ap-4-12.dvi


Acta Polytechnica Vol. 52 No. 4/2012

Fundamental Research on Hydraulic Systems Driven

by Alternating Flow

Ioan-Lucian Marcu1, Daniel-Vasile Banyai1

1
Technical University of Cluj-Napoca, Department of Mechanical Engineering, B-dul Muncii Nr. 103-105 Cluj-Napoca
Romania

Correspondence to: Lucian.Marcu@termo.utcluj.ro

Abstract

This paper presents a new approach to rotary hydraulic systems, and the functional principles of rotary hydraulic

systems that can work using alternating flows. Hydraulic transmissions using alternating flows are based on bidirectional

displacement of a predefined volume of fluid through the connection pipes between the alternating flow, the pressure

energy generator and the motor. The paper also presents some considerations regarding the basic calculation formulas,

the design and testing principles for a hydraulic motor driven by alternating flow, and also a three-phase rotary hydraulic

motor.

Keywords: hydraulic, alternating flow, star or delta connection.

1 General considerations

All known methods of power transmission by a fluid,
and their applications, are based on continuous pres-
sure and flow circulation, which are achieved by the
pump and collected by final elements (actuators), and
the fluid is considered incompressible.

In conventional hydraulic transmissions, the fluid
performs a unidirectional motion between the energy
converters in the power transmitting process. In hy-
draulic transmissions driven by alternating flow, the
fluid executes an alternating periodical motion be-
tween the energy converters. In the case of a hy-
draulic system in which every working volume of an
alternating motor is connected independently, by a
phase pipe, with the corresponding working volume
of a generator, any modification to the volume of the
generator will produce an alternating flow and pres-
sure transmitted along the phase line to the motor.

Figure 1: Principle schema of a rotary hydraulic sys-
tem with alternating flows and pressure drive

Generally, a hydraulic transmission driven by a
rotary alternating flow consists of a generator of al-
ternating flows and pressures (G) and a motor (M).
The connection between them is realized by a num-
ber of pipes equal to the number of phases (Phase 1,
Phase 2 and Phase 3), the pipes being filled with
fluid at a certain (pre-established) pressure, Figure 1.
When the system is functioning, the pressure and
the flow within each pipe varies in a sinusoidal way,
around an average value [1].

In order to have proper functioning, the average
pressure from each pipe must necessarily have the
same value and have a constant value in time. To
obtain the correct functionality, we therefore either
create from the beginning a pressure in each phase
that is higher than the maximum value of the ampli-
tude, or we let the pressure modify itself while it is
functioning.

This result is obtained by using a series of hy-
draulic resistances (Rz1, Rz2, Rz3), rigorously cal-
culated, which interconnect all the phases, and also
a hydraulic accumulator (Ac) connected to them at
point C, Figure 1. The resistances must eliminate
the maximum average pressure rising value in one
second, when the diminution of the flow amplitude
from a phase does not exceed 1 %.

The presence of the accumulator maintains the
pressure in a connection approximately constant at
all time, being able to take over the oil surplus from
the dilatations, and at the same time to supplement
any oil loses. In the case of slow working cycles, as in
this case, the correct transformation for the accumu-
lator is the isotherm transformation, which leads us,

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Acta Polytechnica Vol. 52 No. 4/2012

knowing also the system pressure, to the necessary
volume of the hydraulic accumulator.

This hypothesis was verified using an experi-
mental stand consisting of a three-phase hydraulic
transmission driven by an asynchronous alternating
flow.

2 Design principles of
hydraulic energy converters

driven by alternating flow

As presented in Figure 1, a hydraulic transmission
driven by alternating flow is a closed circuit that does
not require a tank for the working fluid.

The generator was a modified axial piston pump
(wobble-plate type) with three active pistons placed
at angles of 120◦, which allows variation of the plate
angle, thus obtaining different strokes for the pistons.

