Abstract

Fluid power technology uses a pump to deliver pressurized fluid to a cylinder, motor, or rotary actuator. Output speed and direction is controlled by varying flow rate from the pump or through valves within the fluid power circuit. Likewise, output force and torque are regulated by controlling pressure within the circuit. Engineers should understand what the possibility of using fluid power can bring.

Article

Caterpillars 6090-FS mining excavator showcases the power and controllability of hydraulics in mobile equipment. This giant’s five-circuit hydraulic system controls virtually every aspect of operation —the ground drive, swing function, arm and bucket control, and even engine cooling.

Photo: Caterpillar

Caterpillars 6090-FS mining excavator showcases the power and controllability of hydraulics in mobile equipment. This giant’s five-circuit hydraulic system controls virtually every aspect of operation —the ground drive, swing function, arm and bucket control, and even engine cooling.

Photo: Caterpillar

Engineers learn about mechanical and electric power transmission as part of their formal education.

Mechanical power transmission controls different machine aspects such as force, torque, and speed from a prime mover—usually an electric motor or internal—combustion engine—to a driven element using gear, belt, or chain. Electrical power transmission regulates current, voltage, and frequency to deliver controlled speed, force, and torque to an output shaft. These technologies usually work together to produce a power transmission system that capitalizes on the unique strengths of both technologies.

Unfortunately, most engineering schools in North America do not offer a curriculum covering the third type of power transmission—fluid power. The term fluid power was adopted more than 60 years ago to describe hydraulic and pneumatic systems for transmitting power.

Analogies between fluid power and electrical systems can be an effective tool in teaching how fluid systems operate and can be designed. Both systems have their own distinct set of advantages and disadvantages compared to mechanical and electrical drives. Engineers should understand what the possibility of using fluid power can bring.

Fluid Power System Characteristics

Fluid power technology uses a pump to deliver pressurized fluid to a cylinder, motor, or rotary actuator. Output speed and direction is controlled by varying flow rate from the pump or through valves within the fluid power circuit. Likewise, output force and torque are regulated by controlling pressure within the circuit.

In fluid power systems, the molecules of fluid act much like electrons in electrical circuits to transmit power from an input to an output. For example, pressure and flow in a fluid power system are analogous to voltage and current in an electrical system. Fluid power systems also use a variety of directional controls valves, much like electrical circuits use switches. However, valves allow fluid current to pass when open, whereas switches enable electrical current to pass when they are closed.

Most hydraulic fluids exhibit a very high bulk modulus—a measure of resistance to compression, making fluid stiff so highly responsive motion and positioning can be achieved. Oil is the most widely used fluid in hydraulic systems, with water a distant second and other fluids for specialized applications.

Compressed air is the most widely used in pneumatic systems and is highly compliant, which can produce sluggish or spongy operation, depending on the load and system design.

The high-power density is the main reason why hydraulic power transmission continues to be widely used in commercial and military aircraft. The systems provide a compact and lightweight solution for operating flight surfaces, landing gear, steering, and other aircraft systems. Certain hydraulic systems can transmit more power from a smaller output actuator than electromechanical devices can. For example, a hydraulic motor rated for 25 hp typically is one-quarter the size and weight of an electric motor. Specifying a low-speed, high-torque hydraulic or pneumatic motor also eliminates the need for a gearbox to reduce speed and multiply torque output of the electric motor, increasing fluid power’s higher power density even more.

Hydraulic and pneumatic systems also excel at transmitting linear motion. A simple hydraulic or pneumatic cylinder transmits fluid pressure and flow directly into controllable force and speed with no need for a screw or other rotary-to-linear motion conversion device. Furthermore, a properly sized hydraulic system can deliver a force measured in hundreds of tons with motion and position accuracy measured in thousandths of an inch.

Because pneumatic systems typically operate at much lower pressures than their hydraulic counterparts, their power density advantage is not as great. However, compressed air is highly compliant, so pneumatic systems can operate with inherent cushioning to reduce shock transmitted to workpieces from impact.

Pneumatic actuators can cycle back and forth rapidly with high repeatability and do so without the heat-buildup issues of electromechanical actuators or the potential hazards of electrical shock or ignition—offering clean operations. This makes them especially well-suited to pick-and-place, sorting, assembly, food processing, and other factory automation applications.

Limitations of Fluid Power

As with any other technology, fluid power systems have limits. Even though fluid power actuators themselves have a much higher power density than electromechanical actuators, hydraulic and pneumatic systems require a central power unit to convert electrical or mechanical power into pressure and flow. The power unit generally consists of a motor, pump, reservoir, valves, and filters. In most cases, the power unit is installed in a remote location, where generated heat and noise can be isolated and periodic maintenance performed.

Fluid power systems generally operate with lower efficiency than equivalent electromechanical systems. An ongoing trend in fluid power system design incorporates electronic sensors and controls to improve efficiency and performance. A more recent trend uses a variable-speed electrical drive powering the hydraulic pump instead of a fixed-speed motor.

Fluid power systems—especially hydraulics—generally require more intensive maintenance, mainly to control leaks. Repeated pressure cycles can cause hydraulic fluid to leak from dynamic seals and fittings. However, leaks in pneumatic systems do not result in oil to accumulate; rather each leak represents wasted power, which reduces system efficiency.

Fluid power components are designed with tight tolerances and narrow passageways, which makes them sensitive to contamination. Specifying filters to exclude and remove contamination is a critical to avoid erratic operation and a reduction in reliability.

Widespread Application

The application of hydraulic systems often brings to mind rugged and dirty environments. Hydraulics in construction and mining equipment—the technology’s largest sector— involves heavy loads, heavy shock, and dirty environments. Stationary machines, such as industrial presses and cutting machines for example, also involve heavy loads and hostile environments. But hydraulics can also be found in much more delicate operations, such as aircraft and aerospace drives and even medical equipment.

Pneumatics is also used in presses, but it is most prominent in conveying, packaging, labeling, and food processing automation. It’s also widely used in medical equipment because of its clean operation and gentle application of force. Pneumatics is also inherently explosion-proof because it requires no electricity to operate—a popular choice for use in proximity to volatile substances.

Hydraulic and pneumatic technologies are both considered mature industries, but they are by no means stagnant. New product developments, variable-speed drives, energy-efficient developments, and continued integration of IoT and Industry 4.0 capabilities ensure that fluid power technology will remain relevant for future generations.