Non-rubbing seals (Fig. 28.22)
Non-rubbing seals depend for their effectiveness on the sealing efficiency of narrow gaps, which may be arranged axially, radially or combined to form a labyrinth. This type of seal has negligible friction and wear and is not easily damaged. It is particularly suitable for high speeds and temperatures.
Bearings must be protected by suitable seals against the entry of moisture and other contaminants and to prevent the loss of lubricant. The effectiveness of the sealing can have a decisive effect on the life of a bearing.
Many factors must be considered when deciding on the best sealing arrangements for a given bearing application, e.g. the type of lubricant (oil or grease), peripheral speed at the sealing surface, misalignment of the shaft, available space, friction of the seal and resultant temperature rise and cost. Two basic designs are normally used for rolling bearings.
This simple gap type seal which is sufficient for machines in a dry, dust free atmosphere comprises a small radial gap formed between the shaft and housing
(a). Its effectiveness can be improved by providing one or more grooves in the bore of the housing cover
(b). The grease emerging through the gap fills the grooves and helps to prevent the entry of contaminants. With oil lubrication and horizontal shafts, right or left hand helical grooves can be provided in the shaft or
the seal bore (c). These serve to return any oil which may tend to leak from the housing. However, with this arrangement it is essential that the direction of rotation does not vary.
Single or multiple labyrinths give appreciably more effective sealing than gap seals but they are generally more expensive to manufacture. They are chiefly used with grease lubrication. Their effectiveness can be still further improved by providing a grease duct connecting with the labyrinth passage and periodically pumping in a quantity of water insoluble grease, e.g. a calcium soap base grease. In solid housings the tongues of the labyrinth seal are arranged axially (d), and in split housing, radially (e). The radial clearance between the shaft and the housing seal components is not affected by axial displacement of the shaft during running and can be made very small. If angular misalignment of the shaft relative to the housing has to be accommodated, labyrinths of the form shown at (f) are normally used.
An inexpensive and effective labyrinth seal can be made using pressed steel sealing washers (g). The effectiveness of this type of seal increases in direct proportion to the number of washers used. To increase the sealing efficiency of non rubbing seals, the shaft can be fitted with rotating discs (h) and in case of oil lubrication, flinger rings (i) are often used. The oil flung from the ring is collected in a channel in the housing wall and returned to the sump through suitable ducts.
Rubbing seals (Fig. 28.23)
Rubbing seals rely for their effectiveness essentially on the elasticity of the material exerting and maintaining
a certain pressure at the sealing surface. The choice of seal and the required quality of the sealing surface depend on the peripheral speed.
Felt washers (a) are mainly used with grease lubrication, e.g. in plummer blocks. They provide a simple seal suitable for peripheral speeds up to 4 m/s and temperatures of about l0o°C. The effectiveness of the seal is considerably improved if the felt washer is supplemented by a simple labyrinth ring (b). The felt washers or strips should be soaked in oil at about 80°C before assembly.
Where greater demands are made on the effectiveness of the rubbing seal, particularly for oil lubricated bearings, lip seals are often used in preference to felt seals. A wide range of proprietary lip type seals is available in the form of ready to instal units comprising a seal of synthetic rubber or plastics material normally enclosed in a sheet metal casing. They are suitable for higher peripheral speeds than felt washers. As a general guide at peripheral speeds of over 4 m/s the sealing surface should be ground, and above 8 m/s hardened or hard chrome-plated and fine ground or polished if possible. If the main requirement is to prevent leakage of lubricant from the bearing then the lip should face inwards (c). If the main purpose is to prevent the entry of dirt, then the lip should face outwards (d).
The V-ring seal (e) can be used for grease or oil lubricated bearing arrangements. It comprises a rubber ring with a hinged rubber lip which is pressed axially against the sealing surface. It is easy to fit, can accommodate fairly large angular misalignments of the shaft relative to the housing at slow speeds, and in certain circumstances is suitable for high speeds. The effectiveness of the seal owes much to the fact that dirt and liquids tend to be flung off by the rotating seal. The V ring seal is normally fitted on the inside rotating seal. The V ring seal is therefore normally fitted on the outside of the housing when grease lubrication is used and on the inside with oil lubrication.
Spring steel sealing washers provide a cheap and space saving seal, especially for grease lubricated deep groove ball bearings. They can either be clamped against the outer ring (f) or against the inner ring and are designed so that the sealing face is constrained to press against the face of the other bearing ring.
In difficult operating conditions and where severe demands are placed on sealing, e.g. large amounts of dirt or water, rubbing and non rubbing seals are often combined. In such cases the non rubbing seals (labyrinths, flinger rings, etc.) are arranged to supplement the rubber seals and protect them from wear.
Simple space saving arrangements can be achieved by using bearings incorporating seals or shields at one or both sides which are supplied lubricated with the correct quantity of grease. Relubrication is not normally required and they are primarily intended for applications where sealing is otherwise inadequate or where it cannot be provided for reasons of space.
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