Motion Control Resources
Troubleshooting Tips: Lubrication
by Kristin Lewotsky, Contributing Editor
Motion Control & Motor Association Posted 12/12/2012
The right lubricant applied the right way will protect your system from wear, dissipate heat, and give you the greatest possible lifetime.
For want of a horseshoe nail, the kingdom was lost, the saying goes; in other words, sometimes the details that seem unimportant can lead to catastrophic failure. In the area of motion control, lubrication is one of those essential support technologies that doesn't always get the attention it should. It’s easy to focus on controls, communications, motor sizing, and a host of other details. You can design the perfect motion system with sophisticated safety algorithms, zero vibration, and ultra high-speed operation, but if your grease breaks down under a tough duty cycle or your oil doesn’t splash up to reach the top gears in your powertrain, your system will fail. We gathered a group of experts ranging from system designers to gearbox manufacturers to lubrication specialists to talk about lubrication challenges and techniques to follow to get the greatest lifetime from your system.
There are so many choices of lubricants – dry, wet, petroleum-based, synthetic, oils, greases, different weights. What are the most important points an engineer should keep in mind when choosing a lubricant in order to help them decide?
The most important point is to understand what the original equipment manufacturer (OEM) recommends and under what conditions they recommend it. The OEM has typically done extensive lubricant studies to ensure the lubricant will provide the optimum equipment operation. The recommendations typically have specified temperature, load, and speed ranges, though, so if you are running the equipment outside of those ranges, you may need to adjust your lubricant accordingly . Also, when it comes to oils, viscosity is the most important characteristic to get right.
—Tim Stiers, Application Engineer, Castrol Industrial Americas
Temperature, load, contamination, and other factors need to be considered and are often ignored. Additionally, there are many new developments in lubricants today. I feel that the users are much better at using the resources available to them, like the oil suppliers’ representatives, to help to educate them. I would encourage the equipment designers to do the same. Most of the oil suppliers offer design help and support. They will normally offer seminars and support design efforts free of charge.
—James Cartellone, President, Gear Technology Solutions LLC
To attain maximum efficiency in applications, all components in the tribological system should be considered and optimized. There are many factors that influence the best lubricant selection for an application, and several of these influence each other. For instance, for applications that are exposed to a large amount of dust/dirt, you may want to consider a dry lubricant; however, dry lubricants may not provide adequate wear protection under high loads and/or speeds.
—Steve Mazzola, Director of Engineering and Technical Services, Klüber Lubrication North America
In general, gearboxes are supplied with a standard lubrication designed specifically for the type of gear technology being used. For example, spur gears found in planetary gearboxes are generally designed for grease lubrication, as centrifugal forces distribute the lubricant evenly and ensure constant contact. Hypoid gears, on the other hand, are typically filled with an oil type designed for splash lubrication. Besides the fact that certain gears are designed for certain lubricants, speed is the next important factor. Any deviation from the standard should be verified by the gearbox manufacturer to make sure that the new lubrication is compatible with the gears.
—Mike Parzych, Technical Marketing Manager, GAM Gear
The proper selection of the lubricant starts with loading and the required film thickness to keep the components from metal-to-metal contact, but it doesn’t end there. The entire life cycle of the lubricant system needs to be considered.
The first priority when it comes to lubricating a gearbox is making sure that the gear mesh is properly lubricated. Gear meshes often have a lot of sliding contact and they also have extreme pressure on the actual teeth as it turns against itself.
—Rachel Braddock, Senior Applications Engineer, SKF America
When selecting a lubricant for any gear application, the following issues must be considered: type and materials of gear; operating conditions, including rolling or sliding speed, type of steady load, and temperature; method of lubricant application; environment; and type of service. Ambient and operating temperatures also determine the selection of gear lubricants. Oils operating at high temperature require good viscosity and high resistance to oxidation and foaming. Oil for gears operating at low ambient temperatures must be able to flow easily and provide adequate viscosity. Therefore these gear oils must possess high viscosity indices and low pour points.
—Isaac Diggs, Engineering Project Manager, MicroMo
Many customers wish to consolidate and reduce the total number of lubricants used in their facility. While this can usually be done with excellent results, you need to be very careful. Trying to consolidate too much and too quickly can lead to mistakes. I have seen this several times. Don't forget to consider all factors for lubricant selection such as water resistance, speed, loads, bearing types, gear types, starting and running torques, seal and paint compatibility, etc. Sometimes there is a benefit to using a couple more lubricants than initially desired in order to avoid lube-related problems.
