Hydrostatic transmission as an alternative to conventional gearbox

IJRET: International Journal of Research in Engineering and Technology eISSN: 2319-1163 | pISSN: 2321-7308
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Volume: 03 Issue: 05 | May-2014, Available @ http://www.ijret.org 136
HYDROSTATIC TRANSMISSION AS AN ALTERNATIVE TO
CONVENTIONAL GEARBOX
Sumair Sunny1
, Sunny Pawar2
, Siddhesh Ozarkar3
, Sandeepan Biswas4
1
B.E. Mechanical, Maharashtra Institute of Technology Pune, Maharashtra, India
2
3
4
Abstract
This project involves the replacement of a conventional gearbox transmission system by a Hydrostatic Transmission (HST) system.
The applications being considered are those of typical off-road vehicles such as forestry, construction site or mining & quarry
vehicles. In these applications speed is of less importance as compared to torque and load carrying capacity. To create a small scale
simulation of the performance comparison between the two transmission systems, we are selecting a buggy as our test vehicle. The
same engine (Briggs & Stratton Model 20, 10H.P.), as well as wheel base and wheel track being used for both simulations.
Geometrically, both vehicles will remain exactly the same and under running conditions the weight required to be hauled is being
considered as 350 kg. The only variation will be in the mountings of the two different transmission systems. The above criteria enables
a fair simulation as well as comparison of the two transmission systems.
The vehicle will be single seated with a curb weight of around 260 kg. The governor setting of the engine will remain unchanged for
both simulations. 25” kings tire will be used with the tread pattern being KT168. The gearbox being considered is a ‘Mahindra Alpha
Champion’ 4 speed gearbox. The purpose of choosing this as the transmission of our baseline vehicle is that it can operate its four
gears in reverse as well. This is important as a HST system shows symmetrically opposite characteristics for both forward and
reverse, hence making it a fair comparison. The hydrostatics pump and motor will be selected from Eaton product catalogues as this
project has been offered to us, and is being guided by Eaton. The performance analysis is done on basis of Velocity, Acceleration and
Grade-ability. The aim is to design a HST circuit which can closely match up to the performance offered by a gearbox transmission so
as to verify whether a HST system can be a suitable replacement to a gearbox transmission for the above mentioned applications. This
project involves valid database, data flow, algorithm and out reports. The actual fabrication would be too expensive and hence
unfeasible. All self-designed parts will undergo Finite Element Analysis (FEA) using Altair Hypermesh. This will validate the design
calculations. All the calculations have been done using Microsoft Excel and the final simulation has been done on MATLAB
SIMULINK. All the modelling has been done using SolidWorks.
Keywords: HST, gearbox, engine, pump, motor, alternative, SIMULINK, hydrostatic, hydraulic, differential, worm
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1. POWERTRAIN DESIGN
1.1 Objective
The engine has a limited speed range between which it
develops maximum torque. Our aim is to design a system to
extract maximum torque from the engine to develop optimum
tractive effort in order to overcome the various resistances and
propel the vehicle.
Graph 1:- Torque v/s Engine Speed

