Hydraulic motor: linear and radial movements

Generally speaking, a motor is a device that converts power into motion. In hydraulics, such a device is called a “hydraulic motor” in accordance with the power used—i.e., a hydraulically operated drive machine. A distinction is made between two types of hydraulic motors:

• Linear motors
• Radial motors

Hydraulic motor as a linear motor for directed movements

Linear motors refer to hydraulic cylinders. They convert applied hydraulic pressure into a linear movement. In addition to the very high force with which linear motors can be loaded, their precise controllability is a particularly notable feature.

This distinguishes them, for example, from linear motors in pneumatics. Due to the compressibility of gases or compressed air, pneumatic motors always have a certain cushioning effect. This effect is eliminated in hydraulic linear motors due to the incompressibility of liquids.

In simple and small systems, the required pressure is built up by a master cylinder. This usually narrow but long cylinder moves into a tank. The resulting overpressure is transmitted to the extending slave cylinder, which then extends proportionally to the cross-section of the master cylinder. The force also increases proportionally to the cross-sectional area. This design is found, for example, in hydraulic, manually operated jacks.

In larger hydraulic systems, the pressure for the linear motors is applied via a hydraulic pump. Depending on the pressure level used, different designs are employed.

The simplest hydraulic pumps for relatively low-power systems are vane pumps. They consist of two interlocking rotors.

The most powerful hydraulic pumps are piston pumps. They are offered as radial piston pumps or axial piston pumps and can generate pressures of up to several thousand bar. The connected linear motors are correspondingly powerful. They are used, for example, for scrap presses in the recycling industry, for heavy lifting platforms, or as actuators in heavy industry.

Hydraulic motor as a radial motor for maximum torque and high efficiency

Radial motors are hydraulic motors driven by hydraulic pressure. In terms of their design, they are practically reversed hydraulic pumps.

Simple radial motors also have a vane-based mechanism. They are used for non-critical applications, such as driving a fan wheel. For powerful rotary motion, however, the “orbit motor” design has become established. This hydraulic motor is suitable for converting the pressure present in the hydraulic system into maximum torque. Orbit motors are particularly often used as drives for the undercarriages and slewing gears of construction machinery.

Maximum efficiency through the consistent use of hydraulic motors

Every form of energy experiences heat loss when converted into another form of energy. The chemically bound energy of fuel can only be converted into mechanical kinetic energy by an internal combustion engine at a rate of about 50–70%.

If this mechanical energy is converted back into electrical energy by connecting a generator, energy is again lost in the form of heat. If this electrical energy is converted back into mechanical energy by an electric motor, this also cannot be done without a loss of performance.

It is therefore advisable to execute as many forms of movement as possible using hydraulic motors in hydraulic systems. At the manufacturer “SENNEBOGEN – Baumaschinen”, practically everything that is in motion on a material handler or excavator is driven by a hydraulic motor. This even applies to the drives for the radiator fans. With such consistent use of hydraulic motors, maximum efficiency of the entire unit is achieved.