Elevators move people between floors using one of two basic systems: a traction system that pulls the car up and down with steel ropes, or a hydraulic system that pushes it from below with pressurized fluid. The traction design dominates in mid-rise and tall buildings, while hydraulic elevators are common in low-rise structures of roughly six stories or fewer. Both rely on a surprisingly elegant set of mechanical and electronic components working together.
Traction Elevators: Ropes, Sheaves, and Counterweights
The most common elevator type uses a grooved wheel called a sheave, mounted to an electric motor at the top of the shaft. Steel hoist ropes loop over this sheave with the elevator car hanging on one side and a heavy counterweight hanging on the other. When the motor turns the sheave, friction between the ropes and the grooves moves the car up or down, with the counterweight traveling in the opposite direction.
The counterweight is the key to the system’s efficiency. It typically equals the weight of the empty car plus about 40 to 50 percent of the car’s maximum passenger load. This means that when the car is half full, the system is nearly balanced, and the motor barely has to work. When the car carries less than half its rated load, the counterweight is actually heavier than the car side, so the motor uses less energy lifting passengers up. When the car is more than half full, the car side becomes heavier, and the motor works harder. This balancing act dramatically reduces the energy needed compared to lifting the full weight of the car on every trip.
The rope forces on each side of the sheave are almost never equal. That tension difference is what allows the sheave’s friction to grip the ropes and move them. If both sides weighed exactly the same, the system would be in perfect equilibrium and the ropes could slip freely over the wheel with minimal input.
How Hydraulic Elevators Work
Hydraulic elevators take a completely different approach. An electronic pump pressurizes hydraulic fluid and forces it into a cylinder buried in the ground or mounted beside the shaft. That pressure pushes a piston upward, and the piston lifts the elevator car. To go down, a valve releases pressure from the cylinder, and the car’s own weight pushes the fluid back into a reservoir. There are no ropes or counterweights involved.
The jack module, which holds the piston, cylinder, and jack head, is the central component. Because the piston needs physical room to extend, traditional hydraulic elevators are limited to buildings of about six floors. They also move more slowly than traction elevators, but they’re less expensive to install and don’t require heavy machinery at the top of the building, making them a practical choice for smaller structures like medical offices, parking garages, and low-rise apartments.
The Safety Brake That Started It All
Before 1854, hoisting platforms existed, but nobody trusted them enough to ride in one. If the rope snapped, the platform fell. Elisha Otis changed that by inventing a safety brake: a tough steel wagon-spring that meshed with a ratchet built into the guide rails running along the shaft. If the rope broke, the spring engaged the ratchet and the platform held fast. He demonstrated this publicly by having the rope cut while he stood on the platform. The elevator industry was born.
Modern safety systems have evolved far beyond that original design, but the core idea is identical. Wedge-shaped safety devices sit beneath the car, ready to clamp onto the guide rails and bring everything to a controlled stop if something goes wrong.
Overspeed Governors and Emergency Brakes
Every traction elevator has a speed governor: a spinning mechanism connected to the car by a separate rope. As the car moves, the governor sheave rotates. Inside it, weighted arms respond to centrifugal force. During normal operation, these weights stay tucked in. If the car exceeds a safe speed threshold, the centrifugal force pushes the weights outward, and they engage a locking mechanism.
That locking action triggers a chain reaction. It activates a safety switch or trips a lever that engages the emergency brakes, which clamp directly onto the guide rails and stop the car. In modern systems, the governor also sends electronic signals to the control system to cut motor power simultaneously. The result is two independent stopping forces: the electronic shutdown and the purely mechanical rail clamp. Even in a total power failure, the mechanical system works on its own.
Buffers at the Bottom of the Shaft
At the very bottom of every elevator shaft sit buffers, the last line of defense if all other systems fail. For slow elevators traveling at 1 meter per second or less, these are typically spring buffers or polyurethane buffers that compress to absorb kinetic energy. For faster elevators, hydraulic oil buffers are required. These use fluid resistance to cushion and gradually decelerate the car, providing a smoother stop that would be impossible with a simple spring at higher speeds.
How the Doors Know When to Stop
Elevator doors are powered by a small motor called a door operator, mounted above the car doors. The doors slide on a track, and the car doors mechanically engage the landing doors at each floor so both sets open and close together. This is why landing doors can’t normally be opened from the hallway: they have no motor of their own and only unlock when the car arrives and a clutch mechanism connects the two.
To prevent the doors from closing on passengers, most modern elevators use light curtains. These consist of multiple photoelectric beams that crisscross the door opening, creating an invisible net. A passenger only needs to break a single beam to trigger the doors to reopen. Older elevators used mechanical safety edges, rubber bumpers on the door’s leading edge that reversed the motor when physically pushed. Light curtains are more sensitive because they detect a hand or bag before it contacts the door. Hall effect sensors track the exact position of the doors throughout their travel, telling the control system precisely how far open or closed the doors are at any moment.
Machine Room-Less Elevators
Traditional traction elevators require a dedicated machine room, usually above the shaft, to house the motor, drive sheave, and control equipment. Machine room-less (MRL) elevators eliminate this entirely. The motor and governor are mounted inside the hoistway itself, and the electronic controllers sit in a cabinet on the exterior wall near the shaft. Both traction and hydraulic systems can be configured this way.
The primary advantage is space. In a building where every square foot has value, reclaiming an entire mechanical room is significant. MRL designs also simplify installation for technicians since all the equipment is accessible from the hoistway or the landing. These systems have become the default for new mid-rise construction in many markets.
How Smart Dispatch Systems Assign Cars
In a traditional elevator, you press “up” or “down” and the system sends whichever car can reach you soonest. You select your floor only after you’re inside. Destination dispatch systems flip this process. You enter your desired floor on a keypad or touchscreen in the lobby before you board. The system then tells you which elevator to take.
This seemingly small change unlocks a powerful optimization. Because the system knows everyone’s destination before assigning cars, it groups passengers headed to the same or nearby floors into the same elevator. This reduces the number of stops each car makes during a trip, which shortens round-trip times significantly. In a building with, say, eight elevators, the system can divide the building into zones, with each elevator serving a contiguous range of floors. The maximum efficiency comes from balancing the passenger load across zones so no single elevator is overwhelmed while others sit idle.
The result is handling capacity during the busy morning rush that approaches the efficiency of the evening exodus, when everyone is simply going to the ground floor. In conventional systems, the morning rush is far less efficient because passengers scatter to dozens of different floors. Destination dispatch largely solves this by sorting people before they step into the car.