You’re probably already familiar with some of the basic ways a hydraulic system works. From your experience, you probably know that solids are typically impossible to squish. If you pick up a solid object like a pen or piece of wood and try to squeeze it, nothing’s going to happen to the materials. They won’t compress or squish. Liquid works in the same way. It is incompressible, meaning it won’t squeeze when you apply pressure to it. It takes up the same amount of space as it did when pressure wasn’t applied to it. Picture water in a syringe. If you cap the end of it with your finger and try to press down, neither the water nor the plunger will go anywhere.

Where hydraulics are concerned, that incompressibility is a major player in making them work. In that same syringe, if you press down on the plunger normally, you’ll release the water at high speed through the narrow end, even if you didn’t apply that much pressure. When you push down the plunger, you apply pressure to the water, which will try to escape however it can — in this case, at high pressure through a very narrow exit. This application shows us that we can multiply force, which we can then use to power more complex devices.

Open and closed systems of hydraulics refer to different ways of reducing pressure to the pump. Doing this can help reduce any wear and tear.

In an open system, the pump is always working, moving oil through the pipes without building up pressure. Both the inlet to the pump and the return valve are hooked up to a hydraulic reservoir. These are also called “open center” systems, because of the open central path of the control valve when it is neutral. In this case, hydraulic fluid returns to the reservoir. The fluid coming from the pump goes to the device and then returns to the reservoir. There may also be a relief valve in the circuit to route any excess fluid to the reservoir. Filters are usually in place to keep the fluid clean.

Open systems tend to be better for low-pressure applications. They also tend to be cheaper and easier to maintain. One caution is that they can create excess heat in the system if the pressure exceeds valve settings. Another location for added heat is in the reservoir, which needs to be big enough to cool the fluid running through it. Open systems can also use multiple pumps to supply power to different systems, such as steering or control.

Almost all hydraulic pumps are positive displacement pumps, meaning they deliver a precise amount of fluid. They can be used in high-power applications of over 10,000 psi. Non-positive displacement pumps depend on pressure for the amount of fluid they move, while positive displacement pumps do not. Non-positive pumps are more common in pneumatics and low-pressure applications. They include centrifugal and axial pumps.

Positive displacement pumps can have either fixed or variable displacement. Most pumps fall under fixed displacement.

• In fixed displacement, the pump provides the same amount of fluid in each pump cycle.
• In variable displacement, the pump can provide different amounts of fluid based on the speed it is run at or the physical properties of the pump.

A gear pump is inexpensive and more tolerant of fluid contamination, making them suitable for rough environments. They may be less efficient, however, and wear more quickly.

• External gear pumps: These make use of two tight-meshed gears within a housing. One is the driving, or powered, gear, while the other is driven, or free-flowing. The fluid is trapped in the space in between the gears and rotated through the housing. Since it cannot move backward, it is forced through the outlet pump.
• Internal gear pump: The internal gear design places an inner gear, possibly with a crescent-shaped spacer, inside of an outer rotor gear. The fluid is moved via eccentricity — the deviation of the gear from circularity — between the gears. The inner gear, with fewer teeth, turns the outer gear, and the spacer goes in between them to create a seal. The fluid is drawn in, moved through the gears, sealed up and discharged.