Fluid power 101

Fluid power is just what the name says: fluids generate or transmit power to where it’s needed. Specifically, fluid power is divided into hydraulics and pneumatics technologies:

• Hydraulics: the fluid is a liquid, usually oil or water
• Pneumatics: the fluid is a gas, typically compressed air. Nitrogen or argon can also be used, but the expense of producing and processing these gasses usually precludes their use

Fluid power systems

Are used in a variety of applications from braking systems on cars to robotics to heavy machinery. These systems produce linear motion using either hydraulic or pneumatic cylinders. Compare fluid power with two other methods of power transmission, electrical and mechanical, which usually need a mechanical device to convert rotational motion to linear motion.

Fluid power’s cylinders are compact, fitting smaller spaces, yet they have high-energy potential. Typically, fluid power systems use valves to control direction, speed, force and torque.

The advantages of hydraulics and pneumatics

Hydraulics	Pneumatics
High power-to-weight ratio	Inexpensive
High torque at low speed	Lightweight
Ability to hold torque constant	Simple control systems
Ruggedness	Clean & non-reactive in magnetic environments
Reliability	Speed and precision

Perhaps the best-known application of Pascal's law is the hydraulic press. Engineering.com gives us the math:

A force F1 is applied to a small piston of area A1. The pressure is transmitted through a liquid to a larger piston of area A2. Since the pressure is the same on both sides, we see that P = F1/A1 = F2/A2. Therefore, the force F2 is larger than F1 by multiplying factor A2/A1.

 

You’ll find this principle working in hydraulic brakes, car lifts, hydraulic jacks, and forklifts, for example.

Hydraulic brakes

Without hydraulics, pressing the brake pedal with your foot would not produce enough force to apply all four brakes hard enough to quickly stop. Hydraulic brakes rely on pipes filled with fluid that multiply force and transmit it where it’s needed.

How it works:

• When you press on the brake pedal, you’re moving a lever that forces a piston into a long, narrow cylinder, which is filled with hydraulic fluid (usually glycol-ether, mineral oil or silicone-based).
• The piston plunges into the cylinder, squirting hydraulic fluid through a long, narrow pipe at the end.
• This pipe feeds into wider cylinders located next to the car’s four brakes.
• When force is applied, it multiplies, because the cylinders near the brakes are wider than the one near the brake pedal. The multiplied force is able to clamp the brakes and stop your car.

Top hydraulics markets

According to the National Fluid Power Association (NFPA), these are the markets that use hydraulics the most:

Construction machinery	24%
Agricultural machinery	14%
Automotive (including light trucks)	9%
Class 4-8 trucks (including vocational trucks)	6%
Material handling (including conveying)	6%
Oil & gas machinery	4%
Lawn & garden	4%

 

Hydraulics are also widely used in heavy industrial equipment in factories.

 

• Typically, pneumatic systems use a continuous supply of compressed air, which is provided by a compressor.
• The compressor sucks in air from its environment and stores it in a high-pressure tank known as a receiver.
• The compressed air feeds the system via pipes and valves.
• An advantage to pneumatics is that air in the atmosphere is free and plentiful – you have an unlimited supply.

The designs of pneumatic-system components make it ideal for simple automatic controls, which is why it’s often used in automation systems. You can use linear movement or angular rotational movement, with variable operational speeds.

Another place you’ll find pneumatic systems is in HVACs systems, where compressed air is used as a tool for communication:

• From a controller, compressed air flows through copper and plastic tubes to a control device, normally a damper or valve actuator.
• Control is based on sensors and the building’s thermostats that bleed or retain the line pressure, from sensor to control device to actuator.
• Each sensor reacts to changes in temperature, static pressure, humidity and other conditions.
• The sensors feed back in a control loop to open or close the actuator in order to maintain the control set point.
• The actuators, which contain diaphragms, act in sequence with the control signal.
• The building’s thermostat is connected to one or more air lines from the primary source of compressed air, and to a valve or other final device.

Top pneumatic markets

According to the NFPA, these are the markets that use hydraulics the most:

Class 4-8 trucks (including vocational trucks)	13%
Automotive (including light trucks)	5%
Food processing	4%
Medical equipment	4%
Packaging machinery	3%
Material handling (including conveying)	3%
Semiconductor	3%

 

Best practice & Advantages of Fluid Power

The NFPA recommends these practices to cut energy usage by nearly a third:

Hydraulics	Pneumatics
Effective flow control	Eliminating pressure drop
Smart use of accumulators	Repairing leaks
Pressure reducing valves	Reducing pressure
Load-sensing pumps	Evaluating plumbing and storage
Pressure compensated pumps	Optimizing air blow
Servo valves	Employing intermediate bore-size cylinders
Hydraulic power units vs. electric motors	Correctly sizing components
Eliminating parasitic losses

 

 

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