Martin Gregorie wrote:
John Wilton wrote:

Why is the maximum possible force Pi*D*14.7 ?

Physics 101. Area of the suction cup in sq. in. multiplied by
atmospheric pressure at sea level.


John, the formula for area is Pi*r*r. Pi*d is circumference.

I'm not a physicist, but I think you're looking at this problem all
wrong. Which is rigid, the canopy or the suction cup? The suction cup
which Paul was recommending earlier is soft rubber. So the deformation
is there. The only force on the canopy is that caused by the
deformation of the rubber, plus the torque or weight applied to the mount.

I happen to have that suction cup in my hand right now, and I estimate
that it takes about 10 pounds of force to compress it. Then, after
compression, that same 10 pounds is trying to spring back, but is being
prevented by the vacuum. This load is continuously applied as a bending
load trying to deform a disk of plexiglass the size of the cup.
Plexiglass is pretty stiff, I've never seen any deformation of the
canopy when the cup is on. I'm certain that at no time is the stress
greater than when you apply the cup. If one were to put their hand on
the outside of the canopy at the same time to provide counter pressure,
that stress could be reduced to almost nil. That cup is 4 inches in
diameter, or 2 inches radius. The release force would be 2*2*3.14*14.7,
or about 185 pounds, assuming the suction was perfect. Now, my Dell
Axim which I use this way weighs about 1/2 pound, and when hooked to the
cup, it has a moment arm of about 6 inches with the mount I bought from
Paul. That's 1/2 pound * 1/2 foot, or 1/4 foot pound of torque being
applied to the mount. And, I don't think that is increasing the overall
force exerted, it's just redistributing the 10 pounds of compression
force over the disk area.

And if you put the suction cup on properly, there is almost no air
inside there, certainly less than 10%, so it is probably 90% of a
perfect vacuum. Now, it you tried to pull the suction cup straight off,
that would apply the maximum force to the canopy, and as the cup started
to pull off, the volume inside would try to increase, making the vacuum
even greater. But if you remove it by pulling the little tab, then it's
released with almost no force because the vacuum is broken.

The bottom line for me in this is that I think I'd be most careful when
installing and removing the cup. I won't worry too much about the loads
it applies in use.

Ed