The right stuff
Hydraulic hose failures can soon be a thing of
the past, if engineers reconsider the rubber
compound design, suggests Jan Goossens
When a hydraulic hose assembly failure
can cause significant equipment
downtime, choosing the hose that
minimises the risk of failure becomes
of extreme importance. OEM design engineers
generally take performance criteria, such as pressure
rating and minimum bend radii, into consideration,
but many often underestimate or even overlook the
importance of the rubber compound and its influence
on key factors such as hose performance, reliability
and lifetime. Gates Hose and Connectors invests
significant time and resources in rubber compound
development and testing. As such, it is well placed to
share its expertise on ensuring that your hoses are
made from the right stuff.
What is rubber?
Synthetic rubber compounds are used in the
production of hydraulic hoses. Unlike natural rubber,
the chemical structure of which is quite variable, a
synthetic rubber compound consists of a very tightly
controlled composition of man-made polymers (also
sometimes known as elastomers), 'genetically'
engraved with various functional groups to obtain
specific properties. The polymer is the key
determinant of the hose compound's physical
properties and its compatibility with different media.
Once a rubber compound has been mixed, it
becomes a polymer network with viscous behaviour
that can then be moulded or extruded into its final
shape. The rubber is then vulcanised at elevated
temperatures to create chemical cross-links between
the different polymer chains, resulting in a permanent
elastic structure - the hose. The polymers that are
mainly used for hydraulic hose compounds are NBR
(nitrile-butadiene rubber), which is sometimes blended
with PVC for ozone resistance, CR (polychloroprene
rubber), CPE (chlorinated polyethylene rubber) and
CSM (chlorosulphonated polyethylene rubber).
Hydraulic hoses consist of three structural
elements - the inner tube to convey the fluid, a steelwire
reinforcement that withstands the hydraulic
pressure and the outer cover to protect the hose
against external influences. The basic construction is
similar to that of a braided hose, except that the
reinforcement is spiralled around the tube and can
consist of up to six layers of steel wire with additional
friction layers in between.
The hose compound fulfils a different function in
each of these structural elements. Firstly, the inner
tube rubber compound must be compatible with a
broad range of potentially aggressive hydraulic oils
and has to resist high oil temperatures. It must also be
strong enough to bridge any gaps in the
reinforcement, formed when the hose is bent. The
inner tube must also provide the required coupling
hold to ensure the lifetime of the hose assembly.
Secondly, the different layers of reinforcement are
interspersed with thin layers of rubber compound
applied between them. This thin but vital layer of
compound, called friction, transfers the load onto the
different reinforcement layers and also binds them
together. In addition, it also fills any gaps in the
reinforcement pattern.
Finally, hose-cover compounds must have a strong
resistance to potentially damaging external elements,
such as ozone, UV light, hot air and abrasion. They
must also adhere firmly to the wire reinforcement,
especially so in the case of hoses intended for use
with no-skive couplings.
....CONTD
