Hydraulics – Fundamentals for Exam Preparation

Vocabulary flashcards covering the core terms, definitions and factual knowledge required for the hydraulics fundamentals exam.

Advantages of hydraulic drive systems

High power density, stepless speed control, overload protection, easy force reversal, simple load holding.

Disadvantages of hydraulic drive systems

Leakage, noise, temperature sensitivity, fire risk with some fluids, need for filtration and maintenance.

Fluid (hydraulics)

A substance that can flow; in hydraulics mainly liquids, in pneumatics mostly air.

Hydrostatic drive

Energy is transferred by pressure in a stationary fluid; system can work at standstill.

Hydrodynamic drive

Energy is transferred by the motion of the fluid; fluid must flow continuously.

Example – hydrostatic drive

Hydraulic cylinder or positive-displacement pump.

Example – hydrodynamic drive

Torque converter in a vehicle power-train.

Viscosity

Measure of a fluid’s internal friction or ‘thickness.’

Dynamic viscosity

Absolute viscosity; ratio of shear stress to velocity gradient (Pa·s).

Kinematic viscosity

Dynamic viscosity divided by density (mm²/s); used primarily in hydrostatics.

Temperature effect on viscosity

Rising temperature lowers viscosity.

Pressure effect on viscosity

Rising pressure increases viscosity.

Viscosity group (VG)

Kinematic viscosity of the fluid at 40 °C, expressed in mm²/s.

Viscosity index (VI)

Indicates how little viscosity changes with temperature; desirable value: high.

Low-viscosity fluid example

VG-15 or VG-22 hydraulic oil.

Advantage of low-viscosity fluid

Good cold-start behaviour.

Disadvantage of low-viscosity fluid

Poor lubrication/film strength at high temperature.

Compressibility modulus

Bulk (elasticity) modulus; quantifies how compressible a fluid is.

Parameters influencing air solubility

Pressure and time of exposure.

Problems caused by dissolved air

Re-release at pressure drop, higher system compliance, vibration, oil ageing, cavitation wear.

Causes of accelerated oil ageing

Contamination, excessive heat (≈10 K doubles ageing), oxidation (air), hydrolysis (water), high pressure.

Oxidation inhibitors

Additives that slow oxidative ageing of the oil.

Viscosity-index improvers

Polymer additives that flatten viscosity-temperature curve.

Demulsifiers

Additives promoting rapid water separation from oil.

Detergents/Dispersants

Keep contaminants in suspension and surfaces clean.

Anti-wear (EP) additives

Increase load-carrying capacity under high pressure.

Foam inhibitors

Reduce surface tension to prevent foam formation.

Corrosion inhibitors

Form protective film to prevent rust and corrosion.

Stock-point depressants

Additives that lower pour/flow point of the oil.

Total pressure components

Static, dynamic and hydrostatic (gravitational) pressure add to total pressure.

Simplified Bernoulli equation

Total pressure remains constant along a streamline (p + ρv²/2 + ρgh = const.).

Bernoulli insight for hydrostatics

Pressure components interchange but total remains predictable.

Parameters determining piston diameter

External load and chosen system pressure.

Nominal bore (pipe)

Inside diameter of a hydraulic line.

Sizing of pipe bore

Depends on volume flow and permissible velocity (plus pressure class).

Laminar flow

Ordered flow at Re <2300; higher pressure loss, poorer heat transfer.

Turbulent flow

Chaotic flow at Re >2300; preferred for better heat exchange and velocity profile.

Reynolds number threshold

≈2 300 in hydraulic lines marks transition to turbulence.

Pump rated size

Displacement volume per revolution; also called ‘geometric displacement.’

Displacement-controlled drive

Motor speed is set by varying pump/motor displacement.

Resistance-controlled drive

Motor speed is set by throttling (flow resistance).

Higher-efficiency drive

Displacement control has better efficiency.

Higher-dynamic drive

Resistance control offers higher dynamic response.

Pump pressure generation principle

Pump delivers flow; system resistance dictates pressure.

System pressure at any point

Equals local static pressure influenced by load and losses.

Pressure losses manifestation

Convert hydraulic energy into heat within the system.

Volumetric efficiency

Ratio of delivered to theoretical flow; reduced by leakage.

Mechanical (hydromechanical) efficiency

Ratio of hydraulic to mechanical power; reduced by friction.

