LINK Systems System 1100 Tonnage Monitor - Manual (Page 44)

THEORY OF OPERATION
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3-7
Figure 9. Structural Ringing.
The positive going signal is generated by machine frame stretch as
the tooling builds up force on the material. The signal increases
until the tooling breaks through the partly sheared material,
suddenly releasing the machine load to zero. The sudden release of
load causes the machine structure to rebound, creating the dying
oscillations (ringing) in the machine structure after material
breakthrough. The first negative peak of the oscillation is the
largest strain caused in the machine frame due to ringing, and is
often referred to as the "snapthrough" force. In effect, the
energy stored in the machine's frame under load creates a reverse
load on the machine when suddenly released.
Depending on structural mass and stiffness, ringing generally dies
out in a few milliseconds to a few hundredths of a second. This
short response normally has no effect on tonnage monitor accuracy
as long as machine speeds are less than 600 strokes per minute. At
speeds greater than 600 strokes per minute, the ringing may not die
out between load signals, and can cause a small offset in tonnage
monitor readings because the continuous ringing between load
signals interferes slightly with automatic zeroing circuits.
Ringing normally doesn't occur in forming, coining, forging, or
powdered metal compacting operations. The load on these operations
is released slowly by the machine ram or slide in the upstroke.
3.4.3 Structural Nonlinearity and Eccentric Loading