Hydraulic Glossary Vol. 3: Technical Parameters

In previous chapters, we analyzed the component architecture (Vol. 1) and the logical intelligence of the system managed through the valves (Vol. 2). In this third volume, we delve into the heart of diagnostics and design: the quantitative grammar of hydraulics.

Technical parameters are the grammar of hydraulics. Knowing how to read pressure, interpret pressure drops, or recognize cavitation signals is the difference between a functioning system and one that wears out, wears out, and stops. This guide is the practical reference the field has been missing.

1. Pressure

What's this: Pressure is the force exerted by the fluid per unit area. It is the parameter that defines the "power" with which the system can overcome a load.

• Unit of measure: bar, MPa, PSI 
• Conversion factors: 1 bar = 0,1 MPa=14,5 PSI.

Formula:

P = F / A × 10

• P = Pressure [bar]
• F = Force [N]
• A = Surface area [mm2]

Industrial Operating Classes:

• Light/Medium Applications: 50−150 bar (standard agricultural machinery, light automation).
• Heavy industrial: 150-350 bar (earthmoving machinery, industrial presses, rolling mills).
• Extremely high pressure: Up to 700 bar (rescue equipment, special jacks, test benches).

Common operational error: Confusing the safety valve setting pressure with the working pressure. The setting should be 10–15% higher than the maximum working pressure as seen in Flight. 2 which finds its numerical foundation here.

2. Scope

What's this: Flow rate is the volume of fluid passing through a section in a unit of time. It governs the speed of the actuators: higher flow rate means faster movement.

Unit of measure: liters per minute [l / min], cubic meters per hour [m3 / h]

Formula:

Q = A × v × 6

• Q = Flow rate [l/min]
• A = Internal section of the duct [cm2]
• v = Linear velocity of the fluid [m/s]

Reference standards:

• CETOP 03 (NG6): Indicative flow rates up to 40−60l/min.
• CETOP 05 (NG10): Indicative flow rates up to 120l/min.
• High flow systems: Special configurations and cartridge valves operating between 200 and 600l/min.

Common error: Size the pump only for peak flow, without considering the actual cycle. A system that uses maximum flow only 20% of the time can benefit from a variable displacement pump, reducing consumption by up to 40%.

3. Hydraulic power

What's this: Hydraulic power is the energy transferred by a fluid per unit of time. It is the direct product of pressure and flow rate: the parameter that connects mechanics to hydraulics and electrical engineering.

Unit of measure: Kilowatt [KW]

Formula:

P[kW] = (Q × p ) / 600

• PkW​ = Hydraulic power [kW]
• Q = Flow rate [l/min]
• p = Pressure [bar]

Practical example: A 200 bar system with a flow rate of 60 l/min absorbs: (60 × 200) / 600 = 20 kWThis is the minimum power that the electric motor must supply, before applying the performance coefficients.

Common error: Do not apply the pump's overall efficiency (typically 0,85–0,92) when calculating the motor's absorbed power. The mechanical power required is always greater than the hydraulic power produced.

4. Fluid velocity

What's this: The speed at which fluid flows through pipes and components. Often overlooked in the design phase, it is a major cause of noise, erosion, and excessive pressure drops.

Unit of measure: metri per second [m / s]

Formula:

v = Q / (A × 6)

• v = Fluid velocity [m/s]
• Q = Flow rate [l/min]
• A = Internal cross-sectional area of ​​the tube [cm2]

Recommended limit values:

• Suction pipes: ≤ 1 m/s (critical speed for cavitation)
• Delivery pipes: 2–4m/s
• Return pipes: 1–2m/s

Common error: Undersize the suction line diameter to save on material. Exceeding 1 m/s at the suction line is the most direct route to pump cavitation.

5. Pressure Drops

What's this: Pressure losses are the reduction in pressure experienced by fluid as it flows through pipes, fittings, valves, and any element that resists flow. They are energy converted into heat without producing any useful work.

Unit of measure: bar

Type:

• Distributed losses: along the pipes, proportional to the length and speed
• Concentrated losses: in valves, bends, fittings — calculated with local resistance coefficients (ζ)

Rule of thumb: The overall pressure drop balance within an industrial circuit must not exceed 5%-10% of the nominal operating pressure. Every bar lost in the pipeline is an additional bar that the pump must generate, resulting in a direct energy cost. Every bar dissipated along the line forces the pump to increase its generation pressure, directly impacting energy costs and component wear.

6. Oil viscosity

What's this: Viscosity is the fluid's internal resistance to flow. It is the parameter that most influences the health of the system: it governs lubrication, internal leakage, and the ability to form the film that separates moving surfaces.

Unit of measure: centiStokes [cSt] at 40°C and 100°C

Most common ISO VG classes in hydraulics:

• ISO VG 32: high-speed systems, low operating temperatures
• ISO VG 46: industrial standard, the most widespread
• ISO VG 68: heavy systems, high temperatures, low speeds

The golden rule: The optimum operating viscosity for most pumps is between 25 and 54 cStBelow this range, the lubricating film and internal sealing are lost. Above this range, pressure drops increase and cold starting becomes critical.

Common error: Choose oil based on price, not the pump's specifications. Manufacturers always specify the permissible viscosity range: adhering to it will extend the component's life by years.

7. Operating Temperature

What's this: Oil temperature is the most immediate indicator of the health of a hydraulic system. It's the system's thermometer: excessively hot oil signals inefficiencies, excessive internal leaks, or inadequate cooling.

