Hydraulic Elevators

The concept of an elevator is incredibly simple -- it's just a compartment attached to a

lifting system. Tie a piece of rope to a box, and you've got a basic elevator.

Of course, modern passenger and freight elevators are a lot more elaborate than this.

They need advanced mechanical systems to handle the substantial weight of the

elevator car and its cargo. Additionally, they need control mechanisms so passengers

can operate the elevator, and they need safety devices to keep everything running

smoothly.

There are two major elevator designs in common use today: hydraulic

elevators and roped elevators.

Hydraulic elevator systems lift a car using a hydraulic ram, a fluid-driven piston

mounted inside a cylinder. You can see how this system works in the diagram below.

The cylinder is connected to a fluid-pumping system (typically, hydraulic systems

like this use oil, but other incompressible fluids would also work). The hydraulic

system has three parts:

A tank (the fluid reservoir)

A pump, powered by an electric motor

A valve between the cylinder and the reservoir

The pump forces fluid from the tank into a pipe leading to the cylinder. When the

valve is opened, the pressurized fluid will take the path of least resistance and return to

the fluid reservoir. But when the valve is closed, the pressurized fluid has nowhere to

go except into the cylinder. As the fluid collects in the cylinder, it pushes the piston

up, lifting the elevator car.

When the car approaches the correct floor, the control system sends a signal to the

electric motor to gradually shut off the pump. With the pump off, there is no more

fluid flowing into the cylinder, but the fluid that is already in the cylinder cannot

escape (it can't flow backward through the pump, and the valve is still closed). The

piston rests on the fluid, and the car stays where it is.

To lower the car, the elevator control system sends a signal to the valve. The valve is

operated electrically by a basic solenoid switch (check out How Electromagnets

Work for information on solenoids). When the solenoid opens the valve, the fluid that

has collected in the cylinder can flow out into the fluid reservoir. The weight of the car

and the cargo pushes down on the piston, which drives the fluid into the reservoir. The

car gradually descends. To stop the car at a lower floor, the control system closes the

valve again.

This system is incredibly

simple and highly effective,

but it does have some

drawbacks. In the next

section, we'll look at the main

disadvantages of using

hydraulics.

How a Jet Engine Works

This is a picture of how the air flows through a jet engine.

Jet engines move the airplane forward with a great force that is produced by a

tremendous thrust and causes the plane to fly very fast.

All jet engines, which are also called gas turbines, work on the same principle. The

engine sucks air in at the front with a fan. A compressor raises the pressure of the air.

The compressor is made up of fans with many blades and attached to a shaft. The

blades compress the air. The compressed air is then sprayed with fuel and an electric

spark lights the mixture. The burning gases expand and blast out through the nozzle, at

the back of the engine. As the jets of gas shoot backward, the engine and the aircraft

are thrust forward.

The image above shows how the air flows through the engine. The air goes through

the core of the engine as well as around the core. This causes some of the air to be

very hot and some to be cooler. The cooler air then mixes with the hot air at the engine

exit area.

A jet engine operates on the application of Sir Isaac Newton's third law of physics: for

every action there is an equal and opposite reaction. This is called thrust. This law is

demonstrated in simple terms by releasing an inflated balloon and watching the

escaping air propel the balloon in the opposite direction. In the basic turbojet engine,

air enters the front intake and is compressed, then forced into combustion chambers

where fuel is sprayed into it and the mixture is ignited. Gases which form expand

rapidly and are exhausted through the rear of the combustion chambers. These gases

exert equal force in all directions, providing forward thrust as they escape to the rear.

As the gases leave the engine, they pass through a fan-like set of blades (turbine)

which rotates the turbine shaft. This shaft, in turn, rotates the compressor, thereby

bringing in a fresh supply of air through the intake. Engine thrust may be increased by

the addition of an afterburner section in which extra fuel is sprayed into the exhausting

gases which burn to give the added thrust. At approximately 400 mph, one pound of

thrust equals one horsepower, but at higher speeds this ratio increases and a pound of

thrust is greater than one horsepower. At speeds of less than 400 mph, this ratio

decreases.

In a turboprop engine, the exhaust gases are also used to rotate a propeller attached to

the turbine shaft for increased fuel economy at lower altitudes. A turbofan

engineincorporates a fan to produce additional thrust, supplementing that created by

the basic turbojet engine, for greater efficiency at high altitudes. The advantages of jet

engines over piston engines include lighter weight with greater power, simpler

construction and maintenance with fewer moving parts, and efficient operation with

cheaper fuel.