Factors reducing volumetric efficiency

Internal leakage through clearances, slots, gap wear.

Factors reducing mechanical efficiency

Bearing and gear friction, fluid shear, directional changes in flow.

Parameters affecting motor torque

System pressure and motor displacement volume.

Radial piston pump

Positive displacement pump with pistons arranged radially around a crankshaft.

Axial piston pump

Pump with pistons parallel to the drive shaft (swash-plate or bent-axis).

External gear pump

Two external gears mesh; simple, robust, up to ~300 bar.

Internal gear pump

Internal and external gear set; quieter, up to ~350 bar.

Vane pump

Slotted rotor with radial vanes running in a cam ring.

High-pressure machine feature

Contains pistons as characteristic mechanical element.

Constant-displacement pump

Fixed displacement per revolution; flow varies only with speed.

Variable-displacement pump

Adjustable displacement; flow varies with swash/excentric angle.

Axial/radial clearance compensation

Reduces leakage in gear pumps, raising volumetric efficiency; too much increases friction losses.

Typical use of gear machines

Constant-flow units for auxiliary functions.

Nominal pressure – internal gear pump

Approximately 350 bar maximum.

Nominal pressure – external gear pump

Approximately 300 bar maximum.

Main noise sources in hydraulics

Pumps (flow pulsation and mechanical action).

Origin of air, fluid and structure-borne noise

Flow pulsation caused by kinematic displacement irregularity.

Irregularity coefficient

Quantifies amplitude of flow pulsation (noise excitation).

Measures to reduce noise excitation

Increase number of displacement chambers; use odd gear/slot counts.

Axial piston machine designs

Bent-axis (oblique-axis), swash-plate, wobble-plate types.

Functions of case drain oil

Cools, lubricates bearings/seals and carries away leakage.

Two-stroke vane pump – advantage

Hydrodynamically balanced; lower bearing loads, quieter.

Two-stroke vane pump – disadvantage

Single-stroke version allows displacement adjustment; two-stroke does not.

Over-centre adjustment of vane pump

Changing eccentricity through zero to reverse flow direction.

Effect of over-centre adjustment

Pump delivers to the opposite port, enabling bi-directional flow.

Difference pump vs motor

Pump converts mechanical to hydraulic power; motor converts hydraulic to mechanical power.

Problem at excessive pump speed

Insufficient filling → cavitation, noise, wear.

Anti-cavitation measures

Boost pump, elevated/pre-pressurised tank, large short suction line.

Problem at too low pump speed

Flow less than leakage → no pressure build-up; high flow ripple.

Low-speed countermeasures

Use larger displacement, tighter clearances or leakage compensation.

Effects of no end-stroke energy dissipation

Mechanical shock, pressure spikes, noise, piston rod rebound.

Soft landing methods for cylinders

Integrated cushioning, throttling with servo/proportional valve, external shock absorbers.

Valve type for cylinder braking

Continuous (proportional/servo) throttling valve.

Energy form after braking

Mostly converted into heat.

Advantage of direct LSHT motor

Eliminates gearbox; hydraulic compliance damps shock loads.

Heat removal from motor

Flush/case-drain cooling circuit or external cooler port.

Main valve groups

Directional (switching), directional (continuous), pressure, flow, and check valves.

Basic valve function

Control energy/power flow in the circuit.

Valve element types

Seat, spool (slide), plate/disc elements.

Spool vs seat element drawback

Spool has clearance leakage; seat is leak-tight line contact.

Motion control with switching valve

Selects flow path and closed position but never completely leak-free.

4/3 directional valve

Four ports, three switching positions.

Valve overlap

Axial sealing land length between spool and bore controlling initial opening.

Element without overlap concept

Seat valves (binary open/closed).

Overlap types

Positive, negative, zero; positive & negative commonly used.

Purpose of overlap selection

Defines neutral function: blocking, float, pressure-free return, etc.

Internal leakage factors

Overlap length, clearance height, spool diameter, Δp, viscosity.

Directional valve actuation types

Hydraulic, mechanical, pneumatic, electrical (solenoid).

Advantage of pneumatic actuation

Intrinsically safe in explosive atmospheres.

Direct-operated directional valve

No pilot stage; spool moved directly by actuator.

Main disturbance force

Flow (hydrodynamic) force on the spool.

Parameters influencing flow force

Orifice geometry (ε), pressure drop, volume flow.