Circuits for Closed-circuit Hydro-static Transmissions (HSTs)

In a closed-circuit hydrostatic transmission (HST) system, a hydraulic pump drives a hydraulic motor. In the closed-circuit HST system, the fluid discharged from the hydraulic motor outlet flows directly to the pump inlet, thus forming a power transmission loop. The transmission loop has a high-pressure side and a low-pressure side. The pressure on the high-pressure side is determined by the load on the motor. A closed-circuit HST system consists mainly of a pump, a motor, a charge pump, check valves, a shuttle valve, pressure relief valves, accumulators, and filters.

A Basic Circuit of the Closed-circuit HST

The basic circuit of a closed-circuit HST with a pump and hydraulic motor is shown in Figure 1. Generally, a variable displacement axial piston pump with a swashplate, whose position can be infinitely varied, is used to drive a fixed-displacement axial piston motor hydraulically. Case drain lines must be provided in the pump and motor for relieving leakage flows.

Figure 1 | Two positions of the basic circuit of a closed-circuit HST





A Closed-circuit HST with a Charge Pump, a Flushing Valve, and High-pressure Relief Valves (Alternative Circuit)

As we are aware, the charge pump circuit is used for compensating for the leakage flows and boosting the pressure on the inlet side of the pump. A flushing circuit is used for flushing and lubricating the working parts of the motor and pump cases and increasing cooling and filtering

Figure 2 | An circuit for a closed-circuit HST with an alternative connection of high-pressure relief valves PRV3 and PRV4

As described in the previous section (in the reference book) with a circuit configuration, cross-port high-pressure PRVs are used to limit the maximum operating pressure of the entire system and prevent an inadvertent overload on the hydraulic motor. An alternative circuit for connecting high-pressure relief valves (PRV3 and PRV4) is given in Figure 2. The pressure relief valves are connected back-to-back and are linked to the charge pump circuit, as shown in the figure.

When an over-pressure condition occurs, the flow passes from the high-pressure side to the low-pressure side and back to the inlet side of the main pump. It can be seen that the flow always takes an easier path through a high-pressure relief valve and the check valve that gets opened depending on the pressure conditions in the transmission loop.

HST Sizing

Component Typical Sizing criteria / Recommendation
Flow rate, charge pumpAt least 20% that of the main pump
Total reservoir volume, (in lpm)Should be between 0.5 to 1.5 times the maximum flow delivered by the charge pump (for closed-circuit applications)
Fluid volume in the reservoir, litreShould be approximately 80% of the total reservoir volume in lpm
Pressure setting, charge pump PRV10-35 bar (150 – 500 psi)
Pressure setting, Flushing circuit PRVPressure setting, charge PRV  minus 2 bar (30 psi)
Pressure setting, High-pressure PRV20% higher than the main pump compensator setting
Strainer, charge pump suctionMesh width may be greater than 150 microns
Pressure filter, charge pumpMesh width of 3 to 10 microns
Pressure filter, mainlineA high-pressure fine filter can be used in the main transmission loop
Heat ExchangerAn HST with a capacity greater than 10 kW is typically provided with a heat exchanger


Joji Parambath

Author (Amazon Author Page)

Reference: Textbook ‘Hydraulic Circuits – Identification of Components and Analysis

Table of Contents – Hydraulic Circuits – Identification of Components and Analysis

ChapterDescriptionPage No
List of Control Tasksv
1An Overview of Hydraulic Systems and Circuits1
2Hydraulic Circuits with Directional Control Valves3
3Hydraulic Circuits with Check Valves22
4Hydraulic Circuits with Flow Control Valves35
5Hydraulic Circuits with Flow Dividers and Combiners56
6Hydraulic Circuits with Pressure Control Valves66
7Hydraulic Circuits with Accumulators82
8Circuits for the Series and Parallel Connections of Hydraulic Motors88
9Relay-based Electro-hydraulic Circuits90
10Circuits for Closed-circuit Hydro-static Transmissions (HSTs)119
11Hydraulic Circuits with Variable Displacement Pumps129
12Hydraulic Circuits for Load-sensing Systems135
13Hydraulic Circuits with Proportional and Servo Valves140
14Electro-hydraulic Circuits, Wiring Diagrams, and Ladder Programs of PLC-based Systems151
15Hydraulic Circuits with Cartridge Valves154
16Hydraulic Circuits with Pressure Intensifiers169
17Layouts of Hydraulic Reservoirs173
18Application-specific Hydraulic Circuits180

Book Description

The textbook explores a variety of typical hydraulic circuits in multiple positions and with color graphics. The presentation of the hydraulic circuits is structured with well-thought-out chapters. Each chapter presents circuits from simple to complex levels. Relevant symbols are portrayed chapter-wise for quick understanding. Most of the symbols used are as per ISO 1219. The types of hydraulic circuits include circuits for conventional hydraulics, electro-hydraulics, closed-circuit HSTs, PLC systems, proportional/servo valve systems, and cartridge valve systems.

The book is meant for hydraulic professionals to refresh their circuit ideas and know more about hybrid hydraulic circuits. This book is specially written for professionals who are confused with many types of complex hydraulic circuits. They can systematically learn the critical areas in simple or complex hydraulic circuits. Teachers and students may also make use of this book for enhancing their hydraulic knowledge. The reader can build up a strong foundation for circuit ideas and may apply these ideas to a hydraulic application taking into account the operating and environmental conditions, the orientation of the actuators, the type of materials used, and many supplementary factors.

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