What is hydraulics?

Figures 1 and 2

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.


Hydraulic applications

Off-highway equipment is probably the most common application of hydraulics. Whether it’s construction, mining, agriculture, waste reduction, or utility equipment, hydraulics provides the power and control to tackle the task at hand and often to provide motive power to move equipment from place to place — especially when track drives are involved. Hydraulics is also widely used in heavy industrial equipment in factories, in marine and offshore equipment for lifting, bending, pressing, cutting, forming, and moving heavy work pieces. Following are case histories housed on websites of industry publications describing the use of hydraulics in a variety of applications:

Red Arrow Bullet imageTraction is King on Grape Harvester 
Red Arrow Bullet imageAccumulators Beat Boom Bounce

Red Arrow Bullet imageSlipform Paver has all the Smarts 
Red Arrow Bullet imageHydraulics Gives Multi-Articulated Excavator Wide Range of Motions

Red Arrow Bullet imageElectrohydraulics Runs Giant Elephant 
Red Arrow Bullet imageSpider-Man Musical Relies on the Power of Hydraulics to Control and Lift Stages and Platforms

Marine & Offshore:
Red Arrow Bullet imageCrab Boat Catches Huge Fuel Savings 
Red Arrow Bullet imageWave Energy Presents New Challenges

Waste & Recycling:
Red Arrow Bullet imageHydraulics Make Refuse Truck Quick, Quiet, and Efficient
Red Arrow Bullet imageCompact Motors Keep Sweepers Simple 

Other industries where hydraulics is advantageous:

  • Energy
  • Machine Tools
  • Metal Forming
  • Military & Aerospace
  • Mining
  • Utility Equipment

red arrow bulletAdditional Hydraulic Applications

red arrow bulletMore Hydraulic Case Studies

red arrow bulletPrinciples of Hydraulics Online Training


Fluid Power Components

Fluid power systems consist of multiple components that work together or in sequence to perform some action or work. People well versed in fluid power circuit and system design may purchase individual components and assemble them into a fluid power system themselves. However, many fluid power systems are designed by distributors, consultants, and other fluid power professionals who may provide the system in whole or in part.

 The major components of any fluid power system include:

  • a pumping device — a hydraulic pump or air compressor to provide fluid power to the system
  • fluid conductors — tubing, hoses, fittings, manifolds and other components that distribute pressurized fluid throughout the system
  • valves — devices that control fluid flow, pressure, starting, stopping and direction
  • actuators — cylinders, motors, rotary actuators, grippers, vacuum cups and other components that perform the end function of the fluid power system.
  • support components — filters, heat exchangers, manifolds, hydraulic reservoirs, pneumatic mufflers, and other components that enable the fluid power system to operate more effectively.

Electronic sensors and switches are also incorporated into many of today’s fluid power systems to provide a means for electronic controls to monitor operation of components. Diagnostic instruments are also used for measuring pressure, temperature and flow in assessing the condition of the system and for troubleshooting.

NFPA Fluid Power Product Locator

    where you can find hydraulic and pneumatic components and products available from NFPA member companies.

Additional Education and Training Sessions offered by NFPA and its members can be found at
   Learning Resources.