Ed Winchester wrote:
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 should have spotted that: my bad. However my point, that the maximum
possible force that can be exerted is the pressure per unit area times
the area, is still valid.

This is the highest force that can be applied if you try to pull the cup
straight off the canopy without twisting it, sliding it, or lifting its
edge.

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.

Quite true unless you pull on it at right angles to the panel.

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.

I'd restate that a bit. The Axim is applying a downward force of 1/2 lb
at the end of a 6 inch lever, so that is, as you say, 1/4 foot-lb. But,
that is being resisted by a counterbalancing torque thats conventionally
represented as a point force acting at the center of the cup at the end
of a lever that is pivoted at the cup's lower edge: visualize the cup
peeling off the canopy: it cones unstuck and detached by swinging round
its lowest edge. This is also a 1/4 foot-lb, but the lever is only 2
inches, so the outward force on the cup is 1.5 pounds weight. Higher
than your estimate, but still tiny compared with the force needed to
detach the cup.

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.

Agreed.


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