What is Fluid Power?

Fluid power is a term describing hydraulics and pneumatics technologies. Both technologies use a fluid (liquid or gas) to transmit power from one location to another. With hydraulics, the fluid is a liquid (usually oil), whereas pneumatics uses a gas (usually compressed air).

Both are forms of power transmission, which is the technology of converting power to a more useable form and distributing it to where it is needed. The common methods of power transmission are electrical, mechanical and fluid power.

Although they sometimes are viewed as competing technologies, no single method of power transmission is the best choice for all applications. In fact, most applications are served by a combination of technologies. However, fluid power offers important advantages over the other technologies.

Fluid power systems easily produce linear motion using hydraulic or pneumatic cylinders, whereas electrical and mechanical methods usually must use a mechanical device to convert rotational motion to linear.

Fluid power systems generally can transmit equivalent power within a much smaller space than mechanical or electrical drives can, especially when extremely high force or torque is required.

Fluid power systems also offer simple and effective control of direction, speed, force, and torque using simple control valves. Fluid power systems often do not require electrical power, which eliminates the risk of electrical shock, sparks, fire, and explosions.

To visualize a basic hydraulic system, think of two identical syringes connected together with tubing and filled with water (see Figure 1). Syringe A represents a pump, and Syringe B represents an actuator, in this case a cylinder. Pushing the plunger of Syringe A pressurizes the liquid inside.

This fluid pressure acts equally in all directions (Pascal’s Law), and causes the water to flow out the bottom, into the tube, and into Syringe B. If you placed a 5 lb. object on top of the plunger of Syringe B, you would need to push on Syringe A’s plunger with at least 5 lbs. of force to move the weight upward. If the object weighed 10 lbs., you would have to push with at least 10 lbs. of force to move the weight upward.

If the area of the plunger (which is a piston) of Syringe A is 1 sq. in., and you push with 5 lbs. of force, the fluid pressure will be 5 lbs./sq. in. (psi). Because fluid pressure acts equally in all directions, if the object on Syringe B (which, again has an area of 1 sq. in.) weighs 10 lbs., fluid pressure would have to exceed 10 psi before the object would move upward. If we double the diameter of Syringe B (see Figure 2), the area of the plunger becomes four times what it was. This means a 10 lb. weight would be supported on 4 sq. in. of fluid.

Therefore, fluid pressure would only have to exceed 2.5 psi (10 lbs. ÷ 4 sq. in. = 2.5 psi) to move the 10 lb. object upward. So moving the 10 lb. object would only require 2.5 lbs. of force on the plunger of Syringe A, but the plunger on Syringe B would only move upward ¼ as far as when both plungers were the same size. This is the essence of fluid power. Varying the sizes of pistons (plungers) and cylinders (syringes) allows multiplying the applied force.

In actual hydraulic systems, pumps contain many pistons or other types of pumping chambers. They are driven by a prime mover (usually an electric motor, diesel engine, or gas engine) that rotates at several hundred revolutions per minute (rpm). Every rotation causes all of the pump’s pistons to extend and retract — drawing fluid in and pushing it out to the hydraulic circuit in the process. Hydraulic systems typically operate at fluid pressures of thousands of psi. So a system that can develop 2,000 psi can push with 10,000 lbs. of force from a cylinder about the same size as a can of soda pop.

The principles of pneumatics are the same as those for hydraulics, but pneumatics transmits power using a gas instead of a liquid. Compressed air is usually used, but nitrogen or other inert gases can be used for special applications. With pneumatics, air is usually pumped into a receiver using a compressor.

The receiver holds a large volume of compressed air to be used by the pneumatic system as needed. Atmospheric air contains airborne dirt, water vapor, and other contaminants, so filters and air dryers are often used in pneumatic systems to keep compressed air clean and dry, which improve reliability and service life of the components and system. Pneumatic systems also use a variety of valves for controlling direction, pressure, and speed of actuators.

Most pneumatic systems operate at pressures of about 100 psi or less. Because of the lower pressure, cylinders and other actuators must be sized larger than their hydraulic counterparts to apply an equivalent force. For example, a hydraulic cylinder with a 2 in. diameter piston (3.14 sq. in. area) and fluid pressure of 1,000 psi can push with 3140 lbs. of force. A pneumatic cylinder using 100 psi air would need a bore of almost 6½ in. (33 sq. in.) to develop the same force.

