This month’s blog is to promote the thinking that when drive trains are aligned they should be aligned to the bearing tolerances and not the coupling tolerances. In addition how many people receive an alignment report with a soft foot check? We have found that some companies allocate their employees a laser alignment kit tell them what buttons to press and send them in the field. Without proper training and mentoring how will these employees learn correct Precision Alignment? Without correct training they will not know how to fix problems if they don’t understand fully what they are doing.
This month’s blog shows the importance of Precision Alignment including soft foot check and that the users of laser alignment equipment should be properly trained and mentored in Precision Alignment.
This survey was conducted by a great friend of mine and recent VCAT 3 Certified Seasoned Analyst James Pearce. linkedin.com/in/james-pearcevibrationanalysis
We were called to investigate an apparent increase in vibration levels after a high pressure hot water pump was replaced with a new pump end and a reconditioned drive motor. The operator felt that it was not running as smooth as the old pump set.
For this survey James used the CSI 2140 Dual channel Machinery Health Analyser. Data analysis was carried out using the CSI AMS Machinery Health manager software V5.61.
Vibration data including Velocity, Acceleration and bearing condition unit PeakVue was collected from each bearing location as close as possible to the source. Where applicable additional data including high resolution vibration data was collected.
There are elevated directional Velocity vibration levels when running at 2680 RPM (Low speed). This is due to a coincidence of a system natural frequency being excited by a motor Soft Foot condition.
- Check/inspect condition of the foundation, looking for looseness and any deterioration in the base plate.
- Perform precision alignment that must start with a soft food check and soft foot elimination. Followed by precision laser alignment.
- If these actions do not resolve the issue then stiffening of the base may allow for improved precision alignment and may move the system resonance out of the running speed range.
- There are increased directional Velocity vibration levels at the motor when running at 2680 RPM.
- After reviewing the vibration data it was decided to perform further checks and the motor holding down bolts was loosened one time when in operation, this is to check for distortion of the motor rotor to stator air gap. During this test it was and found that the Velocity amplitude reduced. The amplitude reduced to its lowest level when the motor non-drive end foot bolt (bolt closest to pump #1) was loosened (see figure 1&2). This indicates there is a soft foot issue.
- In addition an overall vibration coast down test & resonance bump test was performed. This data confirmed a natural frequency at 2X 2680RPM (see figure 3).
HPHW Pump #2 Motor Non-Drive End
The motor has elevated directional Velocity vibration levels. By loosening one motor fixing foot bolt at a time, the Velocity amplitude reduced. The amplitude reduced to its lowest level when the motor non-drive end foot bolt (bolt closest to pump #1) was loosened.
Figure 1 compares the Velocity spectra when running at 2680RPM, for the one order levels, as found state (4.332mm/sec RMS) & where the amplitude decreased the most after the motor foot bolt was loosened (2.651mm/sec RMS).
HPHW Pump #2 Motor Non-Drive End
Figure 2 is a photo of the motor indicating which foot bolt was loosened which resulted in the best decrease in amplitude.
HPHW Pump #2 Motor Drive End
Figure 3 is the data from a resonance bump test & overall vibration coast down test, performed at the motor drive end (DE).
The top plot bump test result indicates a system natural frequency that will coincide with twice the running speed (when running at the low speed) and amplify the vibration levels.
The bottom plot amplitude peak from the coast down test also confirms this condition with a peak at 5336 RPM, twice the running speed at the low speed setting.