Bearings need considerable, continuous, sometimes nearly seamless protection – you can't just pack on the grease and leave 'em run
Bearings are refined components that often sustain massive loads at high speeds. But without a clean, continually lubricated condition between contacting surfaces, even the best metals will quickly break down. Bearing seals keep lubrication where it’s needed, and help isolate working parts from the effects of abrasive particles and corrosive moisture.
A seal for the occasion
Sealing elements can be integral or external to the bearing component. Each has its own special considerations.
Integral seals are compact, and eliminate the need for grease cavities and shaft seals, which can require more space and impose a mechanical burden on the shaft. Bearings sealed this way are truly self-contained, and therefore safe from the possibility of contamination from handling or from a shared lubricant.
The three basic kinds of integral seals are metal shield (non-contacting), noncontact rubber, and contact rubber. The metal shield is perhaps the most common and inexpensive configuration. The metal is pressed into the outer ring of the bearing, while the groove on the inner ring and the lip on the inside diameter of the shield assemble with some clearance, but with a labyrinth cross section that blocks the entrance of particles. And the inner ring groove is such that any exiting grease tends to be thrown back into the bearing by centrifugal force. Since there is no contact, these shields don’t add to the bearing torque, and are great for high-speed operations, but are best used in clean environments only.
Non-contact rubber seals are actually synthetic rubber bonded to a steel ring that’s fixed to the outer bearing ring. They don’t seal by contacting the bearing elements; instead, both of the seal edges have a circular concave surface that provide labyrinth gaps along the V-shaped groove of the inner ring sealing surface. This rubber non-contact configuration provides very low friction and excellent resistance to contaminants – good for high speeds or low torque in conjunction with moderately dusty conditions.
Contact seals also use synthetic rubber fixed to the bearing outer ring the same way as the non-contact rubber version. At the inner edges, however, there is a double lip. The inner lip makes contact with the V-slot of the inner ring’s sealing surface, while the outer lip provides a labyrinth – if the inner lip wears down over time, the outer lip comes into play, maintaining the seal. This arrangement provides exceptional resistance to foreign particles, but the frictional contact increases bearing torque and reduces the speed allowance.
External seals are frequently used with oil rather than grease; oil is often seen in high-speed applications and where lubrication is shared between multiple components. Other common places for external seals are where regreasing or a large lubrication cavity are necessary.
There are external non-contact as well as external contact seals. The noncontact variety includes oil grooves, labyrinths, and flingers.
Oil groove seals consist of grooves in the shaft or housing. Grease or oil fills the grooves, blocking out contamination. When oil is used, leakage can be prevented by screw thread grooves, as long as the shaft rotation doesn’t change direction. With screw threads, the fluid film protects against contamination while the machinery is running, but after shutdown and during startup, particles and dust can worm their way into the bearing mechanism. Therefore in such an arrangement additional attachments like a dust shield may be necessary.
Labyrinth seals come in axial or radial designs. The axial version is generally easier to assemble, but doesn’t seal as well as the radial design. Since these seals maintain a clearance, (which is often the same as the clearance for oil grooves), they are good for high speeds. Filling the labyrinth grooves with grease (preferably using grease fittings) is a highly effective way to keep particles out of the bearing; grease is particularly an option at lower speeds, since its high viscosity imparts a drag on the shaft.
A flinger is an external mechanism that takes advantage of shaft rotation, collecting oil that is flung off the shaft and returning it to the reservoir by holes in the flinger housing. Additional measures may be necessary to effectively block particulates, and therefore a flinger might be located outside of other sealing equipment.
As for external contact seals, oil seals are the most common. They have an elastomer such as rubber fixed to the housing and pressing against the shaft. In the basic form, protection is only in one direction, and depending on which way the element faces, the seal can be used to keep oil in or dirt out. To do both, oil seals with multiple lips are available. A spring is a common built-in feature, used to keep the seal in contact with the shaft as the element wears down.
With oil seals, key performance and wear factors are the roundness and surface finish of the contacting shaft. Surface roughness should generally fall between 0.20 to 0.80 mm Ra. For shaft surface speeds in excess of 394 in./sec, 0.20 μm Ra or lower is necessary. After assembly, the eccentricity between the oil seal inner diameter and the shaft diameter should be under 0.004 in. The dynamic eccentricity between the shaft and seal lip should be under 0.002 in. For the shaft itself to resist wear, a surface hardness of at least HRC 40 is recommended.
Another type of external contact seal, a V-ring, rotates with the shaft and makes seal contact with a surface perpendicular to the shaft axis. The axial sealing direction lets a V-ring operate effectively with low contact pressure, so wear is minimized, and high speeds are allowable.
John Husemann is Supervisor of Applications Engineering with NTN Bearing Corp., Mt. Prospect, Ill.