Difference between revisions of "Hydraulic fluid"

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The '''hydraulic brake''' is an arrangement of [[brake|braking]] mechanism which uses [[hydraulics|hydraulic]] fluid, typically some type of light-[[viscosity]] [[silicone]] oil, to transfer pressure from the controlling unit, which is usually near the operator of the vehicle, to the actual brake mechanism, which is usually at or near the wheel of the vehicle.
 
  
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The most common arrangement of hydraulic brake, found on most automobiles, consists of a brake pedal, a [[master cylinder]], hydraulic lines, a "[[slave cylinder]]", and the braking unit.
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'''Hydraulic fluids''' are a large group of fluids used as the motive medium in [[hydraulic machinery]]. Fluid types include synthetic compounds, [[mineral oil]], water, and water-based mixtures. The fluids are found in machinery and equipment ranging from [[brake]]s, [[power steering]] systems, and [[transmission (mechanics)|transmissions]] to [[backhoe]]s, [[excavator]]s, [[garbage truck]]s and industrial [[shredder (device)|shredders]]. Hydraulic systems are very common in [[aircraft]] [[Flight controls|flight control systems]].
  
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==General operation==
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Hydraulic systems like the ones mentioned above will work most efficiently if the hydraulic fluid used has low [[compressibility]].
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When the brake pedal is depressed, [[leverage]] multiplies the force applied from the pedal to a [[piston]] in the master cylinder.
 
  
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As force is applied to this piston, pressure in the hydraulic system rises, forcing fluid through the lines to the slave cylinders.
 
  
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The two most common arrangements of slave cylinder are a pair of opposed pistons which are forced apart by the fluid pressure, (drum brakes) and a single piston which is forced out of its housing (disc brakes.)
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== Composition ==
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=== Base stock ===
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Base stock may be any of: [[castor oil]], [[glycol]], [[ester]]s, [[ether]]s, [[mineral oil]], [[organophosphate ester]], [[polyalphaolefin]], [[propylene glycol]], or [[silicone]].
  
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The pistons then apply pressure to the braking mechanism, whether shoes inside a drum, or pads which compress on a disc.
 
  
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==Component specifics==
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=== Other components ===
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(For typical light duty automotive braking systems.)
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Hydraulic fluids can contain a wide range of chemical compounds, including: [[oil]]s, [[butanol]], esters (e.g. [[phthalate]]s, like [[Bis(2-ethylhexyl) phthalate|DEHP]], and [[adipate]]s, like [[bis(2-ethylhexyl) adipate]]), [[polyalkylene glycol]]s (PAG), [[organophosphate|phosphate esters]] (e.g. [[tributylphosphate]]), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g. [[polyisobutene]]s), [[corrosion inhibitor]]s, etc.
  
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The brake pedal is a simple lever. It is attached at one point to the framework of the automobile, a rod extends from a point along its length to the master cylinder, and the pedal is at the end of the lever.
 
  
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The master cylinder is divided internally into two sections, each of which pressurizes a separate hydraulic line. The forward segment applies pressure to the front brakes, and the rearmost segment to the rear brakes. This arrangement is made partly to balance the braking effect between the front and rear wheel sets, and partly for safety reasons: If one system fails, all braking ability is not lost; the other set can stop the vehicle.
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=== Biodegradable hydraulic fluids ===
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A master cylinder may use differing diameters between the two pistons to allow for increased fluid volume to one set of slave cylinders or the other.
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Environmentally sensitive applications (e.g. [[tractor#power and transmission|farm tractors]] and marine [[dredging]]) may benefit from using biodegradable hydraulic fluids based upon [[rapeseed]] ([[Canola]]) [[vegetable oil#industrial uses|vegetable oil]] when there is the risk of an [[oil spill]] from a ruptured oil line. Typically these oils are available as [[International Organization for Standardization|ISO]] 32, ISO 46, and ISO 68 specification oils. [[ASTM]] standards ASTM-D-6006, Guide for Assessing Biodegradability of Hydraulic Fluids and ASTM-D-6046, Standard Classification of Hydraulic Fluids for Environmental Impact are relevant.
  
