Rotary Seals - Dilanda.it

Radial Oil Seal
Overpressure
When the sealing element is exposed to pressure it is
forced against the shaft and the area of the lip in contact
with the shaft increases. The friction as well as the
generated heat, increases. As a result, when the seal is
under pressure, the specified values of peripheral speed
cannot be maintained but must be reduced in relation to
the magnitude of the pressure. At high peripheral speeds
even overpressures of 0.01 to 0.02 MPa may cause
difficulties. By fitting a separate back-up ring the types
(TRA/CB, TRC/BB, TRB/DB) can be used for overpressure
above 0.05 MPa. The separate back-up ring shape follows
the rear profile of the sealing element but without mutual
contact when no pressure differential exists. See Figure 10.
However, the back-up ring requires much accurate fitting.
Please ask your local B+S company for suitable back-up ring
drawing. The cases of seal type TRU are formed to support
the sealing element. See Figure 10. The type TRP/6CC is
designed with a short and sturdy sealing lip, which allows
overpressures without using a separate back-up ring. When
a back-up ring is installed or when the types TRU, TRP/6CC
are used, overpressures of 0.4 to 0.5 MPa are permissible at
moderate peripheral speeds.
At very high pressures, the seals with rubber-covered cases
should be employed in order to avoid leakage between the
periphery of the seal and the housing bore. When the seal
is under pressure there is a risk of axial movement in the
housing bore (pop-out). This effect can be prevented by
locating the seal against a shoulder, with a spacer ring or a
circlip.
1.0
0.9
TRU
0.8
ø 25
Pressure, MPa
0.7
0.6
0.5
0.4
ø 50
ø 100
ø 150
TRP/6CC
0.3
0.2
TRA/CB with back-up ring
0.1
0
1000 2000 3000 4000 5000 6000 7000
Shaft speed, min -1
Figure 10
Permissible overpressure for supported lip seals and for pressure seals
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Edition April 2006