June 6, 2017
Volume 7 Issue 3
A ‘Solid’ Concept for Bearing Lubrication
For most typical applications involving ball or roller bearings, standard greases or oils will serve to adequately lubricate a bearing’s rolling elements and raceways. However, in instances of extreme or especially demanding operating environments, conventional lubricants may not be able to make the grade, adversely impacting both the performance and service life of the bearings.
For example, heavily contaminated process areas subject to frequent washdowns, such as those in the food and beverage industries, will severely test bearing greases. During the intensive hygienic cleaning of equipment, the lubricant in bearings can potentially absorb water or chemicals, lose its lubricating properties, and/or completely wash out, leading to bearing corrosion and oil leakage – significantly reducing bearing service life.
And, in cases where access to bearings for necessary relubrication may be an issue, challenges will arise in keeping bearings properly lubricated and performing as intended. Bearings located in hard-to-reach (or even dangerous) locations can make relubrication procedures difficult or virtually impossible.
Such issues have led to the development of nontraditional bearing lubricant technologies. A notable example: “Solid Oil,” primarily developed to resolve problems associated with wash downs, cold temperatures, and rapid temperature changes and to resolve inaccessibility for relubrication of bearings operating at normal temperature.
This technology contributes additional application benefits, too, which collectively can help to extend bearing service life, increase machinery uptime, and even eliminate any need for bearing relubrication.
How It Works
Conventional bearing grease consists of a high-quality, perhaps synthetic lubricating oil and a thickener system. In heavily contaminated environments or applications requiring frequent washdowns, this combination can present problems ranging from absorption of moisture to total loss of lubricant. Contaminants can invade, corrosion can develop, and bearing failures can follow. In applications dominated by extremely cold and fluctuating temperatures (-15 degrees Celsius to -45 C), normal bearing greases often will be unable to withstand the conditions.
The composition of this new technology provides a practical and highly reliable lubricant alternative. The bearings are lubricated with a polymer material with a porous structure – millions of micro-pores – which is saturated with high-quality synthetic lubricating oil. The pores in the polymer material are so small that the oil is retained in the material by surface tension.
Here’s how it works: The oil-filled polymer material (solid but elastic) is placed into the bearing to completely fill the bearing’s cavity and encapsulate the bearing’s cage and rolling elements, and then cured. In the curing process, a very narrow gap forms around the rolling elements and raceways to enable the bearing components to rotate freely.
In service, the polymer material employs the bearing’s cage as the reinforcing element and, when the bearing rotates, the oil seeps into the created gaps and onto the rolling elements and raceways, providing good lubrication right from the start. When operation stops, excess oil is re-absorbed, ready to be released again when needed.
By completely filling the bearing’s internal space, this solid oil technology minimizes the “breathing” phenomenon seen in a bearing when air expands and contracts internally due to rapid temperature changes. It also protects against entry of water, moisture or chemicals during hygienic cleaning that otherwise could lead to corrosion or grease emulsification in conventionally lubricated bearings.
Among corollary benefits, the polymer matrix contains two to four times more lubricating oil than standard grease-filled bearings, which ultimately makes relubrication unnecessary – especially advantageous for those bearing locations where routine manual relubrication would be difficult.
In addition, the lubricating oil is virtually impossible to remove from the bearing, even during washdowns, and can withstand high centrifugal forces without leaking or otherwise contaminating the environment. Disposal issues, too, become moot.
This technology further adds effective sealing capabilities against potential contaminants, since the completely filled bearing cavity leaves no room for ingress. The use of bearing seals complements the protection.
And with food safety being the number one driver in the food and beverage processing industries, the new polymer-and-oil bearings developed to meet NSF H1 standards for food-grade lubricants offer unprecedented solutions.
SKF is able now to supply most standard bearings as solid oil bearings, with a few exceptions. The exceptions include bearings fitted with large-volume cages made of polyamide or machined brass, because the space available for the matrix within the bearing is too limited. The basic dynamic load ratings for solid oil bearings are identical to those for corresponding standard bearings; and additives, such as corrosion inhibitors, can be applied to provide an extra assist for protection.
Wide Range of Applications
Notable applications for these new bearings in the food and beverage industries include packaging and bottling machines, mixers, blenders, conveyors, separators, grinders, freezers and yeast ovens. In the metal and material-handling industries, typical applications include overhead cranes, chains, crane wheels, support rollers, rope sheaves and conveyor systems.
These bearings are in the field now, as seen by these real-world examples:
- The bearings in bottle filler equipment for carbonated drinks consumed excessive maintenance time and money, due to grease washout, the need for weekly relubrication, and an overall unacceptable MTBF (mean time between failure). The plant made the decision to replace conventional bearing grease with food-grade SKF solid oil bearing technology. The outcomes: Grease washout and weekly relubrication were eliminated, the lifetime of the bearings was extended by more than 24 months, increasing line efficiency.
- A bakery yeast oven operating at 60 degrees C with humidity close to 100 percent experienced constant bearing failure and relubrication was not an option. The operation was routinely replacing more than 800 bearings several times per year, and oven reliability became a paramount and costly issue. An upgrade to solid oil bearings was made. These bearings have been running successfully for three years and the replacement of hundreds of bearings has been unnecessary. Uptime has increased, costs have lowered, unplanned production downtime has ebbed, and enhanced oven reliability has been achieved.
- The traveling wheels on a converter crane at a steel plant were leaking bearing grease onto the crane rails, affecting the braking performance, creating safety issues, and resulting in reliability problems. As well, locations for bearing relubrication were hard-to-reach, excessive grease consumption and the need for disposal presented environmental concerns for the end user, and heavy contamination from metal particles added to the challenges. Solid oil bearings were introduced, which resolved the grease-leakage issues and promoted braking performance and safety; provided a protective barrier against the ingress of dirt, water and metal particles; and eliminated the need for bearing relubrication. In this case, the bearings have been in service for more than six years.
Solid oil bearings can represent suitable solutions for critical operating applications where conventional bearing grease or oil traditionally will fall short. As with any technology, however, it is instructive to note that every application can present its own set of unique challenges. Our recommendation is to partner with an experienced resource and provider equipped to add perspective, educate about the most viable approach consistent with an operation’s needs, and help to oversee application success for the good of equipment reliability, uptime and productivity.
Lube Report Asia occasionally republishes articles from its sister publications. This article originally appeared in the September 2013 issue of Lubes'n'Greases magazine (Volume 19, Issue 9), under the headline "A ‘Solid’ Concept for Bearing Lubrication."