WHAT IS A LEPRECHAUN?
Leprechauns are mythical creatures, often represented as small mischievous old men. Much like undetected wear to mechanics, leprechauns adore playing tricks on the credulous man. Hidden degradation often does not generate any sign until it is too late. Like the leprechaun, hidden failure haunts maintenance personnel and always strikes at the worst time, e.g., when production is needed and parts are not available. It is possible to counteract the leprechaun with effort, knowledge and data. These are key elements to beat hidden failure and find the pot of gold link to reliability!
The centrifuge decanter is essential equipment to the production of ethanol. It is used to separate solid residue from whole stillage. Given the volume of product that needs to be separated, the centrifuge only can be stopped for a short period of time. For this reason, it was decided during commissioning of the Greenfield Ethanol plant in Quebec, Canada, to maximize the use of predictive technology with vibration, oil analysis and ultrasound as part of a preventative maintenance plan on these machines. To optimize the quality of the vibration signal, a triaxial accelerometer was installed directly on the gear drive torque arm to collect the gearbox signature. The predictive technician thoroughly researched the particular design of these machines to come up with all failure frequencies. Good care was also given to oil sampling, but since the gear drive is rotating with the bowl, collection time after shutdown is critical before particles settle out.
FIRST SIGN OF DEGRADATION
Two years after commissioning, vibration trends started to rise. The vibration technician identified the problem from the second reduction of the planetary gear train. With good historical data in hand and no degradation sign from the oil analysis, time was taken to order a new drive and repairs were made during a planned shutdown. The centrifuge was back to the normal vibration value and everybody (expect the leprechaun) was happy with the result of the maintenance process. To be proactive, the maintenance manager had his personnel repair the defective gearbox. This allows the vibration technician to confirm his diagnosis.
The first discovery was one planet on the secondary gear train had a broken tooth. This explained the higher value of gear mesh and confirmed the predictive technician’s diagnosis. But maintenance personnel found a defect that was not foreseen: a Leprechaun! Not a real leprechaun, but a hidden failure that was showing no dynamic sign of degradation.
The last reduction sun was missing metal flake on the loading zone. This had to be further investigated because the gear mesh of the third reduction did not show much vibration energy, making it almost impossible to detect. Root cause analysis was performed to identify causes and solutions.
Figure 1: General design
WHAT IS SPALLING?
Flaking of metal surfaces comes mainly from two sources on the gear drive: micropitting and fatigue spalling. Micropittings are small craters (pits) that form on the load zone of the teeth, while fatigue spalling deteriorates the loading surface of the gear. Micropitting is a form of lapping that occurs during the first stages of commissioning; it corrects small machining error and usually stops after the commissioning stage of the equipment. Spalling, on the other hand, is a serious degradation of gear surface. Fatigue spalling may be a normal aging process, but after several million loading cycles, metal becomes brittle, much like a small wire that breaks when bent back and forth. Similar surface defects happen to gears. Steel selection, tooth surface, machining quality, minor involutes profile errors, teeth hardness, load and load cycle, damping, oil and oil viscosity, speed, and temperature all affect fatigue of the steel. We have to go back to the basics to really understand the roots of the failure.
Figure 2: Planetary sun, third reduction, planetary sun gear after two years with ISO 100 PAG oil
WHY AND HOW?
Centrifugal decanters are, by design, a complex machine that rotates at high speed and is filled with viscous material that causes some imbalance. Vibration adds load on the oil film by adding force on the teeth as the parts try to keep moving due to vibration forces. The inertia of the internal gear adds load to the already present driving load during the imbalance period, especially with planetary gears that float inside the inner ring gear. The design of this gear drive combines all three stages inside a unique housing containing oil for all three stages. Since polishing of the load zone is present, we assumed a mix lubrication regime. German engineer Richard Stribeck defines a mixed lubrication regime as a situation where surface roughness is about the same as the thickness of the oil film. This regime permits higher surface peak to disrupt counterpart. In fact, a mixed lubrication regime polishes mating surfaces and consumes extreme pressure additives. Unbalance contributes to the problem by adding pressure onto the load zone during every vibration cycle.
Fatigue in gears is a perverse effect of cyclic load in gear operation. Each time a tooth enters the mesh, it is slightly bent under the pressure as it transmits power, then, relieves as it exits the mesh. The stress acts on the root of the teeth and improper design may lead to early breakage of the teeth. A larger teeth profile angle has been developed to palliate this problem, however, it reduces the bending action of the teeth, thus resulting in higher pressure on the oil film. Teeth bending and oil film play an important role in gear mesh damping. Oil acts as a shock absorber by cushioning the impact, while teeth bending distributes load over longer periods of time like a spring. Unloaded, the film is thick, but as the load is transferred on the teeth, the film thins out, distributing the load over a larger area of the surface. Pitch line speed and oil viscosity prevent lubricant from flowing out of the mesh and keep parts separated. This phenomenon is essential for relieving stress on the contact zone. Fatigue spalling is a surface degradation due to lack of damping. First, small sub-surface cracks grow in half circles similar to fish scales. They develop into larger and deeper cracks that eventually break, creating large pits. These pits, or spalls, increase load on the surrounding area and accelerate the degradation process of the contact zone. Another phenomenon, called micropitting, helps correct profile imperfections from machining. It is a form of minor spall that develops over an imperfection to rectify the tooth profile during the breaking in stage of the machine. The difference between micropitting and spalling is major. Spalling affects gear life expectancy, while micropitting is merely a breaking in process.