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Table 1: Engine Specifications
Engine Specifications
Model Number I/C MODEL 20
Type 202400
Displacement 305cc
Bore 3.120”
Stroke 2.438”
Compression Ratio 8.0 to 1
1.2 Gearbox Selection:
We have calculated the required ratios based on the mass of
the vehicle, the rated power of the engine & the overall
efficiency (assumed). A number of iterations were carried out
using various tire sizes and assuming that max. Speed of
vehicle in 1st & 4th gear to be 15 kmph & 60 kmph
respectively.
The resulting ratios are tabulated below and accordingly we
have selected the Mahindra Champion Alfa transmission (4
forward 1 reverse) for our ATV.
Table 2: Gear ratios
Gear Calculated Ratios Selected Ratios
1 31.92:1 29.84:1
2 20.1:1 17.71:1
3 12.67:1 10.83:1
4 7.98:1 7.26:1
Reverse - 55.08:1
1.3 Gearbox Performance Curve
Graph 2:- Engine Speed v/s Road Velocity (HST)
2. HYDROSTATIC TRANSMISSION SYSTEM
The primary function of any hydrostatic transmission (HST) is
to accept rotary power from a prime mover (usually an
internal combustion engine) having specific operating
characteristics and transmit that energy to a load having its
own operating characteristics. In the process, the HST
generally must regulate speed, torque, power, or, in some
cases, direction of rotation. Depending on its configuration,
the HST can drive a load from full speed in one direction to
full speed in the opposite direction, with infinite variation of
speed between the two maximums - all with the prime mover
operating at constant speed.
The operating principle of HSTs is simple: a pump, connected
to the prime mover, generates flow to drive a hydraulic motor,
which is connected to the load. If the displacement of the
pump and motor are fixed, the HST simply acts as a gearbox
to transmit power from the prime mover to the load. The
overwhelming majority of HSTs, however, use a variable-
displacement pump, motor, or both - so that speed, torque, or
power can be regulated.
Fig.1:- Basic Hydrostatic Transmission system
The circuit above shows the basic arrangement of all the
components in the HST system. The engine is coupled to a
single direction gear pump (having pressure control). The
pump is connected to a 4/3 DCV. The DCV is connected to a
bi-directional gear motor which is further connected to the
worm differential.
This is an open circuit so as to avoid the use and added
expenses of a charge pump. Thus a reservoir is also required.
An OEM reservoir was not available (in the required size) so
we have designed our own reservoir.
The purpose of using Gear Pump and Motor is that as per the
calculated power demand and supply available (from the
10HP engine), we require very small displacements. Such
small displacements are available in Gear Pumps and Motors.
If larger displacements are required one could always select an
axial piston pump and motor as well. Also the cost of Gear
Pump and Motors is also less as compared to axial piston
pumps and motors.
The basic circuit and its configurations have been designed
using Automation Studio however, we have done the
simulation using MATLAB SIMULINK as additional details
& parameters can be added using SIMULINK giving the
simulation greater accuracy.
1500
2500
3500
4500
7.2 10.1 12.2 17.0 19.5 27.8 31.8 35.6 44.5 53.4
Enginerpm
Road Speed (Kmph)
Engine RPM v/s Road
Speed
1 2n
d
G
e
ar
3
r
d
G
e
a
4
th
G
e
a
r

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The configurations of forward, reverse and lock are as
follows:
2.1 Forward Configuration:
Fig -2: HST system in Forward Configuration
Here the DCV is put into 1st position using the mechanical
lever. This lever can be hand operated and can be made to
latch similar to the hand brake in a car.
2.2 Reverse Configuration:
Fig -3: HST system in Reverse Configuration
Here the DCV is put into 3rd position, thus switching the
direction of flow to the motor and thereby reversing the
direction of rotation of the motor output shaft.
2.3 Hydraulic Locking (Braking):
Fig -3: HST braking circuit
Here we see that the DCV has been put into 2nd position. The
fluid enters and exits the DCV without flowing through the
Gear Motor. The fluid in the motor gets locked. In other words
it prevents the motor from rotating freely. Hence we call this
position as hydraulic locking or braking.
In all the three configurations the fluid is returned to the
reservoir from where it will once again be sucked into the gear
pump.
2.4 Sizing Summary
Table 1: Gear Pump Sizing
GEAR PUMP MODEL NO.: 26002
Displacement min. VD 0.49121 in3/rev
Displacement max. VD 0.55679 in3/rev
Volumetric Efficiency ηpv 90.00% %
Table 2: Gear Motor Sizing
GEAR MOTOR MODEL NO.: 26001
Displacement VD 0.4 in3/rev
Volumetric Efficiency ηmv 90.00% %
Mechanical Efficiency ηmm 90.00% %
Table 3: Max. Motor Speed and Max. Flow Rate
Max. Flow Rate 5.741417 gpm
Max. Speed 2984.102 rpm
Differential Reduction Ratio Required: 7:1
System Pressure: 2300 PSI
3. PERFORMANCE COMPARISON:
3.1 Velocity v/s Time:
Graph 3: Comparison of Velocity v/s Time Graphs (Gear Box
v/s HST)