A hydraulic rotary motor driven by alternating
flow was realized using the principles presented in
Figures 2a and 2b. [4]

The simplest solution for the construction of a ro-
tary hydraulic motor working with alternating flow
is to use in the same assembly three different oscillat-
ing motors with a gear rack (see elements 1 and 3 in
Figure 2a and 2b). Because each piston moves inde-
pendently, corresponding to its phase pipe, each gear
wheel must have the possibility to act individually
on the output shaft. The correct solution for this
is to use an intermediary element, e.g. a drawn cup
roller clutch, which provides a unidirectional rotation
movement (see element 2 in Figures 2a and 2b).

In this construction, the three-phase generator
provides an alternating flow only for the active stroke,
and the retraction stroke of the motor pistons (the

idle stroke) must be made also using an alternating
flow provided by the generator, taking into account
that each piston movement of the generator has a
phase alteration with an angle of 120◦. If this solu-
tion is taken into consideration, we can obtain two
separate ways to interconnect for the working vol-
umes of the star or delta motor.

Figures 2a and 2b show that each phase pipe
(line) is connected individually to a working volume
of the motor cylinders, in this way providing the
active stroke of the pistons. The working volumes
(chambers) of the cylinders are interconnected.

In this way, if we consider that the movement of
each piston has a phase difference of 120◦, the ad-
vance of one motor piston will generate a similar flow
in the star connection, which provides the retraction
stroke (the idling stroke) of the next two pistons. A
characteristic of the star connection is that the sum
of the alternating flows in the connection point is
theoretically zero, Figure 2a.

In the case of the delta connection, each first
working volume of a cylinder, which provides the ac-
tive stroke, is connected with the second working vol-
ume of the next phase cylinder, providing in this way
its retraction stroke (the idling stroke); the pressures
in these chambers are considered, of course theoreti-
cally, to have the same values, Figure 2b.

According to these functioning principles, a pro-
totype of a rotary hydraulic three-phase motor was
realized, working with alternating flows, Figures 3a
and 3b. [2]

In principle, the designed rotary hydraulic mo-
tor consists of three associated hydraulic double end
cylinders, with small dimensions, which act individ-
ually to an output shaft, thus obtaining a continuous
rotational movement of the shaft.

a) star interconnection b) delta interconnection

Figure 2: Functioning principle of a rotary hydraulic motor driven by alternating flow

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Acta Polytechnica Vol. 52 No. 4/2012

a) b)

Figure 3: The design of the three-phase rotary hydraulic motor driven by alternating flow

The hydraulic motor was designed in two versions.
The compact model, Figure 3a, has gear rack conver-
sion mechanisms associated to each small hydraulic
cylinder group. Figure 3b presents an experimental
model of the rotary hydraulic motor driven by alter-
nating flow with a pulley and elastic steel sheet con-
version mechanism. Each designed version contains
three double end hydraulic cylinders which individu-
ally actuate some pulleys (or gear), having drawn cup
roller clutches mounted inside, and in this way the ro-
tational motion and also the torque are transmitted
unidirectionally to the output shaft of the motor.

3 Basic calculation

We assume that the governing equations for the in-
stantaneous flow and pressure are:

Qi = Qa max · sin(ωt + φ0) (1)
And for the pressure:

pi = pst + pa max · sin(ωt + φ0) (2)
in which: (Qi) is the instantaneous flow in [m

3/s],
(Qa max) is the alternating flow amplitude in [m

3/s],
(pi) is the instantaneous pressure in [N/m

2], (pst) is
the initial static pressure of the system in [N/m2],
(pa max) is the alternating pressure amplitude in
[N/m2], (ω) is the angular frequency in [rad/s], (t)
is the time in [s], and (φ0) is the initial phase angle
in [rad].

Making a complete analysis of a hydraulic system
working with alternating flow requires that we take
into account some parameters that define the pres-
sure drop along the line due to the friction, inertia
and elasticity (compressibility) of the fluid, and also
the fluid leaks.

