Contamination Control for Enclosed Gears
Most contamination control strategies for gear drives focus too much on coarse particles. In this context, coarse refers to particles in excess of 20 microns in size. At this size range, even though particles are invisible to the naked eye and are, on average, three to four times smaller than the cross section of a human hair, damage can still occur. But it is particles in the sub-10 micron size range that cause most of the damage in most gearboxes. This stands to reason when you consider the typical dynamic clearances in a gearbox range from a few tenths of a micron to around five to 10 microns, depending on load, speed and design. Particles in the one to 10 micron size range are often referred to as “silt-sized” particles.
To illustrate the effect of silt-sized contaminants, consider the graph shown in Figure 1 that shows the effects of fluid cleanliness on gearbox life expectancy. For many industrial gear drives running in typical plant environments with no silt control, the level of fluid cleanliness is often 22/20/17 (c) or dirtier. Based on the data presented in Figure 1, by maintaining fluid cleanliness at or below the optimum levels of ISO 18/16/13 (c) or cleaner, the life expectancy of the gears and other oil wetted components of the gear drive should be at least twice as long.
Figure 1: Relationship between gear oil fluid cleanliness and life expectancy
For water, a similar relationship holds between the level of contamination and the mean time between failures (MTBF). While the hygroscopic nature of oil makes it next to impossible to keep gear oil completely free from water, keeping water at or below the saturation point is the key. For many conventional gear oils, the saturation point of the oil at typical gearbox operating temperatures ranges around 400 ppm-600 ppm of water (0.04%-0.06% by volume). For a gearbox that holds approximately five gallons (20 liters) of oil, that equates to as little as 1½ teaspoons of water. Once the saturation point is exceeded, water will come out of solution into either the free or emulsified state. In this condition, the deleterious effects of water, which include loss of film strength, rust and corrosion, increase exponentially, seriously impacting equipment life.
This problem is most pronounced in gear drives that operate intermittently at low ambient operating temperatures. While 500 ppm of water in a gearbox operating at 140 F (60 C) typically will be all in the dissolved phase, shutting down the gearbox and allowing the oil to cool to 32 F (0 C) will cause most of the water to come out of solution.
Water also has a secondary effect on gear oils. Many of the additives used in gear oils are either water soluble or react with water. As such, whenever gear oil is left saturated with moisture due to either an extended shut down period or inappropriate new oil storage, additives can be either stripped or rendered ineffective. For most gear oils, water levels need to be kept dry enough so that any water that may be presented is completely dissolved. While not possible in some circumstances, practical limits for water in gear oils should be below 200 ppm-300 ppm (0.02%-0.03% by volume).
Gearbox Maintainability and Contamination Control
Controlling contaminants within gear drives requires a concerted effort to assess each possible ingression source. Even something as simple as changing the oil can result in a significant amount of particle and moisture ingress unless the utmost care is taken. The first step in controlling contaminants is to review all possible ingression sources. These include both contaminants introduced from the outside, as well as contaminants created internally. Some of the more common sources include:
- Airborne dirt and moisture
- Water from wash down/sanitation
- Water from the production process
- Unfiltered new oil
- Internally generated wear debris
- Byproducts of oil degradation.
With any contamination control strategy, the first place to start is to look at external sources of ingression. Most external contamination ingression in gearboxes comes from the breather/vent port. This stands to reason since many gearbox designs utilize a combination breather and fill port. Careful examination of the fill port/breather cap often reveals little more than a course sponge or wire wool/mesh to restrict contamination ingress. Wherever possible, older style breathers or combination breather/fill ports should be replaced with modern, high efficiency breathers (See Figure 2). In very low humidity environments, standard particle removing breathers should be used. These should be sized based on the anticipated air flow requirements and rated to remove silt-sized particles in the sub-five micron range. However, in most plants and industrial environments, moisture is an issue, especially since many gear oils are hygroscopic. Where airborne humidity or process water ingression is an issue, it’s necessary to remove not just silt-sized particles, but also moisture from the air as it enters the gearbox headspace. This requires the use of desiccant breathers, which include both a particle removal element capable of eliminating silt-seized particles and a desiccating media, often comprised of silica gel, to remove all traces of moisture from the air as it enters the gearbox.
Figure 2: Old style breather/fill ports should be replaced with desiccant breathers to prevent particle and moisture ingress, as well as quick connects to all new oil to be added and offline filtration to be performed non-intrusively
While desiccant breathers are effective for removing particles and moisture from the air, in some environments where a lot of moisture and humidity is present, the life expectancy of the silica gel can be a little more than a matter of weeks. Under these circumstances, a more cost-effective solution may be the use of a hybrid breather that remains sealed when no air exchange is required. In this case, thermal expansion and contraction of the headspace as the gearbox heats up or cools down is controlled through a bladder that expands or contracts to equalize pressure. If a significant pressure differential exists, for example during start-up, a series of check valves on the bottom of the breather open to equalize pressure between the gearbox headspace and the environment. Unlike standard desiccating breathers, the advantage of hybrid breathers is that the system is nominally sealed, preventing contamination ingress and preserving the life of the breather. Depending on application and environment, these so-called hybrid breathers can last as much as five to 10 times longer than the life of a conventional desiccant breather.