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The gearbox we have chosen is a 4 speed gear box. Form the
graph we can see initially the velocity of Hydrostatic system is
more than gear box system, but after 25 seconds the velocity
of gearbox exceeds the hydrostatic system. This is because the
hydrostatic system is limited to a top speed of only 50kmph
whereas the gearbox system can achieve a top speed of 58
kmph.
3.2 Acceleration v/s Time:
Graph 4: Comparison of Acceleration v/s Time Graphs (Gear
Box v/s HST)
From acceleration v/s time graph we can see that hydrostatic
transmission system has greater acceleration i.e. 2.5m/s2 as
compared to gearbox system. This is because power given to
both the systems by the engine is same but the gearbox
initially provides more torque than HST system. The initial
torque provided by the gearbox is 524Nm at the wheels and
that of HST is 254Nm.Therefore angular velocity of gearbox
is less than HST system.
3.3 Grade-ability v/s Time:
Graph 5: Comparison of Grade-Ability v/s Velocity Graphs
(Gear Box v/s HST)
From the grade-ability v/s velocity graph we can see that
gearbox system has more grade-ability than HST system. This
is because gearbox is a positive drive system. The torque
provided by the gearbox system initially is 524Nm at the
wheels as compared to 254Nm provided by HST system.
Therefore the gearbox system has more grade-ability than
HST system.
Graph 6: Velocity & Acceleration v/s Time Simulation (Gear
Box v/s HST)
The above graph shows the behavior of velocity and
acceleration of both HST and Gearbox driven vehicles. In a
test of 0-50 kmph. We notice the pick-up of the HST driven
vehicle is much greater than that of the gearbox driven
vehicle. Distance covered is area under the velocity/time
graphs which is greater for the HST driven vehicle by the time
they both are moving at 50 kmph. It can also be seen that the
HST driven vehicle only takes 15 sec to reach a max speed of
50 kmph whereas the gearbox driven vehicle takes nearly 25
sec to attain a speed of 50 kmph.
Fig -4: Simulink Circuit

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4. Simulink Circuit Component Description
4.1 Engine Input Signal Block
This is a signal of engine output speed (rpm) v/s time
(seconds). It simulated the engine output that is fed to the gear
pump.
4.2 Simulink-PS Converter Block
Converts the unit less Simulink input signals to a Physical
Signal.
4.3 Rotational Coupling Block
A rotational coupling has been used to couple the engine with
the pump. The rotational coupling is basically used to couple
mechanical component and hydraulic and hence it has been
used in our circuit wherever required.
4.4 Ideal Angular Velocity Source
The block represents an ideal source of angular velocity that
generates velocity differential at its terminals proportional to
the physical input signal. The source is ideal in a sense that it
is assumed to be powerful enough to maintain specified
velocity regardless of the torque exerted on the system.
4.5 Gear Pump
We have selected a single direction, fixed displacement gear
pump in our simulation.
The three ports of the pump are:
port – it is the suction port of the pump which is connected to
the engine output via the rotational coupling.
port – this port is connected to the reservoir.
P port – this port is connected to the Directional control Valve.
4.6 Directional Control Valve
We have used a 4/3 DCV which is actuated by the 2 position
valve actuator. With the help of this valve we can move in
forward & reverse as well as brake.
4.7 Hydraulic Fluid Block
This block represents the type of fluid used and helps in
simulating its properties in the circuit.
Hydraulic fluid used is SAE DTE 24 in our actual practice.
Although in Simulink we have used Oil SAE 30 because of
the similarity in properties of the two oils & the unavailability
of SAE DTE 24 in the software.
4.8 Solver Block
A Solver block has been added to the circuit to provide global
information and provide parameters so that simulation can
start.
4.9 Hydraulic Reference Block
This block simulated the reservoir as we have design our
system as an open circuit.
4.10 Pressure Relief Valve
A Pressure Relief Valve is employed in the circuit to relieve
any excess pressure in the circuit.
4.11 Hydraulic Motor:
A bi-directional gear motor is used in our project. This
hydraulic motor block is used to represent the motor in
Simulink.
Another rotational coupling has been used between the motor
and the differential.
4.12 Drive Shaft Inertia Block
It is connected to the output coming from the motor to
simulate the rotational inertia of the drive shaft.
4.13 Vehicle Dynamics Block:
The two tires are connected to the scope via a longitudinal
Vehicle Dynamics block.
This block is used to input the parameters like mass of the
vehicle, horizontal distance from C.G. to the front axle,
horizontal distance from the C.G. to the rear axle, C.G. height
from ground, frontal area and drag coefficient.
Embedded components of the Vehicle Dynamics block
include:
4.14 Differential block:
This block represents the differential which has been used in
our design. The differential has a reduction of 7 (7:1).
4.15 Tire blocks (L & R):
The tire blocks are used to simulate the tire size as it reflects
upon the on road velocity
4.16 Vehicle Body Block:
The vehicle body block includes parameters such as vehicle
weight, air drag, etc. It is used to create a realistic simulation
of the actual vehicle.

