The Three-Barrier Solution

The sight of a heavily contaminated bearing may not involve an emotional reaction for most people, but for many in the maintenance sector, this picture does evoke a sense of concern, since it’s know that 34% of bearings fail due to contamination.

Bearings are exposed to tough conditions in various industries, such as mining, mineral processing, cement, pulp and paper, metals, and other heavy and process industries. Contaminants may vary in type and size, from steam and moisture or other corrosive media, to fine dust or heavy rocks. As such, the use of an appropriate sealing solution is of great importance.

In applications with harsh environments, and high maintenance costs due to short bearing lifetimes, one way to get excellent sealing is to use the ‘three-barrier solution’. This involves the use of:

1. Labyrinth seals as a primary barrier

2. A grease-filled cavity in the housing as a secondary barrier

3. Bearing seals as the third and final barrier.

It is important to ensure that the primary barrier is grease-purgeable, to ensure that when the housing is relubricated, the old grease (with some contaminants) is purged out during relubrication.

The cavity in the housing should be filled about 70% to 80% with grease to start out with. This will ensure an adequate grease fill to act as a secondary barrier.

A different type of grease than what is used inside the sealed bearing may be used in the housing. Since this grease will purge out (depending on the relubrication interval and amount), it is suggested you use an environmentally friendly grease, to reduce the effect of grease spillage and/or disposal on the environment.

The final barrier is to be the bearing seal itself. A sealed spherical roller bearing is often used in such a case, since these bearings are capable of supporting heavy radial and axial loads, and are suitable for integrated seals due to their non-separable design.

Using sealed bearings virtually eliminates the common incidence of introducing contamination into the bearing by way of installation or relubrication practices, since the bearing is lubricated and sealed at the factory.

Besides reducing contamination in the bearing, and increasing its operating life, there are other advantages of this approach. The grease used in the housing does not have to be chosen based on the NLGI grade or operating viscosity required by the bearing, since the bearing is sealed. The grease leakage from the bearing itself is virtually eliminated (although there can be a small amount of leakage of the grease from the sealed bearing, mostly during initial start-up). Therefore, an immediate savings in grease consumption is experienced, which can add up quickly.

The use of the sealed bearings also makes it possible to consider downsizing the bearings in many applications, since a lower contamination level will result in a higher bearing lifetime, as per the life modification factors used in bearing life calculations (ISO 281:1990/Amd 2:2000).

As an example, the contamination factor of an open bearing in the range of 0.1 to 0.4 may be improved to as much as 0.6 to 0.9 (depending on the size and application). This higher factor contributes to the longer service life and increased uptime of a sealed bearing.

Finally, ensure that when using the three-barrier solution, it is appropriate for your individual case, paying attention to the selection of each part of the assembly. If you are unsure about any detail, err on the side of caution, and contact your bearing distributor or manufacturer of choice.

Stay clean, live long and prosper.

MRO John Melanson is the engineering manager at SKF Canada.

Find the original article on MRO’s website.