18 MAY/JUNE 2017
costs and potential weight reduction. The firmly bonded
connection between the two end caps and the housing is
carried out with electromagnetic pulse forming technology
(EMPT). It is based on the contactless forming of
electrically conductive materials using strong magnetic
fields. Depending on the energy and material, the reshaping
itself takes place within 10 to 200 micro-seconds
Figure 4: Principle electromagnetic pulse technology
Freudenberg is using this technology for the first time for
the firmly bonded joining of aluminum accumulators.
Unlike mechanical crimping, there is no tool wear
due to the contactless process, and there is much less
contamination during production. High stability is another
advantage, as is the precise dosing of the amount of
energy, which in turn allows highly accurate replicability.
In addition, a new gas-tight anti-friction coating was
developed for the aluminum housing to achieve optimal
guide and wear behavior.
The low weight of the new steel high-pressure
accumulator was achieved with a variety of measures:
a housing made of high-strength steel, dome-shaped
end caps, an aluminum piston as media separator, and
a reduction in the thickness of the pressure vessel’s
material. The housing wall’s thickness is only marginally
greater in certain areas, for example at the ends where the
end caps are attached. Even the shape of the end caps
was optimized. With their new design, they are the best
possible compromise between weight and axial length
under high-pressure loads.
If one assumes the standard industry design for a
20-liter piston accumulator weighing about 110 kg, the
weight can be reduced to 45 kg with a conventional
material (lightweight design using steel). This is equivalent
to a weight savings of 59 percent for a high-pressure
accumulator 800 mm in length, designed for a maximum
pressure of 350 bar. The firmly bonded connection of the
caps and housing is reliably executed with electron beam
welding. The advantages are low energy expenditure,
low heat input, and reduced deformation, along with an
oxidation-free connection of the materials. Still more weight
reduction for high-pressure accumulators is possible with
other plastics reinforced with carbon and glass fibers
(CFK/GFK) as described below.
The noise generation that takes place when the piston
strikes the end stop during a full discharge has also been
optimized. In the new high-pressure accumulators, this is
handled with hydraulic damping. Wide-ranging simulations
of piston speeds were carried out to achieve this. Special
sealing systems (elastomer/PTFE) with low friction are
used in both hydraulic accumulators, which has a positive
effect on efficiency and hysteresis. The result is good
dynamic behavior with high efficiency and low permeation.
The monitoring of the two accumulators’ condition takes
place with a combined pressure and temperature sensor.
At the gas end, safety is assured with pressure and
temperature sensors. On the liquid side, there is also a
Figure 5: Overview of weight reduction Figure 6: Prototype validation plan