# Power Hydraulics - The Foundation of Every Machine

> Hydraulic systems in construction machinery: components, pumps, valves, cylinders, open and closed circuits, fault symptoms, diagnostics and rebuilds.

![Piston pump parts: retaining plate with pistons in brass slippers, gear and sealing ring on a workbench](https://wibako.com/media/articles/article-90/slot1-e-efc48e00.webp)

**Hydraulic power systems** transmit energy by means of a pressurised fluid, which in construction machinery is hydraulic oil. A pump driven by the diesel engine converts mechanical energy into pressure and flow. Hydraulic cylinders and motors convert it back into working motion: raising the boom, slewing the upper structure or travelling.

In this guide we explain how a hydraulic system is built and how open and closed circuits differ. We also describe how to recognise a fault and when a component is worth rebuilding rather than replacing. Finally, we share tips on operating the system to reduce the risk of costly breakdowns.

## What hydraulic power systems are and where they are used

Every modern **construction machine** – excavator, wheel loader, dozer, roller or mobile crane – has a hydraulic system that determines its force and productivity. Power hydraulics are also used in mining, agricultural, forestry and municipal machinery (sweepers, refuse trucks, implement carriers), as well as in presses, lifts and gate drives.

What sets it apart from plumbing hydraulics is above all the pressure level. Excavator working circuits typically operate at 300–350 bar. That is why the requirements for leak-tightness, oil cleanliness and component machining accuracy are very high.

## How a hydraulic power system works

Put simply, pressure generated at one point is transmitted to another through a fluid that is practically incompressible. The pump is the energy source, while cylinders and hydraulic motors are the consumers. The oil not only transmits energy but also lubricates and cools the mating parts.

Three simple relationships are useful for troubleshooting:

- cylinder **force** depends on pressure and piston area – lack of force usually points to a pressure problem (relief valve, internal leakage, pump regulator),
- movement **speed** depends on flow rate (l/min) – slow movements most often point to the pump, its adjustment or a restriction in the system,
- the pump generates flow; pressure only builds up against resistance, i.e. under load.

## Hydraulic machine design – system components

![Axial piston hydraulic pump with regulator on a workshop bench](https://wibako.com/media/articles/article-90/slot2-e-172b5674.webp)

The main components of the system are: hydraulic pump, consumers, control valve block, valves, reservoir, hydraulic oil, oil cooler, filters, control elements and hoses. Pressure gauges and flow meters are used to monitor operation.

### Hydraulic pumps

Both fixed and variable displacement pumps are used. Gear pumps are simple and robust, so they supply auxiliary circuits, pilot control and smaller machines. In excavators and loaders the main role is played by axial piston pumps (e.g. Rexroth, Linde, Sauer-Danfoss, Liebherr, Kawasaki) with power and pressure control, often in a Load Sensing system.

After installing a pump, its housing must be filled with oil and the system **bled**. A dry start can seize the pump within moments.

![Hydraulic cylinder with extended piston rod mounted on a machine arm](https://wibako.com/media/articles/article-90/slot3-e-5083f414.webp)

### Control valve and valves

The control valve block directs oil to individual consumers according to joystick movements. The individual valves perform different tasks:

- relief valves limit pressure in the whole system and in individual sections,
- check valves block reverse flow,
- throttle valves control movement speed,
- brake and safety valves protect the travel and slew motors.

### Hydraulic cylinders and motors

Cylinders convert the energy of the oil flow into **linear motion** (boom, arm, bucket), while hydraulic motors convert it into **rotary motion** (travel, slewing, winches). Replacement components, such as a [Volvo EC240 hydraulic cylinder](https://wibako.com/shop/hydraulics/cylinders/29016X35551-hydraulic-cylinder-volvo-ec240) or a [set of Liebherr R934C cylinders](https://wibako.com/shop/hydraulics/set-of-hydraulic-cylinders/65922X77538-set-of-cylinders-liebherr-r934c), are selected by part number and mounting dimensions.

