Reliability tip of the month

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This monthly bulletin features short technical articles produced by SKF Reliability specialists.

Focusing on practical and technical matters, the Tip of the Month covers various topics such as bearing maintanance, damage analysis, condition monitoring and lubrication.

Aug 2016

Traditional Power
Vibration analysis: resonance

August 2016
Description:

High vibration at one times running speed was noted on the exciter end of an 18 MW, 2-pole Gas Turbine Generator. The vibration amplitude was highest in the axial direction, and had been a problem for 3 years.

During a pre-analysis meeting, the first step in a 6-step analysis process, it was found that the generator bearing had been replaced multiple times during the preceding three years, with little or no effect. Following recommendations of the OEM and several vibration specialists, the unit’s alignment was checked and adjusted three different times, again with little or no effect. These unsuccessful correction attempts cost the company approximately $500,000 over a three year period.
Analysis of the machine during start-up showed a large increase in the vibration amplitude beginning at 3550 RPM on up to normal operating speed of 3600 RPM. These vibration levels were observed without the field being applied, eliminating any generator/electrical problems as contributors to the problem.
During coast down, a 900 phase shift was observed within a very small change in speed. These two symptoms:
1. Large change in amplitude with a small change in speed
2. 900 phase shift during coast down are very strong indicators of aresonant condition.

Solution:
A vibration survey was conducted on the exciter end of the generator to locate the resonance. Because of interference from the collector ring covers the resonance could not be located. The unit was shut down and the covers were removed, and with appropriate safety precautions, the generator was restarted. The resonant condition now occurred at 3400 RPM. The vibration amplitudes at running speed, 3600 RPM, returned to the low values previously observed prior to the sudden increase three years earlier.
The combination of mass and stiffness of a mechanical structure determine its natural frequencies. Increasing mass decreases a natural frequency and adding stiffness increases a natural frequency. Removing these covers reduced both mass and stiffness. Since the resonant frequency decreased, the loss of the stiffness of the covers obviously had a more significant impact than that of the decreased mass.
At this time maintenance recalled removing a 1/2" rubber gasket three years earlier during a routine cleaning and replaced it with standard gasket material. The rubber gasket was originally installed by the OEM to decrease the stiffness of the exciter assembly.  By replacing the rubber gasket with standard gasket material, the natural frequency of the exciter assembly was raised to near the normal operating speed.
A resonance was created that amplified the generator vibration to its high levels. Replacing the hand made gasket with the 1/2” rubber gasket returned the natural frequency to its correct value. The unit then ran successfully. Further improvement was achieved by performing a balance on the generator rotor.

Moral:
Resonance is often the least understood characteristic of vibration, yet is estimated that 20% or more of machines are affected to a degree by resonance. Since resonance is an amplifier, it creates confusing symptoms in the vibration spectrum, resulting in frequent and reoccurring misanalysis using normal vibration techniques. Identifying resonance can be relatively easy using practical techniques.
This case history also emphasizes the importance of determining the proper and full history of the problem.  Seemingly small changes can occasionally have a large impact on a machine’s operation. These problems can be easily solved with a systematic approach. “Non-vibration specialist” technicians solved this problem, after attending the CM 103 Machinery Inspection & Evaluation course.

Vibration analysis: resonance

August 2016

Traditional Power

Description:

High vibration at one times running speed was noted on the exciter end of an 18 MW, 2-pole Gas Turbine Generator. The vibration amplitude was highest in the axial direction, and had been a problem for 3 years.

During a pre-analysis meeting, the first step in a 6-step analysis process, it was found that the generator bearing had been replaced multiple times during the preceding three years, with little or no effect. Following recommendations of the OEM and several vibration specialists, the unit’s alignment was checked and adjusted three different times, again with little or no effect. These unsuccessful correction attempts cost the company approximately $500,000 over a three year period.

Analysis of the machine during start-up showed a large increase in the vibration amplitude beginning at 3550 RPM on up to normal operating speed of 3600 RPM. These vibration levels were observed without the field being applied, eliminating any generator/electrical problems as contributors to the problem.

During coast down, a 900 phase shift was observed within a very small change in speed. These two symptoms:

1. Large change in amplitude with a small change in speed

2. 900 phase shift during coast down are very strong indicators of aresonant condition.


