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Gearbox applications present many challenges when it comes to achieving and maintaining an aggressive level of oil cleanliness. A balance must be maintained between what is financially feasible to achieve and what is absolutely best for the machine.
For many years, a high ISO cleanliness level as measured through particle counting was considered normal by many laboratories and end users as it relates to industrial gearboxes. This was considered to be so much of a fact that oftentimes, particle count testing was not even performed on gearbox applications. Through education and countless case study presentations, the importance of a clean gearbox as it relates to contamination is now widely understood. Now we must learn how to properly achieve our target cleanliness levels while still keeping within a justifiable budget.
It has been estimated that it can cost nearly 10 times more to remove contamination than what it takes to keep contamination out in the first place. In order to keep contamination out of our gearboxes, we must consider the technologies that are available on the market today.
One of the most common types of contamination control devices for gearboxes is the breather. It is common for a gearbox to be sent by default from the manufacturer with a simple vent port/plug. These plugs are not sufficient in keeping out contamination at the size levels that can cause harm to the internal components. Aftermarket breathers are generally a viable option:
Spin-On Filters - Spin on filters are a good choice for a breather provided the appropriate micron level of filter is used. It is generally recommended that a 3um filter be used in a breather application. These types of filters are good choices in applications where water-based fluids are used.
Desiccant Breathers - These breathers allow for air exchange in a gearbox. While the gearbox breathes, the air passes through a 3um particulate removal phase as well as a desiccant moisture absorbing phase. These breathers help to keep particles out as well as help to remove any moisture from the environment as well as any moisture that may build up internally due to condensation.
Expansion Chambers - These components allow for a complete enclosure from the environment while still allowing the component to “breathe.” Expansion chambers have an internal bladder that expands and contracts with the requirements of the gearbox.
Hybrid Style Breathers - These are a combination between the desiccant breathers and the expansion chambers described above. The advantage to these breathers over expansion chambers is that the presence of the desiccant media will allow for water vapor removal that may have become present through condensation. These types of breathers are ideal for applications where frequent wash downs occur, high humidity areas, and those applications located outside.
One often overlooked option for gearbox contamination control is the upgrading of seals. A common approach to seals on high speed and low speed shafts of gearboxes is the use of lip seals. Occasionally these lip seals will utilize grease in order to help keep out contamination. While lip seals are capable of keeping out contamination, they are certainly inferior to labyrinth style seals (Figure 1).
Figure 1- Lip Seal vs. Labyrinth Seal
Labyrinth style seals are most often associated with pump applications. The move to these types of seals for gearboxes is becoming more of a common decision as industry learns of the overall destructive nature of contamination and the superior performance of labyrinth seals.
One other source of contaminant ingress is through the process of oil level checking. Unfortunately, the two most common methods for checking oil levels in gearbox application include either using the supplied dipstick or via a level port that must be removed for level confirmation. Both of these methods have the potential to introduce unwanted contamination to the system.
Modifications that may be considered for level checking include the addition of a bull’s eye style sight glass into those areas where a level port exists or adding a stand pipe style level gauge to the drain or auxiliary side port of the gearbox. Simply adding a stand pipe level gauge does not fully address the possibility of contamination, however, as it is fully possible to experience contaminant ingress through the vent hole of the level gauge itself. Applications that utilize an external level gauge should also have the gauge vented back to the case or to the breather assembly via a T-style fitting.
Once the decision to keep out contamination is made, we then must consider how to remove the present level of contamination and any future contamination that may occur. In performing this task, we must consider two basic approaches.
The first option is to utilize portable filtration. In order to correctly use this technology, some minor equipment modifications must be made. Ideally, quick connect fittings will be used in order to hook up the filter cart without opening the box to the environment. A common choice of quick connect fittings is the ISO B industrial interchange. The best approach to this is to use a female coupling on either the discharge or return and use a male coupling on the opposite end. This will help to reduce the chances of hooking up the filter cart lines in reverse. It has been found, however, that the female ISO B industrial interchange fittings are twice the cost of the male. This has resulted in many end users simply using male fittings on both the suction and returns for the gearboxes and applying female fittings to both lines on the filtration unit.
The second option to consider is mounting a permanent offline kidney loop filter system on the gearbox. This will allow for continuous condition and contaminant removal from the lubricating oil. During the installation of this type of filter system, it is important to include the installation of an appropriate sample port for overall condition monitoring via oil analysis.
While having a basic understanding of the technologies available to keep out contamination and what is available to remove contamination is important, the process by which equipment is chosen for modification is equally important so precious dollars aren’t lost modifying non-essential equipment.
Many manufacturing facilities have undergone some type of criticality assessment of their equipment. There are many different ways of assigning criticality ratings. So long as the site completely understands the level of criticality assigned, selecting appropriate target cleanliness levels should be fairly uniform across the board.
Figure 2 - Cleanliness Levels
In many facilities and contamination control programs, a common approach is to assign a “blanket” target cleanliness level simply based on component type. Assigning target cleanliness levels in this manner is certainly not consistent with the goal of component specific objectives. As can be seen in Figure 2, using a target cleanliness code of 16/13 for industrial gearboxes suggests that the desire to have a “very clean” status is prevalent. While this is certainly the case for high criticality, process critical components, do we truly need to spend the same time and effort on components that are a bit lower on the criticality ranking?
