Dasar Sistem Hidraulik

Tergantung dari fungsi dan rancangannya, setiap system hidraulik mempunyai beberapa komponen dasar bagaimana cairan disalurkan.

Sistem yang menggunakan pompa tangan

Reservoir menyimpan cadangan cairan hidraulik untuk bekerjanya system.dan mengisi ulang cairan bila diperlukan, menjadi ruangan untuk perkembangan suhu dan pada beberapa system merupakan penyaluran udara dari system.

Sebuah pompa diperlukan untuk menciptakan aliran fluida, tetapi pada pesawat terbang biasanya menggunakan pompa yang digerakkan oleh mesin atau oleh motor listrik.

Katup pemilih/selector valve digunakan untuk mengarahkan aliran fluida. Normalnya katup ini digerakkan dengan menggunakan solenoid atau juga dapat dioperasikan secara manual, baik langsung atau tidak langsung dengan menggunakan sambungan mekanik.Suatu tabung/silinder penggerak (Actuating Cylinder) merubah tekanan fluida menjadi kerja/usaha baik secara gerakan liniar atau gerakan naik turun, dimana sebuah motor mengubah tekanan fluida menjadi gerakan yang berguna dengan gerakn putar secara mekanikal.

Pada posisi katup pemilih/selector valve seperti pada gambar, cairan hidraulik mengalir melalui katup pemilih keujung kanan dari actuating cylinder. Tekanan cairan/fluida menekan piston kearah kiri, dan pada waktu yang sama cairan yang berada disebelah kiri dari piston/torak ditekan keluar, keatas melalui katup pemilih dan kembali ke reservoir melalui pipa balik.

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Ketika katup pemilih digerakkan/dipindahkan keposisi yang berlawanan, cairan dari pompa mengalir kesebelah kiri actuating cylinder(actuator) dan melakukan proses sebaliknya. Gerakan dari piston dapat dihentikan setiap saat dengan menempatkan katup pemilih pada posisi netral. Pada posisi ini keempat lubang tertutup dan tekanan cairan tertahan pada pipa penyalur.

Sistem tenaga penggerak pompa

Gambar diatas menunjukkan sistim dasar tambahan dengan pompa yang digerakkan dengan suatu tenaga dan juga saringan, pengatur tekanan, accumulator, pengukur tekanan, relief valve/katup pengatur tekanan,dan dua check valves/katup pengarah aliran.

Filter atau saringan menyaring partikel-partikel kecil dari cairan hidraulik, mencegah debu, pasir atau benda benda lain memasuki system.

Pengatur tekanan/pressure regulator, mengatur/mengurangi tekanan dari pompa sampai tekanan yang diinginkan tercapai. Pengatur ini biasa disebut juga katup pembuang. Ketika satu actuator dioperasikan dan tekanan dalam pipa antara pompa dan katup pemilih telah mencapai titik yang diinginkan, suatu katup dalam pengatur tekanan secara otomatis terbuka dan fluida dikembalikan langsung ke reservoir seperti terlihat pada gambar.

Banyak system hidraulik tidak menggunakan pengatur tekanan, tetapi mempunyai suatu cara untuk menyalurkan kelebihan tekanan kembali kepompa pengisap, Pada banyak pesawat, tekanan atmosfir merupakan prinsip dasar  tekanan yang menyebabkan fluida kembali ke pompa pengisap. Tetapi banyak pesawat yang tidak menggunakan tekanan atmosfir ini karena dianggap terlalu rendah untuk menjadi tekanan penggerak bagi cairan/fluida memasuki pompa, dan reservoir harus mendapatkan tekanan tertentu.

Ada beberapa cara untuk memasukan tekanan ke reservoir. Beberapa system menggunakan tekanan udara langsung dari sistim tekanan kabin pesawat; atau dari engine compressor pada pesawat yang menggunakan turbine engine. Cara lain adalah dengan menggunakan aspirator atau venture tee. Pada system lainnya suatu pompa hidraulik tambahan dipasang pada pipa penyalur yang keluar dari reservoir untuk menyalurkan fluida bertekanan  ke pompa hidraulik utama.

Penekanan dengan udara dicapai dengan memasukan udara bertekanan kedalam reservoir diatas permukaan cairan/fluida. Udara bertekanan 100 psi didapatkan dari engine. Tekanan ini dikurangi sekitar 5 sampai 15 psi, tergantung dari jenis system hidraulik dengan menggunakan pengatur tekanan.

Reservoir yang digunakan menggunakan tekanan hidraulik berbeda dengan reservoir yang menggunakan tekanan udara

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Pompa tangan

Pompa tangan digunakan pada beberapa pesawat terdahulu dan pada sistem yang baru hanya digunakan sebagai cadangan/back up.

Pompa yang digerakkan dengan suatu daya

Pompa-pompa yang digerakkan dengan suatu daya, baik dengan menggunakan putaran engine atau menggunakan motor listrik diperlihatkan pada gambar dibawah :

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Pressure Regulation/Pengaturan tekanan

Tekanan hidraulik harus diatur agar dapat digunakan untuk bermacam-macam tujuan yang dikehendaki. Sistem pengaturan tekanan selalu menggunakan 3 alat ; Katup pengaman tekanan,pengatur tekanan dan alat penunjuk tekanan.

