External sealing

Performance and operating conditionsBearing type and arrangementBearing sizeLubricationOperating temperature and speedBearing interfacesBearing executionSealing, mounting and dismounting

Bearing arrangements generally include a shaft, bearings, housing(s), lubricant, associated components, and seals. Seals are vital to the cleanliness of the lubricant and the service life of the bearings.
Section Components and materials gives a general description of the integral seals used in capped bearings. For detailed information, refer to the relevant product sections.
This section describes seals outside the bearing, and how they affect bearing performance. Because of their importance for bearing applications, this section deals exclusively with non-contact and contact shaft seals, their various designs and executions.

Seal selection criteria
Seals for bearing applications should provide maximum protection with a minimum amount of friction and wear, under the prevailing operating conditions. Because bearing performance and service life are so closely tied to the effectiveness and cleanliness of the lubricant, the seal is a key component. For additional information on the influence of solid contaminants on bearing performance, refer to Contamination factor, ηc.

Many factors must be considered when selecting the most suitable seal for a particular bearing-shaft-housing system. These include:
  • the lubricant type: oil or grease
  • the contaminant type: particles or fluid or both
  • the circumferential speed at the seal lip
  • the shaft arrangement: horizontal or vertical
  • possible shaft misalignment or deflection
  • run-out and concentricity
  • available space
  • seal friction and the resulting temperature increase
  • environmental influences
  • cost
  • required operating time
  • maintenance requirements
For additional information, refer to Power transmission seals.

Seal types
The purpose of a seal is to retain lubricant and prevent any contaminants from entering into a controlled environment.

There are several basic seal types:
  • non-contact seals
  • contact seals
  • static seals
Non-contact radial shaft seals form a narrow gap between the stationary and the rotating component. The gap can be arranged axially, radially or in combination. Non-contact seals, which range from simple gap-type seals to multi-stage labyrinths (fig. 1), do not wear.

Seals in contact with sliding surfaces are called contact seals and are used to seal passages between machine components that move relative to each other, either linearly or circumferentially.

The most common contact seal is the radial shaft seal (fig. 2), which is installed between a stationary and a rotating component.

Seals between stationary surfaces are called static seals. Their effectiveness depends on the radial or axial deformation of their cross section when installed. Gaskets (fig. 3) and O-rings (fig. 4) are typical examples of static seals.

Non-contact seals
The simplest seal used outside a bearing is the gap-type seal, which creates a small gap between the shaft and housing cover (fig. 5). This type of seal is mainly used for grease lubricated applications that operate in dry, dust-free environments. To enhance the effectiveness of this seal, one or more concentric grooves can be machined in the housing cover bore at the shaft end (fig. 6). The grease emerging through the gap fills the grooves and helps to prevent entry of contaminants.

With oil lubrication and horizontal shafts, helical grooves can be machined into the shaft or housing bore, either right-handed or left-handed, depending on the direction of shaft rotation (fig. 7). These grooves are designed to return emerging oil to the bearing; therefore, it is essential that the shaft rotates in one direction only.

Other shapes can be machined into the shaft. Non-helical grooves may be used on the shaft and in the housing; these function as disruptors/flingers. Additional shaft collars can prevent oil leakage, whatever the direction of rotation.

Single or multi-stage labyrinth seals, typically used with grease lubrication, are considerably more effective than simple gap-type seals, but are also more expensive. Their effectiveness can be further improved by periodically applying grease, via a duct, to the labyrinth passages. The passages of the labyrinth seal can be arranged axially (fig. 8) or radially (fig. 9), depending on the housing type (split or non-split), mounting procedures, available space, etc. The radial gaps of the labyrinth (fig. 8) remain unchanged when axial displacement of the shaft occurs in operation; therefore, the gaps can be very narrow. Where angular misalignment of the shaft relative to the housing can occur, labyrinths with inclined passages can be used (fig. 10).

Effective and inexpensive labyrinth seals can be made using SKF sealing washers (fig. 11). Sealing effectiveness increases with the number of washer sets and can be further improved by incorporating flocked washers. For additional information on these sealing washers, refer to Power transmission seals.

Rotating discs (fig. 12) are often fitted to the shaft to act as a shield. Flingers, grooves or discs are also used with oil lubrication. The oil from the flinger is collected in a channel in the housing and returned to the housing sump through suitable ducts (fig. 13).

Contact seals
There are four common types of contact seals: The seal type selected for a particular application typically depends on:
  • the primary purpose of the seal (to retain lubricant or exclude contaminants)
  • the type of lubricant (oil, grease or other)
  • the operating conditions (speed, temperature, pressure and environment)

Radial shaft seals

Radial shaft seals (fig. 14 and fig. 15) are contact seals that are used for oil and grease lubricated applications (→ Radial shaft seals). These ready-to-mount components typically consist of a metal reinforcement or casing, a synthetic rubber body, a seal lip and a garter spring. The seal lip is pressed against the shaft by the garter spring. Depending on the seal material and medium to be retained and/or excluded, commonly used materials for radial shaft seals can be used at temperatures between –55 °C (–65 °F) and +200 °C (390 °F).

