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Discrete Control Elements - Fluid Power Systems

Given the ability of pressurized fluids to transmit force over long distances, it is not surprising that many practical “fluid power systems” have been built using fluid as a mechanical power-conducting media. Fluid systems may be broadly grouped into pneumatic (gas, usually air) and hydraulic (liquid, usually oil).

Although there is no particular reason why a fluid power system must be discrete and not continuous, the majority of fluid power systems operate in an on/off control mode rather than throttling, which is why this subject is covered in the “Discrete Control Elements” chapter. As usual for technical specialties, fluid power has its own unique symbology for describing various components. The following diagram shows some common symbols used in fluid power system diagrams:

Fluid Power System Common Symbols


Many of these symbols are self-explanatory, especially the pumps, motors, and cylinders. What seems to cause the most confusion for people new to this symbology are the spool valve symbols. A “spool” valve is a special type of flow-directing valve used in pneumatic and hydraulic systems to direct the pressurized fluid to different locations. The symbology for a spool valve is a set of boxes, each box containing arrows or other symbols showing the intended direction(s) for the fluid’s travel. Take for instance this pneumatic reversing cylinder control system:


Double Acting Cylinder Circuit

The proper way to interpret a spool valve symbol is to see only one “box” active at any given time. As the actuator (in this case, a hand-actuated lever) is moved one way or the other, the boxes “shift” laterally to redirect the flow of fluid from source to load.

For example, when the spool valve in this reversing control system is in its center position, the outer boxes in the symbol are inactive. This is represented in the following diagram by showing the outer boxes in the color grey. In this position, the spool valve neither admits compressed air to the cylinder nor vents any air from the cylinder. As a result, the cylinder holds its position:


Double Acting Cylinder Circuit Holding Its Position


If the spool valve is actuated in one direction, the spool piece inside the valve assembly shifts, directing compressed air to one side of the cylinder while venting air from the other side. This is shown in the following diagram by shifting the boxes to one side, lining up the “active” box with the cylinder and air supply/vent connections:


Double Acting Cylinder Circuit Extends


If the spool valve is actuated in the other direction, the spool piece inside the valve assembly shifts again, switching the directions of air flow to and from the cylinder. Compressed air still flows from the supply to the vent, but the direction within the cylinder is reversed. This causes the cylinder to reverse its mechanical travel:


Double Acting Cylinder Circuit Retracts


Hydraulic systems require more components, including filters and pressure regulators, to ensure proper operation. Shown here is a simple uni-directional hydraulic motor control system:


Simple Uni-Directional Hydraulic Motor Control System showing where constant hydraulic pressure is maintained

Note the placement of the pressure relief valve: it is a shunt regulator, bleeding excess pressure from the discharge of the hydraulic pump back to the reservoir1. A “shunt” regulator is necessary because hydraulic pumps are positive displacement, meaning they discharge a fixed volume of fluid with every revolution of the shaft. If the discharge of a positive-displacement pump is blocked (as it would be if the spool valve were placed in its default “off” position, with no shunt regulator to bleed pressure back to the reservoir), it will mechanically “lock” and refuse to turn. This would overload the electric motor coupled to the pump, if not for the pressure regulating valve providing an alternative route for oil to flow back to the reservoir. This shunt regulator allows the pump to discharge a fixed rate of oil flow (for a constant electric motor speed) under all hydraulic operating conditions.

An alternative to using a shunt regulating valve in a hydraulic system is to use a variable-displacement pump. Variable-displacement pumps still output a certain volume of hydraulic oil per shaft revolution, but that volumetric quantity may be varied by moving a component within the pump. In other words, the pump’s per-revolution displacement of oil may be externally adjusted. If we connect the variable-displacement mechanism of such a hydraulic pump to a pressure sensing element such as a bellows, in a way where the pump senses its own discharge pressure and adjusts its volumetric output accordingly, we will have a pressure-regulating hydraulic system that not only prevents the pump from “locking” when the spool valve turns off, but also saves energy by not draining pressurized oil back to the reservoir:


Simple Uni-Directional Hydraulic Motor Control System with displacement pump

Note the placement of a filter at the inlet of the pump in all hydraulic systems. Filtration is an absolute essential for any hydraulic system, given the extremely tight tolerances of hydraulic pumps, motors, valves, and cylinders. Even very small concentrations of particulate impurities in hydraulic oil may drastically shorten the life of these precision components.


Pneumatic fluid power systems require cleanliness as well, since any particulate contamination in the air will likewise cause undue wear in the close-tolerance compressors, motors, valves, and cylinders. Unlike hydraulic oil, compressed air is not a natural lubricant, which means many pneumatic power devices benefit from a small concentration of oil vapor in the air. Pneumatic “oilers” designed to introduce lubricating oil into a flowing air stream are generally located very near the point of use (e.g. the motor or the cylinder) to ensure the oil does not condense and “settle” in the air piping.


Fluid power systems in general tend to be inefficient, requiring much more energy input to the fluid than what is extracted at the points of use2. When large amounts of energy need to be transmitted over long distances, electricity is the a more practical medium for the task. However, fluid power systems enjoy certain advantages over electric power, a few of which are listed here:

  • Fluid power motors and cylinders do not overload at low speeds or under locked conditions
  • Fluid power systems present little hazard of accidentally igniting flammable atmospheres (no sparks produced)
  • Fluid power systems present little or no fire hazard
  • Fluid power systems present no hazard of electric shock or arc flash
  • Fluid power systems are often easier to understand than electric systems
  • Fluid power systems may be safely used in submerged (underwater) environments
  • Pneumatic systems are relatively easy to equip with back-up energy reserve (e.g. liquefied nitrogen serving as a back-up gas supply in the event of compressor shut-down)
  • Pneumatic systems are self-purging (i.e. enclosures housing pneumatic devices will be naturally purged of dusts and vapors by leaking air)


1Note also how identical reservoir symbols may be placed at different locations of the diagram although they represent the exact same reservoir. This is analogous to “ground” symbols in electronic schematic diagrams, every ground symbol representing a common connection to the same zero-potential point.

2Close-coupled hydraulic systems with variable-displacement pumps and/or motors may achieve high efficiency, but they are the exception rather than the rule. One such system I have seen was used to couple a diesel engine to the drive axle of a large commercial truck, using a variable-displacement pump as a continuously-variable transmission to keep the diesel engine in its optimum speed range. The system was so efficient, it did not require a cooler for the hydraulic oil!


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