Wear Particle Analysis of Gearboxes

1. How Do You Determine Which Gear Boxes to Monitor?

The criteria for determining which gearboxes to monitor with oil analysis are dominated by one overriding consideration, criticality. The definition of a critical piece of equipment is which would cause the most disruption if it broke down. This could be a large gear reducer on a turbine set or a small gearbox on a waste water pump. Even if it has only two quarts of oil in it, it may be deemed critical and therefore requires monitoring. Traditionally, however, we take the criticality of the gearbox and the quantity of lubricant in the sump into consideration. Small, non-critical gearboxes (less than 5 gallons of oil) are not sampled routinely, as the sampling effort is not cost effective.

2. Frequency of Monitoring.

Large critical gearboxes are best monitored on a monthly basis. Examples of these types of boxes would be those employed in extruder trains at polymer plants or gearboxes driving calender rolls at pulp and paper facilities. Gearing which may be used intermittently or has redundancy may be monitored on a quarterly basis. Small gearboxes (less than 5 gallons of oil) may be monitored on an annual or biannual basis, but in this particular case, the object of sampling is not for routine monitoring, but as a planning tool for preventive maintenance (PM) functions. An excellent example of this is a chemical plant with several hundred small gearboxes that need oil change outs per the preventive maintenance schedule. Normally the maintenance crew would just change all the oil, a time consuming process, as well as quite expensive. By sampling all the boxes prior to scheduled maintenance, the oil analysis can determine:

• If the oil needs to be changed
• If the pump needs to be replaced
• No changes are necessary

Now the PM function is directed at the problem units rather than working on every box. This enables maintenance planners to use their resources with greater efficiency.

3. Where to Take Oil Samples

An oil sample is only as good as the person and method used to take it. To this end, it is imperative that the sampling be performed in a way that will obtain the most representative sample. Taking a sample from a drain pipe at the bottom of the sump serves no purpose, as the sludge and sediment present do not indicate what the current wear conditions are. The “quarter up-quarter in” rule is the most desired way to get a representative sample of the gearbox sump, (i.e., quarter of the height of the reservoir up and a quarter of the way in from the sidewall). The easiest way to obtain a good representative sample is by using a thief pump and a tube from the sump cap; however, safety concerns to the sampling person prevent it from being the method of choice. Because of this, sampling valves have become quite popular and are in widespread use. Originally aimed at taking pressurized oil samples, attachments nowadays make it easier to take a sample from a non-pressurized reservoir with a thief pump. Sampling is performed when the gearbox is in operation. Make sure to always flush any dead leg or tubing that you are using to extract the sample. Additionally, rinse the sample bottle with approximately a third of its capacity, just to ensure you have no cross contamination.

4. How Do You Set Alert or Alarm Limits? Have There Ever Been Modifications?

Most gear oil applications are concerned with the effects of shearing and high load bearing characteristics and so, wear metals and viscosity testing is of primary importance. Although all gear manufacturers give lubrication selection guidelines, most do not publish guidelines for wear condition. Some progressive companies do publish some absolute alarm limits for wear, but most leave the wear monitoring to the end user. The main reason is that their equipment is used in such a wide variety of applications and environments that it is difficult to establish universal alarm limits. Because of this, gearbox users rely heavily on baseline trends and statistical alarming techniques to determine alert and alarm limits. With the above-mentioned difficulties, this is especially suited for wear debris monitoring. In the laboratory, we consider the size of the gearbox (sump capacity), application, criticality, environment, load and lubricant when we begin to apply alarms. It is very important to set your alarms for practicality. A “cry wolf ” condition-monitoring program jeopardizes its existence by wasting time and money. Suggested alarming limits for gearboxes are outlined in Table 1.

Wear Particle Analysis techniques, such as ferrography and patch testing, can yield information about size and morphology of wear particles, with the goal of determining the source of wear and degree of severity. Wear Debris Analysis relies on qualified analysts to make subjective observations and subsequent recommendations.

5. Example of Gearbox Problem Identified and Resolved.

A Tennessee plastics facility began using an oil analysis program on their critical extruder gearboxes. Product demand was high and the reliability team was concerned about unscheduled work stoppages. Vibration analysis was also part of the program. Samples were taken on a monthly schedule, and the oil analysis test package performed to determine wear, and if further analysis was needed. Oil analysis (and vibration analysis) showed normal operation and a baseline trend was established. Several months later, spectrometric analysis showed abnormal increases in iron and aluminum with an increase in the level of silicon.(See figure 2). Recommendations were to filter the oil to remove sand/dirt contaminants and inspect the gearing for signs of abrasive wear. The oil was changed and the next sample revealed a dramatically reduced level of wear and contaminant debris. The same problems returned in April and similar recommendations were issued. New sampling valves were installed at this time; to ensure that poor sampling concerns were minimized. Results for oil analysis wear were so high in July that the Reliability Engineers took the unit out of service. It is worth noting that vibration analysis did not indicate problems. As part of the root cause analysis investigation as to why the wear was high, ferrographic analysis revealed severe sliding ferrous gear wear particles up to 50 µm diameter and large amounts of aluminum particles. (See figure 1). The oil filter debris was also analyzed and it revealed aluminum particles up to 200 µm in diameter present. Visual inspection of the gear housing correlated with what was found in the oil (i.e. that the thrust bearings on the output shaft were worn away well beyond operational tolerances). It was suspected that there was movement of the shaft in the axial direction. There was no redundancy for this unit. After a quick check to see that production was not affected directly by the damage, it was placed back in service and a new box ordered.

Equipment: Extruder Gear Box Make & Model: Werner Pfeiderer W/P Z58 Lubricant: Texaco Meropa 320 Sampling method & location: valve, reservoir Reservoir capacity: 100 gallons Cost of new unit: $70,000 Lead-time to delivery: 3 months Estimated lost production time: $432,000 Oil analysis cost for the year on this unit: $315
	Figure 1: Large Aluminum Particle
Table 2: Benefits to Cost Ratio

A new unit was quoted at $70,000, with a three-month lead-time. In summary, the oil analysis and combination wear debris analysis pointed to serious problems, well enough in advance that the company could prepare for an expected failure. This is borne out by the potential losses in production time that would have arisen if such a problem had occurred. Table 2 illustrates the very high benefit to cost ratio of oil and wear particle analysis.

Figure 2: Wear Metal Trend

6. What Are The Greatest Challenges to Effectively Monitoring Gearboxes?

The most significant challenges effectively monitoring gearboxes are:

1. Establishing and maintaining a proper sampling technique which will ensure that a good sample is taken repeatedly and consistently.
2. Ensuring that the analysis performed on the sample is sufficient for the equipment being monitored. Like all tools, oil and wear particle analysis can be misapplied. Too often, a very basic oil analysis performed by a lubricant supplier is considered to be adequate. Unfortunately, not all the tests necessary to determine if a fault are run, thereby lulling the gearbox owner into a dangerous false sense of security. When a fault goes undetected, everybody suffers- management feel the reliability program is an unnecessary cost center, and the maintenance people involved immediately blackball the entire oil analysis technology as unhelpful and futile.
3. Continuing to educate reliability personnel about oil and wear particle analysis, and the benefits it can afford is also a challenge. Greater effort in this area will reduce the possibilities of applying the misapplication of the technology. Greater effort by oil analysis providers is needed here.