Concept of active counterweight system for medium excavator

Concept of active counterweight system
for medium excavator
ARTUR GAWLIK
STANIS¸AW MICHA¸OWSKI
Excavators and loaders present the biggest chance
in the increase of efficiency through the recuperation
of hydraulic energy. It is a result of repeatability of
work cycles and potential energy of lowering excavator’s
linkage possibility to use. Counterweights are
assembled quite often in the construction equipment
structures but these elements are normally fixed. In
the specialist machines like the pipe layer crane is
used counterweight driven by the hydraulic cylinder.
It is however, only a passive solution which works
apart from the boom mechanism. This system can
increase only the static stability of the machine.
However, level luffing jib cranes have movable counterweights
which have a motion connection to the
motion of the crane boom. The counterweight is
connected with boom mechanism through a link
mechanism. This solution unloads boom drive in
approximate mode and increases the stability of the
crane. The idea of an active counterweight is propagated
by Volvo research team in his “SfinX Project” as
well. This is a conception of how excavators could
evolve over the next two decades. The active counterweight
unloading boom mechanism is only one of
many conceptions.
Idea of active counterweight
The suggested solution is that a movable counterweight
is connected with a hydraulic drive system of
Mgr in˝. Artur Gawlik, prof. dr hab. in˝. Stanis∏aw
Micha∏owski, Cracow University of Technology Institute
of Machine Design.
58
a work linkage. To begin with, few assumptions were
made: the mass of an active counterweight should
not be bigger than the mass of the standard counterweight
by reason of additional resistance to the
motion of a machine and necessary modification of
kinematics pairs. Moreover, the movable link of the
counterweight should not exceed the superstructure
contour. After an initial analysis, of the co-operation
among the connected hydraulic cylinders of excavator
and an active counterweight, it was shown that
their connection should not be direct. Some additional
valves are a necessary in this system.
The center of mass vector for excavator linkage and
counterweight shows formula:
The first part of the numerator of a formula (1)
presents sum of gravitation force moments, which
act on linkages and counterweight. The second part
of the numerator shows the sum of a potential
energy for excavator and active counterweight links.
The correlation between the counterweight center
of a mass and the machine linkage center of mass
could be calculated by a manner where components
of the center of the total mass vector are constants
( = const.). This situation gives two crucial effects:
– a static overturning moment will be constants
for different linkage set up,
– a connection of excavator hydraulic circuit and
hydraulic driven counterweight mechanism allows for
ROK WYD. LXVIII ZESZYT 7-8/2009
(1)

Fig. 1. Homothetic transformation of
excavator and active counterweight
system for scale factor k = 0,5
energy flow between described
systems and produces mutual
static unloading from links load.
Vector of center of mass position
in point P for movable counterweight
describes equation (2):
Taking into consideration that
= const., result from equation
(1) that point P of center of mass
for active counterweight trajectory should be
homothetic to point O of center of mass for excavator
–k
linkage trajectory. Homothetic transformation J has s
center in point S and scale factor –k (Fig. 1). It is
possible to achieve it only if the counterweight
mechanism is homothetic to the linkage mechanism.
Simplified active counterweight mechanism in relation
to typical linkage is presented in Fig. 2. First
link, which is connected to excavator frame, is a result
of homothetic to boom. The second link represents
a bucket and an arm.
As far as the excavator kinematics is concerned, the
biggest energy saving effect has a system with boom
cylinder which is connected to the main cylinder of
the counterweight. A static moment round the pin of
the boom and the pin of the first link of counterweight
allows calculated equation F o = k · F p . The connected
hydraulic system should ensure velocity for counterweight
cylinder as the formula shows v p = k · v o . The
same goes for areas of cylinders A 1 = k · A 4 .
In Fig. 3 the concept of hydraulic systems for connection
excavator and active counterweight mechanisms
is shown. It is the main part of the system. The
similar scheme could be prepared for an arm and
a second link of the counterweight cylinder.
Conclusion
The efficiency of an active counterweight system
requires a simulation verification and proof test during
the stand research. Presented hydraulic system does
not change the functional properties of the excavator
and allows the operator to shut down the movable
counterweight system when it is necessary (for
(2)
Fig. 2. Structure of excavator with active counterweight
example while working as a lift). Taking into consideration
the assumption about the movable counterweight
– the scale factor of homothetic transformation
was determined k = 0,5. Value of this factor limits the
group of machines in which an active counterweight
system could be applied to medium size excavators.
Fig. 3. Concept of main part hydraulic system with active
counterweight: 1 – boom cylinder, 2 – distribution valve 3/4,
3 – relief valve, 4 – filter, 5 – pump, 6 – valve 2/2, 7 – main
counterweight cylinder, 8 – hydro-pneumatic accumulator
ROK WYD. LXVIII ZESZYT 7-8/2009 59

List of notations
A 1 , A 2 , A 3 , A 4 – Area of piston and head side of each
cylinder, m 2
F o , F p – Boom and counterweight cylinder force, N
K – Scale of homothetic transformation (k = m o /m P )
m o , m P – Total mass of excavator linkage and counterweight
linkage, kg
v o , v p – Linear velocity of boom and counterweight
cylinder, m/s
x o , y o – Center of mass excavator linkage coordinates
in point O, m
x p , y p – Center of mass counterweight linkage coordinates
in point P, m
60
REFERENCES
1. Liang X., Virvalo T.: Energy reutilization and balance analysis
in a hydraulic crane. IHA, Tampere University of Technology,
2003.
2. Cetinkunt S., Pinsopon U., Chen C., Egelja A., Anwar S.:
Positive flow control of closed-centerelectrohydraulic implement-by-wire
systems for mobile equipment applications.
Mechatronics 14/2004, pp. 403 – 420.
3. Qing X., Qingfeng W., Yanting Z.: Control strategies of
power system in hybrid hydraulic excavator. Automation in
Construction 17/2008, pp. 361 – 367.
4. Sobczyk A., Buczak G.: Hydraulic-mechanical energy saving
system in cranes and tranportation equipment. Proc. Of
4-th FPNI-PhD Symposium, Sarasota 2006, pp. 641 – 648.
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