IS THERE ANYTHING TO DO?
Early spalling of gears is a serious reliability issue that cannot always be avoided. It is important to determine the causes of spalling to reduce its progression. These are the possible causes of early spalling:
- Number of cycles
- Surface quality
- Temperature and oil viscosity
- Proper gear maintenance (surface quality, cleanliness, etc.).
As indicated, most factors cannot be modified. This leaves only oil viscosity and proper maintenance as options. In the Greenfield Ethanol plant case, the oil recommendation from the manufacturer was a polyalkylene glycol (PAG) with a viscosity of only 100 cSt. With the reduction ratio being 3.11 to 1, we decided to confirm oil viscosity for all three gear sets since they share the same oil.
Oil type is selected with application requirements and oil viscosity is computed from the pitch line velocity. The slower the linear speed of the gear, the higher the viscosity required to maintain a moving parts gap. The American Society of Materials (ASM) oil viscosity formula computes oil grade requirements for gear applications.
V40 = Viscosity at 40°C
V = Pitch line velocity in ft/min
Figure 4: Three stages of a planetary gear train
Pitch line velocity is quite simple to compute from a parallel shaft gearbox, however, it is much more complicated for a multiple stage planetary gear. Figure 4 shows a simplified drawing of a three stage planetary gearbox. The planetary gearbox needs at least one input speed, one output speed and one fixed part. The upper block represents the ring gear, the middle block is the planets and the lower block is the sun gear. Lines connecting each block together are the mechanical links with the speeds included. Figure 4 assists in the understanding of energy path and helps calculate the speed of the various components. It is then possible to calculate the speed from those elements with these formulas:
With the known speed, we can then calculate an oil viscosity of 358 cSt at 40°C using the ASM formula. The calculation is formulated for mineral oil and the viscosity is rectified at 40°C. It does not state the required viscosity at running temperature, which, in this case, is around 60°C, nor does it apply to synthetic oil. To palliate this problem, we created a graph with the available data from another source: AGMA 9005-E02. This American Gear Manufacturers Association (AGMA) standard recommends an oil viscosity from pitch line velocity at various temperatures. The standard uses standard oil viscosity with a known viscosity index. Since none of those oils match the physical properties of the PAG oil used, we had to build a viscosity chart with temperature data versus pitch line velocity. We then calculated the oil viscosity from the ASM formula at various temperatures:
loglog (V+C)=a-b logT
Once the AGMA viscosity was plotted, the recommended viscosity at running temperature matches the formula:
This formula calculates the required oil viscosity at running temperatures and is derived from computed data. Once the viscosity is determined, the oil has to be plotted over its viscosity curve to confirm the proper viscosity at running temperatures. With the AGMA formula, the required oil viscosity is 138 cSt at running temperatures. This is the required viscosity for the sun gear, but what about the output gear? The result is 373 cSt at running temperatures. Quite a difference! As you can see, the required oil viscosity can be very different from gear set to gear set in a single gearbox. Also, the oil has to lubricate everything, including bearings, seals and plain bearings. A large pitch line velocity difference between stages and high running temperatures presents a tough challenge for the lubrication of this application.
TESTING THE SOLUTION
We came to the conclusion that the recommended oil grade is adequate for the first and second reduction, but the viscosity is too low for the third reduction. Given the need to lubricate all three stages, we decided to test ISO 220 PAG oil from the same manufacturer. ISO 220 PAG oil is two viscosity grades higher than recommended oil, therefore, we closely monitored the torque as well as the temperature during the test. The results after two years have been excellent. New sun gear results from the third reduction show little sign of spalling and without any effect on the first and second reduction. Running conditions during the test were similar. We measured only a 5°C increase of the running temperature under the same load.
Figure 5: Planetary sun gear after two years with ISO 220 PAG oil
Hidden degradations are a maintenance manager’s leprechaun because they cause unwanted mischief. They cause mechanical failure at any moment and often at the worst possible time. The maintenance staff has to be aware of the consequences of these hidden degradations and always be on the lookout for signs of a defect. In this case, an unfortunate failure of the second reduction forced the mechanics to inspect the gearbox. The inspection revealed the fatigued state of the third reduction. After two years of testing, selection of a higher viscosity grade reduced the rate of fatigue under the same operating conditions. This case demonstrates the importance of proper personnel training in a modern industrial setting. Fixing machinery is not enough; maintenance personnel have to be looking for ways to improve machinery reliability and prolong machine life. It is the only way to defeat the maintenance leprechaun!
- Taylor James I. The Gear Analysis Handbook, Chapter 9, Tampa, Vibration Consultant, 2000.
- Kuhnell, Bruce T. Wear in Rolling Element Bearings and Gears - How Age and Contamination Affect Them, Machinery Lubrication, Sept. 2004, www.machinerylubrication.com/Read/664/wear-bearings-gears.
- Wikipedia, http://en.wikipedia.org/wiki/Gear
- Gearbox Efficiency And Lubrication, Maintenance technology, www.mt-online.com
- TLT magazine, April 2008, Lubricant selection: bearing, gear drives and hydraulics