Labyrinth sealing operates on the principle of clearance. It ensures sealing performance through long and tortuous clearances, where the sealing effect depends on the labyrinth's form, actual clearance, and leakage path. The basic form of a labyrinth can be provided by rings (on the shaft) and grooves (in the bore), but a more effective design features interleaved teeth or staggered serrated projections. It is generally believed that sharp "teeth" are more effective in reducing leakage under given clearance and sealing length, especially when sealing gases.
The effectiveness of labyrinth sealing improves inversely with the size of the clearance—thus, the smaller the clearance, the better, provided that contact within the seal can be avoided under operating conditions. The clearance must be larger than the axial or radial clearance of the rolling bearings supporting the shaft, or larger than the possible elastic deformation in sliding bearings. Typical fits for sliding bearings are: bore tolerance grades ISO K7, M7, and N7; shaft tolerance grades ISO h6, j6, or k6. When determining the appropriate clearance, the effect of thermal expansion at operating temperatures— which reduces the clearance—must be considered.
On the other hand, if there is a significant risk of contact within the seal, the teeth can be made of sublimable materials instead of fusible ones to prevent seizing or scuffing of mating parts in case of contact. Another solution is to manufacture one set of "teeth" from graphite when the consequences of occasional running-in contact are minimal (prolonged running-in contact will wear the graphite surface into a light "bearing" fit).
Another form of sealing that operates on the pure labyrinth principle is the metal honeycomb ring with slight running-in contact. In this case, the labyrinth path is provided by the honeycomb structure. Such seals, made of stainless steel or similar honeycomb materials, can withstand abrasion and erosion under pressure in applications such as rotor tip seals.
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