It is fully feasible for some equipment to be considered critical in nature simply due to component replacement cost, labor charges, etc., yet not be so critical as it relates to overall process. Equipment that falls in this category may very easily function at what would be a “clean” level in Figure 2, or a target of 18/15. Arguably, it is much easier to achieve and maintain a cleanliness level of 18/15 than what it is to achieve and maintain a 16/13 cleanliness level.
Figure 3- Noria Reliability Penalty Factor Worksheet1
One sure way to help determine the optimum level of cleanliness for a gearbox is to calculate the Reliability Penalty Factor1 (Figure 3) and the Contaminant Severity Factor1 for each specific component. These factors can then be used to help suggest an optimum target level of cleanliness using the Target Cleanliness Grid (Figure 4). Once this is gathered for each component, a review of the required cleanliness levels can be made and then a value assigned utilizing various levels of criticality as the cut off points.
Figure 4 - Noria Target Cleanliness Grid
Using this method of assigning target ISO cleanliness codes will certainly help to fine tune the lubrication and contamination control program. By assigning appropriate levels of cleanliness, equipment such as portable filter carts can be used where they are truly needed rather than trying to clean up a semi-critical gearbox to the same level that a process/highly critical gearbox would need to achieve.
A food processing plant applied a blanket approach to cleanliness. The approach used stated that gearboxes should have a cleanliness level of 22/20/18. Once an understanding of the effects of contamination on machines and oils had taken place, the decision was made to change this target cleanliness level to an 18/16/13.
As expected, 99% of the monthly average of 75 oil samples tested were returned with a high level of alarm for cleanliness. In rather short order, the CMMS became inundated with work orders for some type of contamination removal technology. What had not been considered, however, was that few of the gearboxes were actually equipped with the appropriate modifications for portable filtration.
After 6 months of sampling, the process by which the target cleanliness levels were assigned was addressed. Taking a criticality approach, those gearboxes which were considered process critical were assigned the target cleanliness level of 18/16/13. Based on sump volume, some of these gearboxes were fitted with permanent offline filtration while the remainder of the process critical gearboxes were set up on a regular schedule for portable filtration and sampling.
The gearboxes that were deemed critical due to other means than process were assigned a target cleanliness value of 18/14. These gearboxes were then modified appropriately to be able to achieve and maintain this level of cleanliness
While the initial drive to improve plant cleanliness was there, the initial approach was wrong. This resulted in a severe increase in work orders that were ultimately impossible to manage and deploy. Taking a well thought-out, planned approach allowed for a much easier and meaningful transition to overall equipment cleanliness.
A mining facility decided to implement oil analysis on many of the site gearboxes. The average monthly sample volume was 120. The maintenance team failed to mention the lack of contamination control measures in place. Additionally, the oil sample reports were going through a third party before being received onsite.
Due to the operating environment and the overall lack of any contamination control measures or proper sampling techniques, every single sample report went back to the customer in a red/critical condition. While the recommendations on the reports were to modify the gearboxes for proper sampling and portable filtration, the work orders added to the CMMS were for oil changes.
Consequently, this resulted in an average of 120 oil change work orders added to the system each month. Obviously this was not the best approach.
This facility eventually realized that the ability to receive viable recommendations on their equipment was very limited. This resulted in a suspension of the oil sampling and an adoption of lubrication best practices as it relates to proper sampling modifications.
It should be noted that this case study is ongoing and an update can be expected at a later date.
Generally speaking, when deciding to perform portable filtration on a component, a lower limit sump volume is generally put into place of one gallon. In other words, components with sump volumes less than one gallon generally do not get recommended for portable filtration.
A recent walk through and review of a grain facility’s lubrication program suggested the contrary. This facility has twelve spouts for ship loading and unloading. Each spout has hoist and sleeve gearboxes. The sleeve gearboxes have a sump volume of approximately ½ gallon.
With careful consideration to the importance of the sleeve gearboxes and the impact of failure, it was determined that these components were critical enough to maintain a strict level of cleanliness. Consequently, these gearboxes were modified with quick connects for portable filtration, a hybrid style breather for contamination control and an external standpipe style level port for inspections.
In the initial criticality assessment, these boxes were considered fairly low on the list. The deciding factor for these boxes lay more on the criticality of the entire spout rather than the criticality assigned to that specific gearbox.
Study after study has been made indicating the benefits of contamination control in lubricants both new and in service. It is important to understand that cleanliness can be achieved in gearbox applications just as it can be achieved in hydraulic applications. The difficult part is being able to make that financial justification as it needs to be made on a case by case basis.
The era of applying a blanket standard must be gone. While it is perfectly feasible and recommended for high process critical gearboxes to have a target cleanliness level of 16/13 or better, it is also perfectly acceptable for other gearboxes in the facility to have looser targets. This allows for the most attention to be spent on the highly critical components yet still improve on the reliability of our less critical applications.
1. Noria Fundamentals of Machinery Lubrication Course Manual