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Many modern hydraulic brake systems have a "[[Vacuum]]-assist" module which is attached between the master cylinder and the brake pedal and multiplies the braking force applied.  Typically described as a vacuum booster, these units consist of a hollow housing with a moveable rubber [[diaphragm (mechanics)|diaphragm]] across the center, creating two chambers.  When attached to the low-pressure portion of the throttle body or intake manifold of the engine, the pressure in both chambers of the unit is lowered. The equilibrium created by the low pressure in both chambers keeps the diaphragm from moving until the brake pedal is depressed.  A return spring keeps the diaphragm in the starting position until the brake pedal is applied.  When the brake pedal is applied, the movement opens an air valve which lets in atmospheric pressure air to one chamber of the booster.  Since the pressure becomes higher in one chamber, the diaphragm moves toward the lower pressure chamber with a force created by the area of the diaphragm and the differential pressure.  This force, in addition to the driver's foot force, pushes on the master cylinder piston.  The diaphragm will stop moving when the forces on both sides of the chamber reach equilibrium.  This can be caused by either the air valve closing (due to the pedal apply stopping) or if "runout" is reached.  Runout occurs when the pressure in one chamber reaches atmospheric pressure and no additional force can be generated by the now stagnant differential pressure.  After the runout point is reached, only the driver's foot force can be used to further apply the master cylinder piston.
 
  
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The fluid pressure from the master cylinder travels through a pair of steel lines to a compensator, which performs two functions: It equalizes pressure between the two systems, and it provides a warning if one system loses pressure. The compensator has two chambers (to which the hydraulic lines attach) with a piston between them. When the pressure in either line is balanced, the piston does not move. If the pressure on one side is lost, the pressure from the other side moves the piston. When the piston makes contact with a simple electrical probe in the center of the unit, a circuit is completed, and the operator is warned of a failure in the brake system.
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== Brake fluid ==
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[[Brake fluid]] is a subtype of hydraulic fluid with high [[boiling point]] and low [[freezing point]]. It is intentionally hygroscopic, so that it will absorb water which could otherwise cause corrosion of brake system components.
  
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From the compensator, steel lines carry the pressure to the brake units at the wheels. Since the wheels do not maintain a fixed relation to the automobile, it is necessary to use flexible lines at the pivoting areas. Allowing steel lines to flex invites [[metal fatigue]] and, ultimately, brake failure.
 
  
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Steel lines are preferred for most of the system for their rigidity; any amount of bulging or pressure induced distortion in the lines when pressure is applied results in less useful volume and pressure of fluid reaching the slave cylinders, and thus, reduced braking effectiveness. [[Bundy tube]] is often used in cars.
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== Safety ==
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Flexible lines are usually rubber hoses surrounded by steel braid, then coated with rubber to avoid weather damage to the steel.
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Because industrial hydraulic systems operate at hundreds to thousands of PSI and temperatures reaching hundreds of degrees Celsius, severe injuries and death can result from component failures and care must always be taken when performing maintenance on hydraulic systems.
  
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Finally, the fluid pressure enters the Slave Cylinders and use one or more pistons to apply force to the braking unit.
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Fire resistance is a property available with specialized fluids.
  
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==Special considerations==
 
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Hydraulic systems are used where space restrictions must be considered. Air brake systems are bulky, and require [[air compressor]]s and reservoir tanks for their operation. Hydraulic systems are smaller and less expensive.
 
  
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[[Hydraulic fluid]] must be non-compressible. Unlike [[air brake]]s, where a valve is opened, and air is allowed to surge in to the lines and brake chambers until the pressure rises sufficiently, hydraulic systems rely on a single stroke of a piston to force hydraulic fluid through the system.
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== Trade names ==
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If any sort of vapor is introduced into the system, it will compress, and fluid pressure in the system may not rise sufficiently to actuate the brakes. This can lead to loss of control of the vehicle.
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Some of the trade names for hydraulic fluids include Durad, Fyrquel, Houghton-Safe, Hydraunycoil, Lubritherm Enviro-Safe, Pydraul, Quintolubric, Reofos, Reolube, and [[Skydrol]].
  
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Hydraulic braking systems are sometimes subjected to high temperatures during operation in extreme environments such as when descending steep grades. For this reason, hydraulic, or [[brake fluid]] must resist vaporization under temperature extremes.
 
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Water vaporizes easily with heat, and can corrode the metal parts of the system. If it gets into the brake lines, it can degrade brake performance dramatically. This is the reason for the common use of light oils as hydraulic fluids; oil displaces water and coats metal parts, protecting them against corrosion, and it can tolerate much higher temperatures before vaporizing.
 
