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Fluid Power Hydraulics

Hydraulics (Hydrostatics )

Important: The notes below relate to the area of hydraulic identified as hydrostatics.    In Hydrostatics the Pressure energy of the fluid is converted into mechanical motion.    This differs with the alternative area known as Hydrodynamics whichi is concerned with the kinetic energy of the fluid.    Hydrostatics is also generally based on oil as the fluid although, in the recent past water was the prime fluid used.

Hydraulics are used for providing high torques and high forces with a high level of control of the motion.  Hydrualic fluid is virtually incompressible so controlling the flow of fluid provides accurate control of the motion of the relevant actuator..    Typical uses of Hydraulic drives include..

  • Car braking systems
  • Vehicle drives in agricultural and civil applications
  • High power low weight motors (high speed and low speed)
  • Elevators
  • Aircraft motion control- wings - undercarriage etc
  • Hydraulic Hoists
  • Automation actuators
  • Machine tool drives
  • Flight Simulators- For training pilots
  • Motion Simulators- For vibration seismic testing

The primary advantage of hydraulic systems compared to pneumatic and electric systems is that high forces and torques can be developed with comparatively compact motors which are very convenient for mountingin different locations and orientations.   Very accurate motion controls are possible using sophisticated servo valves and servo controlled motors and pumps.

The disadvantages of hydraulic systems include the relatively high cost of components and the need to condition and contain the hydraulic fluid and also the need maintain the system ( regular filter changing /checking for leaks/ chaning oil).

The best examply of a simple, reliable bring system is in the braking system of all conventional cars Depressing the brake pedal operates on the piston in the brake's master cylinder/s.    Slave pistons, one at each wheel,are then actuated to press the brake pads against the brake rotor to stop the car

Notes on Hydrostatic Drives are provided on webpage    Hydrostatic Dives


Hydraulic system Components

Hydraulic systems are generally confined to local areas e.g. a vehicle, a machine tool. They are not designed as distributed systems as are pneumatic systems.   However large distributed systems have been employed. In times past, in cities, very extensive systems have been engineered to serve diverse hydraulic units elevators - bridges etc.

Hydraulic systems ...

Operating Conditions.
Hydraulic power systems systems use hydraulic fluids at pressures between 35 barg and 350 barg.
The hydraulic fluid must be maintained within safe temperature regime by natural radiation/convection or by use of coolers.
The hydraulic components have close clearances so the fluid must be filtered.

Suggested velocities in hydraulic systems are as follows.


  • Pump Suction...0,6 - 1,2 m/s
  • Delivery flow...2,0 - 5,0 m/s
  • Return flow...1,5 - 4,0 m/s


Power Pack
The hydraulic power pack impart the essential potential energy to the hydraulic fluid using appropriate pumps e.g. Gear, Vane, Piston pumps are used. The power pack normally includes the following features.

  • Reservoir for hydraulic fluid - with instrumentation, air filter and strainer over outlet
  • Protection for pump including filter
  • Pump located below reservoir to minimise suction feed requirement
  • Accumulator to maintain steady pressure against flow variations
  • Cooler for removing waste heat from the system (Air cooler /Water cooler)
  • Control valves, relief valve, unloading valve, isolation valves
  • Provision for containing any leaks with suitable instrumentation

Typical power pack duties obtained from a suppliers literature
10 kW ....45 l/m......120 bar...3 kW Oil cooler
50 kW ....120 l/m.... 200 bar...15 kW Oil cooler


Interconnecting Pipework
The interconnecting piping on hydraulic systems feeds should include for pressurised piping from the power pack to the equipment and also return piping from the equipment to the reservoir through the cooler.   The interconnecting pipework is very important in hydraulic systems because it is essential to have no fluid leakage against an environment with a high degree of continuous vibration

Connections systems are available using compression fitting, O-ring fittings, and at the highest level welded fittings.   It is often necessary to include flexible hoses connecting to the actuators . These are generally the most unreliable part of the system and must be carefully designed in strict accordance with the suppliers instructions

Hydraulic Valves
Hydraulic valves are generally highly engineered components.  Valve types include

  • Pressure Control Valves - e.g Relief valves, unloading valves, pressure maintaining valves
  • Flow Control Valves -
  • Direction Control Valves -
  • Servo Valves - providing high accuracy fast response control of pressures and flowrates.

The control valves are often controlled using electronic /computer control systems...

Hydraulic Actuators
Hydraulic actuators generally based on single or double acting cylinders and motors.   Small cylinders of less than 10mm dia are available.  At the other extreme hydraulic press tools are available capable of forces in excess of 5000 te.

The motors can be low speed (less than 0.5 rpm ) through to high speed (more than 10,000 rpm). Motors can be provided with torque ranges for 1Nm (high speed)to 20000 Nm.(Low Speed)..

The diagram below is a very simplified generalisation of a hydraulic system circuit.     The cooler and the accumulator are shown although on the majority of applications these components may not be required




Useful Links

  1. Parker .. Supplier of wide range of hydraulic and pneumatic components
  2. Hydraulics Technical Library.. Online hydraulics library containing application formulas, product data, maintenance tips and "how-to" tutorials.
  3. A short downloadable paper providing basic information <
  4. How stuff works Hydraulics.. Clear useful basic information..
  5. Eaton .. A supplier of hydraulic equipment..
  6. Lift Systems...High capacity lifting using hydraulics
  7. Hydraulics...A detailed document on all aspects of Hydraulics

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Last Updated 25/01/2013