Hydraulic systems are systems that use changes in pressure to control how fluids move in driving machinery like tools or moving mechanical components such as gears. There are many different ways of classifying hydraulic systems through the different means of using fluid power under high pressure to lift or support a load.
Every hydraulic system, no matter its design or purpose, takes fluid from a reservoir through a pump to a selector control valve. This converts the mechanical energy into hydraulic energy.
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Hydraulic systems can be classified by their purpose and function into classes of industrial hydraulics, mobile hydraulics and aircraft hydraulics as well as into fixed displacement systems and variable displacement systems. The types of pumps are internal gear pumps, external gear pump and screw pumps (which are fixed displacement pumps) and bent axis hydraulic pumps, axial piston pumps, radial piston pumps and rotary vane pumps (which are variable displacement pumps.
Different Types of Hydraulic Systems
The general hydraulic system components involve fluid flowing from the valve to an actuator of a hydraulic system. At the high end of the actuating cylinder there's a piston. High pressure drives the piston down, forcing fluid out of the piston's lower side before returning it through the selector valve back to the reservoir, where the cycle continues as needed.
Fixed displacement types of hydraulic systems are systems in which the amount of displacement that the pump produces cannot be changed. Instead, you can change the drive speed that the pump uses. Gear pumps are among the simplest and most frequent pumps in use today, and they fall under this category. Screw pumps also fall under this category.
Hydraulic systems can also be categorized as open loop or closed loop. When hydraulic fluids flow continuously between the pump and the motor without entering a reservoir, you can call the system "closed." In other cases, when the fluid from the cylinder first enters a reservoir then the pump inlet, the system is "open." Open loop hydraulic systems can typically perform better by producing less heat, and closed loop hydraulic systems have more precise responses of the components with the pump's reservoir.
Internal Gear Pumps
Internal gear pumps or Gerotor pumps use one gear internal to the pump and one external gear that can suit a wide range of uses. They're generally used with thin liquids like solvents and fuel oil, but they can also pump thick liquids like asphalts. They can handle a wide range of liquid thicknesses and a wide range of temperatures.
These pumps only have two moving parts (the rotor is the large exterior gear and the idler the smaller one) and can operate in both forward and reverse directions. This makes them affordable and easy to maintain. Despite the advantages, these pumps generally only operate at moderate speeds with pressure limitations.
The internal gear and external gear versions are examples of these. Internal gear pumps operate with the following steps:
- The suction port between the teeth of the rotor and the idler lets liquid flow into it. The gears turn, and the liquid flows through.
- The crescent shape of the pump divides the liquid and seals the area between the suction and discharge ports.
- When the head of the pump is almost completely filled with water, the intermeshing gears of the idler and rotor create locked pockets for the liquid to keep its volume under control.
- The rotor and idler teeth mesh together to create a seal between the discharge and suction ports to force liquid out in the discharge step.
Internal gear pumps are used in a myriad of purposes for lube oil and fuel oils. They're used in producing resins, polymers, alcohols, solvents, asphalt, tar and polyurethane foam.
External Gear Pumps
External gear pumps, on the other hand, use two external gears and are typically used for lubrication in machine tools, in fluid power transfer units and as oil pumps in engines. They can use either one set of gears or two, and can be found in spur, helical and herringbone gears. The helical and herringbone arrangements allow for smoother flow of liquids than spur gears do.
External gear pumps can run at high pressures because they have close tolerances and shaft support on both sides of the gears. This arrangement of the external gear lets the pump create suction at the inlet to protect fluid from leaking back from the side that discharges fluid. These characteristics also make external gear pumps a great choice for precise transfer of liquids and creating polymers, fuels and chemical additives.
External gear pumps work with the following steps:
- The volume of the pump expands into the pump as the two gears or two pairs of gears emerge from one side of the pump.
- Liquid flows through into the container of the pump. The gear teeth trap the liquid while the gears rotate against the casing of the pump.
- The fluid moves from the inlet to the exit as part of the discharge step.
- The teeth of the gears interlock themselves with each other to reduce the volume and expel the fluid from within.
External gear pumps can operate at high speeds, high pressures, and use many different materials all while operating quietly compared to other pump designs. They're useful for pumping fuel water, alcohol, solvents, oils, lube oils, chemical additives and acids. Engineers also use them for industrial and mobile hydraulic applications.
Screw pumps are another type of fixed displacement pump. They use two helical screws that create shafts that interlock with one another inside a container, with one shaft that drives the pump. As fluid passes through the pump in a single direction, the output is displaced.
The two primary screw pump designs are the two/double screw pump (or twin screw pump) that use two interlocking screws as described and the three screw pump (or triple screw pump) that use a single screw that interlocks with two other screws to move fluid. In both of these designs, the pressure difference by the screw's motion drives the water to move.
