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We have a cooling tower with multiple fans driven by Amarillo 1712.5 gearboxes using Mobil SHC 630 oil. We have had multiple failures which I believe are initiated by scuffing during high torque startups. Would going to a higher viscosity (SHC 632) oil help? We already operate at around 215 degrees F and if the higher viscosity oil doesn't relieve the friction we will operate at even higher temps.

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I'm a bit surprised at your operating temperature of 215Β°. That would be excessive in my experience for CT drives which include those made by Amarillo. I would initially do everything in my control to reduce that temeperature before considering a lubricant change. How is that temperature being determined? Are the drives properly filled with care to make sure they are not overfilled.
RM
From catalog data (for the 1712.5 which you specified, not the 1712.5W) it appears as if the shaft centerline is 12.0 inches up from the bottom and the specified oil level (bullseye) location is 10.25 inches up. See attached graphic which isn't necessarily to scale. The likely oil level is about 1/3 of the way up on the helical gearing. Does that all fit with your machine?

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  • 1712_5_CT
RM
John, I for one would be interested in what would "truly impact temperature" but there doesn't seem to be an attachment to your original post.

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RM
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 β€œstart 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.

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RM
In all my experience, which includes 20 years of direct gear related experience, I have to say that I have never seen a lubricant that made a night and day difference in the operating temperature of a gearbox, assuming the baseline lubricant is a properly selected lubricant as in your case (Mobil SHC 630). I suggest you review "High-Performance Industrial Gear Lubricants for Optimal Reliability" located at http://www.powertransmission.c...ues/1010/mckenna.pdf. The graphics as well may help you in identification of your failure mode.
RM
John, that would be rgf12's decision to try your product. He still should check with Amarillo because the Amarillo lube spec sheet (added 8/8/2013) for this type unit specifically states "Extreme Pressure additives are not recommended" and I see your product does have such an additive package. In any event, the temperature is excessive from my experience with this type unit and the potential reasons need to be investigated. Most manufacturers of gearboxes for cooling tower service specifically recommend against the use of EP additive type lubricants. It is very easy to trade one problem thought to be addressed by the EP additive package for another problem. Water in the lubrication of a cooling tower drive is virtually a given due to the nature of the service. Beyond the normal consideration of its effect on viscosity and potential for increased corrosion, when we combine the effects of moisture, high temperature and most EP additives we can seriously decrease the life of a cooling tower gearbox. This is where the chemistry end of lubricant selection, which is all too often not considered, enters into the picture. Most EP additives are compounds of phosphorous and sulfur. Elevated operating temperatures of a lubricant with EP additives, in the presence of moisture can seriously break down a typical EP mineral oil creating corrosive acids and other compounds that can cause distress to a gearset, often in very short periods of time.

rgf12, examine the attached lube sheet (side view) and note the presence of two 1" NPT pipe plugs on the same side of the casing, only one of which is called out as the oil drain. The lower of the two, 1-11/16" off the bottom of the unit is the oil drain. There is a 2nd 1" NPT plug, 5-1/2 inches away from the oil drain but 2-3/4 off the bottom. Just a wild speculation but if by mistake the upper plug is used as the drain and 22 gallons of oil is added (by measure) the unit will be overfilled, hence the reason to carefully judge oil level with the provided bulls eye. Over filling is a common cause of overheated units.

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RM
Last edited by Registered Member
quote:
Yes across the line starts, we have looked in to soft starts but that is a design team project. I would like to see that in the spec for all cooling towers but $$$ rules not reliability.


Unfortunately across the line starts can be a real and significant issue and even a lubricant change won't likely help. You either have to go with soft start methods or design the gearbox with a significantly higher service factor to accommodate the starting torques of the motors. Higher service factors often aren't always practical since they can drive up the physical size and weight of the gearbox. You might approach Amarillo and see what you can do as far as replacement gearing as you move forward. You basically want a heat treatment method that can maximize the durability and strength ratings of the pinion.

By the way a couple of my questions remain unanswered. What is the total ratio and input RPM of this drive so I can look up its thermal rating? Is it equipped with the optional pump and an external cooler?
RM
I suspect that you are near or possibly exceed the thermal rating of the gearbox, although that is not the cause of your tooth distress. Amarillo isn't clear on their thermal ratings but the mechanical rating of that drive with a SF = 2 is 244 HP. There used to be an AGMA spec 420.04:1975 Practice For Enclosed Speed Reducers Or Increasers Using Spur, Helical, Herringbone And Spiral Bevel Gears that defined the thermal rating for gearboxes that did not use external cooling. It was defined as the horsepower a unit could transmit continuously for 3 hours or more without exceeding a sump temperature of 200Β° F or a rise of 100Β° F over ambient. It appears with your operating temperature of 215Β° F that both of those limits have been exceeded. That spec stated that if the thermal limit was exceeded, external cooling should be supplied. AGMA 420.04 was superceded by AGMA 6010:1997 (Revised 2003) Spur, Helical, Herringbone, and Bevel Enclosed Drives. I unfortunately don't have a reference copy but likely the definition of thermal rating doesn't vary much, if at all, from that of AGMA 420.04.

Beyond what I had already mentioned of pursuing methods to maximize the rating of replacement gearsets or providing soft start methods I don't think there is much you can do that is either easy or inexpensive.
RM
I guess what compounds the heat issue is the restricted airflow in the vicinity of these boxes. The GB's are mounted underneath the fans and the fans have a large hub pan that restricts the airflow up and around the GB's. I haven't seen this arrangement in-person but this is one of the field guys theories. Thanks for all your help. I believe we have a copy of 6010:1997 floating around here that I will check.
RM