Page 271 - The Tribology Handbook
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B22  Labyrinths, brush seals and throttling bushes





               VISCOSEAL
               (Also called a screw seal or wind back seal)
               Resembles a bush seal in which a helical groove has been   SEALED
                                                                                  L
               cut in the bore of the bush or on the shaft (Figure 22.9). As   LIQUID  r-p
               the shaft rotates  the helix  pumps any leaking fluid  back                             (m/s or ft/sec)
                into the sealed system. There is no sealing action if the shaft
               is not rotating: an auxiliary seal can be fitted  to prevent                           3-
               static leakage and may be arranged to lift off automatically
               when the shaft rotates, thereby reducing wear.
                 Viscoseals are used with ViscousJuids, or at high rotational
               speeds, to seal low or moderate presures. When the pump-   DENSITY OF FLUID = p
                                                               VISCOSITY OF FLUID = q
               ing action just balances the leakage flow with the helix full
                of liquid there is no net leakage-this  is the sealing pressure.
                If the system pressure exceeds the sealing pressure the seal
                leaks. At lower pressures  the helix runs partially dry.
                                                                    Figure 22.9  Viscoseal or wind-back seal
                Optimum design and performance
                prediction

                Helix angle, p   = 15.7"                         Typical performance
               Clearance ratio, h/c  = 3.7
                Land ratio b/a   = 0.5                           A  25 mm dia.  seal with  a clearance of 0.025 mm, length
                Groove profile rectangular                       25 mm, sealing water with a shaft speed of 2.5 m/s would
                                                                 be expected to seal at pressures up to 0.67 atm.
                 The 'sealing  pressure'  for a seal of these dimensions, is
                given  by:
                   671 VL
               PI=-.     k  (71 = absolute viscosity)
                     2                                            BARRIER VISCOSEAL
                  (i)  k = 0.1 if (Vcp/q) <500  (Le. laminar flow)   By  installing  a  pair  of  conventional  viscoseals  back  to
                or  (ii)  k is  given  by  Figure  22.10  if  (Vcplq)  >500  (Le.   back, so that pressure is built up between them, a pressure
                     turbulent flow).                             barrier forms to prevent the sealed fluid escaping (Figure
                                                                  22.11).  The sealed  fluid  must  not  be  miscible  with  the
                 The  optimum  design  varies  with  Reynolds  number   barrier fluid. Typically the former is a gas and the latter a
                [(Re) = Vcp/71] for turbulent  flow; in particular  a smaller   liquid or grease.
                helix angle than the optimum laminar value is better.
                  Eccentricity will lower the sealing pressure with laminar
                flow but may raise it for  turbulent  flow.
                 A  problem  may  arise  due  to  'gas  ingestion'  when
                running in the sealing condition. This is due to instability
                at the liquid/air interface and is not easily prevented. The            BUFFER  LIQUID
                effect is to cause air to be pumped into the sealed system.








                Y

                                                                           Figure 22.11  Barrier viscoseal


                     I             I   I   I  III   I   I   I   I   n n m J  Typical performance
                    lo2                 103                10'
                                                                  In tests using grease as the barrier fluid, gas pressures  up
                                       (Re)
                                                                  to  10 atm  have  been  sealed  by  a  13 mm  dia.  seal  at
                Figure 22.10  Variation of k with Reynolds Number   1000 rev/min.
                                                             B22.6
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