Brakes
Last updated: December 13, 2009.
You're driving along quite happily
when, all of a sudden, a dog
runs out into the road just in front of you. You have a split second to
react to what's happened. When you stamp on the brakes, you confidently
expect they'll bring you to a halt in time. How can you be so sure?
Because brakes use the power of science and thankfully, for the most
part, science doesn't let us down!
Photo: The brake disc on a car is the small, metal wheel just
inside the silver spokes of the outer, alloy wheel. When you put the brakes on,
a brake pad clamps onto this metal wheel to slow you down.
The science of stopping
If you're moving, you have energy—kinetic
energy to be precise.
Kinetic energy is simply the energy an object possesses because it has
both mass and velocity (speed in a certain direction).
The more mass
you have (effectively, the heavier you are) and the faster you're
going, the more kinetic energy you have.
That's all well and good, but what if you suddenly need to stop? To
change from moving quickly to not moving at all, you have to get rid of
your kinetic energy.
If you're jumping from an aeroplane, the best
way to lose energy is with a parachute. This giant sack of fabric drags
behind you, slowing you down, reducing your velocity, and therefore
helping to get rid of your kinetic energy. That means you can land
safely. Drag-racing cars and land speed record cars also use parachutes
to stop but, in practice, bicycles and cars simply use brakes.
Photo: Coming in to land: a parachute "brake" reduces your velocity
and kinetic energy so you can land more safely.
Photo by Senior Airman Micky Bazaldua courtesy of
US Air Force.
How brakes work on different machines
Bicycle
If you ride a bicycle, you know all
about brakes. If you want to
stop suddenly, you squeeze the brake levers on the handlebars. Thin
metal cables running to the back and front wheels pull on small
calipers, forcing thick rubber blocks to press against the wheels. As
they do so, friction between the blocks and the metal wheel rims
generates heat, reducing your kinetic energy, and bringing you safely
to a stop.
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Steam locomotive
The brakes on a steam locomotive work the same way as a car's and are even more obvious. You can see the brake just behind the wheel in this photo. It clamps against the locomotive's driving wheels to slow them
down. Since there are no tires on the wheels, the friction that stops the train comes from the immense weight of the locomotive pressing the metal wheels down onto the track.
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Motorcycle
Motorcycles typically have disc brakes comprising a rotor and a brake block. The rotor is a disc with holes (or slots) in it mounted on the side of the wheel. A brake block, operated by a cable, jams against the rotor to slow it down by friction. The holes in the rotor help to dissipate the heat generated.
Photo by Matthew Lemieux courtesy of US Marine Corps and Defense Imagery.
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Airplane
Airplanes have brakes inside their wheels to help bring them to a stop on the runway, but they can also use
air brakes to
increase drag (air resistance) and slow themselves down—a bit like parachutes.
Photo by Kenny Holston of US Air Force (follow this link for a much bigger version of this picture).
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Wind turbine
Wind turbines have brakes to stop their rotors (propellers) turning too quickly. The brake
is mounted inside the nacelle (the square-shaped casing behind the propeller that contains the gearbox and generator).
Most turbines have an anemometer on them to measure the wind-speed. If it rises above a safe level,
the brakes come on automatically and bring the rotors to a standstill. It's a shame, because higher wind speeds
mean more energy could be produced. But safety always comes first!
Photo by Matthew Bates courtesy of US Air Force (follow this link for a much bigger version of this picture).
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Car brakes: disc, drum, and hand brakes
Most cars have two or three different types of braking systems. Peer
through the hubcap of a car's front wheels and you can usually see a
shiny metal disc just inside. This is called a disc brake. When
the driver steps on the brake pedal, a pad of hard-wearing material clamps
onto the brake disc and rubs it to make it slow down—in a similar way
to bicycle brakes.
Some cars have disc brakes on all four wheels, but many have drum
brakes on the back wheels, which work in a slightly different way.
Instead of the disc and brake block, they have shoes inside the hollow
wheel hub that press outwards. As the shoes push into the wheel, friction
slows you down.
A car's handbrake applies the two rear brakes (disc or drum)
in a slower, less forceful way when you pull on a lever located between the
front seats.
A speeding car has loads of energy and, when you
stop, virtually all of it is converted into heat in the brake pads. The brakes
can heat to temperatures of 500°C (950°F) or more! That's why
brakes have to be made of materials that won't melt, such
as alloys, ceramics, or composites.
How hydraulic pipes increase your braking power
Imagine how much force you need to stop a fast-moving car. Simply
pressing with your foot would not generate enough force to apply all
four brakes hard enough to bring you quickly to a stop. That's why
brakes use hydraulics: a system of
fluid-filled pipes that can multiply
force and transmit it easily from one place to another.
When you press on the brake pedal, your foot moves a lever that
forces a piston into a long, narrow cylinder filled with hydraulic
fluid. As the piston plunges into the cylinder, it squirts hydraulic
fluid out through a long and narrow pipe at the end (much like
squirting a syringe). The narrow pipe feeds into much wider cylinders
positioned next to the car's four brakes. Because the cylinders near
the brakes are much wider than the one near the brake pedal, the force
you originally applied is multiplied greatly, clamping the brakes hard
to the wheels.
Caption: When your foot presses the brake lever, brake fluid
squeezes out of a narrow cylinder, through a tube, into a much wider cylinder.
This system, known as hydraulics, greatly increases the pushing force.
