The Working Mechanism of Hydraulic Cranes

Hurricane Danny's torrential rains caused severe flooding across Charlotte, N.C., leading to the rapid rise of Little Sugar Creek. The sudden surge weakened a train trestle's foundation, causing a diesel locomotive to plunge into the creek. Weighing 412,000 pounds (186,880 kilograms), the locomotive required significant effort to recover. Post-flood, emergency crews deployed three hydraulic truck cranes—500-ton, 300-ton, and 175-ton models—to hoist the train from the creek and reposition it onto the tracks.

Despite their straightforward design, hydraulic cranes are capable of performing extraordinary feats, such as lifting massive bridge beams, heavy industrial machinery, and even entire beachfront homes onto pilings. These cranes are also instrumental in transporting marine life, like Shamu the killer whale, between locations such as SeaWorld.

Observing a hydraulic truck crane in operation is awe-inspiring, as it effortlessly handles objects weighing several tons. The lifting capacity of these cranes varies, with their names indicating their maximum load—for instance, a 40-ton crane can lift 40 short tons (80,000 pounds or 36,287 kilograms).

Step into the cab, and we’ll guide you through the operation of these powerful machines.

It's All About Hydraulics

As explained in How Hydraulic Machines Work, hydraulic cranes operate on a fundamental principle: transferring force through a fluid. These systems typically rely on incompressible fluids, such as oil, which is widely used in hydraulic cranes. When a piston compresses the oil, the force is transmitted to another piston, causing it to move upward.

In a basic hydraulic setup, pushing one piston down forces another piston up. Click the arrow to see a demonstration.

The pressure driving the pistons is generated by a hydraulic pump. There are two primary types of hydraulic pumps responsible for creating this pressure.

The majority of hydraulic truck cranes rely on two-gear pumps, which utilize interlocking gears to pressurize hydraulic oil. To increase pressure, the operator accelerates the engine via the foot throttle. In gear pumps, achieving high pressure requires running the engine at maximum capacity.

A 70-ton hydraulic truck crane is powered by a 12.7-liter diesel engine, delivering up to 365 horsepower. This engine drives three two-gear pumps, which include:

In the following section, you’ll discover how the hydraulic system interacts with the other components of the hydraulic truck crane.

Parts of a Hydraulic Truck Crane

While the hydraulic system is the core of the truck crane, the machine consists of numerous parts working together to lift loads. Below are the fundamental components of every hydraulic truck crane:

The boom is the most distinctive feature of any crane, serving as the steel arm that supports and lifts heavy loads. Positioned just behind the operator's cab, the boom is the crane's core component, enabling it to hoist objects to impressive heights.

Many hydraulic truck cranes feature booms with multiple telescoping sections. For example, a 70-ton Link-Belt hydraulic truck crane includes a boom with three telescoping sections, extending up to 127 feet (38.7 meters). Some booms are also fitted with a jib, a lattice extension at the boom's end. On the 70-ton crane, the jib adds an extra 67 feet (20.4 meters), bringing the total length to 194 feet (59.1 meters). The telescoping sections expand to reach the required height during lifting operations.

Reinforced-steel cables, each capable of supporting up to 14,000 pounds (6,350 kilograms), run from a winch behind the cab, over the boom and jib. A 70-ton hydraulic truck crane can utilize up to 10 cables, providing a combined capacity of 140,000 pounds (63,503 kilograms), or 70 tons. These cables are anchored to a 285-pound (129 kilograms) metal ball, ensuring they remain taut when no load is attached to the hook.

To position the load accurately, the boom must move both horizontally and vertically. Beneath the cab, a Rotex gear mounted on a turntable bearing rotates at 2 revolutions per minute (rpm). This gear is powered by a bidirectional hydraulic motor, enclosed in a protective metal casing to prevent accidents. The rotation is managed by a foot-operated hydraulic pedal inside the cab.

Hydraulic truck cranes are designed to lift heavy objects to significant heights, requiring the truck to remain perfectly stable during operations. Since the tires alone cannot provide sufficient stability, the crane uses outriggers to maintain balance and prevent tilting. These outriggers, powered by hydraulics, lift the entire truck off the ground, including the tires. Each outrigger consists of a beam (the leg) and a pad (the foot). In some cases, "floats"—typically wooden planks—are placed under the pads to distribute the crane's force and the load's weight more evenly, especially on concrete or pavement.

Outriggers are just one part of the crane's balancing system. Detachable counterweights are also used, positioned at the rear of the crane beneath the cab. These counterweights prevent the crane from tipping forward during lifts. The required counterweight depends on the load's weight, the boom's radius, and its angle during operation. For example, the 70-ton Link-Belt truck crane uses counterweights in 4,000-pound (1,814-kilogram) sections. These counterweights are removed before the truck is driven.

In the Operator's Seat

In the previous sections, you explored how the hydraulic systems and other components of the hydraulic truck crane function. All these systems are managed by the operator inside the cab, situated atop the deck. Using various controls, the operator can raise and lower the boom, rotate the cab and boom, operate the winch, and manage additional equipment.

The 70-ton Link-Belt hydraulic truck crane features two primary types of controls for handling loads:

Joysticks and foot pedals are linked to hydraulic hoses that connect to spool valves and hydraulic rams. The spool valve is connected to the hydraulic pump via a central hose positioned between the two hoses running to the hydraulic ram. Pushing a joystick in a specific direction closes one hose and opens the other, determining whether the piston in the hydraulic ram extends or retracts.

The spool-valve system enables the crane operator to precisely control the machine's movements.

the hydraulic pistons.

Before initiating a lift, the operator inputs critical data into the cab's computer, such as the load's weight and the desired lifting height. This computer acts as a safeguard, alerting the operator if the crane approaches its operational limits. Using a set of charts stored in the cab, the operator calculates the boom's angle and radius. Once these parameters are set, the computer monitors the lift, issuing warnings if the crane nears its capacity. If the boom is raised too high for the load, warning lights for the Load Moment Indicator (LMI) illuminate above the front window.

Properly executing a lift requires at least two additional personnel: the oiler and the signalman. The oiler ensures all crane components are secure and in place before lifting begins, and acts as a spotter during the operation. The signalman communicates with the operator using hand signals to guide the load's movement. Click here to view common hand signals used during lifts.

Hydraulic truck cranes deliver immense power to move heavy objects, machinery, and even large animals with ease. By leveraging the fundamental principles of hydraulics, these machines effortlessly handle thousands of pounds, making them indispensable in construction and a testament to the effectiveness of basic physics.