Construction machine

A construction machine is a piece of equipment used in earthworks, road construction, civil engineering, or building construction, whose primary drive and control functions are based on hydraulic systems. Hydraulic construction machines convert fluid pressure into mechanical force and motion, thereby handling loads that would be virtually impossible to manage with purely mechanical drives.

Hydraulics as a Drive Technology in Construction Machinery

Modern construction machinery would be inconceivable without hydraulics. Hydraulics provides the high power density required to lift heavy loads, loosen soil, or move entire machines via track drives. An internal combustion engine alone cannot directly perform these tasks. It is only through the conversion of mechanical power into hydraulic pressure energy and its distribution via valves and lines that the diverse working movements of a construction machine become possible.

Almost every construction machine features a central hydraulic circuit fed by one or more pumps. The pump draws hydraulic oil from the tank and delivers it under high pressure to the consumers. Depending on the machine type and task, different circuit types are used, ranging from simple open circuits to load-sensing systems that adjust the flow rate as needed, thereby saving energy.

Major Types of Construction Machinery and Their Hydraulic Systems

The world of construction machinery is diverse, and each type of machine places its own demands on the hydraulic system. The following sections provide an overview of the most important types and their hydraulic characteristics.

Hydraulic excavator

The hydraulic excavator is the most common type of construction machinery. Its characteristic working equipment consists of a boom, stick, and bucket, which are moved exclusively by hydraulic cylinders. A double-acting hydraulic system supplies pressure oil to the cylinders on both sides of the piston, enabling powerful forward and backward movements. The rotation of the upper structure is driven by a swing motor, and the track drive is also hydraulically powered. Modern excavators use variable-displacement pumps with load-sensing control to distribute the flow rate as needed and reduce fuel consumption.

Wheel loaders

Wheel loaders primarily use hydraulics for the lifting and tilting movements of the loading bucket. Unlike excavators, the focus here is on fast work cycles. The hydraulic pump must deliver high flow rates so that the cylinder can quickly lift and tilt the bucket. In many wheel loaders, a hydraulic motor also drives the undercarriage, allowing for stepless speed control without a mechanical transmission.

Cranes and Hoists

Cranes are among the construction machines with the highest safety requirements for their hydraulic systems. Here, it is not just about power, but also about precision and reliability. The hoist is often driven by a hydraulic motor, while boom adjustment and outrigger deployment are controlled by cylinders. Brake valves and load-holding valves secure the load in the event of a pressure loss and prevent uncontrolled lowering.

Bulldozers and Rollers

Bulldozers use hydraulic cylinders for adjusting the height and angle of the blade. In rollers, hydraulics are used for steering and vibration. A hydraulic motor drives the flyweight, which causes the roller to vibrate and thus compacts the soil. Operating pressures in these machines are typically in the range of 200 to 350 bar.

Hydraulic Components in Construction Machinery

Regardless of the machine type, the hydraulic systems in construction machinery consist of the same basic components. The selection and sizing of these components determine the machine’s performance.

Hydraulic pumps

The pump is the heart of every hydraulic construction machine. In heavy machinery such as excavators and cranes, variable-displacement axial piston pumps are the norm because they can continuously adjust the flow rate to meet demand. In simpler machines, gear pumps are also used; they are cost-effective and robust but have a fixed displacement volume. Typical operating pressures range from 250 bar in light machines to over 400 bar in hydraulic tools such as demolition hammers.

Hydraulic cylinders

Hydraulic cylinders convert fluid pressure into linear motion. They drive booms, arms, buckets, and outriggers. In construction machinery, double-acting cylinders are used almost exclusively because they must generate force in both directions of movement. The cylinders must withstand high loads: shock loads, lateral forces, and extreme temperature fluctuations on the construction site.

Hydraulic motors

Hydraulic motors generate rotational motion from pressurized oil. In construction machinery, they drive track systems, swing mechanisms, winches, and blowers. Low-speed radial piston motors are used in applications requiring high torque at low speeds, such as cable winches. Axial piston motors cover the medium-speed range and are frequently found in travel and swing drives.

