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, most 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 unique symbology for describing various components. The following diagram shows some common symbols used in fluid power system diagrams:
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 is the spool valve symbols. A “spool” valve is a particular 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:
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 piston within holds 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:
If the spool valve is actuated in the other direction, the spool piece inside the valve assembly shifts again, switching the directions of airflow 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 piston to reverse its mechanical travel:
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 use. When large amounts of energy need to be transmitted over long distances, electricity is 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 and troubleshoot than electric systems
- Fluid power systems may be safely used in submerged (underwater) environments
- Pneumatic systems are relatively easy to equip with backup energy reserve (e.g. liquefied nitrogen serving as a backup 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 dust and vapors by leaking air)
Another important consideration for fluid power systems is the ongoing maintenance work they require for reliable operation. Hydraulic power systems will suffer rapid wear if the hydraulic oil is not clean and chemically stable. The fluid in a hydraulic system not only transmits mechanical power, but it also lubricates and stabilizes the temperature of components as they transfer that power between different forms. Regular filter changes and oil changes (especially if the fluid is subject to contamination from the process) are necessary for a long service life of any hydraulic system.
Instrumentation Basics 