Case of Missing Gear Mesh Frequency

Question:

“Why don’t we see the Gear Mesh Frequency (GMF) on the output side of a splash lubricated slow speed gear box?”

This is quite puzzling since common sense dictates that such peaks should be present.

My Answer:

The principles involved are the following:

1. Air, water and oil produce turbulence when worked on by machines like pumps, gears, fans, propellers etc.
2. Such turbulence creates damping force.
3. This is proportional to the square of the velocity.
4. But this damping force acts in quite a funny manner.
5. For slow speed machines (say below 750 rpm; slower the better) damping is positive that is it goes against the motion and so neutralizes the entropy as seen by the decrease in the vibration levels. Hence the gear mesh frequencies vanish. Coriolis Effect on the output side of the gear box also helps in attenuating the vibration.
6. But for high speed machines damping is negative. That is it goes in the direction of the motion and therefore enhances the entropy as seen by the increase in the vibration levels.
7. So, for low speed machines it goes against the motion and suppresses the GMF. In some cases it suppresses the fundamental peak as is found in the case of the vertical Cooling Water Pumps of Power Plants. GMF is produced when the fundamental frequency is superimposed onto the vibration generated through gear impacts.
8. It therefore follows that for high speed gear boxes it magnifies both fundamental and GMF peaks.

Missing peaks therefore indicate fluid turbulence, which might also be indicated by other peaks like vane pass frequencies. The condition monitoring of such gear boxes might best be done through Wear Debris Analysis/Ferrography.

So, this is the mystery of the missing GMF in splash lubricated slow speed gear boxes.

Therefore, splash lubrication for a low speed gear box is a good idea. It enhances the life of the gear box since it balances the entropy in the system.

But at the same time, with higher oil level in a splash lubricated high speed gear box the vibration level would increase, specially the fundamental and the GMF. That would spell trouble.

Similarly, it is better to have a turbulent air flow in low speed fans and blowers. It suppresses the vibrations and therefore enhances the life of bearings.

Nature also uses these principles of fluid turbulence and damping? Applications?

1. Bird’s nest are made up of loosely placed twigs and leaves usually not bound to each other. But these don’t break up or fall off in turbulent winds. Damping keeps them in place and provides the necessary security to birds.

2. Swift flowing rivers allow fishes to grow bigger and better.

3. Winds, storms etc neutralize the increase in entropy.

Design Ideas for Reliability & Sustainability?

1. Low speed gear boxes might best be lubricated by splash lubrication.
2. High speed gear boxes might best be lubricated by spray lubrication
3. Hotter and turbulent air might best be handled by low speed fans and blowers.

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Corolisis Effect & Negative Damping – a Report

Report on Thaisen Fan (Scrubber)

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Brief description of the phenomenon:

After cleaning of the fan blades, vibration of the fan gradually increases during operation and in a span of 10 to 14 days vibration level reaches an unacceptable level, which necessitates the next cleaning cycle. However, for so long, this matched the scheduled production window provided by operation. However, after the recent changing of the rotor and the bearings, the fan now reaches unacceptable level of vibration within a short span of time that does not coincide with the scheduled “production window” of the operation, which causes “unplanned downtime.”

Goal of the investigation: To correct the imperfection in the system so that the fan cleaning cycle coincides with.the scheduled production window.

Result of the investigation:

 

1. The problem of rising vibration within a short period of time is an inherent problem (a birth defect) of the fan. The main reason is the Coriolis effect on the fan. Coriolis force is a force exerted by a moving fluid on the disc or impeller rotating in the fluid. If the rotation is CCW (counter clockwise) then the fluid moves to the right of the impeller and away from the centre. Similarly, when the impeller moves in the CW (clockwise direction) the fluid moves towards the left of the impeller and away from the centre.

In this case, with the fan moving in the CCW direction the Coriolis force moves toward the right of the impeller in the same direction as the damping force. This effect (the fan moves in the CCW direction) produces negative damping (since the two forces are in the same line of action).

Negative damping is a phenomenon, when damping force, which usually opposes the driving force, acts in the same direction as the driving force. In such a case the vibration of the system is amplified.

Combination of negative damping and Coriolis effect produces this phenomenon of gradually rising vibration of the fan in a short period of time, which goes away upon regular cleaning. In the present context nothing can be done to eliminate the phenomena of Coriolis Effect and Negative damping. However, if a similar system is to be installed in the future, we would be pleased to provide necessary suggestions and recommendations so that such phenomena are eliminated right from the start.

2. Present signatures indicate misalignment and dynamic imbalance

3. Weak foundations

Actions to be taken to increase the cleaning cycle to match scheduled “production window.”

Countermeasures

1. Take care to align the rotor properly. Care to be taken while putting shims.

2. Dynamically balance the fan in two planes to eliminate the imbalance

3. Cleaning cycle can be initiated when vibration of the fan on the bearings reaches 7 mm/sec (rms). It is safe to run the fan upto this point.

4. Monitor the condition of the foundation by taking vibration measurements in displacement and acceleration modes. Displacement should be taken in the horizontal direction on the topmost accessible point of the columns and at the base.  Acceleration should be taken in both vertical and horizontal directions. Displacement should not cross 50 microns in the horizontal direction or at the base of the columns. Similarly acceleration both in the vertical and horizontal directions must not cross 1.5 g. This would ensure safety of the equipment. In case it crosses corrective actions are to be taken to rectify the foundation.

Result:

After alignment and dynamic balancing in two planes vibrations came down to below 1 mm/sec and maintained its reliability till the next cleaning cycle (10 to 14 days) which matched the scheduled production window of operation — thus avoiding unplanned downtime.
Dibyendu De
dde@rgbwaves.com
9836466678