The instantaneous pressure drop due to the fric-
tion along the line will be:

ΔpiR = Rf · Qa max · sin(ωt + φ0) (3)
where (Rf ) is the flow resistance, in [N · s/m5].

Eq. 3 demonstrates the fact that the instanta-
neous flow is in phase with the pressure drop through
the resistance.

The pressure drop due to fluid inertia is:

ΔpiL = ω · L · Qa max cos(ωt + φ0) (4)

where (L) is the fluid inertia, in [N · s2/m5].
The phase vector of the inductive pressure drop is

advanced by 90◦ in comparison to the instantaneous
flow.

If we consider the elasticity of the transmitting
medium, the capacitive pressure drop is:

ΔpiC = −
Qa max
Ch · ω

· cos(ωt + φ0) (5)

where (Ch) is the hydraulic capacity [m
5/N].

It can be noted that the vector of the capacitive
pressure drop is 90◦ behind the instantaneous flow.

4 Testing principles and
experimental results

The functioning prototype was tested in the idling
state and in the loaded state. The load was ob-
tained by using a braking device connected to the
output shaft and actuated by a small hydraulic cylin-
der. This way, we can obtain different values for the
loading torque. The preliminary experimental data,
representing the functional parameters, was obtained
taking into account the monitoring protocol, the dis-
position of the sensors, and using a data acquisition
board, Figure 4.

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Acta Polytechnica Vol. 52 No. 4/2012

Figure 4: Pressures, motor piston stroke and angular speed evolution

In order to acquire the experimental data during
the testing process we had to control the mechani-
cal and hydraulic parameters precisely, using proxim-
ity sensors mounted on the rotation shafts, pressure
sensors mounted on representative points of the hy-
draulic pipes, and displacement sensors mounted on
the motor cylinder pistons. [3]

A series of functional parameters of the hydraulic
system driven by alternating flow was obtained di-
rectly, as: angular speed of the shafts, generator and
motor; pressures (on the generator, on the motor —
loaded, on the motor - idle, in the phase line pipes or
in the interconnection pipes); and the piston strokes
of the motor.

Some of the diagrams were obtained indirectly,
using recorded data referring to: transmitted torques
and phase alteration between alternating flow and
harmonic pressure or the rotational position of the
generator and motor shafts.

5 Conclusion

The objective of this research was to study a new
approach to hydraulic drives, in which the pressure
and flow is not continuously transmitted between the
energy converters (pumps and motors). The paper
describes the construction principles of hydraulic sys-
tems driven by alternating flow.

Within these systems, the active stroke of the
pistons is produced by the flow of pressurized fluid
from the generator, while, for the retraction stroke, a
supplementary connection to a pressure generator is

necessary, working in the opposite phase to the first.
This means that the pistons can be interconnected in
star or delta configuration.

The experimental results, combined with the
mathematical model of this system, demonstrate a
way to adjust, during the functioning, several input
parameters (e.g. the initial static pressure and the
angular speed of the generator), in order to obtain
the anticipated output values of some parameters, or
whether the system load is modifying.

Knowing the limits of this system, the next de-
velopment step is automatic control of the main me-
chanical and hydraulic output parameters.

References

[1] Constantinescu, G.: Theory of sonics. Bucureşti :
Published by Romanian Academy, 1985 (in Ro-
manian).

[2] Marcu, I.-L.: Researches and contributions re-
garding the improving of the acting systems func-
tioning with pressure waves. PhD. Thesis, Cluj-
Napoca, 2004.

[3] Marecu I.-L.: Functional parameters monitoring
for an alternating flow driven hydraulic system.
In IEEE Proceedings of International Conference
AQTR, Cluj-Napoca, 2008, Tome III, p. 239.
ISBN 978-1-4244-2576-1.

[4] Pop, I., a.o.: Sonics Applications. Experimental
Results. Ed. Performantica, Iaşi, 2007 (in Roma-
nian). ISBN 978-973-730-391-2.

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