Metro elevators can be classified as either electric traction or what other type?

Electric traction elevators are commonly used in modern buildings due to their efficiency and speed. The other principal type of elevator is the hydraulic elevator. Hydraulic systems operate using fluid pressure, which is used to move a piston inside a cylinder. This mechanism allows the elevator car to be raised or lowered smoothly.

Hydraulic elevators are particularly beneficial for low-rise buildings, typically up to five stories, where the cost and complexity of installing a traction system may not be justified. They use a motor to pump hydraulic fluid into the cylinder, which then lifts the car. When the fluid is released, the car descends, thanks to the force of gravity. The design of hydraulic elevators often includes safety features such as a safety valve to prevent uncontrolled descent in case of a system failure.

In summary, hydraulic elevators represent a distinct category of elevator systems, contrasting with electric traction elevators by utilizing hydraulic fluid and pistons rather than cables and pulleys, making them a relevant alternative in building design and functionality.