Hydraulic machinery

Hydraulic machinery uses liquid fluid power to perform work.

These machines are essential for heavy construction and industrial tasks. They rely on an incompressible liquid, meaning the fluid cannot be squished like a gas. This characteristic allows for high power density and massive force multiplication. By pushing fluid through small tubes and flexible hoses, machines can move incredibly heavy loads. This ability to transfer large amounts of power through small spaces is a primary reason for their popularity.

At the heart of these machines is Pascal's law. This scientific principle states that any pressure applied to a fluid inside a closed system transmits that pressure equally in all directions. To move a load, a hydraulic power pack pushes fluid through a hydraulic circuit. This circuit is a network of interconnected components like pumps, pipes, tubes, and hoses. The fluid travels to an actuator, such as a hydraulic motor or a hydraulic cylinder. After performing work, the fluid must return to a reservoir to be filtered and re-pumped.

One of the most important features is force and torque multiplication. This can be achieved by changing the effective areas of two connected cylinders. For example, if cylinder C1 has a one-inch radius and cylinder C2 has a ten-inch radius, the force is multiplied significantly. Because the area of a circle is calculated as $\pi r^2$, the larger cylinder has a hundred times the area. If you apply 10 lbf to the small cylinder, the large one exerts 1000 lbf. However, there is a physical trade-off: you must move the small cylinder 100 inches to move the large one just one inch.

This multiplication also works with rotary components through a hydrostatic transmission. If a pump has a displacement of 10 cc/rev and a motor has 100 cc/rev, the torque is multiplied. The motor provides ten times the torque required to drive the pump, but its shaft speed is only one-tenth of the pump's speed. This is essentially a hydraulic "gear ratio." Engineers often combine these hydraulic ratios with mechanical gear ratios to optimize machine designs. This is common in the boom movements and track drives of large excavators.

Engineers design different types of hydraulic circuits to suit specific needs. An open center circuit uses pumps that provide a continuous flow. When the control valve is centered, it provides an open return path to the tank so the fluid is not at high pressure. In contrast, a closed center circuit supplies full pressure to the valves even when they are not being used. These systems can use variable pumps that change their flow rate.

Another type is the closed loop circuit, often used in mobile applications. In these, the motor-return is connected directly back to the pump-inlet. This setup requires a small charge pump to keep the low-pressure side supplied with filtered oil.

Closed loop systems offer a stepless gear ratio, allowing for continuously variable speed and torque. This provides much more flexible control than traditional mechanical transmissions. However, these systems face challenges with heat. At high vehicle speeds, motor speeds can reach 4000 to 5000 rev/min, which causes significant losses in the motor housing. To prevent high oil temperatures from reducing the transmission's lifetime, operators may need to lower the system pressure during transport. It is recommended to keep transport pressure around 200 to 250 bar.

The history of hydraulics is filled with significant breakthroughs. Joseph Bramah patented the hydraulic press in 1795. Later, Henry Maudslay suggested using a cup leather packing, which improved the press's results. This technology was so powerful that it eventually replaced the steam hammer for metal forging. In the past, large central station hydraulic systems provided power for cranes in European ports. The largest such system was located in London. These systems even operated canal locks, elevators, and rotating bridge sections.

Today, the field is shaped by many specialized tools and figures. Harry Franklin Vickers is known as the "Father of Industrial Hydraulics" by the ASME. Modern earth-moving machines often use an "inch pedal" to manage power. This allows a driver to increase diesel engine rpm while reducing vehicle speed. This increases the available hydraulic power for working at low speeds. While hydrostatic transmissions are great for many tasks, they are often limited to about 200 kW. For even larger machines, like massive wheel loaders, engineers often use converter transmissions instead.