Variable Displacement Hydraulic Pump

A variable displacement hydraulic pump adjusts the amount of hydraulic fluid delivered to the system according to actual flow and pressure requirements. Unlike a fixed displacement pump, a variable displacement pump can change its displacement while operating, helping reduce unnecessary flow, energy losses, and heat generation.

Variable displacement hydraulic pumps are widely used in industrial machinery, mobile equipment, hydraulic power units, construction equipment, manufacturing systems, and other applications where hydraulic load and flow requirements change during operation.

For systems that require improved energy efficiency and precise hydraulic control, selecting the right variable displacement pump can have a significant impact on overall system performance.

What Is a Variable Displacement Hydraulic Pump?

A variable displacement hydraulic pump is a positive displacement pump designed to change its displacement according to operating conditions.

Pump displacement determines how much hydraulic fluid is delivered during each revolution. By changing the displacement, the pump can increase or decrease hydraulic flow without necessarily changing the pump speed.

This makes variable displacement pumps particularly suitable for systems where hydraulic flow demand changes during the operating cycle.

Typical applications include:

• Industrial hydraulic machinery
• Mobile hydraulic equipment
• Construction machinery
• Excavators and loaders
• Hydraulic power units
• Press systems
• Injection molding machines
• Mining equipment
• Material handling equipment
• Heavy-duty industrial systems

Variable Displacement vs. Fixed Displacement Hydraulic Pumps

The main difference between fixed and variable displacement hydraulic pumps is the ability to control pump displacement.

A fixed displacement hydraulic pump delivers a relatively constant volume of fluid for each revolution. If the system requires less flow than the pump produces, excess flow may need to be diverted through a valve or other control arrangement.

A variable displacement hydraulic pump can reduce or increase its displacement according to the hydraulic system's requirements.

This can help reduce:

• Unnecessary hydraulic flow
• Pressure losses
• Heat generation
• Energy consumption
• Cooling requirements

The appropriate pump type depends on the machine's operating cycle, pressure requirements, flow requirements, control architecture, and duty cycle.

How Does a Variable Displacement Hydraulic Pump Work?

A variable displacement pump changes its internal displacement to control hydraulic flow.

For example, when the hydraulic system requires high flow, the pump can operate at a higher displacement. When flow demand decreases, the pump can reduce its displacement.

Depending on the pump design, displacement can be controlled through different mechanisms, including:

• Pressure compensation
• Load sensing
• Electronic control
• Hydraulic control
• Torque control
• Combined control systems

The control method should be selected according to the hydraulic circuit, actuator requirements, operating pressure, and desired machine performance.

Pressure Compensated Variable Displacement Pump

A pressure compensated variable displacement pump adjusts its displacement according to system pressure.

When the system requires hydraulic flow, the pump can increase displacement. As system pressure approaches the compensator setting, the pump reduces displacement to limit unnecessary flow.

This control strategy can be useful in hydraulic systems where pressure requirements change but controlled pressure regulation is important.

Pressure compensated pumps are commonly considered for:

• Industrial machinery
• Hydraulic presses
• Machine tools
• Hydraulic power units
• Manufacturing equipment
• Heavy-duty hydraulic systems

Correct pressure compensator settings are important for achieving the desired system performance and avoiding unnecessary energy losses.

What Is a Load Sensing Hydraulic Pump?

A load sensing hydraulic pump is a variable displacement pump that adjusts its output according to the hydraulic system's actual load and flow demand.

The pump receives a load-sensing signal from the hydraulic circuit and adjusts its displacement to maintain the required pressure difference between pump pressure and load pressure.

A simplified load-sensing relationship can be expressed as:

Pp = Ps + ΔP

Where:

• Pp = Pump outlet pressure
• Ps = Load or system pressure
• ΔP = Required pressure differential

The objective is to provide the flow and pressure required by the system without continuously producing maximum pump output.

Benefits of Load Sensing Hydraulic Systems

Load sensing technology can provide several benefits when properly designed and matched with the hydraulic circuit.

Potential advantages include:

• Improved hydraulic efficiency
• Reduced unnecessary pump flow
• Lower energy consumption
• Reduced heat generation
• Better control of hydraulic actuators
• Improved machine efficiency
• Reduced cooling demand
• Better performance under changing loads

The actual energy savings depend on the complete hydraulic system design. Pump selection alone does not determine overall system efficiency.