### Oil, reservoir, filters and cooler

The oil is the weakest point of the system. Water accelerates corrosion and scoring of working surfaces. Air impairs lubrication and causes uncontrolled temperature rise. Fine contaminants act like grinding paste inside the pump. That is why suction, pressure and return filters, the reservoir breather and a clean oil cooler are just as important as the pump itself.

## Hydraulic circuits – open and closed loop

![CAT crawler excavator with boom and bucket hydraulic cylinders, service van alongside](https://wibako.com/media/articles/article-90/slot4-e-8310348a.webp)

Circuits are classified by oil circulation and the role of the reservoir. In an **open circuit** the pump draws oil from the reservoir and delivers it through the control valve to the consumers. Return oil flows back to the reservoir through the filter and cooler. This is how the working equipment of most excavators and loaders operates.

In a **closed circuit** oil from the hydraulic motor returns directly to the pump inlet, bypassing the reservoir and control valve. Direction and speed are changed by tilting the pump swashplate. A charge pump makes up for leakage and exchanges part of the oil for cooled oil. Such hydrostatic transmissions drive the travel of wheel loaders, rollers and telehandlers.

Troubleshooting requires the hydraulic schematic of the specific model from the service documentation. The symbols on schematics are standardised (ISO 1219). Once you have mastered the basics, an annotated schematic lets you trace the oil path from the pump to the consumer and locate the pressure test points.

## Hydraulics in railway applications

The same principles apply to rail vehicles and track maintenance machines: tampers, ballast regulators, railway cranes and road-rail vehicles such as excavators on road-rail undercarriages. There, hydraulics power the working mechanisms and, in some vehicles, also working travel and cooler fans.

Pumps, motors and cylinders in these machines often come from the same manufacturers as in construction equipment, so diagnostics and rebuilds are carried out in a similar way. For a specific component, it is best to contact us and provide the data from the nameplate.

## Advantages and weak points of hydraulic power systems

Hydraulic systems transmit high forces in a compact package, allow smooth speed control and make it easy to route power to different parts of the machine. This translates directly into productivity and work pace.

The weak points are sensitivity to contamination and oil temperature, as well as system complexity. With many interacting components, quickly and precisely pinpointing the source of a failure requires measurements, not guesswork.

## Fault symptoms and diagnostics

![Disassembled axial piston hydraulic pump: pistons with brass slippers, retaining plate and rockers prepared for inspection](https://wibako.com/media/articles/article-90/slot5-e-e7c18027.webp)

- the machine “has no power” or slows down under load,
- movements slow down once the oil warms up – typical of a worn pump or heavy internal leakage,
- the boom or bucket drifts down on its own – leakage past the piston seal or a valve,
- the pump whines or rattles, the oil foams – cavitation, air ingress, clogged suction filter,
- the oil overheats – dirty cooler, constant flow over a relief valve, worn components,
- metal particles on filters and the magnetic plug – advanced internal wear.

Diagnostics include measuring pressures at the test points, checking valve settings and measuring case drain leakage from pumps and motors. The condition of the oil and filter elements is also assessed. We describe a real-life example in our article on the [Menzi Muck A91 walking excavator that “had no power”](https://wibako.com/about-us/news/835-the-customer-said-it-has-no-power-we-stripped-the-entire-hydraulics-of-a-menzi-muck-a91-walking-excavator).

## Repair and rebuild or replacement

![Mechanic inserting the cylinder block into a hydraulic pump housing during reassembly after rebuild](https://wibako.com/media/articles/article-90/slot6-e-0754a067.webp)

Repair begins with cleaning and disassembly. Each part is then inspected to determine what can stay in service, what can be reconditioned and what must be replaced. In cylinders, the parts that wear most often are the piston rod, the base and rod eyes, the piston guide ring, the barrel, the gland, the seals and the wiper ring. Grinding, chrome plating and honing are used, and an eye, gland or barrel is often manufactured from scratch.

