As strong as ball bearings are made, there are many things that influence their effect on rotating machinery. Poor alignment, rapidly aging bases, shaft voltage, and improper lubrication are the more common reason for bearing failure. If the bearings fail, invariably your rotating machine will fail.
Unless a plant has its own maintenance department, services companies, such as L&S Electric, are contracted to perform laser shaft alignment, make base modifications, and test for shaft voltage. When it comes to lubrication, however, you may be on your own.
Usually, equipment manufacturers provide lubrication schedules to follow for maintenance. For example, Marathon Electric created this lubrication interval procedure for their electric motors. However, what would happen if there were no documentation providing any information regarding lubrication? One result may lead to over-greasing, which may lead to contamination leading to failure. The photo on the right was taken in one of our service centers and provides an excellent example of over-greasing a motor.
In many grease lubricated bearing applications, the service life of the grease exceeds the bearing life, so that no grease needs to be added during the bearing life. For such applications SKF provides greased bearings and bearing units with integral seals or shields, which are suitable for maintenance-free operation. Consider though, to which life would the grease be suitable; is it the life of the machine, the life of the bearing or the life of the grease?
In order to judge if a maintenance-free solution is possible, the grease service life is estimated from operating parameters in open bearings, designed to be re-lubricated, this calculation produces the re-lubrication interval. The main factors that determine the grease service life (re-lubrication interval) are the bearing type and size, speed operating temperature, grease type and the bearing environment (orientation, rotation, cleanliness, ambient temperature, vibration, etc). The same calculation applies to sealed and shielded bearings.
Never expect grease in an operating bearing to last longer than grease on the shelf in a sealed container. Shelf life is a special case and considers the chemical make up of the lubricant as well as storage in a properly sealed container in a properly controlled environment.
Moritz also provides a tip regarding lubricating vertical machines:
Problem: You’re suffering repeated failures of vertically mounted, grease lubricated equipment. The downtime is getting serious! What’s cause those shiny bearing raceways, surface distress and spalling?
Answer: There may be a lubrication problem Gravity forces grease to flow down through the bearing and leak out of the arrangement. Using a higher consistency grease (NLG13) may be preferred to better retain the grease in the bearing. Being vertical, these bearings need to be re-lubricated twice as often to maintain the correct available amount. As well, seals need to be suitably designed for laction and contact to prevent premature grease loss.
How much grease is enough? That is the title of a related article at the Machine Lubrication site.
Several factors influence the quantity of grease that would go into a bearing at the selected interval. Critical factors include:
- Design of the bearing (plain, roller, ball or spherical roller).
- Type of shield used in the bearing.
- Size and speed from which to calculate dN values.
- Viscosity of the lubricant in the grease.
The Dodge Type-E bearing has a shield and lip seal configuration, with an option for an additional two-stage lip seal. This type of seal, by design, will allow for discharge of grease without damage at the outer perimeter of the seal. You could consider this a type of shielded bearing configuration.
A sealed bearing is not designed to be purged. If you apply too much grease too quickly, you can rupture the seal and compromise the life of the bearing.
If you look closely at the manufacturer’s guidelines, you should see both general and specific directions for re-lubrication, including frequency and quantity for a given speed and load. The original equipment manufacturer (OEM) parameters are typically the best starting point for re-lubrication practices.
The OEM will also suggest that if you have a highly aggressive environment, it may be necessary to adjust the interval or volume to increase the amount of grease to the bearing. The SKF formula also provides a good starting point. Again, the calculated value must be adjusted to accommodate the environment.
The decision to flood or purge a bearing should be taken within the context of bearing construction, production environment and OEM guidelines. This is rarely a simple question.
Remember to always stick to the manufacturer’s recommended lubrication schedule for bearings. Too much lubrication is not a good thing and will lead to problems. When in doubt, contact your equipment manufacturer for proper lubrication.