When measuring the torsional vibration of a rotating shaft, a optical pickup (for example, a laser) is often used in conjunction with “zebra tape”.
“Zebra tape” has black and white stripes, and is wrapped around the shaft, while the laser points directly at the surface of the tape.
As the shaft rotates, the laser “sees” each stripe as it passes, and the time between stripes in used to determine the rpm of the rotating shaft.
By using many stripes, very fine fluctuations in the rpm can be measured, including fluctuations within one revolution of the shaft. These fluctuations can be very damaging to the rotating equipment, putting great stress and strain on the rotating parts. For example, the speed changes associated with combustion events in a engine can cause the crankshaft or driveline to fatigue prematurely.
Despite taking great care, it is possible that the stripes are not evenly distributed over the radius of the shaft. When this happens, a “butt joint” is created where the two ends of the tape overlap.
If a gap occurs at the overlapping ends, the shaft will appear to slow down momentarily once per revolution. This instantaneous slow down appears as a sudden dip in the rpm value. It is also possible that two stripes are closer together at the overlapping “butt joint”, causing a momentary increase in rpm speed (ie, a “rpm spike”). This momentary changes in rpm are not real, but are artificial readings caused by the zebra tape overlap.
In Simcenter Testlab Time Signal Calculator, there is a function called “ZEBRA_MOMENTS_TO_RPM”. This function is used to correct for the rpm spikes or dips introduced by the butt joint. “ZEBRA_MOMENTS_TO_RPM” is part of the "Tacho group" of functions in Time Signal Calculator.
“Function1” is used to point to rpm data with the butt joint error (Channel number or Channel name can be used).
The intended number of stripes is entered into the “Pulses_per_rev” field.
After running the ZEBRA_MOMENTS_TO_RPM function, a new data trace is created that has the butt joint effects removed.
With SImcenter Testlab 16A and higher, it is even possible to remove the effects of multiple butt joints.
See the attached Torsional Vibration White Paper PDF at the end of this post for more information on measuring torsional vibration.
Rotating Machinery Links: