Hydraulic Circuit (Forward, Neutral, Reverse) - Maxma · Hydraulic Circuit (Forward, Neutral,...

Hydraulic Circuit(Forward, Neutral, Reverse)

INTRODUCTION

Hydrostatic transmission are hydraulic systems consisting of two main components: hydraulic

pump, hydraulic motor. The pump transforms mechanical energy in hydraulic energy transferred

to the motor, which converts again the hydraulic energy in mechanical one, obtaining finally a

power transmission.

The hydrostatic transmission offers infinite control of speed and direction. The operator has

complete control of the system with one lever for starting, stopping, forward motion or reverse

motion. The lever controls the position of the swash plate of the pump Tilting the swash plate of

the variable pump produces a certain flow from the pump; this flow is transferred through high

pressure lines to the motor. The volume of flow from the pump in connection with displacement of

the motor will determine the speed of the output shaft of the motor.

Moving the swash plate of the pump to the opposite side of the neutral position, the flow from the

pump is reversed and the output shaft of the motor turns in the opposite direction. Therefore

speed of the output shaft is controlled by adjusting the control lever of the pump which acts

proportionally the swash plate, varying the displacement. Working pressure is determined by the

external load on the motor and this, together with flow produced by the pump, establishes the

power demanded by the system.

Standard Close Loop Circuit Diagram

Primary System Components

In order to avoid cavitation problems, it is foreseen a charge pump to boost all possible

suction lines. The charge pump is mounted on the rear of the main pump and draws oil

from the reservoir through a filter and provides a flow of oil under pressure to the main

pump. Charge flow exceeding the need of the circuit is by-passed through an auxiliary

relief valve normally calibrated at 13 bar.

Two check valves permit to boost the low pressure line only, isolating the high pressure

line: A and B ports are alternatively working at high pressure in function of flow direction,

which depends from the position of the main pump swash plate.

There are also two main relief valves calibrated at the highest working pressure of the

circuit: in case the external conditions force the hydrostatic transmission to overcome

the max. allowable pressure, one of the two relief valves corresponding to the high

pressure line (A or B) automatically dumps oil to the other line, preventing sustained

abnormal pressure surges in the high pressure line and cavitation in the other line.

A shuttle valve makes possible to leak a certain amount of flow to reservoir in order to

assure the oil replacement inside the closed circuit, the shuttle valve works in

combination with another relief valve calibrated normally at 11 bar. The shuttle valve

allows the flow from the charge pump to flow through the opposite check valve to

replenish internal leakage beyond what is normally supplied for oil replacement to cool

and lubricate the circuit.

At same time the relief valve combinated with shuttle valve and calibrated at 11 bar

avoids pressure dumping in the corresponding line.

The charge pump also provides oil to the servo-valve which controls the swash plate

angle of the main pump: the main pump is put into operation by a signal to the servo-

valve from the operator, and the pump is stroked into forward or reverse. At this time, the

servo-valve directs the flow from the charge pump to one servo-cylinder, which is linked

to the swash plate. The control pressure forces one of the two servo-cylinders to tilt the

swash plate in one side and it causes a high pressure flow of oil to the motor in one line

which starts rotating in one direction.

The servo-valve determines the amount of tilt of the swash plate in one or the other

direction, and it can be varied by the operator; this allows variable speed and control of

the system.

Hydraulic Circuit

Axial Piston Variable Displacement Pump

General Description

Axial piston variable displacement pump swash plate design for hydrostatic

transmission ir closed circuit.

Flow is proportional to shaft speed and pump displacement, this can be infinitely varied

as per the operator request.

Flow direction is reversed by tilting the swash plate to the opposite side of the neutral

displacement position.

Features

Axial piston variable displacement pumps are welt-engineered and easy to handle, with

high reliability proven in laboratory and field.

The full length shaft with- a highly efficient tapered roller bearing arrangement offers a

high loading capacity for external radial forces.

The hydro-mechanical servo displacement control maintains the selected swash plate

position and hence pump displacement.

The low noise level, the high ratio power / weight permit to approach the widest range of

applications


Servo Displacement Control - Reversing time

Regulated by the control handle on the servo valve, the swashpalte can be infinitely

varied in both directions with the help of the servosystem.

The pump displacement results from the control handle position.

Time for the directional change of max flow in one side to max flow in the other side

(across neutral position) is depending on the size of the control orifice fitted in the supply

port to the servo valve.

The values given assume movement of the control handle directly from one end position

to the other.

Adjustment time of handle: < minimum reversing time

Operating pressure: 210 bar

Speed: 1450 rev / min2Viscosity: 35 mm / sec


Effcien curves

Legend: Series1 = Total efficiency at 350 barSeries2 = Total efficiency at 210 barSeries3 = Volumetric efficiency at 350 barSeries4 = Volumetric efficiency at 210 bar

Outline Drawing of Axial PistonVariable Displacement Pump

Causes high pressure in "A" port in anticlockwise pump

B: Causes high pressure in "B" port in clockwise pump

Causes high pressure in "B" port in anticlockwise pump

A: Causes high pressure in "A" port in clockwise pump

Direction of flow related to position of control lever:

Axial Piston Fixed Displacement Motor

General Description

These types of hydraulic motors are of swash plate design with fixed displacement. The

output speed is proportional to the inlet flow and inversely proportional to displacement

of the motor.

The output torque is proportional to the displacement and the drop pressure across the

motor.

The direction of output shaft rotation depends upon the - flow direction, which port is

boosted by high pressure. These motors are mainly designed for closed circuit

application, because a purge valve is normally installed on the rear of the port block.

Features

These motors have high performances and easy to handle, because of the very high

ratio power / weight.

The shaft is supported by two roller bearings and the torque is transferred by the cylinder

block to the shaft through a splined connection.

High efficiency is guaranteed by the simple construction and the accurate mechanical

design, combined with the constant quality of production and assembling.

High case pressure can be achieved because of the special design of the front shaft seal.

AXIAL PISTON FIXED DISPLACEMENT SWASH PLATE MOTOR

Axial Piston Fixed Displacement Motor

Motor dimensions

Outline Drawing of Axial PistonFixed Displacement Motor (valve block)

VALVE BLOCK

VALVE BLOCK

The valve block is installed on the rear cover of the motor.

This block has induded two relief valves which protect the circuit preventing sustained

pressure surges in the high pressure line and cavitation in the other line.

A shuttle valve makes possible to leak a certain amount of flow to reservoir in order to assure

the oil replacement inside the dosed circuit, the shuttle valve works in combination with

another relief valve calibrated normally at 11 bar, which avoids pressure dumping in the

corresponding line.

VALVE BLOGK WITH BY-PASS VALVE

This valve assures the same performances of the standard one, further on the by-pass makes

possible the connection of two high pressure lines; so that the entire hydrostatic transmission is

in idle position.

Hydraulic Circuit (Forward, Neutral, Reverse) - Maxma · Hydraulic Circuit (Forward, Neutral,...

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