Having a desiccant or hybrid breather and removing other sources of contamination ingress is an excellent first step in any gearbox contamination control strategy. Eventually however, there will be a need to open up the gearbox to change oil, check oil level, etc., and in doing so, it’s easy to undo all the benefits provided by high quality breathers. To illustrate this point, consider the way oil is changed on most splash lubricated gearboxes.
Typical Oil Change Strategy:
Since the oil must be changed with the gearbox shut down, the oil inside the gearbox is typically colder than during normal operation. As the oil cools, the viscosity increases, making it difficult to drain all the old oil out of the drain port. To reduce the amount of time it takes for the oil to be drained, most mechanics are prone to removing the breather or fill port to increase flow rate. However, by doing so, the effect on contamination control can be disastrous. Draining five gallons of oil from a gearbox requires the equivalent volume of air to enter through the open port, which in most ambient plant environments is enough to increase the effective ISO cleanliness code and moisture content within the gearbox by several orders of magnitude.
The solution for controlling contaminants is to configure the gearbox to remain sealed during all phases of normal operation, including routine planned maintenance such as level checks and oil changes. This can be easily achieved by modifying the drain and fill/breather ports with simple adapter kits that permit multiple access points to the gearbox without opening the gearbox sump to the environment. This kit, which is used to replace the breather/fill port, allows for the installation of a desiccant breather and quick connect fittings to facilitate the addition of new oil without opening the gearbox. By combining this adapter with a simple quick connect fitted on the drain, this gearbox can be maintained without ever being exposed to the ambient environment.
Maintaining the correct oil level is also critically important, particularly in smaller, splashed lubricated gearboxes where an oil level variation as small as 1/2-inch can mean the difference between success or lubricant starvation. Because of this, routine oil level checks are an important part of any gearbox preventive maintenance program. To facilitate level checks, many gearboxes are equipped with a dipstick style level indicator. Although these are effective when the gearbox is shut down, they cannot be used effectively with the gearbox running and often times create an ingression source for contamination. Wherever possible, external level gauges should be used. The most common type is a brass fitting with a glass or plastic clear tube. It allows the oil level to be quickly and easily checked without pulling a dipstick or opening up the gearbox. Wherever possible, the top of the level indicator should be plumbed back to the top of the gearbox – ideally to the fill port adapter – so the system can be kept free of contamination but still read the correct level. High and low (shut down and running) levels should be marked on any level gauge to indicate the correct oil level under any operating condition.
The importance of isolating the lubricant from the ambient environment cannot be overstated. Even oil changes can be done without exposing the gearbox to the outside environment by using a filter cart with a manual filter bypass to pump the drain using the quick connect and then using the quick connect on the fill port adapter to add the new oil without removing the breather or adapter.
Even with a good quality desiccant breather, adapter kits and quality seals, gearboxes still need to be filtered to achieve optimum levels of contamination control. But while some gearboxes with circulating oil have full flow filters, most gearboxes have no permanent filtration, and even where filtration is present, most full flow gearbox filters do very little to remove silt-sized particles or moisture.
For gear drives, the secret for precision contamination control is the use of supplemental offline filtration. This simple strategy, which involves taking a small amount of oil from the wet sump, passing the oil through a high efficiency filter and returning it back to the sump, has proven to be very effective at maintaining optimum levels of cleanliness in gearboxes.
The simplest is to use a permanently installed bypass filtration system. This system has a pump and two filter housings: the first housing being used either to remove water or large particles, with the second filter rated to remove silt-sized particles. Flow rates for this type of system should not exceed 10% of the total oil capacity (for example, no more than 5 GPM for a 50 gallon sump), but even with just a small amount of oil passing through the filter at any given time, these types of systems can effectively control particles and moisture to very low levels.
In some instances, installing a permanent filtration system on all critical gearboxes within a plant can be cost prohibitive. Under these circumstances, a portable filter cart can be used in conjunction with quick connects on the drain and fill port adapter to periodically decontaminate the gearbox. Just like permanent systems, flow rates should be controlled to 10% or less of the total volume to prevent foaming, aeration and oil starvation to the gearbox, while the use of two filters in sequence allows for moisture and large particle control, as well as small particle filtration. Ideally, whenever using a portable filtration system, the cart should be left connected for a sufficient time so the oil is passed through the filters at least seven and preferably ten times. For all but the largest gearboxes, this equates to a matter of hours, making the use of a single cart for a multiple gearbox highly practical. Wherever possible, a single cart should be dedicated to a single type and grade of lubricant to avoid cross-contamination.
Gearbox Contamination Control – It Is Possible!
Controlling contaminants in gearboxes can be challenging, but with a concerted effort, it’s possible to control particles and moisture to very low levels. Figure 3 shows the impact that 24 hours of offline filtration can have on a roll drive gearbox on a paper machine, with particle levels dropping from 24/21 (R6/R14) to a remarkable 12/9!
Figure 3: Particle levels over a 24-hour period using offline filtration on a roll drive gearbox on a paper machine
In the third and final part of this series, we’ll examine critical success factors for sampling and analyzing gear oils.