What can a good choice of lubricant do to actually improve system performance (e.g. reduce vibration/noise, etc.)?
Some of the best high-performance lubricants will actually extend the life of equipment, dampen vibration, eliminate noise, and lower operating temperatures as compared to conventional lubricants. In addition, they will last longer and require less maintenance. These lubricants are priced higher but typically provide a lower total cost of operation. Keep in mind that lubrication is a relatively small part of the maintenance budget (about 3%) but can have a huge impact on the other 97% of the budget. Also remember that even the best lubricants can't overcome mechanical/design flaws.
— T. S.
With the possible exception of synthetic lubricants offering reduced friction losses, lubricants are not intended to cure ills. They are intended to assure long term reliability and life of equipment. Sure, it is possible to reduce some noise and vibration by adding a heavier lubricant to some equipment and some applications, but these are masking problems. This is not the purpose of lubricants. The proper selection, introduction into the machine parts—splash vs. forced, for example—plus proper conditioning steps like filtering and cooling, will assure long life of the equipment.
A properly selected lubricant will help a gearbox function better, especially when it is subjected to extreme environments. At cold temperatures, a gearbox must overcome added friction due to the increased viscosity of the lubrication as well as contraction of components (especially from a no-load state). There are lubricants that are designed specifically for low temperatures so they do not add to this friction. Also, at high temperatures, it is very possible for lubricants to burn up or even separate which means they lose their essential lubricating properties.
The ideal lubricant should allow the application to operate at its most efficient state. That typically means lowest friction, lowest operating temperature, minimal wear, lowest noise, and longest life. High efficiency will reduce maintenance, reduce power consumption, and extend component lifetime. All of this will help to reduce costs.
The most critical function performed by lubricants is to extend equipment service life. The important objectives accomplished by lubricants include: reduction of friction and wear, corrosion prevention, reduction of operating noise, improvement in heat transfer, and removal of foreign or wear particles from the critical contact areas of the gear tooth surfaces.
What can a poor choice of lubricant due to make a system vulnerable to failure (e.g. premature burnout, seizing, etc.)?
At the fundamental level, the incorrect lubricant will not properly separate and protect metal surfaces, leading to metal-to-metal contact and accelerated wear. This will in turn lead to higher friction, higher temperatures, metal removal, loss of clearances, and ultimately shortened equipment life/failure.
— T. S.
The wrong choice of lubricant or means of introducing the lubricant will almost certainly result in premature failure. I feel that the use of the wrong viscosity or type of lubricant is secondary to not getting sufficient lubricant to where it is needed. Even a lot of the wrong lube will be better than not enough of the correct stuff. There are gravity effects, as well as aerodynamic effects, like air pressure in rotating machinery that should be considered. For example, in high speed gear drives, the oil is introduced on the exit side of the mesh instead of the more traditional closing mesh side. In this application, oil sprayed into the closing mesh would have to be done at high pressure to overcome the air pressure on the closing side of the meshing gears. On the exiting side of the mesh a vacuum is created that will actually suck the oil into the area. There is also needs to be consideration for draining the excess oil with gears operating at very high speed. There is also a practical limit to the pitch line velocity of gears dipping in oil baths for splash lube applications. Gears rotating at high speeds will churn oil into foam, which doesn’t lubricate very well.
A poorly selected lubricant can have a catastrophic effect on performance. For instance, selecting an oil with too low of a viscosity would lead to boundary friction (insufficient lubricating film) that would generate heat, excessive wear, and eventually failure. Even selecting a lubricant that is incompatible with sealing materials can lead to leakage and lubricant starvation (wear/failure). A grease that is incompatible with an existing grease may "de-gel" which reduces the ability of the mixed thickeners to retain the oil. This would result in excessive oil loss and could lead to immediate failure.
Gear lubricants are formulated and applied to prevent premature component failure, assure reliable operation, reduce operating cost, and increase service life.
