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Introduction
Hydraulic elevator
Cable system elevator
The most popular elevator design is the roped elevator. In roped elevators, the car is raised and lowered by traction steel ropes rather than pushed from below.
­ ­ The ropes are attached to the elevator car, and looped around a sheave . A sheave is just a pulley with a grooves around the circumference. The sheave grips th­e hoist ropes, so when you rotate the sheave, the ropes move too.
The sheave is connected to an electric motor . When the motor turns one way, the sheave raises the elevator; when the motor turns the other way, the sheave lowers the elevator. In gearless elevators, the motor rotates the sheaves directly. In geared elevators, the motor turns a gear train that rotates the sheave. Typically, the sheave, the motor and the control system are all housed in a machine room above the elevator shaft.
Safety system
Elevators are built with several redundant safety systems that keep them in position.
 The first line of defense is the rope system itself. Each elevator rope is made from several lengths of steel material wound around one another. With this sturdy structure, one rope can support the weight of the elevator car and the counterweight on its own. But elevators are built with multiple ropes (between four and eight, typically). In the unlikely event that one of the ropes snaps, the rest will hold the elevator up.
Even if all of the ropes were to break, or the sheave system were to release them, it is unlikely that an elevator car would fall to the bottom of the shaft. Roped elevator cars have built-in braking systems, or safeties, that grab onto the rail when the car moves too fast.
Control system
Many modern elevators are controlled by a computer. The computer's job is to process all of the relevant information about the elevator and turn the motor the correct amount to put the elevator car where it needs to be. In order to do this, the computer needs to know at least three things.
* Where people want to go
* Where each floor is
* Where the elevator car is
Most systems also have a load sensor in the car floor. The load sensor tells the computer how full the car is. If the car is near capacity, the computer won't make any more pick-up stops until some people have gotten off. Load sensors are also a good safety feature. If the car is overloaded, the computer will not close the doors until some of the weight is removed.