In piston pumps and motors, the pistons with slippers, cylinder block, valve plate, swashplate and bearings are inspected. We write more about typical wear in our articles on the [DPVO 165 pump rebuild](https://wibako.com/about-us/news/817-how-often-should-excavator-pumps-be-serviced-rebuilding-a-dpvo-165-from-a-liebherr-a316) and on [overhauls of Liebherr hydraulic pumps and motors](https://wibako.com/about-us/service/431-repairs-of-liebherr-hydraulic-pumps-and-motors).

Replacement can be the more sensible option when the housing is cracked, damaged surfaces cannot be machined, parts are unavailable or the cost of the rebuild approaches the price of another unit. This applies, for example, to Liebherr DPVO and LPVD pumps in Liebherr excavators and loaders or Kawasaki K3V112 pumps in mid-size excavators. In such cases it is worth comparing the rebuild cost with the price of a used or new pump – check the availability of a specific model in our shop or ask us, quoting the number from the nameplate. Whatever you choose, the root cause of the failure must be eliminated. If there are metal particles in the system, the oil and filters must be changed and the lines flushed; otherwise the new component will soon share the fate of the old one.

## How to avoid costly breakdowns

A fault in any component can reduce productivity or even bring the machine to a standstill. The basic duties of the operator and owner are:

- daily checks of the oil level, leaks and hose condition,
- changing oil and filters according to the manufacturer's manual, and more often under heavy-duty operation; oil sample analysis is helpful,
- cleaning quick couplers before connecting attachments and regularly cleaning dust from the oil cooler,
- warming up the oil before applying full load, especially in winter,
- responding to the first symptoms before a worn pump contaminates the entire system.

Scheduled inspections are best carried out by a service shop with proper diagnostic equipment. At WIBAKO a periodic inspection costs PLN 1,400 net (excluding travel and materials), and a mechanic's labour hour costs PLN 230 net. Travel is charged at PLN 3 net per km, counted both ways. Our workshop in Kojszówka is located near Sucha Beskidzka, Żywiec and Myślenice. Contact us for diagnostics, hydraulic system overhauls or help selecting parts.

## Frequently asked questions

### What is a hydraulic power system?

It is a technology for transmitting energy by means of a pressurised fluid, most often hydraulic oil. The pump converts engine power into pressure and flow, and hydraulic cylinders and motors convert it into working motion.

### What components make up a hydraulic power system?

The main components are the pump, control valve, valves, consumers (cylinders and hydraulic motors), reservoir, oil, filters, oil cooler, hoses and control elements. Pressure gauges and flow meters are used to monitor operation.

### What is the difference between an open and a closed hydraulic circuit?

In an open circuit the pump draws oil from the reservoir and return oil flows back to the reservoir. In a closed circuit oil from the hydraulic motor returns directly to the pump inlet, which is used mainly in hydrostatic travel drives.

### Why does an excavator lose hydraulic power?

The most common causes are a worn or poorly adjusted pump, leaking relief valves or internal leakage in cylinders and motors. The cause is identified by measuring pressures and leakage, not by replacing parts on a trial basis.

### Is it worth rebuilding a hydraulic pump or cylinder?

Usually yes, if the housing is intact and the worn parts can be machined or replaced. Replacement may be better with a cracked housing, unavailable parts or damage that rules out machining. The decision is made after inspecting the disassembled component.

### Are hydraulic power systems used in the railway industry?

Yes, in track maintenance machines, railway cranes, road-rail vehicles and auxiliary drives of rail vehicles. The operating principle and component design are the same as in construction machinery.

Published: 2024-03-25

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Source: https://wibako.com/about-us/blog/90-power-hydraulics-the-foundation-of-every-machine
Contact: +48338704200, biuro@wibako.pl