To All,

My 2 cents worth.

I have been using flexures in my seismometer designs for 6 years now and 
would never think of using a pivot point configuration again.

If you are the type who needs to prove to themselves which is the better, a 
piece of music wire or a piece of shim stock, for a flexible hinge try this 
simple experiment. Take a piece of your favorite size music wire (example 
..010" diameter), cut it 1" long. Calculate its cross sectional area (example 
is .0000785 sq. "). Then cut a piece of .001" thick heat treated shim stock, 
length 1". Cut its width to have the same cross sectional area as the wire 
piece (example shim width = .078"). Clamp both pieces in a vice with about 
1/2" of each protruding. Push on the free end of both pieces. You will 
instantly notice which piece flexes the easiest.

The horizontal instruments that I have been making lately have .002" thick 
shims for flexures. The upper flexure is narrower than the lower one. The 
upper is in tension and the lower is in compression. The lower flexure's 
width should be such that it will not buckle under the weight of the 
pendulum. Minimize the width of the flexures to minimize the restoring force 
on the pendulum. An inverted pendulum needs flexures that have sufficient 
restoring force to center the pendulum, otherwise the pendulum will flop off 
to the side.

To reduce any oil canning effect, make sure that the surfaces that the 
fexures are clamped to are really flat and aligned in the same plane. Don't 
clamp them directly to the surface of a piece of extruded aluminum (straight 
from the mill) for example. Place a straight edge across the mounting 
surfaces and hold it up to the light. Run the straight edge backwards and 
forwards to ensure some degree of flatness. If you cannot have the flexure 
mounting surfaces machined coplannar, put a piece of fine emery paper face up 
on a machined flat plate, then slide the part you want flat across the paper 
in a figure 8 pattern, without rocking the part. This should at least knock 
off the high spots. Having non-distorted, burr free edged flexures will not 
perform well if they are forced down on an uneven and/or misaligned surfaces.

Thank you Dean for in depth description of flexures.

Allan Coleman 

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