Guide to Hydraulic Systems

Without hydraulics, modern construction equipment wouldn’t operate as effectively as it does. Because of this fluid-based system, heavy equipment can use small operator motions to create large movements in attachments and things they hold. Knowing when to repair the hydraulics of heavy equipment requires an understanding of the system. This guide provides important information you need to know about hydraulic systems used for heavy equipment.


What’s a Hydraulic System?

The definition of a hydraulic system is an operation that uses pressurized fluid to power motion. The pressure of the fluid multiplies pressure put on it to increase the force at the output. A hydraulic system uses this fluid in cylinders or hydraulic power units to do work such as stop a vehicle through its brakes, lift a crane and its load or move a bucket on a loader.

Within a hydraulic system, there are components that put the incompressible fluid under pressure. Because the hydraulic oil does not press into a smaller space, the force applied to it gets transferred to the other end of the area where the oil is. The pressure exerted from the oil moves a large piston that can work alone or with additional cylinders to move things requiring extra force.

Hydraulic system

When equipping a device with a hydraulic system, you can increase the amount of work the system can do without increasing the effort you put into it. Applied to construction equipment, hydraulic power systems allow a small joystick movement to lift a tractor bucket filled with weighty rubble.

Such an operation would not be possible with humans using their muscles and shovels to lift the same amount of debris. For instance, a steam-driven predecessor of today’s hydraulic-powered construction equipment could move the same amount of substance in a day that two men equipped with a wheelbarrow could do in two weeks.

The improvements made to hydraulic systems have only increased their ability to make today’s equipment capable of the heavy lifting on construction sites that requires long-lasting reliability, power and control.

What Does a Hydraulic System Mean for Equipment?

A hydraulic system offers three main advantages when used within construction equipment:

  • Reliability: Hydraulic systems offer reliable operation as long as the system remains closed and the fluid stays free of contaminants.
  • Power density: The output of hydraulics is many times greater than the force put into the system, reaching close to 7,000 pounds per square inch in some pieces of heavy construction equipment.
  • Versatile control: Innovations allow for hydraulic systems that move in multiple directions and have electronic controls.

These benefits of hydraulics make it the best choice for many types of heavy equipment, including pieces used for construction and earthmoving.

What Does a Hydraulic System Do?

Hydraulic systems operate with one of two methods: cylinders or hydraulic power units. Cylinders are the original components used to multiply force with hydraulic fluid. However, advances in engineering now allow improved operations from larger hydraulic power units that increase the system’s work capability.

Hydraulic Systems With Cylinders

When using cylinders, hydraulic systems have a smaller and a larger cylinder. The smaller one has a piston for work put into the system. The piston presses down on hydraulic fluid in the small cylinder and flows into the bottom of the larger cylinder. The large cylinder also has a piston that moves based on the force of the oil.

To allow for movement of the larger cylinder, the force applied to the smaller cylinder must move farther than the movement created by the output from the large cylinder. The amount the smaller cylinder must move is equal to a multiple of the difference in the area of the cylinders. For instance, if the larger cylinder is five times bigger than the smaller one, the piston on the smaller cylinder must move five times the distance of the needed output. To move the larger piston one inch, the smaller one must travel five inches.

Hydraulic Systems With Hydraulic Power Units

The other type of system used is a hydraulic power unit that increases the capabilities of the system by using a pump and pressurized fluid to replace the small cylinder.

While the distance the small piston travels determines the output in a system that uses cylinders, those that have hydraulic power units do not have the limitation of physical distance. By raising the pressure of the fluid, the need for travel distance disappears, allowing for far higher output forces than with a cylinder system. This type of system is often used on construction equipment to achieve massive amounts of work and lifting capabilities.

How Does a Hydraulic Drive System for Construction Equipment Work?

The use of small and large cylinders and pistons works in some tiny devices, but the heavy loads and large movements required of construction equipment need more robust hydraulic solutions. For these more extensive devices, hydraulic power units replace cylinders for higher power output.

Because hydraulic power units (HPUs) bring in more fluid from a pump at higher pressures, they can create a force that equals a greater travel distance needed by a small piston. Since these HPUs do not have the physical size of a small cylinder to limit them, they can create far more force for the output than two-cylinder systems can.

An HPU contains all the components needed to operate the hydraulic system — including a pump, motor and fluid reservoir — in a self-contained area. The pump and its motor generate a small amount of pressure needed to move the system. When the hydraulic system starts, the pump delivers oil into the accumulator. Once the oil in the accumulator reaches the necessary pressure, the bigger piston moves, and a valve allows pressure to drop as the fluid returns to the reservoir.

Some hydraulic systems use two-stage pumps that permit faster pushing and pulling of a hydraulically operated force by shifting between high pressure and low flow rate and low pressure and high flow rate of the oil. A large reservoir is a requirement for many HPUs on earthmoving equipment. For some pieces of equipment that have multiple cylinders, the reservoirs can store dozens of gallons of fluid.

The hydraulic systems on construction equipment can operate various components. For example, the tracks on tracked backhoes have hydraulic drivers. Loaders often have a pair of pistons to move the bucket vertically, a couple to rotate the bucket to turn out contents and a set to open the bucket’s sides. Dump trucks have a comparatively simple operation, only requiring one or two cylinders to lift the bed.

