CA gluing Grand Pinblocks....My take on it/ Richard Brekne

[Richard Brekne]

Keith Roberts wrote:
> 
> Richard,  the shear force of scissors is two forces that are parallel but
> opposite and they shear the hair off which is at 90 degrees to the forces.
> Nail two blocks together and move them sideways and the nail will shear off.
> The shear strength of the nail is at 90 to the nail. Shear wind is at 90 to
> the direction of travel of the airplane. The string pulling the pin sideways
> trying to cut the top off the pin could be called a shear force. Hard to
> explain but did I make it clear enough?
> Keith R
> 


Ok... obviously either I have misunderstood the concept of
shear force, or something else is wrong with this picture.
Tho the scissors exert a shearing force on an object, does
the object being sheared exert a shearing force in return ?? 

"Definition: A force that acts parallel to a plane rather
than perpendicularly, as with tensile or compressive force."

I have never thought of shear force in this fashion, tho of
course I have heard of "shearing" off a nail, and the
likes.... but I wonder this is actually the same thing as we
are talking about. In our example of the string exerting a
shear force on the bridge pin, the bridge itself on the
other side of the pin have to be the "other half of the
scissors" so to speak. While this exacts a shearing force on
the pin itself, I am confused about how the same can be said
for the two arms of the "scissors". The bridge should suffer
from the results of the pin being pressed against it. This
speaks of a compressive force on the bridge to me... or what
?. 

Attempting to cut a rigidly held nail with the average pair
of scissors, or a cheap pair of nippers will result in dents
in the blades .... at 90 degrees to their cutting planes....
isnt this the result of compressive force exerted on the
blades ?

Shear stress between two surfaces of the bridge pin and
bridge would have to be a result of whatever force was
either pulling up, pushing down, or twisting the pin against
the ability of the tightness of the hole to resist the pin
moveing. Right ???

Thanks muchly for your thoughts... but I am still scratching
my head... grin.. what else is new eh ?

Cheers !

RicB