The seal type selected for a particular application typically depends on its primary purpose (retain lubricant or exclude contaminants), type of lubricant (oil, grease or other) and operating conditions (speed, temperature, level of contamination).
Radial shaft seals
Radial shaft seals (fig. 1 and fig. 2) are contact seals that are used mainly in oil lubricated applications. 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, radial shaft seals can be used at temperatures between –60 and +190 °C (–75 to 375 °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 55 HRC at a depth of at least 0,3 mm. The surface roughness should be in accordance with ISO 4288:1996 and within the guidelines of Ra = 0,2 to 0,8 μm. In applications where speeds are slow, lubrication is good, and contamination levels are minimal, a lower hardness can be acceptable. To avoid the pumping effect induced by helical grinding marks, SKF recommends plunge grinding the counterface.
V-ring seals (fig. 3) 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 +150 °C (–40 to 300 °F). They are simple to install and permit relatively large angular misalignments of the shaft at slow speeds.
A surface roughness of Ra = 2 to 3 μm is sufficient for the counterface. 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.
The good sealing ability of a V-ring seal 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.
Axial clamp seals
Axial clamp seals (fig. 4) 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 roughness Ra = 2,5 μm.
Mechanical seals (fig. 5) are used to seal grease or oil lubricated applications where speeds are relatively slow and operating conditions difficult and arduous. They consist of two sliding steel rings with finely finished sealing surfaces and two plastic cup springs (Belleville washers), which position the sliding rings in the housing bore and provide the necessary preload force to the sealing surfaces. There are no special demands on the mating surfaces in the housing bore.
Felt seals (fig. 6) are generally used with grease lubrication. They are simple, costeffective and can be used at circumferential speeds of up to 4 m/s and at operating temperatures up to 100 °C (205 °F). The counterface should be ground to a surface roughness 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. 7) are simple, cost-effective and space-saving seals for grease lubricated bearings, particularly deep groove ball 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, these seals become non-contact seals by forming a very narrow gap with the rotating ring.
For additional information about seals supplied by SKF, refer to Power transmission seals.