Rotary Seals - Dilanda.it

Rotary Seal
Shaft run out and eccentricity should as far as possible be
avoided as these parameters may cause leakage depending
on the capability of the sealing lip to follow the shaft
movements. Limits can be found in the various chapters.
They may vary for the different seal materials.
For example, leakage classes are defined for oil seals in DIN
3761 Part II (Motor vehicles). Today “zero leakage“ is
demanded in practice for motor vehicle applications. Zero
leakage means that under various conditions the fluid
being sealed must be absolutely separated from the
environment.
n Environment
Leakage control
When defining leakage control one must differentiate
between static sealing (sealing of two surfaces without
motion relative to one another) and dynamic sealing
(relative motion between the two surfaces).
With a moving seal surface, a fluid film separates the
sliding surfaces from one another; a dynamic seal gap
forms. The leakage path is not fully closed off as with static
sealing, so small quantities may escape. Seals in which a
dynamic seal gap forms between the seal body and a
rotating shaft cannot be tight in the physical sense.
Absolute tightness in the physical sense cannot be achieved
with a seal gap alone when sealing moving parts.
For many technical applications, however, it is quite
adequate if the “leakage“ is reduced to such an extent
that there are no negative consequences for the
environment or the operation of the assembly. This is
called technical tightness.
Environmentally-friendly hydraulic fluids (bio oils)
When machines or process equipment is hydraulically
operated, escaping hydraulic oil can pollute surface
waters and the ground. One way of minimising the
danger posed by unwanted leakage, is the use of
biologically degradable, non toxic oils. In many countries,
there are already statutory regulations and catalogues of
requirements for dealing with materials that endanger
water. Hydraulic and transmission fluids that protect the
environment are already specified in some cases. Figure 1
shows the types of biodegradable fluids.
Environmentally-friendly fluids have application in all
systems, which operate for example in mobile and
agricultural machinery and in the water and forestry
industries. In stationary systems, they are employed in
plants where water is endangered, such as locks, water
turbines and for foodstuffs and pharmaceutical products.
An important criterion for biologically rapidly degradable
fluids is their compatibility with seals. In Table II the
resistance of elastomeric materials are given to “bio-oils“.
They are, however, provided with a number of comments.
Technical tightness must be specified by the user or
manufacturer of an assembly, i.e. in some circumstances
maximum permitted leakage rates must be defined.
Biodegradable Hydraulic Fluids
Vegetable
Oils
Synthetic Fluids
Triglyceride
HETG
Synthetic
Ester Oil
Polyglycol
HEPG
Rapeseed
Oil
Polyalphaolefins
PEG
PPG
Figure 1
Biodegradable hydraulic fluids
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Edition April 2006
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