Pressure relief valve/katup pengaman tekanan

Katup pengaman tekanan digunakan untuk membatasi tekanan pada cairan. Hal ini diperlukan untuk mencegah kerusakan pada component atau pipa-pipa terhadap tekanan berlebihan, dan merupakan katup system pengaman. Dalam rancangannya alat ini menggunakan per/pegas yang dapat diatur. Ditempatkan sedemikian rupa untuk mengeluarkan cairan dari pipa kembali ke reservoir bila tekanan melebihi tekanan maksimum yang diinginkan.Per ini kekencangannya yang dapat terdorong oleh tekanan cairan yang berlebihan.

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Pressure gage/ pengukur tekanan

Alat ini digunakan untuk mengukur tekanan dari system hidraulik yang digunakan untuk mengoperasikan unit-unit pada pesawat terbang.Alat ini menggunakan Tabung Bourdon/Bourdon tube dengan susunan mekanik untuk menyalurkan pengembangan tabung ke indicator. Suatu lubang kecil/vent pada bagian bawah wadah, memungkinkan tekanan atmosfir memasuki sekitar tabung bourdon.Lubang ini juga dimaksudkan untuk mengeluarkan kelembaban.

Accumulator

Accumulator adalah suatu wadah dari baja yang ruangannya dibagi menjadi dua oleh suatu diaphragma karet sintetis. Ruangan atas terisi cairan yang bertekanan sesuai system, dan ruangan bawah diisi udara.

Fungsi accumulator adalah :

1. Meredam perubahan tekanan dari system hidraulik yang disebabkan oleh tekanan pompa dan kerja dari unit.
2. Membantu daya pompa ketika beberapa unit bekerja serentak dimana dibutuhkan tekanan ekstra yang telah dikumpulkan oleh accumulator.
3. Menyimpan daya tekanan untuk operasi terbatas dari unit hidraulik ketika pompa tidak bekerja.
4. Memberikan cairan bertekanan untuk mengatasi bila ada kebocoran external maupun internal yang menyebabkan sistim berubah-ubah karena kerja pressure switch /sakelar tekanan yang selalu menendang-nendang /”kicking in”

Dibawah ini ditunjukkan beberapa macam Accumulator:

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Perawatan accumulator

Perawatan termasuk inspeksi, perbaikan kecil, penggantian komponen dan testing. Pada accumulator terdapat element yang berbahaya sehingga harus dilakukan percegahan yang tepat agar tidak terluka ataupun rusak.

Sebelum melepas accumulator. Yakinkan semua udara bertekanan yang dimasukkan(atau nitrogen) telah dikeluarkan. Kesalahan dalam melepas/membuang udara dapat menyebabkan kecelakaan yang serius. Yakinkan jenis katup udara bertekanan tinggi macam apa yang digunakan. Bila sudah diketahui atau diyakini semua udara bertekanan telah dilepas/dibuang, lanjutkan untuk melepas unit. Selalu ikuti petunjuk pabrik untuk jenis unit yang digunakan.

Check Valve /katup pengarah

Agar komponen hidraulik dan system bekerja sebagaimana diharapkan, aliran fluida harus terkontrol. Fluida harus mengalir sesuai rencana tertentu.Salah satu alat untuk melaksanakan control ini adalah dengan menggunakan check valve yang akan mengarahkan aliran fluida kesatu arah saja. Gambar dibawah menunjukkan macam-macam jenis check valve yang umum digunakan :

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Actuating Cylinder / Actuator

Suatu Actuating Cylinder/Actuator merubah energi tekanan fluida menjadi gaya mekanik, atau aksi untuk melakukan kerja. Alat ini digunakan melakukan tenaga gerakan liniar pada suatu benda bergerak atau mekanisme.

Actuating Cylinder terdiri dari tabung, dengan beberapa piston, tangkai piston dan beberapa penyekat/seal.Tabung silinder bagian dalamnya dibuat halus mengkilat dimana piston bergerak, dan beberapa lubang dimana fluida masuk dan keluar dari dalam tabung. Piston bergerak maju/mundur dalam silinder dan suatu tangkai piston bergerak masuk/keluar dari silinder melalui lubang pada salah satu ujung silinder. Penyekat/seal digunakan untuk mencegah kebocoran antara piston dengan dinding dalam silinder, dan antara tangkai piston dengan ujung silinder. Baik tabung silinder dan tangkai piston dapat disambungkan kesuatu benda atau mekanisme  yang harus digerakkan oleh actuating cylinder/actuator. Gambar dibawah menunjukkan macam-macam jenis actuating cylinder/actuator yang umum digunakan :

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Selector Valve/ Katup pemilih

Katup pemilih digunakan untuk mengontrol arah gerakan unit penggerak/ actuating unit. Suatu katup pemilih/selector valve, merupakan jalur aliran cairan hidraulik untuk masuk dan keluar actuator. Alat ini juga merupakan sakelar yang baik untuk mengarahkan aliran cairan agar tangkai silinder dari actuator dapat terjulur atau tertarik masuk. Macam-macam Selector valve yang umum digunakan diperlihatkan pada gambar dibawah.

