What Happens When Elevator Cables Break?

Elevators cables are inspected on the regular, but what happens when one breaks? Learn about the ingenious systems that keep elevators safe when even the worst happens in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:02):
Welcome to brain Stuff from How Stuff Works. Hey, brain Stuff,
I'm Lauren vog O Bomb and you may have heard
about it in the news. In November, six people board
at an elevator at the former John Hancock Center in
Chicago for the ride down from the signature Room bar
on the ninety floor to the lobby, but one of
the cables snapped and the elevator plunged eighty four floors

(00:23):
to the eleventh floor. Amazingly, none of the passengers had
to be hospitalized and there were no serious injuries. The
passengers thought they had only fallen a few floors. However,
they did have to wait three hours to be rescued
by firefighters because there were no openings between the floors.
So how was it possible that one of the worst
things that can happen to people in an elevator occurred
and everyone survived. Elevators in the real world have so

(00:46):
many safety features that the kind of thing you see
in movies where a villain cuts a single cable and
disaster ensues usually never happens. Here's the breakdown. First, let's
look at those cables and a cable elevator system. Steel
cables bolted to the car loop over a sheave. A
sheave is a pulley with a grooved rim surface at
the top of the elevator shaft. The sheaves grooves grip

(01:08):
the steel cables, so when an electric motor rotates the sheave,
the cables move to. The cables that lift the car
are also connected to a counterweight, which hangs down on
the other side of the sheave. The car and the
counterweight both right along on steel rails. Each elevator cable
is made from several lengths of steel material wound around
one another. These cables very rarely snap, and inspectors look

(01:30):
at them for wear and tear. But even a steel
cable can break. So what happens then? Almost all pulley
elevators have multiple cables, between four and eight in total.
Even if one cable snapped, the remaining cables would hold
the elevator car up. In fact, just one cable is
usually enough. But let's say all the cables did snap,
then the elevator's safeties would kick in. Safeties are braking

(01:54):
systems on the elevator car that grab onto the rails
running up and down the elevator shaft. Some safeties clamped
the rails, while others drive a wedge into notches in
the rails. Typically, safeties are activated by a mechanical speed governor.
The governor is a pulley that rotates when the elevator moves.
When the governor spins too fast, the centivigal force activates
the braking system. Even if the cables and the safeties

(02:17):
all failed, sure, you would be plumbering rapidly, but you
wouldn't quite be in free fall. Friction from the rails
along the shaft and pressure from air underneath the car
would slow the car down considerably, though you would feel
a bit lighter than normal. On impact, the car would
stop and you would keep going, slamming you into the floor.
But two things would cushion that blow. First, the elevator

(02:39):
car would compress air at the bottom of the shaft
as it fell, just as a piston compresses air in
a bicycle pump. The air pressure would slow the elevator
car down. Second, most cable elevators have a built in
shock absorber at the bottom of the shaft, typically a
piston in an oil filled cylinder, that would cushion the
impact too. With all of these features in place, you
would have an slint chance of surviving any elevator mishap.

(03:03):
In the case of the Chicago elevator incident, once the
firefighters figured out where the passengers were, the crew put
up struts to make sure the elevator did not drop
any further. Then they broke through a wall, forced to
the elevator door open, and put a ladder into the
elevator to help people up and out. Chicago Fire Department
spokes been Larry Langford told the Chicago Tribune, we don't

(03:24):
like to have to go through walls unless it's absolutely necessary.
The only other way to get to the elevator would
have been ropes from the floor, and that would not
be safe. We don't come down like Batman, so we
must go through the wall. You sometimes hear that you
should jump immediately before an elevator crashes, so that you
would be floating at the second of impact. Would that work. Nah,

(03:44):
Even if you could perfectly time such a leap, it
wouldn't help. Let's say you and the elevator are falling
at a hundred miles per hour. That's around a hundred
and sixty one kilometers per hour unless you have some
superhero powered legs. When you jump up in the elevator,
you'd still be going about a hundred miles per hour,
and then you would hit the ground at a hundred
miles per hour, just like the elevator. Your best bet

(04:06):
would be to lie flat on the floor. This would
stabilize you and spread out the force of the impact
so that no single part of your body would take
the brunt of the blow. Today's episode was written by
Katherine Whitbourne and produced by Tyler Clang. For more on
this and lots of other well backed up topics, visit
our home planet, how Stuff Works dot com.