TP40-240/2 Shaft Seals Manual

13
Shaft seals in general
Shaft seals
According to the figure below, the lubricating film can
be pumped to the pumped liquid side or to the atmos-
phere side, depending on the direction of the scratches
on the surface.
Fig. 14
Leakage rate of seal
The typical surface topography of seal rings is a statis-
tic distribution of scratches in all directions obtained by
means of a lapping process. A shiny surface with a
small roughness can be produced by lapping. However,
where both seal rings are in hard materials, one of the
seal rings should have a dull finish to prevent the seal
rings from sticking together during standstill.
The running-in period for a dull surface finish lapped to
an Ra value of 0.2 may last several days.
With submersible pumps where the shaft seal is
intended to prevent the ingress of water into an oil
chamber, the pressure difference and thus the leakage
rate above the seal is normally relatively small. During
the running-in period, only few millimetres of pumped
liquid enter the oil chamber under normal operating
conditions.
O-ring shaft seals
In an O-ring seal, sealing between the rotating shaft
and the rotating seal face is effected by an O-ring.
The O-ring must be able to slide freely in the axial direc-
tion to absorb axial displacements as a result of
changes in temperatures and wear. Incorrect position-
ing of the stationary seat may result in rubbing and thus
unnecessary wear on the O-ring and on the shaft.
O-rings are made of different types of rubber material
such as NBR, EPDM and FKM, depending on the oper-
ating conditions.
Fig. 15
O-ring shaft seal
Advantages and disadvantages
Advantages:
Suitable in hot water and high pressure applications.
Disadvantages:
Deposits on the shaft such as rust may prevent the O-
ring shaft seal from moving axially.
Bellows shaft seals
A common feature of bellows mechanical seals is a rub-
ber or metal bellows as dynamic sealing element
between the rotating ring and the shaft.
Rubber bellows shaft seals
The bellows of a rubber bellows mechanical seals is
made of different types of rubber material such as NBR,
EPDM and FKM, depending on the operating condi-
tions.
Two different geometric principles are used for the
design of rubber bellows:
• folding bellows
• rolling bellows.
Fig. 16
Rubber bellows shaft seal
Advantages and disadvantages
Advantages:
Not sensitive to deposits such as rust on the shaft.
Suitable for solid-containing liquids.
Disadvantages:
Not suitable in hot water and high-pressure applica-
tions.
TM02 7454 3603
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 0.05 0.1 0.15 0.2
Leakage rate [ml/h]
Roughness [Ra]
TM00 2581 4593TM00 2582 3097
O-ring
Rubber bellows with
folding bellows
geometry