Engineering a better noncontact seal

The seals used for most bearings typically go unnoticed until something goes wrong, which often results in unexpected downtime. However, as lifecycle costs and energy savings become increasingly important, attention is being placed on optimizing bearing seal design. Innovations in labyrinth seal design mean that engineers no longer need to accept the limitations associated with standard seals. The new breeds of hybrid labyrinth seals and advanced centrifugal pressure seals are suitable for use in a wide range of applications, exhibiting longer life, reduced wear, reduced heat buildup, and no parasitic drag.

Seal applications

A typical seal application involves a stationary mechanical element, a non-stationary (or rotating) element, and what's called a controlled fluid cavity that must be isolated from the environment and its contaminants. Traditionally, simple lip seals have been applied in this type of setup.

Initially, noncontact seals were developed as a sealing method for use in environments with extremely high shaft speeds, or in applications where the durability of a traditional lip seal is limited. Here's why: The contact lip seal has a stationary sealing element, or lip, in contact with the rotational element. This constant contact creates wear on both the lip and the rotating element, frequently requires the addition of “wear rings,” and adds significant parasitic drag to the system. Operating speed is limited in these applications due to overheating and wear on the seal's contact lip, as a result of rubbing between the rotating and non-rotating surfaces. Ultimately, the seal lip will melt or centrifugally lift from the contact surface as operating speeds increase.

Noncontact seals

Noncontact seals are designed to seal without any physical contact between the rotating and stationary elements — so their use results in no parasitic drag or wear on the rotating element, long life, reduced heat buildup in the sealing cavity, and extended bearing life. Classified by their design characteristics, noncontact varieties include labyrinth seals, hybrid labyrinth seals, and centrifugal pressure seals. Each design features certain advantages and limitations. To determine the suitability of each design for a given application, engineers must examine the seal's performance during each of three normal operating modes:

• While the shaft is stationary

• While the shaft is accelerating or decelerating to its normal operating speed or speed range (or down to a stop or non-rotating operating mode)

• While the shaft is operating at its normal speed or within its operating speed range

Standard labyrinth seals

Several types of standard labyrinth seals exist. Typical designs incorporate a set of rings, or maze, with seal components in extremely close proximity to capture lubrication and push it back toward the controlled fluid cavity. The rotating and non-rotating components are held close to form a barrier by shearing the fluid, centrifugally slinging lubricants from the rotating seal element to the stationary seal element in a series of steps.

However, the labyrinth seal is constrained in its ability to function with a wide variety of fluids due to its limited ability to centrifugally push the lubricants back into the controlled fluid cavity. Further, it can only handle a limited volume of lubricating fluid in the controlled fluid cavity and it requires high rotational speeds to seal adequately. The seal cannot prevent external environmental contaminants from entering the controlled fluid cavity when the seal is not rotating, and some fluid weepage is expected from the seal. In general, standard labyrinth seals have limited abilities when reviewed in terms of their three operational modes:

Operating mode 1: Non-rotating

When the seal is not rotating, the controlled fluid cavity is open to the outside environment. Fluid contained within the seal can migrate to the bottom and possibly weep out. The length of time the seal is stationary will affect the amount of weepage.

Operating mode 2:
Shaft accelerating or decelerating

When the shaft is accelerating, the seal is accelerating — and the fluid within the controlled fluid cavity must remain in laminar flow or splashing will occur and the seal will leak. Splashing can be somewhat controlled by selecting a lubricant with high viscosity. The seal is designed to push fluid back into the controlled fluid cavity, but it will also push any contaminants that have entered the seal into the controlled fluid cavity. The cavity's fluid level must be limited to ensure that the fluid does not splash and flow past the seal.

Operating mode 3:
Normal operating speed

When rotating at normal operating speed, the labyrinth seal pushes any fluid trying to exit the controlled fluid cavity back into the seal cavity. The normal speed range required depends on the selected seal, but in general, high speeds are required to maintain consistent behavior. During operation, the seal is open to contaminants entering from the outside environment. The seal's rotating and stationary elements must remain axially aligned during operation; any axial movement between the two elements, or vibration, can cause the seal to leak.

Hybrid labyrinth seals

The hybrid labyrinth seal incorporates a plain contact seal component to solve some standard labyrinth seal limitations. In short, the integrated lip seal contacts the shaft at lower speeds, and then centrifugally lifts from the contact surface at higher speeds.

