Fluid Energy

Fluid Power - Definitions

• Fluid Power:– Fluid power is energy that is transmitted by

pressurizing and controlling a contained fluid

• Hydraulics:– Use of liquids to perform mechanical tasks

• Pneumatics:– Use of pressurized gas (air) to perform

mechanical tasks

Pressure

• Pressure =

•Pressure is a force acting on a unit of area of a surface.

Force

AreaIn the metric system pressure is measured in Newtons/area

Atmospheric Pressure is the weight of a column of air pushing down through miles of atmosphere on a measured area of the earth surface.

Open and Closed Fluid Systems

Open systems vent or the fluid does not return to the starting point.

Examples are irrigation systems, forced hot air heating systems, air compressors.

Close systems are designed to have the fluid return to the start or do not allow the fluid to escape or the system.

Examples would be a hot water heating systems

and hydraulic brake system.

Pressure in a confined vessel

• Pascal’s Law states that pressure applied to a confined fluid is transmitted equally in all directions.

• If the stopper of a flask, which is full of a liquid, is pushed in with a Pressure of 100N/cm2 then the pressure will be the same on all sides of the flask.

Pressure in a confined vessel

• If the flask has an internal surface area of 20 cm2, and the force is 100 N, what is the total pressure in the flask?

Total Pressure = 5 N/cm2

Total Pressure =

= 100 N / 20 cm2

Force

Area

Hydraulic SystemsTwo Pistons That Are The Same Size

• A hydraulic system has two or more pistons and the pressure per area inside the system is the same.

• Two pistons with the same diameter have the same surface area. Piston 1 travels a certain distance as indicated by the dotted line. Piston 2 will correspond and travel the same distance in the opposite direction with the same lifting force as the applied force. Link

Hydraulics Two Different Size Pistons

When force is applied to piston C, it will move nine units for every one unit that piston D moves, and the force is multiplied nine times on piston D.

Piston D is 9 times larger than Piston C. Therefore the resultant force is 9 times more than the input piston. Or the resultant force on Piston D is 900 lbs. The Trade-off is the distance on Piston D is 1/9th the applied distance on Piston C.

(900 lbs)C D

Piston D is 9 times larger

Caliper Disc Brakes

Hydraulic Applications

• Bucket loaders, snow plows, wood splitter also use Hydraulic rams to apply a force to do work.

• Car brakes – Step on the brake and piston in the master cylinder pushes the brake fluid down to the wheel cylinder that pushes the

pistons that expand the brake pads.

Drum Brakes

Link

Pneumatic System

• Use of pressurized gas (air) to transmit and control power– Uses air as the working fluid:

• Air is safe for disposal or in case of leaks• Low cost• Readily available• Lower frictional pressure and power losses• Low pressure applications when compared to

hydraulic systems (high pressure applications). Usually less than 300 psi for pneumatics while for hydraulics can go as high as 12,000 psi.

Pneumatics

• When pressure is put on a gas it will compress.

Liquids can not compress which means more force but need to be closed systems.

What is the difference between a gas and a liquid?

• A device that operates using compressed gas is called a Pneumatics.

Pneumatics systems are fast acting and clean air systems do not need a return (vent at work area)

Vent or Exhaust

Single acting (spring return) Reciprocal or Double acting (air return)

Pneumatic Applications

• Pneumatic Clamps or vices

• Car lifts in garage (pneumatic and Hydraulic)

•Air operated tools: Paint sprayer, pneumatic wrench, Nail guns, etc.

A force provided by an air compressor

Pressure is exerted equally in an enclosed static fluid: Pascal’s Law

The force in the small cylinder must be exerted over a large distance. A small force over a large distance is a tradeoff for a larger force with a short distance.

Although the pressure is the same it is exerted over a larger area of the large piston thus the force is multiplied.

Boundary layer buildup in a pipe

Pipe Entrance

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Because of resistance and surface tension along the walls of a pipe the flow of a fluid will be slower near the wall and faster in the center.

Flow rates in relation to diameter changes

High Pressure Low Pressure

• Speed changes as diameter changes• Pressure decreases with increasing speed

Venturi Effect

An area of low pressure will draw a liquid up a tube.

Bernoulli's Principle states that as the speed of a moving fluid increases, the pressure within the fluid decreases.

Elbow Loss in a Pipe

• A piping system may have many minor losses of flow rates on elbows.

• As a fluid flows around the elbow it causes a area of turbulence just after the turn. In rivers this often leads to a sandbar.

• 45° elbows would have less turbulence than 90°

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