- Lip seal reliability is a function of the shaft’s surface geometry and the seal’s material of construction.
- The preload the lip seal exerts on the shaft is a critical variable.
- There are six relevant factors that you should use when selecting a shaft seal.
Shaft seals on pumps, compressors, turbines, and other rotating equipment not only retain the bearing lubricants but also protect them from external contaminants. Both contact lip seals and non-contact bearing isolators can serve this purpose. The best solution for a given application depends largely on service conditions, desired performance, and value, or cost plus delivered performance.
Figure 1. A seal should be riding on thin meniscus of oil.
Contact lip seals are available in a variety of materials, sizes, and configurations. Made of elastomers molded into engineered angles and contours, these seals act as micro-hydrodynamic wedges that raise the lip and recirculate lubricant to form a thin meniscus of oil on which the seal rides. This hydrodynamic action reduces friction between the lip and shaft (Figure 1).
There are three basic types of lip seals. General-service seals, using snap-in springs to provide lip load, are made of commodity-grade elastomers that offer satisfactory performance at lower speeds and have the ability to accommodate misalignment. High-performance lip seals are made of specially engineered synthetic rubber and incorporate molded-in springs for improved performance and service life. Specialty lip seals have custom-engineered designs for pressurized, non-lubricated, and other demanding conditions.
As contact lip seal technology evolves, new materials, geometries, and configurations are being tested for their ability to reduce or overcome frictional drag between the lip and the shaft. Because the hydrodynamic pumping action isn’t continuous, the lip can come in contact with the shaft, especially during dry-running startup. Dry running increases drag (the force needed to overcome it), and the power required to turn the shaft.
Figure 2. Close-up view of seal lip-to-shaft interface.
Direct contact also causes grooving on the shaft and wear on the lip. Repairing damaged shafts is costly and time-consuming, so most contact lip seal manufacturers recommend ranges for shaft hardness and surface finish. Formation of the oil meniscus is subject to a combination of factors, but mostly radial load, which is the sum of the forces the lip exerts on the shaft (Figure 2).
The greater the load on the lip, the harder it is for the oil to lift it; too little load and the oil will leak past the lip, too much might result in higher power consumption and premature lip wear. Among other factors, radial load is a function of lip material and geometry, the type of spring used, and seal interference. The angle of the lip and location of the spring play a critical role in seal design. Interference is built into the lip design to develop preload on the shaft and provide greater misalignment tolerance. Excessive pre-load, however, can cause shaft grooving and make a seal difficult to install. Harder elastomers such as hydrogenated nitrile butadiene rubber (HNBR) are more abrasion-resistant, but can generate too much lip load thereby causing the seal to generate excessive heat.
Contact lip seal performance depends upon a proper match with the application’s requirements. A number of factors affect seal performance, and taking them into account helps ensure selecting the optimal seal in terms of reliability, longevity, and maintenance.
It’s critical to know how a seal is going to be used, whether its primary function is to retain lubricants, prevent ingress of contaminants, or both. If intended solely for lubricant retention, the lip should be directed towards the lubricant. If the purpose is to exclude contamination, it should point towards the contamination and away from the lubrication. In applications that require both, either a dual-lip seal or two single-lip seals placed back to back might be used.
Figure 3. These are the basic components of a bearing isolator.
The media to which the seal will be exposed also might determine the type selected. Dry-running applications might require bearing isolators rather than contact lip seals, which need lubrication to prevent premature wear. Standard bearing isolators also can be used in oil mist or splash applications in which the lubricant surface is below that of the seal. Unlike lip seals, isolator seals are a non-contact means of retaining lubricants and excluding contaminants. Standard labyrinth type seals are characterized by close tolerances and intricate, circuitous paths with abrupt directional changes to prevent leakage (Figure 3).
Bearing isolators also are more energy-efficient than contact lip seals. With minimal dynamic contact, they present little to no frictional drag. As a result, they reduce power consumption by as much as 99% and can last 65 times longer than lip seals (Figure 4). The power savings from a single seal might not be particularly impressive, but it becomes significant when multiplied by the hundreds of pieces of rotating equipment in a typical processing plant.
Figure 4. Test results show the power consumption of lip seals and bearing isolators tested on a 3-hp motor.