The pump, valves, actuators, piping, filtration, cooling system, and control architecture should be evaluated as one system.

Why Does a Hydraulic System Lose Energy?

Hydraulic systems can lose energy when the pump generates more flow or pressure than the machine actually requires.

For example, a fixed displacement pump may continuously produce a certain flow while the actuator requires only a portion of that flow.

If excess flow is controlled through a valve and returned to the reservoir under pressure, the hydraulic power associated with that flow can be converted into heat.

This can increase:

• Hydraulic oil temperature
• Cooling requirements
• Electrical power consumption
• Component operating temperatures
• Operating costs

Variable displacement and load sensing systems can reduce some of these losses by adjusting pump output according to system demand.

Variable Displacement Pumps and Hydraulic Energy Efficiency

Hydraulic efficiency should be evaluated based on the actual operating profile of the machine rather than only the maximum pressure and flow rating.

A machine that operates at maximum flow and pressure continuously has different pump requirements from a machine that frequently changes between high-flow, low-flow, standby, and high-pressure operating conditions.

A variable displacement hydraulic pump can adapt its output to these changing conditions.

This can be particularly valuable in systems with:

• Variable loads
• Variable actuator speeds
• Intermittent operation
• Long operating cycles
• Multiple hydraulic functions
• High installed motor power
• Significant standby periods

For industrial systems, reducing hydraulic losses can also reduce the amount of heat that must be removed from the hydraulic oil.

Variable Flow Hydraulic Pump Applications

Variable displacement hydraulic pumps are used in many applications across industrial and mobile equipment markets.

Construction Equipment

Excavators, loaders, material handlers, and other construction equipment often require different flow rates depending on the hydraulic function being performed.

Load sensing and variable displacement pump systems can be used to manage changing hydraulic demand.

Industrial Machinery

Variable displacement pumps can be used in presses, machine tools, injection molding machines, production machinery, and custom hydraulic equipment.

Mining Equipment

Mining machinery frequently operates under high loads and demanding duty cycles. Pump selection must therefore consider pressure, flow, operating temperature, duty cycle, and reliability requirements.

Hydraulic Power Units

Variable displacement pumps can be integrated into custom hydraulic power units (HPUs) where the required flow and pressure change during the operating cycle.

Material Handling Equipment

Hydraulic systems used for lifting, positioning, clamping, and other material handling functions can benefit from variable flow control.

How to Select a Variable Displacement Hydraulic Pump

Selecting a variable displacement hydraulic pump requires more than simply choosing a pump based on maximum flow.

The following parameters should be evaluated:

1. Operating Pressure

Determine both continuous and peak pressure requirements.

2. Required Flow

Calculate the minimum, nominal, and maximum flow required by the hydraulic actuators.

3. Pump Speed

Pump displacement and operating speed must be compatible with the prime mover and application.

4. Hydraulic Fluid

Oil type, viscosity range, contamination level, and operating temperature should be considered.

5. Control Method

Determine whether the application requires:

• Pressure compensation
• Load sensing
• Electronic control
• Torque control
• Combined control

6. Duty Cycle

A pump designed for intermittent operation may have different requirements from one operating continuously under high load.

7. Heat Load

The hydraulic system's total heat generation should be evaluated to determine whether additional cooling capacity is required.

8. System Compatibility

The pump must be compatible with the hydraulic valves, actuators, filtration system, reservoir, piping, and control system.

Variable Displacement Pump Sizing

Proper pump sizing is essential for hydraulic system performance.

Oversizing a pump can increase equipment cost, motor power requirements, and potential energy losses. Undersizing a pump may prevent the system from achieving the required actuator speed or pressure.

Pump sizing should therefore be based on the actual operating cycle.

A simplified hydraulic power relationship is:

Hydraulic Power = Pressure × Flow

For U.S. customary units, hydraulic horsepower can be estimated using:

HP = PSI × GPM / 1714

This calculation provides a useful starting point for estimating hydraulic power requirements. Actual motor sizing should also consider pump efficiency, mechanical losses, duty cycle, starting conditions, and system losses.

Variable Displacement Hydraulic Pump for Industrial Systems

Industrial hydraulic systems often operate under changing pressure and flow conditions.