Most new and rebuilt equipment is shipped without oil. A gearbox, for example, is not an approved container for shipment of lubricants. I have seen several examples of brand new and rebuilt equipment being put into service without being filled with oil prior to starting them. One of the worst was a scrap chopper rebuild in an aluminum mill that was in service about 8 hours without oil. Attached are some photos of the gears that melted from the excessive heat. This error did several thousand dollars’ worth of damage to the scrap chopper gearbox itself, as well as cost the owner several hours of unnecessary downtime.
What do you wish that your customers knew about lubricants that they typically don't?
Just how important lubrication is to their equipment and that all lubricants are not created equal . The phrase "lifeblood of the equipment" really does hold true. Proper lubrication means the proper lubricant in the proper place at the proper time and in the proper amount. If this task can be mastered, the longest life and most efficient operation of the equipment will occur.
— T. S.
Keeping the lubricant clean is one of the most important things that any equipment owner/operator can do to assure that the machine runs a long time. Without filtration or regular oil changes, contamination and metal particles are recirculated. These metal and dirt particles act as abrasives when caught between moving metal parts, creating fine scratches that accelerate wear. Often times these particles are larger than the film thickness of the oil, so they make contact with both metal surfaces even if the oil is sufficient to do the job.
The amount of lubrication is often an overlooked aspect by customers; however, it is very critical. Customers tend to think more lubrication is better, however (often times especially with grease based lubricants) too much can actually result in leakage. Conversely, not enough lubrication means increased heat and wear on the gear teeth.
All greases are not created equal. The characteristics that you see and feel are not likely to be the most important characteristics for an application. For instance, some customers specify a certain color, a certain consistency, or a certain thickener type. While sometimes there are valid reasons for a specific consistency of thickener type, they are not usually the most important decision factors. Base oil viscosity, shear stability, corrosion protection, thermal stability, and additives could be much more important for a particular application.
Gear speed is a key factor in the selection of proper oil viscosity. The pitch line velocity determines the contact time between gear teeth. High velocities are generally associated with light loads and very short contact times. For these applications, low-viscosity oils are usually adequate. In contrast, low speeds are associated with high loads and long contact times. These conditions require higher-viscosity oils.
The difference between oil and grease is that grease doesn’t dissipate heat as well as oil does so you end up with a slightly lower operating temperature and thus a slightly lower viscosity. In some gears that doesn’t matter but in other gears it does. If you’ve got a gearbox with an input speed of 60 RPM and an output speed of 40 RPM that does not generate any heat, use grease. If you’ve got a gearbox with a 10,000 RPM input speed, use oil.
What are the most common errors machine builders make when choosing a lubricant or lubricating parts?
I think that one of the most common mistakes that engineers make is not to give the lubricant enough attention at the design stage. Oftentimes they use the same lubricant that they have always used. Meanwhile, the machine may have changed or been designed for a slightly different application, but there is no consideration given to the lubricant in that application.
One of the more common errors is to underestimate the conditions under which the equipment could operate. Lubrication is much easier to accomplish under ideal or typical conditions, but once the equipment is experiencing temperature extremes, run under higher than expected loads, operated at different speeds, or subjected to contamination, the challenge becomes much greater.
— T. S.
The most common mistake is to only consider the lubricant at the end of the design cycle...almost as an afterthought. Subtle changes in the actual design could simplify the lubricant selection and optimize the system for the end user. Some examples are: adding relubrication ports (in and out) which allow easy access and effective relubrication to the lube point, considering seal compatibility to allow the ideal base oil viscosity and type for the lube point, using shielded bearings where there is a high potential for grease migration, shielding/locating the lube point away from a high heat source, and many more.
Not considering temperature, duty cycle, not planning for re-lubrication, etc. Any one of these can be detrimental. Know about the environment that the equipment is to operate in; know the duty cycle and plan for it to change—because it will; put visual indicators and switches for remote indication of dirty filters because maintenance is not always done as it should be done. Try to make it as foolproof, but as simple as possible. Most lubrication systems are about getting clean, cool, lube to the place(s) where it is needed. Add a few devices to make sure that the system is working, such as pressure switches, flow switches, temperature switches, etc.
Many customers wish to consolidate and reduce the total number of lubricants used in their facility. While this can usually be done with excellent results, you need to be very careful. Trying to consolidate too much and too quickly can lead to mistakes. I have seen this several times. Don't forget to consider ALL factors for lubricant selection such as water resistance, speed, loads, bearing types, gear types, starting and running torques, seal and paint compatibility, etc. Sometimes there is a benefit to using a couple more lubricants than initially desired in order to avoid lube related problems.