Unit of measure: Degrees Celsius [∘C]

Operating thermal thresholds:

• Optimal range: 40-60 ° C (ensures the chemical stability of the oil and the correct viscosity)
• Maximum limit: 70–80°C (above this, degradation of the oil and seals is accelerated)
• Minimum for startup: depends on the oil, but generally > 10°C for VG 46

Relationship with viscosity: Temperature and viscosity are inversely proportional. At 20°C, a VG 46 may have a viscosity of 200 cSt; at 80°C, it drops to 8-10 cSt—outside the safe operating range.

Common error: Treat overheating as a cooling problem and add a larger heat exchanger without analyzing the cause. Heat is always the symptom: the cause could be a constantly tripping safety valve, oversized oil viscosity, or internal leaks in a worn pump.

8. Performance

What's this: Efficiency expresses how efficiently the system converts input energy (mechanical/electrical) into useful hydraulic energy. There is no single efficiency: there are volumetric, mechanical, and overall efficiency.

Overall performance formula:

η(global) = η(volumetric) × η(mechanical)

Reference values ​​for pumps in good condition:

• Volumetric efficiency: 93-98%
• Mechanical efficiency: 90-95%
• Overall performance: 85-92%

Warning signal: A volumetric efficiency below 90% in a gear pump or below 85% in a piston pump indicates significant wearThe component is “losing” fluid internally, which is converted into heat.

Common error: Don't periodically measure volumetric efficiency (comparing actual and theoretical flow rate). It's the cheapest and most powerful diagnostic test available for assessing a pump's condition without disassembling it.

9. Displacement of motors and pumps

What's this: Displacement is the volume of fluid displaced in a single complete revolution. It is the fundamental design parameter that links rotation speed to the flow rate produced (for pumps) or torque generated (for motors).

Unit of measure: cubic centimeters per revolution [cm3/rev]

Formula (pump flow calculation):

Q = (Vg​ x nx ηv)​​ / 1000

• Q = Effective flow rate [l/min]
• Vg​ = Pump displacement [cm3/rev]
• n = Rotation speed [rpm]
• ηv​ = Volumetric efficiency of the component

Formula (calculation of torque of an engine):

M = (p × Vg​ × ηm) / (20π)

• M = Driving torque [Nm]
• p = Differential pressure [bar]
• Vg​ = Engine displacement [cm3/rev]
• ηm​ = Mechanical-hydraulic efficiency

Fixed vs. Variable Displacement Pumps: Fixed-displacement pumps always deliver the same flow rate at the same speed. Variable-displacement pumps adapt the flow rate to system demand, resulting in significant energy savings during low-demand cycles.

Common error: Select the displacement based only on the maximum required load, regardless of the prime mover's rotation speed. The same load can be achieved with different displacements and speeds: the optimal choice balances quietness, wear, and cost.

10. Cylinder force

What's this: Force is the effective thrust a hydraulic cylinder can exert on a load. It depends on the pressure and the effective area of ​​the piston—the values ​​are always different during extension and retraction due to the rod.

Unit of measure: Newton [N] or KiloNewton [kN]

Formula (extension stroke – piston side):

F = p × A(piston) x 10

Formula (retraction stroke – rod side):

F = px [A(piston) – A(rod)] x 10

(With F expressed in [N], p in [bar] and areas in [cm2])

Practical example: Cylinder with piston ∅100 mm, stem ∅50 mm, pressure 200 bar:

• Extension force: 200 × 78,5 × 10 = 157.000 N = 157 kN
• Retracting force: 200 × (78,5 − 19,6) × 10 = 117.800 N = 118 kN

Common error: Size the cylinder based only on the extension force and ignore the retraction force. In applications with loads in both directions, the force differential can cause erratic or insufficient movement. 

11. Cavitation

What's this: Cavitation is the most destructive phenomenon that can occur in a hydraulic system. It occurs when the local pressure in the fluid drops below its vapor pressure: the fluid "boils" at room temperature, forming micro vapor bubbles that implode violently when they return to high-pressure areas.

How to recognize it:

• Characteristic noise: crackling, similar to grit hitting metal
• Abnormal vibrations on the pump
• Localized overheating
• Crater erosion on the internal surfaces of the pump

Main causes:

1. Fluid velocity at suction greater than 1 m/s
2. Suction pipe too long or with excessive bends
3. Clogged suction filter
4. Oil too cold (high viscosity) at start-up
5. Excessive geodetic height between tank and pump

Common error: Confusing cavitation with aeration (the presence of dissolved air in the fluid). The symptoms are similar, but the causes and remedies are different. Aeration is resolved by eliminating the air intake points (gaskets, fittings); cavitation requires changes to the geometry of the intake circuit.

The parameters at a glance

FAQ – Doubts on the field

Can I use the same VG 46 oil in summer and winter? 

It depends on the ambient temperature range. If the system operates in environments with highly variable temperatures (unheated workshops, outdoor applications), consider a multigrade oil or a VG 32 for winter and VG 68 for summer, or a single oil with a good viscosity index (VI > 100).

How do I know if my pump is losing efficiency? 

Measure the actual flow rate with a flow meter and compare it to the theoretical flow rate (displacement × rpm / 1000). The ratio is the instantaneous volumetric efficiency. If it is below 90%, the pump is losing efficiency.

Does cavitation always damage the pump immediately? 

No, but the damage is cumulative and irreversible. A few minutes of intense cavitation can permanently erode internal surfaces. Chronic cavitation, even at low intensity, progressively reduces performance and accelerates failure.

What is the most important parameter to monitor during operation? 

Oil temperature. This is the aggregate indicator of all system imbalances: excessive pressure drops, declining efficiency, and constantly unloading valves. A well-designed and well-maintained system maintains the oil between 40 and 60°C.