Frequently asked questions on hydraulics – TFS FAQ

Index of questions

Definitions

• What is hydraulics (oleodynamics)?
• Oleodynamics vs oleohydraulics

How it works

• How a hydraulic pump works
• How a power unit works
• How a hydraulic motor works

Technical differences

• External vs internal gear pumps
• Servo vs proportional valve
• Axial vs radial piston

Repair and maintenance

• Repair or replace a pump
• What is a certified test bench
• How often to service
• How to choose the pump
• What multi-brand overhaul means

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Definitions

What is hydraulics (oleodynamics)?

Hydraulics (oleodynamics) is the technology that transmits power through a pressurised fluid (oil). A pump converts mechanical energy into hydraulic energy; actuators such as cylinders and motors convert it back into motion. It allows very high forces to be controlled in a small space, with precision and continuity.

It is the principle behind presses, excavators, agricultural machines, marine and steel-industry plants. The main components of a hydraulic system are: pump, power unit, valves, motors and cylinders.

What is the difference between oleodynamics and oleohydraulics?

There is no substantial difference: «oleodynamics» and «oleohydraulics» refer to the same technology — power transmission through pressurised oil. In English both are commonly called «hydraulics».

They should not be confused with pneumatics, which uses compressed air instead of oil: air is compressible, so pneumatics offers lower forces and less precise positioning than hydraulics.

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How it works

How does a hydraulic pump work?

A hydraulic pump draws oil from the tank and pushes it into the circuit, generating flow. It does not directly «create» pressure: pressure arises from the resistance the circuit opposes to the flow. By converting mechanical energy into hydraulic energy, it powers the whole system.

The main families are: gear pumps (simple and economical), vane pumps, and piston pumps (high pressure, fixed or variable displacement — e.g. Bosch Rexroth A4VG, Kawasaki K3V). TFS repairs and overhauls pumps of all types on a certified test bench.

How does a hydraulic power unit work?

A hydraulic power unit is the assembly that generates and regulates the flow of pressurised oil for a system. It consists of tank, pump, electric motor, control valves, filters and instrumentation. The pump pressurises the oil; the valves regulate flow, pressure and direction to the actuators.

Power units are sized for the application: pressure (bar), flow (l/min), duty cycle and fluid. TFS designs and builds custom power units and tests them before delivery — see power unit construction and power unit catalog.

How does a hydraulic motor work?

A hydraulic motor is the reverse of a pump: it receives pressurised oil and converts it into mechanical rotation, i.e. torque. The oil flow determines the rotation speed; the pressure determines the torque delivered. It drives winches, wheels, drums, cranes and tracks.

Types include: gear, vane, orbital (low speed, medium torque), axial piston (high speed/pressure) and radial piston such as Hägglunds motors, capable of very high torque at low speed with direct drive. TFS overhauls multi-brand hydraulic motors and is a Kawasaki partner; for radial-piston Hägglunds motors it offers a dedicated overhaul with test-bench certification.

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Technical differences

What is the difference between external and internal gear pumps?

In an external gear pump two external toothed wheels mesh together and carry the oil along the periphery: simple, economical construction, suitable for medium-high pressures but noisier. In an internal gear pump an internal toothed wheel rotates inside a ring gear: quieter, compact and smoother, ideal at low speeds.

	External gear	Internal gear
Noise	Higher	Lower
Pulsation	More marked	Reduced
Cost	Lower	Higher
Typical use	General, medium pressure	Low speed, precision

See gear pumps. TFS handles their repair and overhaul.

What is the difference between a servo valve and a proportional valve?

Both continuously regulate flow or pressure from an electrical signal, but with different performance. The servo valve offers very high precision and dynamic response with internal feedback: ideal for fine position or force control. The proportional valve offers good control, greater robustness and lower cost: ideal for most industrial applications.

	Servo valve	Proportional valve
Precision/dynamics	Very high	Good
Feedback	Yes (internal)	Optional
Contamination sensitivity	High	Medium
Cost	High	Low
Typical brands	Moog, Atos	Bosch Rexroth, Atos

TFS repairs and tests servo and proportional valves on a test bench, partner of Moog and Atos.

What is the difference between axial and radial piston machines?

In axial piston machines the pistons are parallel to the axis of rotation and act on a swash plate: compact, suitable for high pressures and high speeds, often with variable displacement (e.g. Bosch Rexroth A4VG, Kawasaki K3V). In radial piston machines the pistons are arranged radially, perpendicular to the axis: they deliver very high torque at low speed (e.g. Hägglunds motors).

In short: axial = compactness, high speed, high pressure; radial = maximum torque at low speed, ideal for heavy direct drives. TFS overhauls both types — piston pumps and motors.

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Repair and maintenance

How do you know if a hydraulic pump should be repaired or replaced?

The warning signs of a hydraulic pump are: loss of pressure or flow, abnormal noise, overheating, vibration and oil leaks. In most cases overhaul is preferable to replacement: it costs less, uses original spare parts and, with test-bench certification, restores factory performance. Only severe structural damage makes replacement preferable.

A reliable diagnosis is made by disassembling the component and testing it on a test bench: actual flow, pressure and efficiency are measured. TFS provides an overhaul quote after diagnosis, with no obligation.

What is a certified test bench and why does it matter?

A test bench is the equipment that tests hydraulic pumps and motors by reproducing real working conditions — pressure, flow, torque and temperature — and certifies their performance with a measurement report. It is what distinguishes a professional overhaul from a simple repair: it guarantees that the overhauled component performs like new.

TFS operates a Bosch Rexroth certified test bench and is a Bosch Rexroth Certified Excellence Partner in the three areas of Distribution, Solution and Service. Every overhauled pump or motor is delivered with a documented test. See pump and motor repair and testing and certification.

How often should a hydraulic system be serviced?

The frequency depends on operating hours, environment and type of system, but some practical rules apply: continuous checking of level and leaks; oil and contamination analysis every 500-1000 hours (cleanliness class per ISO 4406); filter replacement at the scheduled intervals; checking of pressures and temperatures. Contaminated oil is the leading cause of failure.

Scheduled maintenance reduces unexpected downtime and extends component life. TFS offers scheduled maintenance and on-site support, also for critical sectors such as hydroelectric and oil & gas.

How do you choose the right hydraulic pump for a system?

Choosing a hydraulic pump depends on four main parameters: required flow (l/min), operating pressure (bar), fluid type and temperature, and fixed or variable displacement according to the duty cycle. Rotation speed, efficiency, noise and size must also be assessed. A wrong choice causes overheating, efficiency loss and premature wear.

For high-pressure, variable-displacement applications piston pumps are used; for general use, gear pumps. TFS supports the customer in the choice and in system design.

What does «multi-brand» overhaul mean?

Multi-brand overhaul means the workshop can overhaul hydraulic pumps and motors of different brands, not just those it distributes. It requires cross-brand expertise, spare-part availability and a universal test bench. It is the advantage of an independent centre over brand-tied service.

TFS overhauls Bosch Rexroth, Kawasaki, Hägglunds, Casappa, Parker, Danfoss, Moog, Atos, Hydac and many other components — see the 30 partners. A single point of contact for the entire machine fleet, with certified testing on every part.

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By Angelo Pagano, Owner and Technical Director of Tecno Fluid Service — over 25 years of experience in the repair and overhaul of hydraulic components, on a Bosch Rexroth certified test bench.