CONTENTS

1. ABSTRACT 2. LIST OF SYMBOLS 3. INTRODUCTION 4. HYDRAULIC DRIVES IN OFF-THE-ROAD

VEHICLE 5. EXPERIMENTAL SET UP 6. CONCLUSION

7. ACKNOWLEDGMENTS 8. REFERENCES

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ABSTRACTHydrostatic transmission (HST) system used in heavy earth moving machineries

(HEMMs) has high power density, wide range of speed control and good overall

efficiency. Hydrostatically coupled two motor summation drive is an alternative

power transmission system, compared to existing closed-loop HST system with

low speed high torque motor, used in HEMM. Such drive arrangement has made

the possibility to design the transmission system, used in heavy vehicles, in an

efficient way to cover wide range of torque-speed demand. This article studies the

concept of two motor summation drive and its steady state performance.

Experiments have been carried out to analyze the performance of such system. The

characteristics of single and two motor drive systems are compared at different

load-torque and speed levels. It is concluded that two motor hydrostatic drive

systems is more effective at high load-torque and low speed compared to single

motor drive system.

Keywords:

Hydrostatic transmission (HST),Two motor summation drive , Theoretical range

(TR) ,Steady-state performance , Heavy earth moving machinery (HEMM)

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Seminar Guide

Seminar In-Charge HOD(ME)

LIST OF SYMBOLS

Dp Displacement of the main pump

Dm General displacement of the hydro-motor

Dm1 Displacement of the hydro-motor-1

Dm2 Displacement of the hydro-motor-2

DPp Differential pressure across the main pump

Qp Flow supplied by the main pump

Tl Generalized load-torque

Tld Load-torque on double motor

Tls Load-torque on single motor

am1 Displacement setting of the hydro-motor-1

am2 Displacement setting of the hydro-motor-2

ap Displacement setting of the main pump

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gd Efficiency of the two motor drive system

gs Efficiency of the single motor drive system

x Generalized speed of HST drive

xm1 Speed of the hydro-motor-1

xm2 Speed of the hydro-motor-2

INTRODUCTION:

In a hydrostatic transmission (HST) system, the mechanical energy of the input

drive shaft of the pump is converted to pressure energy in a nearly compressible

working fluid and then reconverted into mechanical energy at the output of the

drive shaft of the motor. It is used to transmit the rotational mechanical power from

one source to another without the use of gears. It provides much softer power

transmission than mechanical gear train. High power density, wide speed range of

speed control and good overall efficiency are important demands of a modern HST

system intended to be used in a heavy off-road vehicles. A systematic overview on

mobile hydraulics from industry perspective is presented by Eckhardet al. With

regard to the overall reduction of fuel consumption of the vehicle, Ossyra has

presented a comprehensive control strategy of hydrostatic drive used in mobile

vehicles. A different hydrostatic driveline concept for off-highway machines

through system modeling and simulation with a wheel loader as an example is

made by Torsten. In such investigation, simulation tool has been developed to

analyze the performance of the drivelines regarding losses in every component of

the transmitting system. Carl et al has made a detailed study to apply power split in

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vehicle drive trains. They have investigated four architectures of power split

transmission illustrating the advantages and disadvantages of each type with regard

to energy consumption, efficiency, system complexity, compactness and control

effort for off-road vehicles. Most vehicles require high tractive forces during

starting and climbing that may be 10–30 times the relatively light load in normal

operation. Vehicle speed and load vary over a wide range and require a maximum

tractive effort when starting under full load. Hydraulic systems using fixed

displacement pump are inefficient in cases where the desired velocity and load

vary over a wide range, since the excess flow is dissipated over a relief valve The

maximum performance of the hydrostatic drive depends on the functioning of its

components that mainly depends on various operating conditions. While the

longterm hydrostatic components development focuses on achieving small gains

by improving efficiency, it is to be focused to maximize the system efficiency by

creative system engineering with standard components. Two motor summation

drive system is one of the creative system engineering. Concepts based on using

more than one motor offer other advantages for cases where high power must be

transferred through the transmission. The obvious way is to connect the motors in

parallel and to control them in parallel. Another way is to control them in

sequence. In such a concept, one or two hydro-motors are powered hydraulically

and connected with the load, depending on the loading conditions or the gradient

of tracks. Various authors have studied the connection and disconnection

procedures to be adopted for its fast, smooth and stable operation has tested a

summation drive using hardware-in-the-loop simulation (HWIL). It shows that the

connection using active control of the zero-motor works best at high load

condition. Very few literature is available regarding the performance investigation

on multi-motor hydrostatic drive system.This article compares the steady state

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performance of single motor and two motor closed-loop HST systems used in the

off-the-road vehicle.

