How elevators work

Elevators work because of several different systems: gravity, electrical power and pulleys. The trick is how they find each floor without falling.

In the earliest days of passenger elevators, a trained elevator operator would be employed to accommodate users. Would-be passengers would activate a mechanical or rudimentary electrical signal and the operator would manually throw a switch to activate the elevator's motor. There were no buttons for individual floors- the operator had to be told which floors to visit during each run. Two separate gates would have to be opened in order for passengers to disembark. The elevator operator would literally spend hours riding from the lobby to the highest floors and back each day.

Modern elevator technology has rendered the position of elevator operator nearly obsolete. Some hotels and high-rise office buildings may still assign employees to assist elevator passengers, but their role is now primarily security or service-oriented.

Modern elevator systems work on a number of engineering principles. Physically, an elevator car is attached to several thick metal cables which are part of an elaborate pulley system. Contrary to popular belief, an elevator motor does not 'winch' each car up and down the cables- such an operation would put tremendous stresses on the braking system and the motor's gears. Instead, gravity assists the electrical winches in the form of counterweights. Heavy lead or steel plates are attached to the other side of a pulley system. As the elevator car rises, the winches pull the cables with some force but the counterweights also fall towards the ground. The total weight capacity of an individual car is usually counterbalanced by a nearly-equivalent amount of metal weights. This counterweight rides inside its own shaft near the cars- waiting passengers can often feel a breeze as the counterweights pass their floor.

Whenever a passenger presses the call button for an elevator, a computer receives the request and logs it for future reference. Most older elevator systems still work on the principle of moving one direction at a time. Elevators heading to higher floors are programmed to ignore calls for lower floors until they have reached the top. Once the car changes direction, the computer orders it to stop at the next down call. An elevator car can recognized individual floors by reading holes in a strip mounted on the sides of the shaft. Once the sensors detect a certain number of holes have been passed, they send a signal to stop the car at the appropriate floor. The computer clears the call signal from its memory and then sends out a command to two electric motors which control the doors.

There are actually two sets of doors which must work in tandem in order to safely allow passengers to exit and enter the cars. One set of doors remains shut until an elevator car's presence is detected. This keeps would-be passengers from walking into an open elevator shaft. The other door is controlled by the elevator's computer and opens sideways as a powerful electric motor pulls the first panel. Once both doors are open, passengers should leave quickly to allow new passengers to board and more calls to be answered. Elevator doors also contain motion detectors and other presence-sensing devices to keep doors from closing prematurely or trapping passengers.

Some newer elevator systems have abandoned the 'either up or down, no exceptions' philosophy. Information from passengers is analyzed by the computer and cars are dispatched to handle the most common destinations at the busiest times, regardless of the direction. If a business on the third floor takes a lunch break at noon and generally goes to the cafeteria on the fifth floor, then more elevator cars are assigned to the third floor at noon, and they will go to the fifth floor regardless of the general direction of the other requests. Some ultramodern systems will also suggest specific cars to take for faster service.

Many elevator riders are curious about the weight capacity of individual cars, especially when they become very crowded. Load sensors located under the floor can weigh each car after stops. When the car has reached its maximum capacity, the computer will often ignore incoming calls until enough passengers leave. The weight of passengers alone would rarely cause elevators to fall- the capacity limits are intended more for passenger comfort and the adjustment of counterweights. In order for a modern elevator to actually fall, many safety systems would have to fail all at once. Once the computer senses an unplanned release from the main cables, a set of emergency brakes is immediately deployed. The car will not move from that location on its own. Even if the brakes did fail mechanically, the counterweights would assist to slow the car's descent. Even a complete power failure would not compromise an elevator's safety measure.

Perhaps the most common problems elevator passengers face are temporary power failures and misalignments between the elevator and the intended floor. Most cars have direct phone lines to qualified technicians or emergency workers, so the only thing left to do is wait for assistance. In the case of a misalignment, a trained elevator technician should be informed of the problem. Getting out of an elevator stuck between floors may be a dangerous thing to do, because the car may decide to move on to the next assignment unexpectedly.