Solution:
A vibration survey was conducted on the exciter end of the generator to locate the resonance. Because of interference from the collector ring covers the resonance could not be located. The unit was shut down and the covers were removed, and with appropriate safety precautions, the generator was restarted. The resonant condition now occurred at 3400 RPM. The vibration amplitudes at running speed, 3600 RPM, returned to the low values previously observed prior to the sudden increase three years earlier.
The combination of mass and stiffness of a mechanical structure determine its natural frequencies. Increasing mass decreases a natural frequency and adding stiffness increases a natural frequency. Removing these covers reduced both mass and stiffness. Since the resonant frequency decreased, the loss of the stiffness of the covers obviously had a more significant impact than that of the decreased mass.
At this time maintenance recalled removing a 1/2" rubber gasket three years earlier during a routine cleaning and replaced it with standard gasket material. The rubber gasket was originally installed by the OEM to decrease the stiffness of the exciter assembly.  By replacing the rubber gasket with standard gasket material, the natural frequency of the exciter assembly was raised to near the normal operating speed.

A resonance was created that amplified the generator vibration to its high levels. Replacing the hand made gasket with the 1/2” rubber gasket returned the natural frequency to its correct value. The unit then ran successfully. Further improvement was achieved by performing a balance on the generator rotor.


Moral:
Resonance is often the least understood characteristic of vibration, yet is estimated that 20% or more of machines are affected to a degree by resonance. Since resonance is an amplifier, it creates confusing symptoms in the vibration spectrum, resulting in frequent and reoccurring misanalysis using normal vibration techniques. Identifying resonance can be relatively easy using practical techniques.
This case history also emphasizes the importance of determining the proper and full history of the problem.  Seemingly small changes can occasionally have a large impact on a machine’s operation. These problems can be easily solved with a systematic approach. “Non-vibration specialist” technicians solved this problem, after attending the CM 103 Machinery Inspection & Evaluation course.

Prevent Roller Bearing Damage on Startup

July 2016
Tapered roller and spherical roller thrust bearings_rolling element_1
Tapered roller and spherical roller thrust bearings design rolling element
Tapered roller and spherical roller thrust bearings have internal flanges that must be considered during mounting.

Ensure proper roller seating during assembly:

The rollers of both bearings have a contact angle that can allow the rollers to drop out of contact with their guiding flanges (red areas in figures) during assembly. To ensure proper roller seating during assembly, rotate the shaft or the outer ring (common in wheel applications). This rotation will ensure proper contact of the large end of the rollers against the bearing internal flanges which is a prerequisite before measuring any end play. (NOTE: The green arrows in the figures at right indicate the direction of roller seating.)


Consequences if not done properly:

If rotation is omitted, smaller end play values will be measured as the rollers have not been properly seated. During startup, excess end play may allow rollers to skew so
severely that they may skid or slip instead of rolling properly. This skidding can result in very early failure of the bearing from extremely high heat generation, lubricant failure, and cage damage that could seize the bearing. Seizure may occur in the first minute of operation! 


Guidance:

Follow manufacturer’s end play recommendations during assembly. If guidance isn’t available, contact SKF Applications Engineering Service for assistance or fill in your request on the right hand side "Contact us".

Bearing Maintenance and Technology

June 2016
Jun 2016_TOM

Am I using the wrong tool with bearings?

How do you know if you’re using the wrong tool to mount or dismount a ball or roller bearing?
Simple check: if you have to reach for a hammer, chisel, punch, screwdriver or torch to install or remove a bearing, you’re generally on the wrong track, and you might actually get hurt.


Here are the details:
Hammer and/or chisel:
applying force directly to bearing parts can cause chipping and cracking of the through hardened steel. Too many people have been injured, some seriously, to take this risk.
Hammer and punch:
only proper for mounting an eccentric locking collar-type bearing. Locking nuts will be damaged and chipped out with this combination of tools.
Screwdriver:
often used to remove bearing seals or shields. This tactic will not improve bearing lubrication! If your bearing isn’t performing, consult SKF for help.
Torch:
on installation, you risk seriously overheating the bearing, losing hardness of the steel which will cause premature failure. On removal, a torch can crack rings (see above) or permanently bend the shaft you’re working on.

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