Hydraulic Elevators

In the 1800s, new iron and steel production processes revolutionized the world of construction. With sturdy metal beams as their building blocks, architects and engineers could erect monumental skyscrapers hundreds of feet in the air.

But these towers would have been basically unusable if it weren't for another technological innovation that came along around the same time. Modern elevators are the crucial element that makes it practical to live and work dozens of stories above ground. High-rise cities like New York absolutely depend on elevators. Even in smaller multi-story buildings, elevators are essential for making offices and apartments accessible to handicapped people.

In this text, we'll find out how these ubiquitous machines move you from floor to floor. We'll also look at the control systems that decide where the elevator goes and the safety systems that prevent catastrophes.

The concept of an elevator is incredibly simple - it's just a compartment attached to a lifting system. Tie a piece of rope to a box, and you've got a basic elevator.

Of course, modern passenger and freight elevators are a lot more elaborate than this. They need advanced mechanical systems to handle the substantial weight of the elevator car and its cargo. Additionally, they need control mechanisms so passengers can operate the elevator, and they need safety devices to keep everything running smoothly.

There are two major elevator designs in common use today: hydraulic elevators and roped elevators.

Hydraulic elevator systems lift a car using a hydraulic ram, a fluid-driven piston mounted inside a cylinder. The cylinder is connected to a fluid-pumping system (typically, hydraulic systems like this use oil, but other incompressible fluids would also work). The hydraulic system has three parts: a tank (the fluid reservoir), a pump, powered by an electric motor, a valve between the cylinder and the reservoir.

The pump forces fluid from the tank into a pipe leading to the cylinder. When the valve is opened, the pressurized fluid will take the path of least resistance and return to the fluid reservoir. But when the valve is closed, the pressurized fluid has nowhere to go except into the cylinder. As the fluid collects in the cylinder, it pushes the piston up, lifting the elevator car.

When the car approaches the correct floor, the control system sends a signal to the electric motor to gradually shut off the pump. With the pump off, there is no more fluid flowing into the cylinder, but the fluid that is already in the cylinder cannot escape (it can't flow backward through the pump, and the valve is still closed). The piston rests on the fluid, and the car stays where it is.

To lower the car, the elevator control system sends a signal to the valve. The valve is operated electrically by a basic solenoid switch. When the solenoid opens the valve, the fluid that has collected in the cylinder can flow out into the fluid reservoir. The weight of the car and the cargo pushes down on the piston, which drives the fluid into the reservoir. The car gradually descends. To stop the car at a lower floor, the control system closes the valve again. This system is incredibly simple and highly effective, but it does have some drawbacks.

The main advantage of hydraulic systems is they can easily multiply the relatively weak force of the pump to generate the stronger force needed to lift the elevator car.

But these systems suffer from two major disadvantages. The main problem is the size of the equipment. In order for the elevator car to be able to reach higher floors, you have to make the piston longer. The cylinder has to be a little bit longer than the piston, of course, since the piston needs to be able to collapse all the way when the car is at the bottom floor. In short, more stories means a longer cylinder.

The other disadvantage of hydraulic elevators is that they're fairly inefficient. It takes a lot of energy to raise an elevator car several stories, and in a standard hydraulic elevator, there is no way to store this energy. The energy of position (potential energy) only works to push the fluid back into the reservoir. To raise the elevator car again, the hydraulic system has to generate the energy all over again.

1. Tie a piece of rope to a _______, and you've got a basic elevator.

2. The concept of an elevator is incredibly simple - it's just a _______ attached to a lifting system.

3. The _______ is operated electrically by a basic solenoid switch.

4. The pump forces fluid from the _______ into a pipe leading to the cylinder.

5. To lower the _______, the elevator control system sends a signal to the valve.

6. Hydraulic system is incredibly simple and highly effective, but it does have some _______.

7. The other disadvantage of hydraulic _______ is that they're fairly inefficient.