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MiniMax Bolt Tension Monitor
12.7 Interpreting the Waveform
One of the most important things to consider, when using the MiniMax, is how to
interpret the waveform in order to choose the proper phase (polarity) and gain
setting. It's also important to understand what's taking place during a basic
measurement. The user has the option of selecting the phase only once during the
L-REF (reference length measurement). Once the phase is selected and the L-REF
measurement taken and stored for a particular bolt, all the elongation and loads
taken for that bolt will assume the sample phase/polarity. Initially, this might not
seem that critical. In most cases, with well prepared bolts, this might very well be the
case. While both phases may look fine at first sight, during the L-REF measurement,
deformations in the joint can occur when a load is applied to the bolt.
When a load is applied on a bolt, the velocity of the fastener decreases. Any
deformations in the joint (bending) will disrupt the sound path and potentially cause
an overall decrease in signal amplitude. When this occurs, the decrease in amplitude
can cause the MiniMax to lose the first cycle altogether. If the first cycle is lost, the
MiniMax will "peak Jump" to another cycle later in time with greater amplitude. Often
times transducers have a very slow ramp up time until reaching maximum energy. In
other words, the amplitude of the first cycle is lower in amplitude than the second or
third cycle. For this reason, selecting the proper phase is very important.
The Auto Set feature built into the MiniMax will do a lot of this work for you.
However, it's always a good idea to understand how to interpret the waveform. The
following diagrams offer a brief explanation:
Correct Phase (Polarity) – Fig. 1
Incorrect Phase (Polarity) – Fig. 2
Refer to the illustrations above. The first thing to note is that the screen shots above
are split into positive and negative regions. The centerline is assumed zero, and the
two regions in the upper portion of the display are positive, while the two lower halves
are negative. Refer to Fig.1. Notice that the height or amplitude of W1+ is greater
than the height of W2+ from the zero crossing (centerline). These are the positive
cycles, as they are in the upper regions. Now consider that the height of W1- is lower
in amplitude than W2-. These are the negative cycles as they are in the lower
regions. Keeping that in mind, the MiniMax uses a zero crossing style of detection.
Therefore, the detection will always occur to the right side of the positive or negative
cycle being detected. With this said, the detection in Fig.1 (B) is occurring on the
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