Zebra Tape Butt Joint Correction for Torsional Vibrations

Siemens Experimenter Siemens Experimenter
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Measuring Torsional Vibration with Zebra Tape?  Be careful about the "Butt Joint" where the tape ends overlap on the shaft!

 

shaft.pngLaser and Zebra Tape

 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.

 

zebra_tape.pngZebra tape is wrapped around the rotating shaft. When the rotating system is operating, the time between the passing of each stripe is used to measure the rpm very precisely.

 

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.

 

overlap.pngA "butt joint" can occur when the ends of the zebra tape do not wrap evenly around the rotating shaft. This causes erroneous readings in the rpm.

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.

 

RPM_Dips.pngUneven overlap in the Zebra tape causes artificial fluctuations in the rpm of the rotating system.

 

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.

 

zebra_moment_menu.pngThe ZEBRA_MOMENTS_TO_RPM function in LMS Test.Lab Signal Calculator can be used to reduce artificial fluctuations in rpm due to zebra tape butt joints.

“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.

 

correct_zebra.pngCorrected torsional rpm (Green) with artificial fluctuations removed versus original butt joint affected data (Red).

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.

 

Questions?  Email scott.beebe@siemens.com, contact Siemens PLM GTAC support, or post a reply to this article.

 

See the attached Torsional Vibration White Paper PDF at the end of this post for more information on measuring torsional vibration.

 

Rotating Machinery Links:

Comments
Experimenter
Experimenter

Can you recommend a source for zebra tape?

Siemens Valued Contributor Siemens Valued Contributor
Siemens Valued Contributor

I suggest you try Optel Thevon, they supply both zebra tape and disks, as well as sensors compatible with the SCADAS mobile.

www.optel-thevon.com

Optel-Thevon hardware can be ordered through your Siemens salesperson.

Here are some relevant part numbers and explanations:

Sensor

A complete torsional sensor consists of two parts: A 152G7 power amplifier for SCADAS and MULTI YO sensor with cable. The sensor is used in conjunction with zebra tape or a zebra disc.

 

 

zebra_tape_1.png

 

 

The MULTI YO sensor and cable plug into the 152G7.  The 152G7 is plugged into the SCADAS.

There are some options with the MULTI YO cable include length, threaded sensor, colored cables, etc.

zebra_tape_2.png

 

 

 

For example, to order the cable, a complete product code example would be: MULTI YO 2M HM6X20 SURG/R, where:

  • 2M – Means two meters. Other standard lengths include 4 meters (4M) and 10 meters (10M).
  • HM6X2 – Threaded connection on sensor for easier mounting.
  • SURG/R – R is for Red, G for green, Y for yellow, and B for blue.

 

Zebra Tape

Zebra tape can be wrapped around shafts, etc. When ordering zebra tape (product code MRL), the following can be specified:

  • S - Stripe size in millimeters
  • W - Width of tape in millimeters
  • L - Length of tape in meters

A complete code example would be: MRL-0.5/10/1.2 (MRL-S-W-L).  In this case, the stripes are 0.5 millimeters, tape width is 10 millimeters wide, and tape length is 1.2 meters.

 

zebra_tape_3A.png

 

Here is a list of valid combinations available:

MRL-0.5/10/1.2, MRL-1/10/1.2, MRL-2/10/1.2, MRL-2/20/1.2, MRL-2/20/1.4,
MRL-3/20/1.4, MRL-5/20/1.4, MRL-10/20/1.4, MRL-20/20/1.4

 

Zebra Disc

Zebra discs are useful at the ends of shafts, for example on pulleys.  The zebra disc has product code MRC, where the following can be specified:

  • PPR – Number of stripes
  • Do – Outer diameter in millimeters
  • Di – Inner diameter

An example of a complete code would be: MRC-120/170/30, where 120 is the number of stripes, 170 millimeters is the outer diameter, and 30 is the inner diameter in millimeters.

 

zebra_tape_4.png

 

 

Valid available codes include: MRC-15/200/00, MRC-36/200/00, MRC-60/200/00, MRC-120/200/00, MRC-240/200/00, MRC-60/250/00, MRC-60/170/30, MRC-120/170/30, MRC-60-1/170/30, MRC-120-1/170/30.