Motion Control Resources
Understanding Lubricants Part II - Avoiding the Pitfalls
by Kristin Lewotsky, MCA Contributing Editor
Motion Control & Motor Association Posted 11/13/2013
From adjusting your maintenance schedule to avoiding cross-contamination, these tips can help you get the longest life from your equipment.
Proper lubrication is probably the single easiest, most direct way for end-users to maximize the return on their equipment investment by supporting key components like gearboxes, bearings, and actuators. In part one of this two-part article, we covered the basics of lubrication, including a discussion of the meaning and the selection criteria for viscosity, consistency, thickeners, and additives. Here, we focus on pitfalls to avoid and best practices to follow to optimize machine life and performance.
Lubricants separate moving parts to reduce friction, minimizing heat, reducing wear, and making components run longer. Each base oil has a viscosity that determines the film strength, which is just the oil’s ability to provide that separation. Well-lubricated systems require a fixed volume of lubricant that lasts for a specific lubrication interval, after which a component needs to be either re-lubricated or replaced.
Oil versus grease
Where suitable, oil generally delivers the best protection. Gearheads, for example, are most frequently lubricated with oil, particularly when they are horizontally oriented. Because they’re generally enclosed in a housing, they work well with delivery methods like splash lubrication, in which oil resting in a pool covering the bottom part of the gears is picked up for distribution during operation. Pumps augment this process in larger systems. For the biggest gears, surface tension may not be enough to maintain lubrication. An oil mist, produced by an atomizer, might be better method for ensuring coverage.
Oil can also do a good job of dissipating heat. Gear oil can bind to certain wear metals (particulates generated by the system) and carry them away from the moving elements, where they can be removed by a filter during recirculation. These filters involve trade-offs, though - too loose a mesh and the contaminants get through the filter, too fine a mesh and the oil cannot pass through efficiently. As with all things engineering, it is a balance.
In the case of non-horizontal gearboxes, it can be difficult to keep the oil on the moving parts and it may leak out of the seal over time. For these types of applications, as well as many bearings and actuators, grease is a better solution. Greases contain thickeners to increase the consistency and hold the base oil in place. Both oils and greases commonly contain additives designed to modify their properties, for example oxidation inhibitors, rust inhibitors, and viscosity enhancers.
Although base oil viscosity plays a key role in choosing the right grease, there are also other important characteristics like the ability of the thickener to survive the environmental conditions and shear forces involved in operations. Another often overlooked issue is the compatibility of the lubricant with the seal material. General recommendations exist for the effect of the different base oil is on rubbers and polymers, but the exact properties of a seal can differ from manufacturer to manufacturer. Testing provides the safest solution.
Although making the right choice of lubricant and delivery method is important, overall performance depends greatly on execution. Users still need to be concerned about how the lubricant is introduced into the component, both initially and during maintenance, how it changes over the lifetime, and the pitfalls that can degrade performance.
There was a time large facilities had techs that specialized in lubrication. Today, that task frequently falls to general maintenance staff, which have many other responsibilities. The problem is that re-lubrication may seem simple but when performed carelessly can be worse than no lubrication at all. It’s also time-consuming. Depending on the gearbox, changing the oil can take an hour, not counting the time required to gather tools. That means that realistically, a single person can only address 10 or so gearboxes in a single day, which is a small number, considering that some machines may have 100 or more axes.
Of course, personnel time is only part of the challenge. To change oil or grease, the machine typically has to be shut down. Production lines that operate around the clock may have scheduled downtimes of only a day or two, during which maintenance staff are focused on repairs and upgrades they consider absolutely essential. It's tempting to push aside preventative maintenance for some future time. Unfortunately, that future time often translates to the 12th of never. Meanwhile, wear caused by contamination or lubricant breakdown continues. The result is more failed components to be replaced in scheduled-or unscheduled-downtime.
In these kinds of situations, so-called maintenance-free components can be very appealing. Typically motors smaller than 25 hp are designed to operate without any re-lubrication. Sealed for life bearings and non-lubricated couplings can also help eliminate lubrication headaches. Indeed, components may deliver better performance when sealed for life simply because all too often maintenance staff uses poor techniques in re-lubrication. “In an average plant, more damage is caused by people [improperly] greasing electric motors then is prevented by it," says Jarrod Potteiger, Education Services Manager and Senior Technical Consultant at Des-Case Corp. “That has led a lot of folks to just stop lubricating motors altogether. I don't advocate that, but I do advocate identifying those cases for which there are no positive benefits. That way you don't spend time unnecessarily and damage components when essentially it costs more to go out and re-lubricate than we’re saving by potentially extending the life."
Best practices for lubrication
Although choosing sealed-for-life components is a helpful strategy, nearly all industrial end-users will have a certain amount of re-lubrication they have to perform. The key is executing a properly. The volume of lubricant is an essential parameter - empty space allows the grease or oil to recirculate. Often, gearboxes have oil level gauges on the outside with high and low fill lines to simplify the process.
Free space is a particular concern for high-speed bearings. For some high-speed applications, a bearing might be packed with grease to fill only 20% of the open volume. In contrast, bearings for low-speed applications can tolerate a fairly high fill size. It’s a balance, of course. Too much grease creates problems but not enough presents the risk of leaving some surfaces unprotected.
To help prevent damage, before the component goes into operation, all surfaces that may come into contact must be coated, a process known as running in. Machine bearings, for example, should be run in when the machine is being assembled. For high-speed machine tool spindle, for example, the installer should introduce a precise amount of the lubricant and then began moving the bearing at extremely low speed while monitoring temperature for spikes to ensure only a gradual increase takes place. "If you can run this machine up to its maximum without a spike in the temperature and it stabilizes at a reasonable temperature then you've completed the run in," says Steve Mazzola, Director of Engineering and Technical Services for Klüber Lubrication. That becomes more and more critical as speeds increase.”