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4.17 Velocity Scope
This scope is connected to the output of the vehicle dynamics
block and tires. It is used to generate the output graph of road
velocity v/s time.
4.18 Derivative Block:
This block derives the output velocity of the vehicle dynamics
block and generated an acceleration v/s time signal.
4.19 Acceleration Scope:
This Scope is connected to the output from the derivative
block so as to produce an acceleration v/s time plot.
5. COMPARISON OF SIMULINK GRAPHS &
EXCEL GRAPHS
This involves the comparison of the graphs generated by
mathematical calculations performed on MS Excel and a
simulation of the HST system modelled on MATLAB
SIMULINK. Although the graphs may be similar in nature
there are some noticeable differences. The SIMULINK graphs
are based on a simulation which takes into consideration
various parameters such as fluid properties, miscellaneous
losses, efficiency of almost all components, etc., whereas in
MS Excel we have only considered pump and motor
mechanical efficiency.
Thus the graphs generated by SIMULINK are more realistic in
nature however it would be very difficult to describe the
behavior as compared to the graphs generated by Excel. The
SIMULINK circuit also factors in the hydraulic fluid
properties, shaft inertias, as well as pressure relief valve
settings which have been neglected in the Excel calculations.
For the sake of evaluating the accuracy of the Excel graphs we
have calculated the Normalized Root Mean Square Deviation
with respect to the SIMULINK graphs, for both (velocity &
acceleration).
Graph 7: - Comparison SIMULINK v/s Excel Graphs
(Velocity v/s Time HST)
Graph 8: - Comparison SIMULINK v/s Excel Graphs
(Acceleration v/s Time HST)
Since the NRMSD lies within a 10% error margin the excel
calculations are quite accurate.
6. CONCLUSIONS
To conclude we would like to draw light to the fact that the
HST shows very similar performance as compared to the
gearbox and with the price difference of less than 5000 Indian
rupees between the two systems it seems like a financially
competitive solution. The HST may not have as good grade-
ability as compared to the gear box but it compensates for this
in terms of haulage capacity. The performance of the HST
doesn’t vary much as we increase the load to be hauled. This
can be proved using the Simulink model and varying the
vehicle weight parameter. In vehicles with torque convertors

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the HST would even prove to be a more fuel efficient system.
Thus we conclude that with further improvements in the near
future the HST systems would be more commonly seen in off-
road vehicles and load carrying vehicles.
ACKNOWLEDGEMENTS
We would like to thank Professor P. D. Sonawane (MIT Pune)
for all his support over the duration of our research and all his
technical guidance, helping us improve the quality and
accuracy of our work.
REFERENCES
[1] Design of Machine Elements, 3rd Edition, V. B. Bhandari
McGraw Hill Publication
[2] A Textbook of Machine Design, R S Khurmi, J K Gupta,
S. Chand Publication
[3] PSG Design Data, PSG Coimbatore
[4] Theory of Machines, R S Khurmi, J K Gupta, S. Chand
Publication
[5] Shigley’s Mechanical Engineering Design, 9th Edition,
Richard G Budynas, J Keith Nisbett, McGraw Hill Publication
[6] Fluid Power with Applications, 6th Edition, Anthony
Esposito, Pearson Publication
[7] Industrial Hydraulics Manual 5th Edition, 2nd Printing
Eaton Hydraulics Training Services Eaton Publication
BIOGRAPHIES
Sumair Sunny, Engg. Student
(Undergraduate)
B.E. (Mechanical), MIT Pune
Sunny Pawar, Engg. Student (Undergraduate), B.E.
(Mechanical), MIT Pune
Siddhesh Ozarkar, Engg. Student (Undergraduate), B.E.
Sandeepan Biswas, Engg. Student (Undergraduate), B.E.

Hydrostatic transmission as an alternative to conventional gearbox

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Hydrostatic transmission as an alternative to conventional gearbox