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Attachment Pads and Quality Data

For ExpertALERT, DCX, TRIO

 

Introduction


This application note covers how to mount the Azima DLI triaxial sensor attachment pad and the basics for collecting quality vibration data.


Vibration Test Basics


Prepare for Data Collection - 'Tools' of the Trade

The Azima DLI data collector is a sophisticated piece of equipment; however, the ability for the Expert Automated Diagnostic System to accurately analyze machine faults, or to progressively trend the development of faults, the person collecting data must understand some basic techniques to ensure repeatable data.

In order to use the data collector effectively and efficiently you must remember to bring along the less sophisticated 'tools' of the trade. These 'tools' all fit inside the data collector's carrying bag; they should include the following:

Flat File or machine grinder tool Drill Bit
Extra Sensor Attachment Pads 5/32 Tap
Balldriver(s) Small Wiping Rag
Two-part Loctite Adhesive Kit Ear Plugs
Prepare to Test a Machine

Before you can collect data, you must make certain everything, including the machine and the data collector, is ready for the test. The following checklists will help:


Sensor Mounting Pads

If there are no attachment pads installed on the machine, you must refer to the VTAG or other documentation for the correct location and orientation before gluing on a new pad.

Prepare the pad location by using the flat file or grinder to remove all of the paint in the area under the new pad. The best vibration transmission for the machine to the data collector is a paint-free metal to metal contact between the sensor and the bearing housing. Layers of paint will dampen the vibration signal and hide small vibrations.

Use the wiping rag to remove any oils or residues on the mounting surface and on the back of the attachment pad.

Gluing the Attachment Pads

Use the two-part adhesive to glue on the new pad.
1. Using the activator, swab an even layer on the machine surface and the back of the attachment pad.
2. Allow a minute for the adhesive to become a little tacky.
3. Apply a pea-sized drop of the adhesive on the bottom of the attachment pad.
4. Firmly hold onto the machine location for about 30 seconds.
5. You should keep constant pressure on the pad.
6. Depending on the temperature of the machine, the glue should be set in about 3-5 minutes.
7. Allow 20 minutes before connecting a triaxial sensor.

If the two part adhesive is not available, or you need to mount a pad in a very wet location, you could use an industrial super-glue such as the speedbonder by Loctite. Keep in mind, however, unlike the two-part adhesive which will adhere the attachment block nearly permanently, super-glue will become brittle under normal operation of the machine and will likely fail within a year.

Top with a “red cap‟ plastic protective cover which will help keep your attachment pad clean and ready for data collection.


There are certain factors which could cause the attachment pad to come off.

1. The shelf life of the Loctite glue is about one year after it has been opened. It is recommended the user indicate the open date on the container and discard the glue after one year. It may continue to form a bond but may take significantly longer to cure.
2. The temperature or humidity in the space will affect the bonding time. Very hot or very cold machines will take longer.
3. Make sure to wipe off debris. It is important to wipe the location on the machine and the back of the attachment pad to prepare the surface for the glue.
4. Be aware of corrosion. If a machine is exposed to the elements, the exposed metal around the area which was filed for the pad should be re-painted to prevent rust and corrosion. After time, the pad may fatigue and fall off.
5. The pad must have adequate surface area to bond and transmit data. If the curvature of the bearing housing allows less than half of the pad to make contact, the area should be ground down with a file or grinder to create a flatter surface or an adapter pad may need to be required.

There are few basic principles to follow when collecting vibration data to ensure quality data is collected and accurate analysis of the data can be conducted.


 Verify the machine is correct per the VTAG. The VTAG must be used to compare the nameplate information on the machine to what is setup in the software. If there is a new machine installed and the information on the nameplate does not match what is in the VTAG, the collected data could incorrectly be diagnosed and false repair recommendations reported.

 Variable-speed machines need to adjusted to set running speed. Turbinedriven
and VFD machines have very specific test operating conditions stated
in the VTAG. It is imperative that the speed is tested using a tachometer and
either set into the collection or the machine adjusted before testing. The
speed also needs to be at a steady speed and not fluctuating or „hunting‟. It is
recommended to test machines with no greater than a ten percent variance
from the established baseline speed as indicated in the VTAG. This will
ensure comparable readings for accurate diagnostics.

 Ensure the machine is warmed up. A cold running machine will inaccurately demonstrate machine faults do to thermal growth. While the machine warms up the various metals heat and expand at varying rates. Shafts will grow and come into alignment as the two shafts and the coupling heat.

 Collect all pickup locations on the machine. The expert system will not automatically analyze spectra from only one component. If there are two pickup locations, both need to be tested. If you are troubleshooting a fault, for example, determining motor imbalance or pump imbalance, then collecting data on only the motor while the two components are uncoupled is expected. However, all analysis must be done manually.

 Collect all pickup locations without delay. Not only does the expert system need all pickup locations to be collected to be processed, it also needs them to be collected in a set amount of time. If a long delay occurs between test locations, the entire machine will need to be retested. Too many variables can occur and the system dynamics can change if you wait between tests.

 Keep the machine in a steady state during entire test. If system parameters change during data collection such as the machine trips offline, a companion pump starts or stops, valve lineup is manipulated, a governor automatically adjusts turbine speed, or flow or pressure is adjusted retest the entire machine. To accurately diagnose a machine you need like comparison
of data. If the machine is changing during data collection, this change will affect the vibration spectrum and accurate analysis cannot be performed.

 Re-attach blocks that are about to fall off. If the attachment pad appears to be loose or there is rust around the base of the pad, remove the old pad and attach a new one. If the connection is weakened, adequate vibration will not transmit through to provide adequate diagnostics.

Several things can occur that will give you “bad data”. The first line of defense for preventing inaccurate results is to ensure the data collection is performed correctly. Reviewing a machine‟s setup and test conditions prior to data collection is paramount. The following is a list of the most common causes of bad data.

 Improper Test Conditions – This includes normal operating temperature, loading, specific discharge pressure, stroke/cycling, and stable speed. All test conditions that are required for data collection are specified in the VTAG. These test conditions are vitally important for the expert system to accurately diagnose machinery faults.

 Wrong Test Speed – Variable speed machines rarely operates normally
at the required test speed specified in the VTAG. To ensure the machine is at
the correct speed, a tachometer (strobotach, optitach, digitach) is required
prior to data collection. Electric motors with multiple speed controls and
should be tested at high speed, as specified in the VTAG.

 Wrong Location – It is easy to get confused as to which location is required for some machine tests. Care must be given when selecting the location from the tree in the data collector to make sure it matches the actual location the sensor is connected to on the machine.

 Incorrect Sensor Orientation – The VTAG needs to be consulted prior to data collection to ensure the attachment pad is oriented correctly.

 Loose Accelerometer – Each accelerometer needs to be firmly attached to the sensor and the sensor to the pad. The balldriver should be used to gently torque the sensor onto the attachment pad. After attaching the sensor, a quick check of tightness should be done by gently attempting to twist it.

 Hot Accelerometer – The sensor can handle temperatures of about 300 degrees F. However, the amplifier and piezoelectric crystal that are contained inside each accelerometer need to equalize in temperature before collecting data. The accelerometer takes only a few minutes to equalize in temperature. Keep in mind that the sensor will be very hot to the touch when collection is completed.


Conclusion

To get the best vibration readings for the Azima DLI trending vibration analysis program, attachment pads should be used. Once attachment pads are installed onto the machines, routine vibration data collection is quick and efficient and with some simple best practices, accurate diagnostics will be achieved.

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