Hybrid labyrinth seals solve the problem of contaminants entering the controlled fluid cavity when the seal is not rotating, and are suitable in some applications. However, caveats exist for the three seal operating modes:

Operating mode 1: Non-rotating

When the seal is not rotating, the controlled fluid cavity is closed to the outside environment, but some weepage is possible because the lifting mechanism must be capable of moving in and out radially. Fluid contained within the seal can also migrate to its bottom and cause weepage, though the amount will be determined by the length of time the seal is stationary.

Operating mode 2:
Shaft accelerating or decelerating

Operating speeds are critical to ensure proper seal operation, because when the shaft is accelerating, the seal is accelerating — and fluid within the controlled cavity must remain in laminar flow, or splashing occurs, and the seal will leak. In fact, splashing problems from acceleration and deceleration are similar to those of the standard labyrinth seal, particularly when the seal is operating at speeds high enough to centrifugally lift the contact element from the rotating shaft.

Splashing can be partially controlled by selecting a lubricant with high viscosity, but the seal also exhibits parasitic drag torque until the contact element inside the seal is centrifugally lifted from the contact surface.

Operating mode 3:
Normal operating speed

When rotating at normal operating speed, the hybrid labyrinth seal pushes any fluid trying to exit the controlled fluid cavity back into the seal cavity.

The speed range required depends on the selected seal, but just like the standard labyrinth seal, high speeds are required to maintain consistent operation. The seal is also open to contaminants entering from the outside, and the seal will push these contaminants into the seal cavity during operation.

The rotating and stationary elements in the labyrinth seal must remain axially aligned during operation; any axial movement between the two elements, or vibration in the application, can cause the seal to leak.

Centrifugal pressure seals

Centrifugal pressure seals utilize a rotating chamber within the seal to develop internal pressures and pump sealing fluid back into the controlled fluid cavity.

The double seal design pumps contaminants out of the seal from the non-fluid side, and pumps lubricating fluid back into the controlled fluid cavity. The rotating chamber method significantly reduces the operational speed requirements of the traditional noncontact seal and allows the use of extremely low-viscosity fluids, even water. How does it work? The more fluid that passes from the controlled fluid cavity into the rotating chamber, the greater the pressure inside that chamber to return the fluid to the controlled fluid cavity. The fluid in the controlled fluid cavity continues to fill the rotating chamber in the seal — until pressure from the fluid in the rotating chamber is in balance with the pressure in the controlled fluid cavity. At this point, the seal continues to move fluid in and out of the rotating chamber to maintain a pressure balance with the controlled fluid chamber.

Centrifugal pressure seals solve the alignment issues of other labyrinth seals, and pump fluids to create a pressure differential between the environment and controlled fluid cavity. This differential prevents lubricants in the controlled fluid cavity from leaking out and contaminants from entering, which is not possible with other noncontact seals. Performance within the three operational modes can be useful for certain applications:

Operating mode 1: Non-rotating

In a double-seal arrangement, the controlled fluid cavity is only partially open to the environment or the non-fluid side of the seal. If contaminants enter the non-fluid area while it is stationary, they will be pumped out of the chamber upon acceleration of the seal. Due to the wrapped chamber design, fluid does not leak from the seal in the stationary operating mode.

Operating mode 2:
Shaft accelerating or decelerating

When the shaft is accelerating or decelerating, the seal is unaffected by any lubricant splashing. A series of pumping discs located in the rotational chamber form a baffle to eliminate splashing and keep the fluid in laminar flow at all times. The centrifugal pressure seal can operate with extremely low viscosity fluids such as water, without any leakage. Because the rotational chamber is located toward the outside diameter of the seal envelope, and the fluid is captured in the seal's rotational chamber, the seal can operate at much lower speeds than typical labyrinth seals.

Operating mode 3:
Normal operating speed

When operating at normal speed, the seal pumps any fluid trying to exit the controlled fluid cavity back in. The speed range required depends on the specific seal design, but in general, high speeds are not required to maintain consistent seal operation. The centrifugal pressure seal pumps contaminants from the non-fluid rotational chamber into the outside environment. This applies to both liquid and gaseous contamination; the non-fluid chamber pumps any type of contaminant out of the chamber. The seal also pumps and produces a pressurized controlled fluid cavity while in operation, to allow the controlled seal cavity to be completely filled with fluid. As long as the seal is rotating, no fluid will leak from the controlled fluid cavity. Further, the design does not require the rotating and stationary elements to remain in precise axial alignment during installation, maintenance, or operation. Some vibration is tolerable as well.

For more information, call (860) 594-7183 or visit centritecseals.com.

Noncontact seals in different operating modes