Typical traction speed characteristics for a wheel loader

Hydraulic Drives in Off-the-Road Vehicle In hydrostatic drive systems, either by

allowing the displacement of the pump and the motor or by controlling the flow

rate supplied from the pump through proportional valve to the motor, the

transmission characteristics are varied. Maximum speed and maximum tractive

effort are usually not required to occur simultaneously in a machine. Generally, it

is accepted for a construction machine to slow down when output torque

requirement goes up and vice versa. The use of variable displacement pump and

motor meets this requirement. Such drive concept is currently the state of the art in

the wheel loader.HEMMs are subjected to extremely variable loading conditions

and hence there is a need for sophisticated control for improved performance. To

cater the requirement of wider torque-speed demand with maximum

performance,high velocity range of the mobile machineries is state of the art in the

wheel loader.HEMMs are subjected to extremely variable loading conditions and

hence there is a need for sophisticated control for improved performance. To cater

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the requirement of wider torque-speed demand with maximum performance,High

velocity range of the mobile machineries is required. This demand is quantified by

Theoretical Range (TR), which is the ratio of highest vehicle speed to lowest speed

at maximum engine power. Increase in TR value fulfills the required demand of

the mobile machineries.A traditional solution for an HEMM is to use a

hydrodynamic transmission system (torque converter) in series with multi-speed

gear box. However, due to limited number of speed stages in an additional gear

box, used in hydrodynamic system, an HST system may be used as an alternative

drive arrangement of HEMM. The characteristics of the drive systems used in

HEMM. In an HST system with the single motor drive, the increase in the load-

torque increases the system pressure that affects the efficiencies of the hydrostatic

components and the overall system. Therefore, these drives will be effective within

a limited range of the TR value. To use such drive for higher TR value with

reasonable efficiency, it is connected with multi-speed gear unit. However, main

using a single larger one. During high load demand, both hydro-motors are used

and develop maximum torque at their maximum displacement. For high speed and

low load operation, one hydro-motor strokes to zero displacement to improve

efficiencies.This article studies the steady state performance of two motor

summation drive both theoretically and experimentally. In this respect, a system

has been designed and fabricated the schematic representation of which is shown

in The performance of the system has been compared with the drive using single

motor transmission system.

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EXPERIMENTAL SET UP:

Description of the Test Set Up A schematic hydraulic circuit diagram of the two

motor summation drive considered in the present analysis.

Simplified representation of the experimental test set up

TABLE.1,LIST OF MAJOR COMPONENTS USED IN THE TEST SET UP:

S/No: Item description:

1. Electric motor (15 kW)

2. Main pump unit 9. Flushing block

3. 3/2 Directional control valve (DCV)

4. High pressure relief valve (4.1–4.4)

5. Bent axis motor

6. Gear box (Gear ratio 1:1)

7. Loading pump unit

8. Pressure relief valve (PRV) at loading

10. Booster pump

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11. Flow control valve

12. Pressure sensor (12.1–12.6)

13. Speed sensor

List of major components and their specifications are given in Table 1 and 2

respectively. The photographic view of the test set up is shown in Fig. 5. The

operation of two motor summation drive is shown in Fig. 4.A 15 kW electric motor

(1) rotating at 1,450 rpm drives the main pump (2), the swash plate angle of which

is varied by the command signal sent from the control panel. Thus the flow

supplied by the pump to the bent axis motors (5.1and 5.2) is varied. The motors

drive the loading pump (7) through the gear unit (6). The displacement of the pump

(7) is varied by the command signal sent from the control panel. The pump (7)

supplies flow through the pressure relief valve (8). The load on the hydro-motor is

varied by adjusting the pressure setting of the relief valve.The experiments were

conducted for the following two conditions:

TABLE 2, SUMMARY OF MAJOR COMPONENTS AND INSTRUMENTS

USED IN TEST SET-UP:

Item no: Name of the component and Summary of specification

2 and 7 Axial piston variable displacement pump Displacement 28 cc/rev Nominal

Pressure 400 bar.

5.1 and5.2 Bent axis motor Displacement 16 cm3/rev Nominal pressure 400 bar

4 and 8 High pressure relief valve Max. set pressure 350 barMax. flow 200 LPM

11. Flow control valve or sensor Max. range 60 LPM Analogue o/p 4–20mA

Linearity 0.42 %

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12. Pressure sensor (12.1–12.6) Pressure range

12.1 and 12.2 :0–150 bar12.3–12.6 :0–200 bar

13. Non contact type optical speed sensorMax. range 0–5,000 rpm Accuracy 0.5 %

• Driving the single motor:

For driving the single motor,power is supplied to the DCVs (A1and

B1).Experimental Procedure The experiment was conducted at wide range of speed

and torque levels, following a standard procedure. A stable source of power supply

was provided to the electric motor driving the main pump that supplies fluid to the

hydro-motors.The motor speed was varied by controlling the swash plate angle of

the pump.Using suitable sensors and instruments, the speed (x) and the torque of

the hydro-motors (Tl), the differential pressure across the main pump (DPp) and

the flow (Qp) supplied by it were measured.During experiment, the viscosity of the

fluid was kept nearly constant by maintaining its temperature at 50 ± 2 _C.