The seal counterface, that part of the shaft where the seal lip makes contact, is of vital importance to sealing effectiveness. The surface hardness of the counterface should be at least 45 HRC at a depth of at least 0,3 mm. The surface texture should be in accordance with ISO 4288 and within the guidelines of Ra = 0,2 to 0,5 μm. In applications where speeds are low, lubrication is good, and contamination levels are minimal, a lower hardness can be acceptable. For oil lubrication, to avoid the pumping effect induced by helical grinding marks, SKF recommends plunge grinding the counterface.

If the primary purpose of the radial shaft seal is lubricant retention, the seal should be mounted with the lip facing inward (fig. 14). If the primary purpose is to exclude contaminants, the lip should face outward, away from the bearing (fig. 15). → Radial shaft seals

SKF can also supply machined polyurethane radial shaft seals. → Machined seals


Safety precautions for fluoro rubber and Polytetrafluoroethylene

Fluoro rubber (FKM) and Polytetrafluoroethylene (PTFE) are very stable and harmless up to normal operating temperatures of 200 °C (390 °F). However, if exposed to temperatures above 300 °C (570 °F), such as fire or the open flame of a cutting torch, FKM and PTFE give off hazardous fumes. These fumes can be harmful if inhaled, as well as if they contact the eyes. In addition, once the seals have been heated to such temperatures, they are dangerous to handle even after they have cooled. Therefore, they should never come in contact with the skin.

If it is necessary to handle bearings with seals that have been subjected to high temperatures, such as when dismounting the bearing, the following safety precautions should be observed:
  • Always wear protective goggles, gloves and appropriate breathing apparatus.
  • Place all the remains of the seals in an airtight plastic container marked with a symbol for “material will etch”.
  • Follow the safety precautions in the appropriate safety data sheet (SDS).

If there is contact with the seals, wash hands with soap and plenty of water and, if contact has been made with the eyes, flush eyes with plenty of water and consult a doctor immediately. If the fumes have been inhaled, consult a doctor immediately.

The user is responsible for the correct use of the product during its service life and its proper disposal. SKF takes no responsibility for the improper handling of FKM or PTFE, or for any injury resulting from their use.

V-ring seals

V-ring seals (fig. 16) can be used with either oil or grease lubrication. The elastic rubber body of the seal grips the shaft and rotates with it, while the seal lip exerts a light axial pressure on a stationary component, such as a housing. Depending on the material, V-rings can be used at operating temperatures between –40 and +200 °C (–40 to 390 °F). They are simple to install and permit relatively large angular misalignments of the shaft at low speeds.

The recommended counterface surface finish (surface texture) depends on the circumferential speed (→ Counterface and table 1). At circumferential speeds above 8 m/s, the body of the seal must be located axially on the shaft. At speeds above 12 m/s, the body must be prevented from lifting from the shaft. A sheet metal support ring can be used to do this. When circumferential speeds exceed 15 m/s, the seal lip lifts away from the counterface and the V-ring becomes a gap-type seal.

V­-ring seals have good sealing abilities, which can be attributed to the body of the seal, which acts as a flinger, repelling dirt and fluids. As a result, these seals are generally arranged outside the housing in grease lubricated applications and inside the housing, with the lip pointing away from the bearing, in oil lubricated applications. Used as a secondary seal, V­-rings protect the primary seal from excessive contaminants and moisture.

For added protection in extremely contaminated applications, SKF also supplies MVR seals. → MVR axial shaft seals and fig. 17.

Axial clamp seals

Axial clamp seals (fig. 18) are used as secondary seals for large-diameter shafts in applications where protection is required for the primary seal. They are clamped in position on a non-­rotating component and seal axially against a rotating counterface. For this type of seal, it is sufficient if the counterface is fine-turned and has a surface texture of Ra = 2.5 μm.

Mechanical seals

Mechanical seals (fig. 19) are used to seal grease or oil lubricated applications, where speeds are relatively low and operating conditions arduous. Mechanical seals consist of two sliding steel rings with finely finished sealing surfaces and two Belleville rubber compound washers, which position the sliding rings in the housing bore and provide the necessary preload force to the sealing surfaces. There are no special requirements for the mating surfaces in the housing bore.

Other seals

Felt seals (fig. 20) are generally used with grease lubrication. They are simple, cost­-effective and can be used at circumferential speeds of up to 4 m/s and at operating temperatures up to 100 °C (210 °F). The counterface should be ground to a surface texture of Ra ≤ 3.2 μm. The effectiveness of a felt seal can be improved substantially by mounting a simple labyrinth seal as a secondary seal. Before being inserted in the housing groove, felt seals should be soaked in oil at about 80 °C (175 °F) prior to mounting.

Metal seals (fig. 21) are simple, cost-effective and space-saving seals for grease lubricated bearings. The seals are clamped against either the inner or outer ring and exert a resilient axial pressure against the other ring. After a certain running-in period, a narrow gap forms and these become non-contact seals.

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