Seals for bearings
Whateverthe bearing arrangement or type of bearing used, the bearings must be sealed to prevent contaminants and moisture entering the bearing in addition to retaining the lubricant. When seals are an integral part of a rolling element bearing, the bearing can be greased and sealed for life. However bearings used on medium and large motors and many small motors have to withstand load and speed conditions for a life which is outside the ability of sealed bearings. Hence the seals are generally part of the bearing housing in all but the smallest motors, because access for oil lubrication or greasing is required.
Shields and seals may be fitted. A shield does not form a complete seal and is fitted to the non-rotating ring with a small gap between the shield and the rotating ring, whereas seals are fixed to one ring and have a low-friction sliding face or fine clearance on to the other ring. Shields and seals may be fitted to one or both sides of a bearing and serve to keep contaminants out of the bearing and the lubricant in the bearing. Seals are usually of a synthetic rubber and thus usually have a temperature limitation of about -40°C to 120°C whereas metallic shields can be used outside this range. Shielded bearings are only suitable where water is not present and contamination is very light. It is more normal for fan bearings, except for very small sizes, to be fitted into housings with seals as part of the housing.
Fan manufacturers will normally have standard bearing housings incorporating suitable seals to cover most applications and the operating conditions of the motor, but if there are particularly harsh operating conditions then special sealing arrangements may be necessary. Seals that form part of the bearing housing can be of non-rubbing or rubbing types. The non-rubbing type has the advantage of very low friction and no wear and is ideally suited to high speed and high temperature. Rubbing seals rely on a rubbing contact with a means of applying a light contact pressure and can provide a much more reliable seal than a non-rubbing type, when running and stationary. However, wear does take place and friction losses are generated, thus making them normally unsuitable for high peripheral speeds. If not fitted correctly, rubbing seals can give problems and contaminants that try to enterthe seal can cause damage.
Non-rubbing seals are simply narrow gaps either axially, radially or a combination of both; the deciding factor being the likely movement of the shaft relative to the bearing housing. For example, a shaft that is likely to move axially either because of load influences or thermal expansion — but is restrained radially, would require a radial gap. Labyrinth seals are more effective than plain gaps and take many forms, examples of which are shown in Figure 10.38.
The third example of Figure 10.38 requires a split outer ring for assembly purposes. All the examples can improve the sealing properties by using a grease within the seal, a water-insoluble lithium or calcium based grease is recommended. The first example can have shallow grooves machined into the shaft adjacent to the seal and these grooves may be helical to drive lubricant back into the bearing, but this is only suitable for one direction of rotation.
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Figure 10.38 Examples of labyrinth seals
Another form of labyrinth seal involves washers with integral spacing flanges which are designed to fit either onto the shaft or into the bearing housing. By alternately placing the washers onto the shaft and into the housing a seal is created, the efficiency improving with the number of washers used.
Rubbing lip seals are generally manufactured from a synthetic type rubber, either of a form that gives a natural pressure from deflection of the seal or enhanced pressure by using a garter spring. Sections through typical rubbing seals are illustrated in Figure 10.39.
Metal support insert Garter spring providing
Flexible seal deflected from position shov«)
Figure 10.39 Examples of rubbing seals
The seal material type determines the operating temperature range, but generally -40°C to 200°C can be achieved without resorting to expensive special materials. The sealing surface on the shaft should be ground for best performance. At peripheral speeds in excess of about 4 m/s this is essential and at speeds higher than about 8 m/s the surface should be fine ground and hardened. As shown in Figure 10.40, the bearing is assumed to be positioned to the left of the seal and the seal is most effective at keeping contaminants from the bearing. If it is more important to keep lubricants in the bearing then the seal should be reversed.
A simple form of rubbing seal is the V-ring seal as shown in Figure 1 0.40. Made from synthetic rubber, it can be stretched over the shaft and provide enough grip to rotate with it, whilst the flexible lip rubs on the fixed sealing surface. Considerable misalignment can be permitted at low speeds and the sealing surface need not be exceptionally smooth. If the peripheral speed exceeds about 7 m/s, axial location is necessary and above about 12 m/s a steel support ring must be used to prevent the seal lifting from the shaft. The sealing lip is likely to lift off the sealing surface and create a small gap at above about 15 m/s peripheral speed.
An inexpensive seal, but limited to low temperatures and peripheral speeds below 4 m/s, is the felt insert. This is a simple felt ring soaked with oil within and located in a suitable retaining groove. It is an effective seal for grease lubricated bearings.
The seals described above are for the retention of grease or oil in bearing housings and to prevent moisture or contaminants entering the bearing. Seals for preventing the egress of contaminants or the ingress of air to fan casings are described in Chapter 7.
Figure 10.40 V-ring seal
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