Experiment was repeated several times to examine repeatability before collecting

the data. Test data were collected at different torque levels by adjusting the set

pressure of the relief valve.

Experimental Observation:

The expression of the TR and the efficiency of two motor summation drive are

derived under ‘‘Derivation of TR Value for the Set Up’’ and ‘‘Expression for

Efficiency of Two Motor Summation Drive’’ respectively.

Determination of TR Value:

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Maximum speed of single motor, xm1ðmaxÞ = 2,450 rpm Pump reaches maximum

input power at hydro-motor speed, xm1N = 130 rpm Therefore,TR ¼ xm1ðmaxÞ_

xm1N ¼ 2450=130 ¼ 18:84 Determination of Efficiency From the data obtained

for the single motor and two motor drive systems, the efficiencies of the systems at

different

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load-torque and speed were calculated using Eqs. 8 and 10, respectively. The

performance characteristics are plotted using best fit lines to the data points. The

characteristics of the two motor and single motor drive systems are shown in Fig. 6

and 7, respectively. From Fig. 6, it is observed that the efficiency of the two motor

transmission system increases with increase in loadtorque and decrease in

speed.However, efficiency of single motor decreases with increase in torque and

speed levels. It is observed that efficiency of single motor is more than two motor

at low torque, whereas, the efficiency of the single motor drive system is

comparatively less than two motor at higher torque levels, shown in Fig. 7.

Comparing Figs. 6 and 7, it may be concluded that at the high torque and low

speed demand, two motor drive system is more efficient than single motor drive

system, whereas for low torque high speed demand, single motor drive system

would be better than two motor system.

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Torque-speed characteristics of two-motor drive system

From the above observations, it may be concluded that machine requiring high

fluctuating load demand as in HEMM, by switching between using single motor or

two motor in the proposed system, the efficiency of the system increases,

throughout its torque-speed range.

Torque versus Speed Curve:

The torque-speed (T-N) curve for single and two motor were plotted at constant

power which is compared with the theoretical T-N curve .From the above study, it

is observed that by combining the single and two motor drive systems, the TR

value of the proposed closed-loop HST system increases.

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CONCLUSION:

The concept of closed-loop HST system using multi-motor drive has been

discussed and its steady state performance is analyzed. The TR value of the

tractive effort versus speed of the proposed hydrostatic transmission system has

been calculated theoretically and validated experimentally.

From the above study, following conclusions are drawn:

• The efficiency of the two motor drive system decreases with increase of speed.

However, the efficiency of this system increases with increase in torque levels.

• The efficiency of the single motor drive system decreases with increase in speed

as well as load-torque.

• The proposed two motor drive system is an alternative drive arrangement,

compared to the conventional closed-loop HST system with low speed high torque.

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Additionally, the two motor summation drive system can have increased TR value

by switching it to operate as a single motor drive system. To meet the fluctuating

load demand of the HEMM, the clutching and declutching methods for switching

between using single and two motor in the proposed multi-motor drive system

should be done quickly and without causing any significant disturbances. Studies

in this regard require more detailed analysis of the system. However, the study

presented in this article may be useful for initial design of the HST system for an

HEMM, where such a drive is incorporated.

ACKNOWLEDGMENTS:

The Research and Development Project Grant for 2009–2012 from University

Grant Commission, Government of India, for carrying out the research work on

this topic is acknowledged. The authors are thankful to the representative of Bosch

Rexroth India, Ltd. for their help in fabricating the Test set up.

REFERENCES:

1. T. Ze-yang, Hydraulic motors and vehicle hydrostatic transmission system of

wheel motor type. Patent No. 4903792, 1990

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2. S. Eckhard, G. Gerhard, Mobile hydraulics—an overview, in Proceedings of the

5th International Fluid Power Conference, 20–22 March, 2006, Aachen, Germany,

vol. 3, pp. 13–24

3. J.C. Ossyra, Control Concepts for Vehicle Drive Line to Reduce Fuel

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2005), ISBN: 3-18-359812-4

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accumulator. J. Mech. Sci. Technol. 24(5), 1163 (2012)

7. M. Sannelius, Hydrostatic Transmissions with Sequence-controlled Motors—

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(Dept. of Mech. Eng., Linkoping University, Linkoping, Sweden, 1997)

8. M. Sannelius, K.O. Palmberg, Control aspects of hydrostatic transmissions with

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transmission with sequence-controlled motors, in Proceedings, 3rd JHPS

International Symposium on Fluid Power, Yokohama, Japan, 1996

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10. M. Sannelius, K.E. Rydberg, Hydrostatic transmissions for wheel loaders-

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Kolloquium, Aachen, Germany, 1996, pp. 389–403

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Hydrostatic Power Transmission, 1983, British Standard Institution,

London

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