Using a properly selected variable displacement pump can help the system respond to these changing requirements while reducing unnecessary hydraulic power generation.

However, pump efficiency should always be considered together with:

• Hydraulic circuit design
• Valve technology
• Actuator sizing
• Filtration
• Oil cleanliness
• Hydraulic piping
• Heat exchanger capacity
• Electric motor efficiency
• Control system configuration

A high-efficiency pump cannot compensate for an inefficient hydraulic circuit.

Hydraulic Pump Heat Generation

Excess hydraulic power can eventually appear as heat within the system.

When hydraulic fluid is continuously circulated at pressure without performing useful mechanical work, energy is lost and the oil temperature can increase.

High oil temperature can affect fluid viscosity, seal performance, component life, and overall system reliability.

For this reason, hydraulic system design should consider both power transmission and heat management.

Variable displacement and load sensing technologies can help reduce unnecessary hydraulic flow and therefore contribute to better thermal management when properly applied.

Variable Displacement Pump vs. Load Sensing Pump

The terms are related but should not be treated as identical.

A variable displacement pump describes a pump capable of changing its displacement.

A load sensing pump describes a variable displacement pump system that uses a load-sensing signal to adjust pump output according to system demand.

Therefore, a load sensing pump is typically a type of variable displacement hydraulic pump, but not every variable displacement pump is a load sensing pump.

Common Questions About Variable Displacement Hydraulic Pumps

What is a variable displacement hydraulic pump?

A variable displacement hydraulic pump changes its displacement to adjust hydraulic flow according to system requirements.

What is a load sensing hydraulic pump?

A load sensing hydraulic pump adjusts displacement based on the hydraulic load and flow demand of the system.

Are variable displacement pumps more efficient?

They can reduce unnecessary hydraulic flow and power losses in applications with changing flow and load requirements. Actual efficiency depends on the complete hydraulic system design and operating conditions.

What is the difference between a variable flow pump and a fixed flow pump?

A variable flow pump can change its output flow by changing displacement or another control parameter. A fixed displacement pump delivers a relatively constant volume per revolution.

Where are variable displacement hydraulic pumps used?

They are commonly used in industrial machinery, construction equipment, mining machinery, hydraulic power units, presses, injection molding machines, and mobile hydraulic systems.

How do I choose the right variable displacement pump?

Pump selection should consider operating pressure, required flow, speed, displacement, hydraulic fluid, temperature, duty cycle, control method, actuator requirements, and complete system design.

Can a variable displacement pump reduce hydraulic oil temperature?

It can help reduce unnecessary energy losses and heat generation when properly applied. However, oil temperature is affected by the entire hydraulic circuit, including valves, piping, actuators, pump efficiency, and cooling capacity.

Engineering Support for Variable Displacement Hydraulic Systems

Selecting the correct hydraulic pump requires an understanding of the machine's complete operating cycle.

Hidroman provides engineering-based hydraulic solutions for industrial and mobile applications, including hydraulic system design, hydraulic power units, hydraulic filtration, hydraulic testing, hydraulic hose solutions, and custom hydraulic equipment.

For variable displacement and load sensing applications, system requirements can be evaluated based on:

• Pressure and flow requirements
• Actuator specifications
• Machine duty cycle
• Load profile
• Pump control requirements
• Motor power
• Hydraulic oil temperature
• Filtration requirements
• Cooling requirements
• Overall energy efficiency

The goal is not simply to select a pump with the highest pressure or flow rating. The goal is to develop a hydraulic system that delivers the required performance under actual operating conditions.

Conclusion

A variable displacement hydraulic pump can be an effective solution for hydraulic systems with changing flow and pressure requirements.

By adjusting pump displacement according to system demand, variable displacement technology can help reduce unnecessary flow, improve hydraulic efficiency, control heat generation, and optimize machine performance.

Load sensing hydraulic pumps take this approach further by using the hydraulic load signal to adjust pump output according to actual system demand.

Proper pump selection requires a complete evaluation of pressure, flow, speed, load profile, duty cycle, control method, hydraulic fluid, filtration, cooling, and the overall hydraulic circuit.

Hidroman supports industrial and mobile hydraulic applications with engineering, custom hydraulic system design, hydraulic power units, filtration solutions, testing equipment, and hydraulic service capabilities.

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