Have you ever seen an action movie where the hero gets in an elevator, but the evil villain has cut the cables? The elevator plummets dozens of stories and disintegrates in a ball of fire on impact. There's even a ride at Disney Land, called "Tower of Terror," where you can actually take a ride in a runaway elevator that falls 13 floors!
Fortunately, elevators in the real world have so many safety features that this kind of stuff never happens. Here's the breakdown:
Snapping Cables
In 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 sheave's grooves grip the steel cables. So when an electric motor rotates the sheave, the cables move too. 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 ride 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 regularly look 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 total. Even if one cable snapped, the remaining cables would hold the elevator car up. In fact, just one cable is enough.
Safeties and Governor
But let's say all the cables did snap. Then the elevator's safeties would kick in. Safeties are braking systems on the elevator car that grab onto the rails running up and down the elevator shaft. Some safeties clamp 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 centrifugal force activates the braking system.
At the Bottom
If the safeties failed, you would be plummeting rapidly, but you wouldn't quite be in a free fall. Friction from the rails along the shaft and pressure from the air underneath the car would slow the car down considerably (you would feel lighter than normal though). On impact, the car would stop and you would keep going, slamming you into the floor.
But two things would cushion the blow. First, the elevator car would compress the 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 these features in place, you would have an excellent chance of surviving any elevator mishap.
Don't Bother JumpingYou sometimes hear that you should jump immediately before the elevator crashes, so you would be "floating" at the second of impact. Would it work? Nah. Even if you could perfectly time such a leap, it wouldn't help. Let's say you and the elevator are falling at 100 MPH. When you jump up in the elevator, you would still be going about 100MPH. You would hit the ground at 100MPH, just like the elevator. That's gonna hurt! Your best bet 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. But, it's still gonna hurt!
قناة التقنية
كيف يعمل نظام الامان في المصعد عندما ينقطع الحبل 
العرض العادي
