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Hydraulic drive system

A hydraulic or hydrostatic drive system or hydraulic power transmission is a drive- or transmission system that makes use of a hydraulic fluid under pressure to drive machinery.

Such a system basically consists of:

Generator part of the transmission, in general a hydraulic pump, driven by an electric motor, a combustion engine or a windmill.
Valves, filters, piping etc. to guide and control the system
Motor part of the transmission a hydraulic motor or hydraulic cylinder to drive the machinery.

Hydrostatic means that the energy comes from the flow and the pressure, but not from the kinetic energy of the flow.

Principle of a hydraulic drive

Pascal's law is the basis of hydraulic drive systems. As the pressure in the system is the same, the force that the fluid gives to the surroundings is therefore equal to pressure x area. In such a way, a small piston feels a small force and a large piston feels a large force.
The same counts for a hydraulic pump with a small swept volume, that asks for a small torque, combined with a hydraulic motor with a large sweptvolume, that gives a large torque.

In such a way a transmission with a certain ratio can be built.

Most hydraulic drive systems make use of hydraulic cylinders. Here the same principle is used- a small torque can be transmitted in to a large force.

By throttling the fluid between generator part and motor part, or by using hydraulic pumps and/or motors with adjustable swept volume, the ratio of the transmission can be changed easily. In case throttling is used, the efficiency of the transmission is limited; in case adjustable pumps and motors are used, the efficiency however is very large. In fact, up to around 1980, a hydraulic drive system had hardly any competition from other adjustable (electric) drive systems.

Nowadays electric drive systems using electric servo-motors can be controlled in an excellent way and can easily compete with rotating hydraulic drive systems. Hydraulic cylinders are in fact without competition for linear (high) forces. For these cylinders anyway hydraulic systems will remain of interest and if such a system is available, it is easy and logical to use this system also for the rotating drives of the cooling systems.

Hydraulic pump

For a hydraulic pump, it is of interest to know what pressures and sizes are available. The smallest gear pumps (except miniature ones) have a swept volume of 1 cm³ and the largest axial plunger pump that is available from stock will have a swept volume of 1000 cm³ (2000 cm³ might be possible for a special delivery). Other pumps are between these two figures. For continuous hydraulic drives, the maximum working pressure will be some 200 bar. The pumps may be used up to 450 bars, but at these pressures special components are required and pump life is limited.

Low-cost hydraulic drives on garbage trucks may use gear pumps with maybe 180 bars, depending on the number of working hours. Small pumps can rotate at high speeds of say 3000-5000 rpm, whereas large pumps only can rotate at 1200 or 1500 rpm. Sometimes it is important to use multiple pumps, so that each motor or cylinder within the hydraulic drive system can have his own pump. In case an adjustable speed is required, it is often better to use adjustable pumps in order to save energy (no throttling). For constant power or constant pressure systems also adjustable vane pumps are used. The simple control system decreases the swept volume automatically if the pressure that is asked for increases.

Sometimes closed loop systems are used, in that case normally axial plunger-pumps are used (with separate leakage connection). These systems can cope with negative forces and because of the boost pressure, the pumps can be rotated at a higher speed.

Hydraulic cylinder

Hydraulic cylinders (also called linear hydraulic motors) are mechanical actuators that are used to give a linear force through a linear stroke. A hydraulic cylinder is without doubt the best known hydraulic component. Hydraulic cylinders are able to give pushing and pulling forces of millions of metric tons, with only a simple hydraulic system. Very simple hydraulic cylinders are used in presses; here the cylinder consists out of a volume in a piece of iron with a plunger pushed in it and sealed with a cover. By pumping hydraulic fluid in the volume, the plunger is pushed out with a force of plunger-area * pressure.

More sophisticated cylinders have a body with end cover, a piston-rod with piston and a cylinder-head. At one side the bottom is for instance connected to a single clevis, whereas at the other side, the piston rod also is foreseen with a single clevis. The cylinder shell normally has hydraulic connections at both sides. A connection at bottom side and one at cylinder head side. If oil is pushed under the piston, the piston-rod is pushed out and oil that was between the piston and the cylinder head is pushed back to the oil-tank again.

The pushing or pulling force of a hydraulic cylinder is:

F = Ab * pb - Ah * ph
F = Pushing Force in N
Ab = (π/4) * (Bottom-diameter)^2 [in m2]
Ah = (π/4) * ((Bottom-diameter)^2-(Piston-rod-diameter)^2)) [in m2]
pb = pressure at bottom side in [N/m2]
ph = pressure at cylinder head side in [N/m2]

Apart from miniature cylinders, in general, the smallest cylinder diameter is 32 mm and the smallest piston rod diameter is 16 mm.

Simple hydraulic cylinders have a maximum working pressure of say 70 bar, the next step is 140 bar, 210 bar, 320/350 bar and further, the cylinders are in general custom build. The stroke of a hydraulic cylinder is limited by the manufacturing process. The majority of hydraulic cylinders have a stroke between 0,3 and 5 metres, whereas 12-15 metre stroke is also possible, but for this length only a limited number of suppliers are on the market.

In case the retracted length of the cylinder is too long for the cylinder to be build in the structure. In this case telescopic cylinders can be used. One has to realize that for simple pushing applications telescopic cylinders might be available easily; for higher forces and/or double acting cylinders, they must be designed especially and are very expensive. If hydraulic cylinders are only used for pushing and the piston rod is brought in again by other means, one can also use plunger cylinders. Plunger cylinders have no sealing over the piston, or the piston does not exist. This means that only one oil connection is necessary. In general the diameter of the plunger is rather large compared with a normal piston cylinder, because this large area is needed.

Whereas a hydraulic motor will always leak oil, a hydraulic cylinder does not have a leakage over the piston nor over the cylinder head sealing, so that there is no need for a mechanical brake.

Hydraulic motor

The hydraulic motor is the rotary counterpart of the hydraulic cylinder.

Conceptually, a hydraulic motor should be interchangeable with hydraulic pump, because it performs the opposite function -- much as the conceptual DC electric motor is interchangeable with a DC electrical generator. However, most hydraulic pumps cannot be used as hydraulic motors because they cannot be backdriven. Also, a hydraulic motor is usually designed for the working pressure at both sides of the motor.

Hydraulic valves

These valves are usually very heavy duty to stand up to high pressures. Some special valves can control the direction of the flow of fluid and act as a control unit for a system.