb]

Hi,

I haven't got a specifically structural question, but similar none-the-less.

I work for an aluminium screw cap manufacturer, and we are interested in knowing the axial force that is generated when we apply a torque to the cap.

Specifically, that torque is 1.694 Nm (15 lbs-in) when the cap is fully screwed on. The cap is connecting onto the top of a wine bottle. The diameter of the bottle (and cap essentially) is 26.1 mm. The thickness of the wine bottle glass at the opening of the bottle is 2.5 mm.

Once I have the axial force I can work out the pressure, as I can work out the surface area of the bottle top (pi*0.0025^2) (in m^2).

Any help appreciated.
Thanks,
James.

 

[CoryPad]

I don't think this is a straightforward analysis.

Are you looking for the axial force that is stretching external threads on the glass bottle? Analytical torque-tension relationships have been developed for screw threads, but they assume elastic loading. I presume your cap is a thin metal one or made from polypropylene. The thick glass threads will cause both of these to plastically deform, rendering the equations less accurate or inaccurate.

I presume there is an elastomeric seal inside the cap. As it compresses, that will generate compressive preload on the glass threads, complicating the analysis.

I don't understand your surface area/pressure comments. What are you planning to do with the axial force calculation?

Regards,

Cory

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

 

[JimmyWeb]

Thanks for your reply Cory.

No, I'm not really interested in the axial forces that are stretching the external threads on the glass bottle. But now that you mention it, it is a consideration...

The cap is thin aluminium. I'm not sure of the thickness, but I guess you could say its the thickness of a Coke can. And the cap is a cylindrical shape with a plastic insert which matches the thread of the bottle.

I just want to work out how much pressure is applied ot the 2.5 mm thick glass rim which is at the top of the bottle and comes into contact with the cap when it is fully torqued.

 

[arto]

The good old bolt force formula is F = 5T/d
F = force
T = Torque
d = diameter

 

[CoryPad]

After your further description, I believe I understand your request better. You should measure the compliance C of the elastomeric seal material. Then, measure the rotation angle [θ] of the cap after the seal has just seated against the glass. Then,

[δ] = [θ] [·] p/360

where

[δ] = displacement
p = thread pitch

and

F = [δ]/C

Regards,

Cory

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

 

[Screwman]

Good catch Corey.
The other thing to do would be to run a load deflection test on the sealing ring, using and unthreaded bottle neck as a mandrel. Compression of the sealing ring after assembly could be checked to give an approximation of the force.
The tough thing here is the friction of the threads and the behavior of the cap material.