  
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"[[Brake fade]]" is a condition caused by overheating in which braking effectiveness fades, and ultimately is lost. It may occur for a number of reasons: The pads which engage the rotating part may become overheated and "glaze over" (Become so smooth and hard that they cannot grip the metal sufficiently to slow the vehicle), vapor may be introduced to the system by vaporization of the hydraulic fluid under temperature extremes, and thermal distortion may cause the pads to change their shape and engage less surface area of the rotating part. Thermal distortion may also cause permanent changes in the shape of the metal parts, resulting in a reduction in braking capability that is irreparable without complete replacement of the affected parts.
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== Aircraft hydraulic systems ==
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The use of hydraulic systems in aircraft almost certainly began with braking systems.{{Fact|date=February 2008}}  As aircraft performance increased in mid-20th century, the amount of force required to operate mechanical flight controls became excessive, and hydraulic systems were introduced to reduce pilot effort.  The hydraulic actuators are controlled by valves; these in turn are operated directly by input from the aircrew (hydro-mechanical) or by computers obeying control laws (fly by wire).  See [[flight controls]]. 
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Hydraulic power is used for other purposes.  It can be stored in accumulators to start an auxiliary power unit (APU) for self-starting the aircraft's main engines.  Many aircraft equipped with the [[M61 Vulcan|M61]] family of cannon use hydraulic power to drive the gun system, permitting reliable high rates of fire.
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The hydraulic power itself comes from pumps driven by the engines directly, or by electrically driven pumps. In modern commercial aircraft these are electrically driven pumps, should all the engines fail in flight the pilot will deploy a propeller driven electric generator which is concealed under the fuselage.<ref>Discovery channel-'seconds from disaster'</ref> This provides electrical power for the hydraulic pumps and control sysytems as power is no longer avaliable from the engines. In that system and others electric pumps can provide both redundancy and the means of operating hydraulic systems without the engines operating, which can be very useful during maintenance.
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===Specifications===
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Aircraft hydraulic fluids fall under various specifications:
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Common petroleum-based:
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*Mil-H-5606:  Mineral base, flammable, fairly low flashpoint, usable from -65F to 275F, red color
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*Mil-H-83282:  Synthetic hydrocarbon base, higher flashpoint, self-extinguishing, backward compatible to -5606, red color
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*Mil-H-87257:  A development of -83282 fluid to improve its low temperature viscosity.
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Phosphate-ester based:
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*BMS 3-11: [[Skydrol]] 500B-4, Skydrol LD-4, Skydrol 5 and [[Exxon]] HyJetIV-A plus - Typically light purple, not compatible with petroleum-based fluids, will not support combustion.
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===Contamination===
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Special, stringent care is required when handling aircraft hydraulic fluid as it is critical to flight safety that it stay free from contamination.  It is also necessary to strictly adhere to authorized references when servicing or repairing any aircraft system. Samples from aircraft hydraulic systems are taken during heavy [[aircraft maintenance checks]] to check contamination.
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== See also ==
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*[[Dexron]]
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*[[Hydraulic brake]]
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*[[Osmosis]]
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== External links ==
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*[http://www.nfpa.com Information about Fluid Power is also available on the National Fluid Power Association web-site nfpa.com]
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*USDA Research. [http://www.ars.usda.gov/is/AR/archive/nov98/oil1198.htm Biodegradable Plant-Based Hydraulic Fluid]
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*[http://www.oiluk.net/industrial_lubricants/hydraulic_oils.html Industrial Hydraulic Oils]
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*[http://www.shell.com/static/aviation-en/downloads/publications/aeroshellbook/aeroshellhydraulicfluids.pdf Aviation hydraulic fluids]
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{{Automobile configurations}}
 
{{Automobile configurations}}
  
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[[Category:Fluid dynamics]]
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[[Category:Gear oils]]
 
[[Category:Vehicle braking technologies]]
 
[[Category:Vehicle braking technologies]]

Latest revision as of 20:18, 12 February 2009


Hydraulic fluids are a large group of fluids used as the motive medium in hydraulic machinery. Fluid types include synthetic compounds, mineral oil, water, and water-based mixtures. The fluids are found in machinery and equipment ranging from brakes, power steering systems, and transmissions to backhoes, excavators, garbage trucks and industrial shredders. Hydraulic systems are very common in aircraft flight control systems.

