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Both hydraulic and pneumatic systems are used to run machinery we encounter in our day-to-day life, from car brakes and construction equipment to elevators and automated doors on commuter trains and buses.

While hydraulic and pneumatic power circuits have a number of mechanical similarities, and even share some integral components including specific pumps and a set of valves that direct flow, providing force and velocity control of the actuators.

Still, end-users of many kinds of machines may be wondering which kind of system is best for their specific applications. Read on to learn the key differences between hydraulic and pneumatic power systems.

How Hydraulic Systems Work

Hydraulic systems apply pressure to fluid in order to generate power. A pump moves mechanical energy into the system by transporting fluid — usually a hydraulic oil or synthetic lubricant — into a reservoir, where the fluid is stored and residual material including air and other moisture particles are removed.

Then, pressure is exerted against one side of the reservoir, forcing the liquid through valves using electrical, manual, hydraulic, pneumatic, or mechanical methods. The fluid is forced against an actuator, such as a hydraulic motor, cylinder, or piston on the opposite side of the reservoir. Energy is transferred to the actuator and turned from hydraulic energy into mechanical energy, forcing the actuator to move.

Due to the pressure exerted through the fluid, the actuator is not able to move in the opposite direction unless the pressure is released by a system operator. If the actuator is a piston being used, for example, to raise a forklift’s prongs, the prongs will remain elevated until the hydraulic pressure is released.

The Advantages and Disadvantages of Hydraulic Systems

Since hydraulic oil is only compressed by , it is considered non-elastic, or non-compressible. The advantage of a fluid’s inability to diminish in volume in response to pressure is that it acts as a more efficient medium for transferring energy. Little energy is lost to compression of the fluid in valves, so the transfer of hydraulic energy back into mechanical energy at the actuator is highly efficient.

Due to its inelasticity and high bulk modulus, hydraulic oil offers greater power density than mediums like air, which translates to high performance at high pressures. , in fact, operating a hydraulic system at 10,000 PSI may do less damage to the machine’s performance than running a similar pneumatic system at 300 PSI.

The major drawback of many hydraulic systems has to do with the challenge of transporting non-elastic oil through valves and plumbing systems.

As compared with the compressed air used in pneumatic systems, hydraulic oil faces high rates of resistance and elevated energy losses when flowing through restricted spaces. As such, if components within a hydraulic system are improperly fitted or do not correlate in size to desired flow rates, operators will suffer major energy and cost losses in every application.

How Pneumatic Systems Work

Pneumatic systems operate in much of the same way as hydraulic systems do. In general, they require an air compressor, which draws atmospheric air in through an intake valve and feeds the air into a receiver tank.

Pressure is then applied to air within the receiver tank. Gases, unlike fluids, can be compressed, making high pressure density easily achievable through the exertion of force. Air is compressed and passed into a network of pipes and valves that direct the flow of air toward an actuator. The actuator will transfer the energy back into mechanical energy in order to initiate motion, as with a hydraulic system.

Why Pneumatic Systems Beat Out Hydraulic Systems

While hydraulic systems may guarantee fewer energy losses during operation, in general, pneumatic systems ensure a lower up-front cost, fewer maintenance demands, greater durability, and a higher degree of mechanical reliability, making them the most cost-effective option in the long-run.

What’s more, compressed air comes with the unique advantage of speed — unlike hydraulic oil, when air is released, it expands at high speed and force, enabling much higher actuation velocities. As such, pneumatic systems offer expanded application possibilities and are the obvious choice for applications requiring high speed and precision.

Pneumatic systems can efficiently facilitate most industrial-scale applications requiring movements such as clamping, positioning, pressing, lifting, sorting, and stacking — such as those seen in manufacturing, assembly, and distribution facilities. Further system modifications can increase the precision with which a pneumatic system operates, enabling applications that require labeling, cutting, crimping, or embossing.

Due to their durability, superior performance, and ability to supply energy to actuators at high speed, pneumatic systems remain among the most reliable choice for powering everything from commuter transit systems and natural gas extraction to agtech devices that can milk a cow autonomously.