How often is poor choice of lubricant responsible for failures?
There are many factors that can contribute to a failure; however when we conduct an engineering evaluation on a gearbox, the lubrication is one of the first things we look at. Things such as discoloration, metal fragments, lubrication amount, and even odor can provide clues as to the possible root cause of the failure.
Across the entire manufacturing industry, the number of failures and associated costs would be fairly significant. Even after the proper choice of lubricant is made, that choice can be destroyed with contamination or lack of maintenance of the lubricant. Studies have shown that improper lubrication is responsible for as much as 40% of bearing failures while contamination accounts for 25%. The remainder is primarily due to installation errors and overloading/misloading.
— T. S.
I think that a poor choice is probably responsible for failure about 1/3 of the time, and that improper maintenance is the other 2/3. These are just my best guesses.
Improper lubrication is one of the leading causes of component failure. It is not only the use of an incorrectly selected lubricants, it is also the use of improper lubrication frequencies and amounts. For instance, you can select the best oil for a gearbox, but not maintain the proper oil level or oil change interval. You can select the best grease for a bearing, but over grease it. These can also lead to what is considered lubricant related failures. Optimizing a lubrication program starts with proper selection and includes best practices such as condition monitoring, re-lubrication frequency, re-lubrication amount, and re-lubrication techniques.
How much of a difference can proper choice of lubricant make in the lifetime of a gearbox or bearing?
The proper choice of lubricant, proper lubrication practices, and proper lubricant maintenance together will ensure you get maximum life out of the equipment. The impact of high performance lubricants is huge and documented in hundreds of case studies we have. The examples include four times longer gear oil drain intervals, improved gear conditions, saving damaged gearboxes from failure, extended bearing re-lubrication intervals, and 50% reduced grease consumption.
— T. S.
I feel that using the correct lubricant can double the life of a machine, but it has to be more than just the proper initial selection. It has to be maintained properly. Proper selection is often a compromise. For example, gearboxes have gears and bearings. Gears typically like to run in heavy oils, but bearings like to run in lighter viscosity oils. The same oil is used to lubricate both. Which do you choose? The choice has to be made based on the component requiring the heavier lubricant, so the bearings in this case have to operate in an oil selected for the gears. This gets back to considering all aspects of the equipment and the lubrication of the entire machine for all conditions.
Lubrication keeps things moving. The wrong lubricant can make things stop...and quickly. If a bearing or gearbox fails, there is likely to be downtime and that can be very expensive. Proper lubrication can allow for many years of predictable service depending on the design and proper selection of the gearbox or bearing. The best bearing grease will not provide ideal service if the bearing is not selected properly.
If a gearbox is sized correctly and properly lubricated, oftentimes it can exceed the estimated life ratings.
What do you wish that you had known about lubricants when you started in the business? What advice would you give users?
I wish that I had known how much of an impact the use of the ideal lubricant can have on actual operational costs. Like most, I was under the impression that there were only subtle differences in the formulation of gear oils or bearing greases, and because of that, I would gravitate towards the lower cost product. I used to think that I was saving money, but it nearly always cost more in the long run. The ideal lubricant can provide huge savings in reduced maintenance, power consumption, disposal costs, reduced emissions, and longer component life.
The key is to identify all of the operational parameters that are important to an application and work with a lubrication specialist early in the design process in order to optimize the system.
It’s important to consider the gearbox orientation, as this can play a large role in the lubrication provided. For example, spiral bevel gearboxes have a significant number of shaft configurations and mounting orientations which means they are easy to integrate into a machine design. However, depending on the orientation, the gearbox lubrication level may need to vary in order to ensure that the gears are actually in contact with the lubrication. For all new spiral bevel gearbox applications, we always require mounting orientation from our customers.
In general, lubricants don’t get the attention they deserve. Common practice is for engineers to provide the bare minimum to keep OEM costs down, when a few more dollars and efforts up front could increase the life of the machine significantly. I would encourage engineering managers and supervisors to make sure that junior engineers have a solid foundation and understanding of the importance of the lube system, and that they devote a sufficient amount of time to it in the design phase. Educate these young engineers through involvement with lubrication supplier’s engineers, seminars, literature, professional societies, and trade shows.