Even though pneumatic systems usually operate at much lower pressure than hydraulic systems do, pneumatics holds many advantages that make it more suitable for many applications. Because pneumatic pressures are lower, components can be made of thinner and lighter weight materials, such as aluminum and engineered plastics, whereas hydraulic components are generally made of steel and ductile or cast iron.

Hydraulic systems are often considered rigid, whereas pneumatic systems usually offer some cushioning, or “give.” Pneumatic systems are generally simpler because air can be exhausted to the atmosphere, whereas hydraulic fluid usually is routed back to a fluid reservoir.

Pneumatics also holds advantages over electromechanical power transmission methods. Electric motors are often limited by heat generation. Heat generation is usually not a concern with pneumatic motors because the stream of compressed air running through them carries heat from them. Furthermore, because pneumatic components require no electricity, they don’t need the bulky, heavy, and expensive explosion-proof enclosures required by electric motors.

In fact, even without special enclosures, electric motors are substantially larger and heavier than pneumatic motors of equivalent power rating. Plus, if overloaded, pneumatic motors will simply stall and not use any power. Electric motors, on the other hand, can overheat and burn out if overloaded. Moreover, torque, force, and speed control with pneumatics often requires simple pressure- or flow-control valves, as opposed to more expensive and complex electrical drive controls. And as with hydraulics, pneumatic actuators can instantly reverse direction, whereas electromechanical components often rotate with high momentum, which can delay changes in direction.

Another advantage of pneumatics is that it allows using vacuum for picking up and moving objects. Vacuum can be thought of as negative pressure — by removing air (evacuating) from the volume between two parts, atmospheric pressure outside the volume pushes the parts together. For example, attempting to pick up a single sheet of paper or a raw egg presents a challenge with conventional grippers. But with a vacuum pneumatic system, evacuating a suction cup in contact with a sheet of paper or eggshell will cause atmospheric pressure to push the paper or egg against the cup, allowing it to be lifted.

Standard electric motors typically rotate at 1,800 or 3,600 revolutions per minute (rpm) — much faster than is practical for most machines. Gasoline and diesel engines also rotate at thousands of rpm when powering equipment. Therefore, some form of power transmission is needed to convert power from the motor or engine to a more useable form — slower speed and, often, linear motion instead of rotary.

Mechanical power transmission methods include gear, chain, belt, and other mechanical drives that convert the high-speed mechanical power from the engine or motor’s output shaft to a slower speed with higher torque (twisting force). Mechanical power transmission components also include ball screws, rack-and-pinion assemblies, chain drives, and other components that convert rotational motion and torque to linear motion and force.

Electrical methods of power transmission regulate electrical power to the motor to control speed and torque. But these methods cannot convert the rotary motion of a motor to linear. When linear output is needed, a linear motor may be used, but, its high cost generally makes a mechanical rotary-to-linear motion device more practical for producing linear motion and force.

In many cases, however, mechanical and electrical methods cannot provide a practical power transmission solution. In these cases, fluid power — whether hydraulic or pneumatic — is used because it can deliver linear and rotary motion with high force and torque within a smaller, lighter package than is possible with other forms of power transmission.

A cement mixer is an example illustrating how different methods of power transmission can be used. Early cement mixers used mechanical drives driven either by the truck’s engine or transmission. A system of gears, chain drives, and drive shafts provided the speed and torque necessary to rotate the heavy drum of concrete, but speed was difficult to control. The rotational speed of the drum depended on the engine speed or transmission speed. As the driver shifted gears, the drum would speed up or slow down, and rarely rotated at the ideal speed. Plus, the complexity and bulk of all the mechanical components were highly maintenance intensive.

An electrical drive could provide good speed control but would require a high-power electric generator, controls, and a motor to drive the drum. Plus, the motor would either be prohibitively large or would require a large gearbox to achieve the low-speed rotation of the mixer drum. Either solution would be much larger and heavier than a hydraulic drive.

Hydraulic drives are the primary choice for cement mixer drives. They use a pump, hydraulic motor, and valves to precisely control speed regardless of the engine or transmission speed. The drum rotates at optimum speed or can be manually controlled. Plus, the components are relatively compact: the pump is tucked away within the framework of the truck, and the hydraulic motor is only a small fraction of the size of a comparable electric motor-gearbox combination.