Cranes also use hydraulics in many ways. For cranes that have outriggers to lift the entire system, hydraulic systems provide the power to hoist the multi-ton vehicle vertically. To turn the crane’s load on a boom, a hydraulic system moves the Rotex gear. The boom also telescopes in or out thanks to the motions of hydraulics. The operator’s controls connect to the hydraulic hoses in a crane and other similarly operated construction equipment.

Using the controls in the operator’s cab changes the flow of hydraulic fluid in the system, allowing for parts of the equipment to move. The fluid routing happens at the spool valves, which link oil lines to the pump and each other. These valves change the direction of the fluid, which moves the hydraulic force to the parts of the equipment where the operator needs it to go.

Because the hydraulic systems in heavy equipment allow these pieces to perform vital work, problems with the hydraulic power unit’s motor, pump or reservoir can hamper productivity on a job site. In these situations, it’s helpful to identify the symptoms of the problem and get the equipment to a service shop for repairs to minimize downtime.

How to Know When a Hydraulic System in Heavy Equipment Needs Repairs

Hydraulic repairs

While reliable, the hydraulic system for earthmoving equipment and other similar vehicles can have problems. Major symptoms of hydraulic system issues include the following:

  • Noisy operation
  • Problems with hydraulic oil flow
  • Faulty hydraulic system operation
  • Incorrect hydraulic pressures
  • Excessive heat

If any of the above begins to happen in your heavy equipment, schedule a service to make necessary part replacements or repairs to the system. These major issues break down into specific problems that hint at possible causes and remedies for the system.

1. Noisy Operation

Noisy operation means there’s excessive noise coming from some part of the equipment. Listen carefully for the source of the sound because this can help a certified technician identify potential problems. For instance, a noisy pump could indicate air in the hydraulic fluid, a worn pump or misaligned couplings.

Noise coming from the pump’s motor could also be a sign of misaligned couplings or wear in the pump’s engine. When you hear extra noise coming from the relief valve, the valve may not have the correct setting or a poppet on it may have worn.

Be cautious about making a diagnosis of your hydraulic system only from the sounds and their locations. A certified technician has the tools and tests to determine the exact cause of the noise and provide a repair for it.

2. Problems With Oil Flow

Problems with hydraulic fluid flow fall into three categories – too much flow, not enough flow or no current flow. Because these groupings cover a wide range of causes, a certified technician may need to conduct additional diagnostic tests or look for other factors to find the exact cause.

For example, if you have no flow from the hydraulic pump, it could be due to any of the following:

  • A faulty pump motor
  • Pump not getting hydraulic fluid
  • A damaged or improperly assembled pump
  • Sheared coupling for the pump drive
  • The pump turning in the opposite direction
  • An improperly placed prime mover direction
  • All hydraulic oil passing over the relief valve instead of into the pump

Many of these problems will require service to the pump or replacement parts in the hydraulic pump. The certified technician may also need to make repairs or part replacements based on the wear sustained by your equipment’s hydraulic system.

3. Faulty Hydraulic Operation

Abnormal movement of a hydraulic system can lead to dangerous situations on a construction site. Hydraulically operated components must move as expected. Erratic, slow, inconsistent or limited movement all indicate serious problems that need immediate repairs. These faulty movements may happen in conjunction with other issues, such as oil flow problems or noisy operation. For example, a lack of hydraulic fluid flowing through the pump may completely restrict movement. Any air in the oil may cause both noisy use and erratic operation.

To avoid causing damage or dangerous scenarios on the work site, instruct all operators to report the faulty operation immediately and keep the equipment out of use until after repairs.

4. Incorrect Pressures

Incorrect pressure in a hydraulic system closely mirrors faulty operation. If your equipment shows faulty operation, a technician may need to look at the pressure of the fluid and determine the cause of the pressure problem first. For instance, air in the oil may cause both erratic pressure and erratic operation of the hydraulic mechanism. Low pressure could happen from a damaged pump or pressure reduction valve and cause the system to operate slowly.

5. Excessive Heat

A hydraulic system may overheat when the fluid overheats. Whichever component of the system runs hot can provide a clue to the source of the excess heat. For instance, if the pump or motor run hot, the system may have too high of a load put on it, the engine may have damage or the relief valve may be set too high.

As with other hydraulic system problems, an expert must conduct a thorough investigation of the entire system to identify the sources of any issues. When looking for an expert, choose a certified technician who has the replacement parts for your equipment’s brand on hand. This combination ensures your heavy equipment will be repaired correctly and as quickly as possible.

Learn About Our Shop Service Capabilities for Hydraulic Systems in Heavy Equipment

learn about hydraulics

If your hydraulic system needs service or replacement parts, connect with us at Prime Source. Our shop service capabilities range from offering replacement parts for all makes of heavy equipment to providing repairs for hydraulic systems. Our certified technicians can inspect your equipment’s hydraulic system as a whole and make any necessary repairs or part replacements.

We aim to give our customers the highest caliber of service possible for their heavy equipment. Our expertise in multiple equipment brands sets us apart from the rest, and our customers trust us for our outstanding service and unparalleled range of parts. Learn more about our shop service capabilities today, whether you own a single tractor or a fleet of heavy equipment.

Contact Us