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How Is Pressure Created In A Hydraulic System?

As opposed to mechanical or electrical drive systems, hydraulic systems rely on fluid to transfer energy. Fluid offers some tangible benefits as a medium through which to transfer power when compared to many mechanical components. These benefits are what make hydraulic systems so widespread in heavy industrial plant machinery. Because hydraulic systems rely on fluid to transfer energy, they are not limited by the same directional constraints as mechanical systems. Fluid lines can be bent and shaped in any number of directions.

Hydraulic systems convert kinetic energy into hydraulic pressure and then convert it back into kinetic energy again to move vital components. At the beginning of this system, transferring the kinetic energy into fluid pressure, are the hydraulic pumps. As the components which convert the kinetic energy, usually supplied by an electric motor, into hydraulic pressure, hydraulic pumps and vital for continued operation of the entire hydraulic system.

Knowing exactly how they work can help many workers who regularly operate hydraulic plant machinery better understand the tools of their trade. Understanding these components also tends to give a better understanding of the need for regular hydraulic maintenance, as without it, key components like hydraulic pumps are much more likely to fail.

How does a hydraulic pump operate?

When a hydraulic pump is in operation it performs two main functions. Firstly, its mechanical action makes a vacuum at the pump inlet that allows atmospheric pressure to force liquid from the reservoir into the inlet line to the pump. Then its mechanical action transports this liquid to the pump outlet and forces it into the hydraulic system.

A pump produces liquid movement or flow in a hydraulic system, its function does not include generating pressure that falls to other elements within the system. The hydraulic pump produces the amount of flow needed for the production of pressure, which is a function of resistance to fluid flow in the system. For example, the pressure of the fluid at the pump outlet is zero for a pump that is not connected to a system. Also, for a pump delivering into a system, the pressure will only rise to the level needed to overcome the resistance of the load. 

Some different types of hydraulic pumps

Just like there are many different kinds of any one specific type of machinery, there are also many different kinds of hydraulic pump. Their individual characteristics and attributes make each of them more suitable for use in certain applications.

Hydraulic pressure

Gear pumps

Because they are cheap and have a very simple design, gear pumps are one of the most widespread types of hydraulic pump found in all sort of machinery. When it comes to simplicity, it doesn’t get much simpler than two gears interlocking. This is the basis of the way in which gear pumps transfer kinetic energy into hydraulic pressure.

As the gears turn, they force the hydraulic fluid past them within the gear housing. This creates pressure which is transferred towards the hydraulic motors which will perform the opposite action to drive the components at the other end of the hydraulic system.

Because gear pumps wear slowly over time, they gradually become less efficient. This gradual loss of efficiency shows itself as a decrease in power being supplied to the hydraulic system, but also means that catastrophic failures are less common. If a pump fails catastrophically, it can easily damage other components in the process, raising the repair bill significantly.

During hydraulic maintenance checks, the gears within hydraulic gear pumps will be checked for wear and tear. If the friction between the interlocking gears has resulted in excessive loss of efficiency, they can be replaced if it is deemed necessary.

Rotary vane pumps

Another type of hydraulic pump is the rotary vane pump. While the principle of a turning object which forces hydraulic fluid through the hydraulic system is the same, the components within rotary vane pumps are very different. Within the motor housing, an eccentrically mounted rotor hub is turned by the drive shaft, which is in turn driven by the power input – usually an electric motor.

A series of vanes protrude from the centre of the rotor hub. To ensure they always maintain contact with the edges of the motor housing, they are spring loaded from the inside. This is vital to ensure pressure can be maintained and transferred into the hydraulic fluid as the hub turns. Rotary vane pumps are not as simple as gear pumps, but they are more efficient. They are also especially effective in high flow, low pressure applications, which means they are often used to drive components which require a high degree of movement.

Piston pumps

There are two main types of piston pump – axial piston pumps and radial piston pumps. The first of these two types relies on the circular motion of a rotating disc known as a swashplate. Attached to the front face of the swashplate is a series of pistons. Because the swashplate is offset at an angle, the pistons move in and out as it turns. This forces hydraulic fluid into and out of the piston chambers, therefore creating pressure within the system.

The second type of piston pump, the radial piston pump, makes use of an eccentrically mounted rotor which has a series of pistons located around the outside. As the offset rotor rotates, the pistons are forced in and out, again creating pressure within the hydraulic system. This particular type of pump is able to handle very high fluid pressures, making it a popular choice in many types of heavy industrial machinery.

Hydraulic maintenance plays a vital role in the smooth operation of hydraulic plant machinery. Just because everything looks alright from the outside, doesn’t mean potential problems aren’t on the horizon. With proper hydraulic maintenance checks, companies can protect the financial investment they have made in their machinery.

What to consider if your hydraulic system is not working properly

Naturally, if you are experiencing problems with any aspect of your hydraulic system, you should contact repair professionals like CJ Plant. However, having some basic knowledge of things to check and questions to consider can help, as this will provide some background information of the issues you’re experiencing and will assist us in quickly and effectively diagnosing the problem for repair.

The first key thing is understanding where the problem is originating and what exactly it is, which will involve asking a few key questions. Some examples of questions to consider include:

  • Have there been any recent changes to the hydraulic system, such as maintenance, modifications to the settings, or repairs
  • When was maintenance last completed on the system?
  • How long has the issue been occurring?
  • What was occurring when you first noticed the problem? (e.g. system start up or shut down, heavy load, drastic temperature change etc)

Common problems

There are some specific problems that can commonly prevent hydraulic systems from working properly such as overheating hydraulic fluid and an inoperative system. Other common faults in hydraulic systems include:

  • Slow operation – when the system is working slower than normal, one cause of this can be hydraulic fluid that is too thick
  • Erratic operation – if it is being unpredictable, a common reason can be trapped air in the system
  • Excessive noise or vibration – if the noise is coming from the pump, check that the oil level is sufficient, the right type of fluid is being used, and that the oil is not foamy


If you would like to know more about hydraulic systems, hydraulic repair or hydraulic maintenance, please get in touch with the experts at CJ Plant Maintenance today.

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