In single screw pumps, the screws do come into contact with each other, which often limits the pump to handling only clean liquids. These pumps don't produce much noise because the contact between the gears is continuous, and they're very reliable in transferring fuels, moving elevators between floors and other applications in industry. With higher viscosity liquids, screw pumps can be less efficient.
Engineers use single screw pumps, also known as Archimedean screw pumps, for moving water in systems for sewage, storm water, drainage, and industrial waste water.
Bent Axis Hydraulic Pumps
Bent axis hydraulic pumps can be either a fixed displacement type or a variational displacement type. The body of the pump contains a rotating cylinder chamber with pistons that act external to it. These pistons add force to a plate on the shaft end such that, when the shaft rotates, the pistons move as well. This force controls the motion of the fluid through the pump.
You can change the piston's stroke by varying the pump's displacement angle making these types of pumps highly reliable and efficient for use especially in mobile machinery.
Axial Piston Pumps
In axial piston pumps, the shaft and pistons are arranged in a radial formation around the area of a circle. This makes the design close-packed, efficient and cost-effective. By applying different pressures, flow and control functions for power, the pump can become suited for different purposes in industry.
An eccentric ring, one that flows from many sources to a single channel, surrounds the arrangement of pistons such that, when the shaft rotates, the distance between the eccentric ring and shaft center changes so that the pistons move through a cycle that creates and dissipates pressure. This drives fluid through the pump.
You can use adjustment screws or a piston to change the amount of displacement that occurs. This makes these types of pump strong, reliable natural candidates for high pressure uses. They produce a low amount of noise, but may not operate well at high pressures.
Radial Piston Pumps
When operating radial piston pumps, you control a rotating shaft much the same way an axial piston pump operates. But, for radial piston pumps, the shaft rotates such that the pistons extend radially around the shaft in different directions as though they were lined on the circumference of a circle. The distance between the eccentric ring and the center of the shaft also causes the differences in pressure that let the fluid flow.
These types of pump have a high amount of efficiency, can operate at high pressures, have a low noise level, and can in general be very reliable. They do have greater dimensions than axial piston pumps do, but the size can be changed for appropriate purposes. They make ideal candidates for machine tools, high pressure units, and automotive tools.
Rotary Vane Pumps
These types of pumps use a rotary displacement pump that has a container, an eccentric rotor, vanes that move radially under forces and an outlet to dispel the liquid. The inlet valve remains open while liquid enters the working chamber that the stator, rotor, and vanes restrict. The eccentricity between the rotor and vanes create divisions of the working chamber that let different amounts of volumes enter.
When the rotor turns, gas flows into the enlarging suction chamber until the second vane seals it off. The pump then compresses the gas inside, and, when the outlet valve opens against the atmospheric pressure, it stops. When the outlet valve opens, oil enters the suction chamber to lubricate and seal the vanes against the stator.
Rotary vane pumps generate little noise and can be reliable. They don't work well with high pressures, though. They're common in machine tools applications as well as applications in vehicles for power steering and as carbonators for soda machine dispensers.
Types of Hydraulic Systems in Aircraft
There are many different types of hydraulic systems in aircraft that perform various functions. They're used to apply pressure when activating brakes on wheels and can even power systems for nose wheel steering, landing gear retraction, thrust reversers, and windshield wipers. These systems sometimes take into account multiple pressures sources for many pumps working together.
Engineers design these hydraulic systems such that they prevent themselves from overheating by determining the maximum temperature at which they can operate. They're designed such that the system doesn't lose necessary pressure through loss of fluid or failure of different pumps. They also take into account the contamination of the hydraulic fluid from external chemical sources.
For aircraft, hydraulic systems consist of a pressure generator (or hydraulic pump), a hydraulic motor that powers the component, and a system plumbing that directs the fluid throughout the aircraft. These pumps can have a range of power sources including manual pumps, engines, electric currents, compressed air and other hydraulic systems.
- Pumpschool: Internal Gear Pumps
- HydraProducts: Different types of Hydraulic Pumps
- Pumpschool: External Gear Pumps
- Michael Smith Engineers: Useful information on External Gear Pumps
- Empowering Pumps and Equipment: Screw Pump Basics
- Skybrary: Hydraulic Systems
- Hydraulics and Pneumatics: Hydraulic Motor Face-Off: Bent Axis vs Axial Piston
- DTA: What is a radial piston pump and why do you need it?
- East Coast Vacuum Systems: What Are Some of The Most Common Uses For Rotary Vane Pumps
- Linkedin: Difference between Open Loop Hydraulic Circuit and a Closed Loop Hydraulic Circuit.
About the Author
S. Hussain Ather is a Master's student in Science Communications the University of California, Santa Cruz. After studying physics and philosophy as an undergraduate at Indiana University-Bloomington, he worked as a scientist at the National Institutes of Health for two years. He primarily performs research in and write about neuroscience and philosophy, however, his interests span ethics, policy, and other areas relevant to science.