Control Valves

Directional control valves, pressure valves, and flow control valves control and regulate the distribution of hydraulic oil. In modern construction machinery, electrohydraulic proportional valves are increasingly replacing purely mechanical controls. They enable finer control of pressure and flow rate and can be operated via joysticks and electronic control units. Load-sensing valves adjust the system pressure to the highest load pressure, thereby reducing losses during partial-load operation.

Operating Pressures and Performance Classes

The design of the hydraulic system depends on the performance class of the construction machine. Light machines such as mini excavators operate at pressures around 250 bar and flow rates in the single-digit liters-per-minute range. Medium-duty machines such as 20-ton excavators operate at 300 to 350 bar. Heavy construction machinery such as large excavators and crawler cranes reach pressures of 400 bar and above. The system pressure limits are set by pressure relief valves, which are set approximately 5 to 10 percent above the nominal pressure to absorb pressure spikes.

Standards and Safety Regulations

Hydraulic systems in construction machinery are subject to strict standards and regulations because a failure can endanger people and machinery. The central standard for safety requirements for hydraulic systems is DIN EN ISO 4413. It governs the design, control, and protection of hydraulic systems in machinery and is supplemented by DIN EN ISO 13849-1 for safety-related control components.

At the European level, the Machinery Directive 2006/42/EC requires that machinery be designed and constructed in such a way as to prevent hazards caused by hydraulic energy. Annex I, Section 1. 5. 3, specifies the requirements for non-metallic hoses, pressure relief devices, and leakage protection. The new EU Machinery Regulation 2023/1230 expands these requirements and succeeds the previous directive.

In Germany, the Industrial Safety Regulation (BetrSichV) and DGUV Rule 113-020, which governs the handling of hydraulic hoses and hydraulic fluids, also apply. It mandates inspections prior to initial commissioning and at regular intervals. The Pressure Equipment Directive 2014/68/EU may also apply to certain pressure components in hydraulic systems.

Maintenance and Servicing

The reliability of a hydraulic construction machine depends largely on the maintenance of the hydraulic system. Construction site conditions, dust, temperature fluctuations, and mechanical stress take a heavy toll on the components.

Oil Quality and Filtration

The cleanliness of the hydraulic oil is critical to the service life of pumps, valves, and cylinders. Particulate contaminants cause abrasive wear on the fine gaps in proportional valves and pumps. Regular oil analyses and timely filter changes maintain oil purity at the required level. The filter fineness depends on the sensitivity of the components; for proportional valves, filters with a fineness in the single-digit micrometer range are common.

Hose lines

Hydraulic hoses are wear parts that must be inspected and replaced regularly. DGUV Rule 113-020 requires a visual inspection of hose lines for cracks, abrasion, deformation, and leaks. If visible defects are found or if the recommended service life has been exceeded, the hoses must be replaced. A burst in a high-pressure hose can lead to injuries from escaping oil under high pressure and to fires caused by hot hydraulic fluid.

Seals and Wear Parts

Seals in cylinders and valves age due to thermal and mechanical stress. Brittle or deformed seals lead to internal leaks, which reduce the system’s efficiency and can cause the machine to become uncontrollable. Regular inspection and replacement of seals are part of the standard maintenance schedule for every construction machine.

Trends and Developments

Hydraulics in construction machinery continue to evolve, driven by pressure to improve efficiency, emissions regulations, and advances in electronics.

Electrohydraulics and Load Sensing

Electrohydraulic systems replace mechanical control chains with electronic signals and proportional valves. This improves operating precision and enables automated workflows. Load-sensing systems adjust the delivery pressure and flow rate of the pump to the actual demand of the consumers. This significantly reduces losses during partial-load operation and lowers fuel consumption.

Hybrid and electric drives

The first manufacturers are offering construction machinery with electric or hybrid-electric drives. In these machines, an electric motor drives the hydraulic pump instead of a diesel engine. Hydraulics remain the power transmission method because they provide the power density required for earthmoving and lifting operations. The combination of an electric drive and intelligent hydraulic control promises significant reductions in emissions on the construction site.

Digital Monitoring

Sensors measure pressure, temperature, flow rate, and wear condition directly on the hydraulic components. The data is fed into higher-level control systems and cloud platforms, enabling condition-based maintenance. Rigidly scheduled maintenance intervals are thus replaced by a demand-driven approach that reduces downtime and increases the availability of the construction machine.