9 - AHP Merkle GmbH

!
When using HFC fl uids, it must be clarifi ed whether the seal materials being used
are suitable. While fl uororubber (FKM) is not suitable in every case, seals made of
acrylnitrile-butadiene rubber (NBR) are unproblematic.
HFDs are water-free fl uids that can be used in the temperature range from +20 °C to +150 °C (+68 °F to +302 °F).
Their compositions differ greatly, which means that a further distinction is made among HFD-R, HFD-S, HFD-T,
HFD-U. These fl uids are fl ame retardant, but can cause problems in the suction performance of pumps, and
they are corrosive to many seal materials.
Quickly biodegradable hydraulic fl uids are created using vegetable-based materials. Their abbreviation HE stands
for Hydraulic Environmental, and also appears in the following designations: HETG (based on triglycerides /
vegetable oils), HEES (based on synthetic ester), HEPG (polyglycols), and HEPR (based on other fl uids / primarily
poly-alpha-olefi ns).
Pure water as a hydraulic fl uid appears in very few applications, because its physical properties are hard to control.
1.13 Rod quality and selection of seals
!
Tip
Due to improvements in seal technology, today hydraulic systems can be designed so that they are completely
leak-free. However, for seals which seal the piston rod relative to the pressure chamber, a minimal „lubricating
fi lm“ is desirable. This improves not only the gliding properties on the piston rod, but also reduces wear.
For this purpose, the special seals have a recirculation function that conveys this microfi lm back to the pressure
chamber. This prevents the microfi lm from forming drops, and thus ensures that no hydraulic fl uid escapes
into the environment.
In order to achieve as long a service life as possible, the seal, microfi lm and rod characteristics have to be
matched to each other optimally. Particular attention has to be paid to the rod surfaces; this can achieved in
the following ways:
● Piston rods hardened, polished and hard chrome plated
● Hardened and polished
● Hardened, polished and hard chrome plated
Even extremely fi ne scoring on the piston rod will inevitably lead to leakage, and will
signifi cantly reduce the service life of seals. Therefore it has to be ensured that piston rods are not
subjected to any external mechanical stresses – either in operation or during maintenance.
The use of hardened, polished and hard chrome plated piston rods like those supplied
by AHP Merkle signifi cantly reduces the risk of damage.
Hardened piston rod (AHP Merkle standard)
Non-hardened piston rod
Basic information about hydraulic cylinders en
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