Forging
ahead fluidly
Electro-hydraulics
and mechatronics are only some of the recent advancements in fluid power
systems, says V Palanisamy
Oil
hydraulics was introduced as a useful medium for power transmission
during the earlier part of this century. Since then, the technology
has undergone continuous evolution by responding to the challenging
demands poised by a wide variety of applications. Over these years,
it has competed with other competitive power transmission technologies
to find its present place. The strengths of hydraulic drives and hydrostatic
transmissions that make them the preferred choice are their high force
and acceleration capability, ability to operate at full torque even
at zero speeds, continuous speed variability and stiffness. These superior
features combined with the development of components that can interface
with complex electronic controls have carved an undisputed domain for
hydraulics in power transmission.
Hydraulic
systems are being used on a wide variety of machines today and they
form sub-systems of many machines deployed in agriculture, construction,
mining, earthmoving, material handling, defence and aerospace. The manufacturers
of these systems and the components have been alert to the needs of
the customers and are vying with each other to offer systems that are
more compact, energy efficient, reliable and less expensive. This endeavour
has resulted in higher system pressures, close centre systems with feedback,
better filtration and components with modular type of construction blocks.
The use of electronic circuits for the control of these systems has
also been on the increase. An overview of these developments would provide
an insight into the present status of fluid power technology in India.
The
power transmitted by a hydraulic system is proportional to the product
of pressure and flow in the system. This means that by raising the pressure
levels, the same power can be transmitted by a lesser flow. The benefits
are two fold; the energy losses are brought down thereby reducing the
heat generated in the system and the components can be made more compact
to handle the reduced flow. The higher pressure levels have been made
feasible by the development of a whole range of components from pumps
and actuators to fittings and hoses assisted by the advancements in
material science and manufacturing technology. In industrial applications,
the highest pressures adopted now are around 210 bar and in mobile applications
they are around 400 bar. There are attempts to raise the working pressures
to 630 bar. But this may not come into widespread use within the next
decade.
Seals
To
cope with the enhanced pressures and the stringent performance requirements
set by the users, the seal manufacturers have been constantly upgrading
the seal materials, their designs and the manufacturing processes. A
few years back, a certain amount of leakage was acceptable in the approval
programmes of the users. But nowadays, the criterion is simply zero
leakage. In addition, the users need greater reliability, less maintenance
and zero downtime. A good seal should have excellent flexibility, high
wear resistance, resistance to permanent set during cycling, resistance
to extrusion after installation, ability to perform over a wide range
of operating temperatures and compatibility with the fluids in contact.
Development of seals, which meet all the above requirements, is indeed
a difficult job. Perhaps it is in this area that we find the most intense
yet silent research in the whole gamut of fluid power. Thermoplastic
polyurethane materials that retain their properties even above 120oC
and elastomers impregnated with materials such as bronze and glass are
the results of this
research.
The
design of the traditional U-cup seals has also undergone changes with
refinement of the profiles, which eliminate contact stress concentration
and the resultant excessive wear during operation. Finite Element Mathematics
is an important tool used to arrive at these patented profiles. The
U-cup seals or slipper seals, being pressure loaded, depend on the sealed
pressure for the lips to extend and make the sealing effective. Hence
they suffer from poor low-pressure sealing. Such a hurdle has been overcome
by incorporating ‘energisers’ in thermoplastic polyurethane materials
with remarkable flexibility. Manufacturers have also sought to reduce
the wear out rate of these seals by introducing the concept of bearing
area in the specification of the surface finish of the hardware, Bearing
area defines the texture of the surface more clearly than is done by
the roughness average value, Ra. It is determined by calculating the
percentage of material relative to the free space on a line established
below the highest peak and parallel to the surface. Thus, surfaces with
flat, blunt peaks but deep fissures will have a good bearing area though
the roughness as defined by the Ra value would be high. For longer seal
life, this ratio has to be above 50 per cent, which can be obtained
by processes like roller burnishing. Grinding and honing processes produce
inferior ratios. The introduction of ceramic coatings for piston rods
for use in corrosive environments allows for higher sliding speeds without
deterioration of seal life. A process for ceramic coatings on piston
rods of larger sizes has been patented by Hunger Hydraulik, a company
based in Germany.
Filtration
Filtration
is an area, which has now received the attention, which was due earlier.
Many users and the system designers have learnt it the hard way. The
closer clearances in the components for high-pressure hydraulics and
the highly sensitive servo controls have placed more stress on the cleanliness
of the fluids. For trouble free operation, some of these components
require cleanliness levels as good as class 4 of National Aerospace
Standards 1638. Filter manufacturers have developed high dirt holding
capacity deep filtration media like glass fibre that are used for making
filters that can give long service life. Filters that can remove water
from the fluids are also available today. Judicious use of these filters
in the pressure line, the return lines and on-off line filtration has
eliminated 70 per cent of the component failures in today’s systems.
But the prices of these filters are not encouragingly low for their
widespread use.
Hydraulic
oils
In
addition to striving for its own advancements, fluid power technology
has shown concern for the environment. The research programmes aimed
at reducing noise levels and developing biodegradable fluids demonstrate
this. These biodegradable oils under development are either vegetable
oil based or synthetic ester based. They contain low toxic, ashless
inhibitors and additives to enhance performance. Though they exhibit
wear resistance characteristics as good as that of petroleum based fluids,
they have poor water tolerance and oxidation resistance, especially
when the base is vegetable oil. Hence they have not yet been considered
as replacement for petroleum based fluids and in the immediate future
they may be used only in environmentally sensitive areas where contamination
of ground water by petroleum lubricants could be a problem.
....CONTD