Hydraulic systems like the ones mentioned above will work most efficiently if the hydraulic fluid used has low compressibility.


Composition

Base stock

Base stock may be any of: castor oil, glycol, esters, ethers, mineral oil, organophosphate ester, polyalphaolefin, propylene glycol, or silicone.


Other components

Hydraulic fluids can contain a wide range of chemical compounds, including: oils, butanol, esters (e.g. phthalates, like DEHP, and adipates, like bis(2-ethylhexyl) adipate), polyalkylene glycols (PAG), phosphate esters (e.g. tributylphosphate), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g. polyisobutenes), corrosion inhibitors, etc.


Biodegradable hydraulic fluids

Environmentally sensitive applications (e.g. farm tractors and marine dredging) may benefit from using biodegradable hydraulic fluids based upon rapeseed (Canola) vegetable oil when there is the risk of an oil spill from a ruptured oil line. Typically these oils are available as ISO 32, ISO 46, and ISO 68 specification oils. ASTM standards ASTM-D-6006, Guide for Assessing Biodegradability of Hydraulic Fluids and ASTM-D-6046, Standard Classification of Hydraulic Fluids for Environmental Impact are relevant.


Brake fluid

Brake fluid is a subtype of hydraulic fluid with high boiling point and low freezing point. It is intentionally hygroscopic, so that it will absorb water which could otherwise cause corrosion of brake system components.


Safety

Because industrial hydraulic systems operate at hundreds to thousands of PSI and temperatures reaching hundreds of degrees Celsius, severe injuries and death can result from component failures and care must always be taken when performing maintenance on hydraulic systems.

Fire resistance is a property available with specialized fluids.


Trade names

Some of the trade names for hydraulic fluids include Durad, Fyrquel, Houghton-Safe, Hydraunycoil, Lubritherm Enviro-Safe, Pydraul, Quintolubric, Reofos, Reolube, and Skydrol.


Aircraft hydraulic systems

The use of hydraulic systems in aircraft almost certainly began with braking systems.Template:Fact As aircraft performance increased in mid-20th century, the amount of force required to operate mechanical flight controls became excessive, and hydraulic systems were introduced to reduce pilot effort. The hydraulic actuators are controlled by valves; these in turn are operated directly by input from the aircrew (hydro-mechanical) or by computers obeying control laws (fly by wire). See flight controls.

Hydraulic power is used for other purposes. It can be stored in accumulators to start an auxiliary power unit (APU) for self-starting the aircraft's main engines. Many aircraft equipped with the M61 family of cannon use hydraulic power to drive the gun system, permitting reliable high rates of fire.

The hydraulic power itself comes from pumps driven by the engines directly, or by electrically driven pumps. In modern commercial aircraft these are electrically driven pumps, should all the engines fail in flight the pilot will deploy a propeller driven electric generator which is concealed under the fuselage.[1] This provides electrical power for the hydraulic pumps and control sysytems as power is no longer avaliable from the engines. In that system and others electric pumps can provide both redundancy and the means of operating hydraulic systems without the engines operating, which can be very useful during maintenance.


Specifications

Aircraft hydraulic fluids fall under various specifications:

Common petroleum-based:

  • Mil-H-5606: Mineral base, flammable, fairly low flashpoint, usable from -65F to 275F, red color
  • Mil-H-83282: Synthetic hydrocarbon base, higher flashpoint, self-extinguishing, backward compatible to -5606, red color
  • Mil-H-87257: A development of -83282 fluid to improve its low temperature viscosity.

Phosphate-ester based:

  • BMS 3-11: Skydrol 500B-4, Skydrol LD-4, Skydrol 5 and Exxon HyJetIV-A plus - Typically light purple, not compatible with petroleum-based fluids, will not support combustion.


Contamination

Special, stringent care is required when handling aircraft hydraulic fluid as it is critical to flight safety that it stay free from contamination. It is also necessary to strictly adhere to authorized references when servicing or repairing any aircraft system. Samples from aircraft hydraulic systems are taken during heavy aircraft maintenance checks to check contamination.


See also


External links


  1. ↑ Discovery channel-'seconds from disaster'
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