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An Introduction to Hydrostatic Drive Systems

Key Takeaways

  • A hydrostatic drive is any system that converts mechanical power into fluid power and then to shaft power. 

  • Based on the transmission ratio, the hydrostatic drive is classified into two types: fixed displacement hydrostatic drive and variable displacement hydrostatic drive.

  • Hydrostatic drive systems are widely used in variable output velocity of torque applications.

Hydrostatic drive with hydraulic pump

A hydrostatic drive consists of a hydraulic pump

Throughout the industry, the hydrostatic drive is used in various applications such as log cranes, conveyors, centrifuges, mobile equipment, chemi-washers, and planers. Hydrostatic drive, otherwise called hydrostatic power transmission, usually consists of a hydraulic pump-motor combination. Hydrostatic drive is preferred over gear-based drive in many applications due to its step-less speed change. 

Types of Power Transmission

Power transmission from one point to another can take place via electric, mechanical, hydromechanical, hydrodynamic, or hydrostatic drive. A drive is a system that enables the controlled transfer of power from one system to another.

  • Electric drive - In electric drive, the mechanical energy is converted into electric energy with the help of an electric generator. An electric motor transforms the electric energy back to mechanical energy.

  • Mechanical drive - Various methods (chains, belts, gears, shafts, torque converters, etc.) are used to convert mechanical energy to kinetic energy in mechanical drives.

  • Hydromechanical drive - The power split scheme used in hydromechanical drives converts energy input to both mechanical and hydrostatic energy. These types of drives are best for heavy-duty applications.

  • Hydrodynamic drive - In hydrodynamic drive, power is generated due to the velocity variations of a fluid passing through a channel. The system consists of a connected hydrodynamic pump and motor.

How Do Hydrostatic Drive Systems Work?

Hydrostatic drive is any system that converts mechanical power into fluid power and then to shaft power. The principle of operation of a hydrostatic drive relies on the energy transfer using hydraulic fluid. Generally, a hydrostatic drive consists of a hydraulic pump. The pump is connected to the prime mover and it generates the fluid flow to run a hydraulic motor, which is interfaced with the load. The hydrostatic drive can be described as a hydraulic system where a hydraulic pump runs the motor using fluids.

Parts of a Hydrostatic Drive 

The hydrostatic drive consists of:

  1. Transmission case - The part of the hydrostatic drive system that holds the components in place. It also holds the fluid transfer as well.

  2. Charge pump - The charge pump is used for providing the initial case fluid pressure and to fill the circuit with fluid.

  3. Input shaft - The input shaft transmits the power from the engine and rotates the charge pump.

  4. Axial piston pump - Input shaft rotation is produced by the axial piston pump. It also supplies the fluid, in most cases oil to the motor.

  5. Hoses - Hoses are used to connect the pump to the motor.

  6. Pressure relief valves - In case of increased fluid pressure, the pressure relief valves provide an alternate path for the fluids.

  7. Motor - The motor is one of the integral parts of a hydrostatic drive system. The motor drives the output shaft.

  8. Swashplate - The displacement of the piston pump can be changed using the swash plate.

Types of Hydrostatic Drive

There are various classifications in hydrostatic drive systems. Based on the configuration of the hydraulic pump and motor, the hydrostatic drive is classified into:

  1. In-line configuration - A variable pump and a constant displacement motor are connected in-line directly.

  2. U-shaped configuration - In this classification of hydrostatic drive, the motor is placed below the pump. The direction of rotation of the input shaft and motor shaft are the same.

  3. S-shaped configuration - Similar to the U-shaped configuration, the motor is placed below the pump or prime mover, but behind the pump.

  4. Split configuration - High-pressure hoses separate the motor and pump in a split configuration.

The hydrostatic drive can be broadly classified into two types based on the transmission ratio:

  1. Fixed displacement hydrostatic drive - In this type of hydrostatic drive, the displacement of the pump and motor are fixed. A hydrostatic drive of this kind can be described as a simple gearbox that transmits the mechanical energy from the prime mover to the load.

  2. Variable displacement hydrostatic drive - In variable displacement hydrostatic drive, the pump or motor (or both) produces variable displacement. The hydrostatic drive can be a variable displacement pump and fixed displacement motor, variable displacement pump and variable displacement motor, or fixed displacement pump and variable displacement motor. The variable displacement hydrostatic drive helps regulate the speed, power, and torque of the system.

Based on the circuit design, the hydrostatic drive can be categorized as:

  1. Closed-circuit drive - The fluid from the motor goes directly into the pump. This type of hydrostatic drive requires a charge pump.

  2. Open circuit drive - The pump sucks the fluid from the reservoir. The fluid from the motor settles in the reservoir.

Advantages of the Hydrostatic Drive

Advantages of the hydrostatic drive include:

  • Compact-sized drive system with high power transmission (high power-to-weight ratio)

  • Low inertia

  • Ease of operation 

  • Safety 

  • Relatively fast response compared to mechanical or electromechanical transmissions

  • Variable speed control, regardless of the load 

  • Self-overload protection capacity

  • Dynamic braking

  • Reverse rotation 

  • Quick-stop in emergencies

  • Smooth and efficient application of various torque-to-speed ratios

  • Anti-stalling feature

  • Power transmission from the single prime mover to multiple locations, without being affected by the change of position and orientation of the location

  • No creeping at zero speed

  • Maximum design flexibility 

Applications of the Hydrostatic Drive

The hydrostatic drive is widely used in variable output velocity of torque applications such as harvesters, golf-course maintenance equipment, tractors, construction machinery, and trenchers. Cadence’s suite of CFD software can help you develop hydrostatic drive systems. Cadence offers a complete set of CFD simulation tools that help simulate the fluid-solid interactions in a hydrostatic drive.

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About the Author

With an industry-leading meshing approach and a robust host of solver and post-processing capabilities, Cadence Fidelity provides a comprehensive Computational Fluid Dynamics (CFD) workflow for applications including propulsion, aerodynamics, hydrodynamics, and combustion.

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