How Hydraulic Cranes Work

By: Kevin Bonsor

In the Operator's Seat

The crane is operated by hydraulic joysticks and foot pedals.

In the previous two sections, you learned how the hydraulics and other pieces of equipment on the hydraulic truck crane work. All of this equipment is controlled by the operator inside the cab, which is located on top of the deck. Crane operators use several control mechanisms to raise and lower the boom, rotate the cab and boom, wind and unwind the winch and control other peripheral equipment.

The 70-ton Link-Belt hydraulic truck crane has two basic types of controls for maneuvering a load:


  • Joysticks - There are two joysticks in the cab. One controls left-to-right movement of the boom, and the other controls forward and aft movement.
  • Foot pedals - These pedals are responsible for retracting and extending the telescoping sections of the boom. They also control the amount of pressure being generated by the pump.

Joy sticks and foot pedals are connected to hydraulic hoses that connect various hydraulic rams to spool valves. The spool valve is connected to the hydraulic pump via a third hose that is placed between the two hoses that run from the spool valve to the hydraulic ram. When a joystick is pushed in one direction, it causes the valve to shut off one of the hydraulic hoses leading to the ram and open the other. Which way the joystick is pushed determines whether the piston in the hydraulic ram slides in or out.

The spool-valve system lets the crane operator control

the hydraulic pistons.

Prior to any lift, the operator enters data into a computer inside the cab, including the weight of the object to be lifted and the height to which it is to be lifted. This computer serves as the operator's backup, warning the operator if the crane is being pushed beyond its capability. Using a binder of charts in the cab, the operator also determines the angle of lift and the radius of the boom. Once all of this is entered, the computer can track the progress of the lift and warn the driver if the crane is nearing its limitations. If the boom is lifted too high for the load amount, a series of lights just above the inside of the front window will begin to light up. These lights are the warning lights for the Load Moment Indicator (LMI).

If the operator tries to lift a load too high, the Load Moment Indicator will light up.

There are at least two other people needed to perform a lift properly, including the oiler and the signalman. The oiler is responsible for making sure that all of the crane's parts are in place and secured prior to any lift. He or she also acts as a spotter during a lift to ensure that the lift is being performed properly. The signalman, as the name suggests, gives hand signals to the operator during the lift to make sure the load is being maneuvered correctly. Click here to see some of the hand signals used during a lift.

Hydraulic truck cranes provide brute strength to move objects, machines and even large animals that would otherwise be very difficult to budge. Using a very simple principle of hydraulics, these machines move thousands of pounds with relative ease, making them an essential component of most construction projects and a great example of the power of basic physics.

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