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

[Keith Roberts]

Richard, I think the shear strength of a 3 1/4 inch (16d) nail is around
2,000lbs. Obviously in wood, the wood will compress and fail long before the
nail gives way. Think of it as two flat plates of metal placed together and
slid in opposite directions. These directional forces are called shear
forces. Pin the plates by drilling a hole through the plates perpendicular
to the plane of motion and inserting a pin or dowel. When the shear forces
exceed the strength of the pin, the pin is said to shear off and the plates
move. Of course, put a cutting edge on the plates and the force required
becomes far less. Scissors or sheep shears wasn't a good example because it
has a cutting edge which adds a multitude of variables. In this case the
bridge top is one plate of metal and the string trying to move sideways
across the bridge is the other.

Keith R


----- Original Message -----
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: <pianotech@ptg.org>
Sent: Wednesday, July 31, 2002 1:18 PM
Subject: Re: CA gluing Grand Pinblocks....My take on it/ 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
>