Contamination is one of the biggest issues in lubrication and can have the greatest impact on machine lifetime. Lubricants can be contaminated by particulates and by moisture. Of the two, particle contamination presents the biggest problem. Industrial applications like CNC tooling or wood processing generate significant amounts of particulates that can penetrate seals and mix with the grease or oil to have an abrasive effect on gearing, actuators, and bearings. "Most lost equipment life is caused by mechanical wear, and most machine wear is caused by particle contamination," says Potteiger. “If you eliminate particle contamination you can to a large extent eliminate machine wear and ultimately extend the component life."
When it comes to moisture contamination, the caustic fluids used in a washdown environments to mind, but even water can have a surprisingly negative effect. You don't need direct exposure to liquid via constant drip or high-pressure water jets. Oil can absorb moisture from the air in the type of high-humidity environments found in printing and paper processing. In the presence of thermal cycling caused by environmental extreme or even daily shutdown, water can condense in the components or even in the containers used to store or deliver oil to the system. Water can emulsify the base oil or form puddles within the housing of lubricated component. Worse, it can react with additives like antioxidants, rust inhibitors, viscosity enhancers, etc., leading to early breakdown.
Oil can be filtered to remove contaminants, assuming that the filters are properly sized and removed or cleaned at an appropriate interval. Grease, on the other hand does not release particulates. They can only be removed from the contact area by displacing the old grease during re-lubrication. When it comes to moisture, for both oil and grease, re-lubrication is the best solution.
Oil tends to break down with oxidation. That means that over time, grease will deposit residues from its base oil as well as from the thickener. Meanwhile, the overall proportion of base oil to thickener in the grease, changes. This can cause the grease to heart and yet lose its ability to dissipate heat. In this case, the best solution is to remove the component, clean it out, and re-grease.
Proper choice of lubricant and proper maintenance are particularly important in high temperature operation. Although thickeners are designed to hold the oil in place, it is not an all-or-nothing process. At higher temperatures or under shear forces, the thickener may release some of the base oil, only to resorb it when the stress disappears. Polyurea thickeners tend to be more effective at controlled release and resorption, making them a good fit for motion applications. Thickeners like lithium or calcium may release the oil too rapidly at high temperatures, resulting in net oil loss.
It might be tempting in the event of oil loss to just add more base oil in the hope of re-emulsifying the grease. That's a tricky solution, cautions Mazzola. Cross-contamination between lubricants presents a huge and frequently ignored problem, especially for greases. Certain grease chemistries can be considered miscible (mixable at any proportion) with other grease chemistries, while others are not. Mixing greases with incompatible thickeners can cause a grease to liquefy, accelerating wear; or to solidify, potentially burning out the component itself. Incompatible greases can cause each other to de-gel, which means that grease loses its ability to hold the base oil in place. The two separate so that the oil runs out of the bearing. Not only will it fail to protect the bearing rollers, it could penetrate the seal of connected components and trigger failure there, as well.
Mixing incompatible greases is an easier mistake than you might think. "The vast majority of bearings used in motors today used polyurea grease for high temperature operation, while most common greases used for the general market are lithium-based. Polyurea and lithium are not compatible, so if a maintenance tech tries to put standard shop grease into the grease fitting for an electric motor, the result could be rapid failure.
“Cross-contamination of grease is a very big problem," says Potteiger. "A lot of component failures are induced by people inadvertently mixing incompatible greases because they don't understand the issues of grease compatibility or sometimes just because they make mistakes." He advocates using color coding or alphanumeric codes to identify different types of greases and minimize the chance of mixing.
Sometimes, the mixing can be avoided, as when the customer needs to change from one lubricant to another. Perhaps they're no longer able to find the recommended lubricant or maybe they hope to save money. The problem is that developing lubricants is an art and a science, and no two are alike. Some might exhibit good shear stability, some might have better corrosion resistance, and some might flow better at the target operating temperature. It is difficult for a new supplier to know which aspects of the original lubricant made it appropriate for the application. Rather than simply looking for a set of properties, users should provide as many details about the application as possible. “A customer might call and say, ‘Hey, I'm using a synthetic lithium number two grease, I want to buy your equivalent,’ but it's not that easy. We may have 80 different greases that meet that basic description and all of them are a bit different," says Mazzola. “It's really important to understand exactly what the requirements of the application are, from environmental conditions, to speed, load, duty cycle, and so on to understand the differences between the lubricant you're trying to replace and the new one.”
The run in and the initial operating period after the changeover is a very critical time. The component should be watched carefully for temperature increases, vibration, or leakage. Perhaps the seal material is incompatible with the new lubricant but if it's not possible to track down that manufacturer, the end user won't know until they failure occurs. The safest approach is to test out the new lubricant in a single component to uncover any problems before making the substitution plant wide.
Properly selected and maintained, lubrication is probably the best investment you can make in maximizing the lifetime performance of your capital assets. All it takes is a bit of research and discussion with your vendors to develop a preventive maintenance plan. “From my standpoint it's all about education," says Mazzola. "I take the opportunity any time I'm talking to a customer or potential customer to make sure they understand how critical it is to not only use the right lubricant but to really lubricate at the right frequency and take care of the asset their equipment.” If you avoid the pitfalls and commit to executing your maintenance plan, you can expect to derive peak performance from your equipment for a long time to come.