Lubrication and Oil Analysis

As you mention high torque at startup can be a key factor. If you go to

To compound this issue these drives were utilized at a nuclear power plant and were started across the line. [comment: Who cares about in-rush current when you are next to a utility?] Another compounding effect was the geographical location of the utility; suffice it to say just south of central Canada. That meant these drives would frequently ice up and be reversed to clear the ice. However no means was present to tell the operators the drives were still turning in a forward direction. They would hit the OFF button, wait a certain period of time, then hit the REVERSE button. Frequently these drives would still be turning at 20% speed (forward) and decelerate rapidly through zero-speed, then ramp up to full speed reverse. During that entire time very high transient torques, far in excess of design were observed.

These high torques that are exhibited in high inertia systems (load inertia in excess of 10X motor inertia) are not unknown but are not well documented in the literature. One of the best papers on the phenomenon is "The Nature of A. C. Machine Torques" written by G. L. Godwin and published in the IEEE Transactions on Power Appartus and Systems, Volume PAS-95, no. 1, January/February 1976. If I can locate the paper I'll attach it here. There are also some ASME papers (1980's) by E. Rivin titled "Role of Induction Driving Motor in Transmission Dynamics".

It should be noted that the ultimate fix for the instance I cite was the installation of reduced voltage starters for the motor and also appropriate devices to indicate to the operators the direction and extent (RPM) of the rotation.

Attachment(s)

cooling_tower_starting_torque_vs._time.pdf  

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rgf12, are you certain that you have typical scuffing? Go to http://www.nrel.gov/docs/fy12osti/53084.pdf and review page 20, specifically "High-Speed Stage Pinion (Severe Scuffing)". Even though the resource is about wind turbine failure modes, the graphic of the scuffing failure is typical. Scuffing or scoring is essentially a lubrication failure. Unlike fatigue failures, which occur after many cycles of operation, scoring is apt to occur as soon as new gears are first brought up to full speed and load. Hence the importance of an internal inspection through inspection covers shortly after a machine is placed in operation. Although it is often a lubrication failure, the blame can often be placed elsewhere. For instance the design or finish on the tooth flanks are such that no lubricant could be expected to adequately perform its function although that is not likely in this instance.As you mention high torque at startup can be a key factor. If you go to http://www.geartechnology.com/issues/0609x/drago.pdf there is a good cooling tower failure analysis by Ray Drago (excellent consultant should you need one). He discusses high torque as a potential cause of issues on page 8 of the article. Having said that Drago makes the statement "Since the system was not started from zero speed during this testing, we still do not know what the actual “start from zero speed” load would be, but we can make a projection using the data available to us." I can shed some light on the "zero-speed" starting torque in a cooling tower drive. Many years ago I successfully defended my employer in a multimillion dollar lawsuit concerning numerous cooling tower drives that had failed in a very short period of time after installation. The cooling tower manufacturer blamed the presence of the 3-node torsional near the helical mesh frequency as the primary cause. Upon investigation, which included strain gages on the input shaft, it was discovered that when the motor starting circuit was closed, there was impressed on the gearbox a starting torque of 6.2X rated in the positive direction and 3.1X in a negative direction. These torques were present for six to eight cycles and then decayed to the normal starting torque of a NEMA B induction motor. At the time AGMA 490.02 permitted transients of 200% of rated torque based on a 1.0 service factor. The torque vs. start time curve can be seen in the attachment.To compound this issue these drives were utilized at a nuclear power plant and were started across the line. [comment: Who cares about in-rush current when you are next to a utility?] Another compounding effect was the geographical location of the utility; suffice it to say just south of central Canada. That meant these drives would frequently ice up and be reversed to clear the ice. However no means was present to tell the operators the drives were still turning in a forward direction. They would hit the OFF button, wait a certain period of time, then hit the REVERSE button. Frequently these drives would still be turning at 20% speed (forward) and decelerate rapidly through zero-speed, then ramp up to full speed reverse. During that entire time very high transient torques, far in excess of design were observed.These high torques that are exhibited in high inertia systems (load inertia in excess of 10X motor inertia) are not unknown but are not well documented in the literature. One of the best papers on the phenomenon is "The Nature of A. C. Machine Torques" written by G. L. Godwin and published in the IEEE Transactions on Power Appartus and Systems, Volume PAS-95, no. 1, January/February 1976. If I can locate the paper I'll attach it here. There are also some ASME papers (1980's) by E. Rivin titled "Role of Induction Driving Motor in Transmission Dynamics".It should be noted that the ultimate fix for the instance I cite was the installation of reduced voltage starters for the motor and also appropriate devices to indicate to the operators the direction and extent (RPM) of the rotation.