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GENERAL – hydraulic system have many advantages as a power source for operating various aircraft units. The advantages of hydraulic; light weight, ease of installation, simplification of inspection, and minimum maintenance requirements. Hydraulic operations are also almost 100% efficient, with only a negligible loss due to fluid friction.

Hydraulic fluid, - are use primarily to transmit and distribute forces to various units to be actuate, they are almost incompressible. Pascal’s Law states that pressure applied to any part of a confined liquid is transmitted with undiminished intensity to every other part. Thus, if a number of passages exist in a system, pressure can be distributed through all of them by means of the liquid.

Viscosity, - one of the most important properties of any hy­draulic fluid is its viscosity. Viscosity is internal resistance to flow. A liquid such as gasoline flows easily (has a low viscosity) while a liquid such as tar flows slowly (has a high viscosity). Viscosity increases with temperature decreases.

A satisfactory liquid for a given hydraulic system must have enough body to give a good seal at pumps, valves, and pistons; but it must not be so thick that it offers resistance to flow, leading to power loss and higher operating temperatures. These factors will add to the load and to excessive wear of parts. A fluid that is too thin will also lead to rapid wear of moving parts, or of parts which have heavy loads.

The viscosity of a liquid is measured with a viscometer. The Saybolt universal viscosimeter or viscometer instrument measures the number of seconds it takes for a fixed quantity of liquid (60 cc. (cubic centimeters)) to flow through a small orifice of standard length and diameter at a specific temperature. This time of flow is taken in seconds, and the viscosity reading is expressed as SSU (Sec­onds, Saybolt Universal). For example, a certain liquid might have a viscosity of 80 SSU at 130⁰ F.

Flash point, - is the temperature at which a liquid gives off vapor in sufficient quantity to ignite mo­mentarily or flash when a flame is applied. A high flash point is desirable for hydraulic liquids be­cause it indicates good resistance to combustion and a low degree of evaporation at normal temperatures.

Fire point, - is the temperature at which a sub­stance gives off vapor in sufficient quantity to ignite and continue to burn when exposed to a spark or flame, like flash point, a high fire point is required of desirable hydraulic liquids.

Type of hydraulic fluids

To assure proper system operation and to avoid damage to non-metallic components of the hydraulic system, the correct fluid must be used. When adding fluid to a system, use the type specified in the aircraft manufacturer's maintenance manual or on the instruction plate affixed to the reservoir or unit being serviced.

There are three types of hydraulic fluids cur­rently being used in civil aircraft:

Vegetable base hydraulic fluid (MIL-H-7644) is composed essentially of caster oil and alcohol. It has a pungent alcoholic odor and is generally dyed blue. Although it has a similar composition to automotive type hydraulic fluid, it is not inter­changeable. Natural rubber seals are used with vegetable base hydraulic fluid. If it is contami­nated with petroleum base or phosphate ester base fluids, the seals will swell, break down and block the system. This type fluid is flammable.

Mineral base hydraulic fluid (MIL-H-5606) is processed from petroleum. It has an odor similar to penetrating oil and is dyed red. Synthetic rub­ber seals are used with petroleum base fluids. Do not mix with vegetable base or phosphate ester base hydraulic fluids. This type fluid is flammable.

PHOSPHATE ESTER BASE FLUIDS

Non-petroleum base hydraulic fluids were intro­duced to provide a fire-resistant hydraulic fluid for use in high performance piston engines and turboprop aircraft. These fluids were fire-resistance tested by being sprayed through a welding torch flame (6000⁰). There was no burning, but only occasional flash of fire. These and other tests proved non-petroleum base fluids (Skydrol ®) would not support com­bustion. Even though they might flash at exceed­ingly high temperatures, Skydrol ® fluids could not spread a fire because burning was localized at the source of beat. Once the heat source was re­moved or the fluid flowed away from the source, no further flashing or burning occurred.

Several types of phosphate ester base (Sky­drol ® ) hydraulic fluids have been discontinued. Currently used in aircraft are Skydrol ® 500B-­a clear purple liquid having good low temperature operating characteristics and low corrosive side effects; and, Skydrol ® LD-a clear purple low weight fluid formulated for use in large and jumbo jet transport aircraft where weight is a prime factor.

Intermixing of Fluids, - due to the difference in composition, vegetable base, petroleum base and phosphate ester fluids will not mix. Neither are the seals for any one fluid useable with or tolerant of any of the other fluids. Should an aircraft hydraulic system be serviced with the wrong type fluid, immediately drain and flush the system and maintain the seals according to the manufacturer's specifications.

Health and Handling, - Skydrol ® fluid has a very low order of toxicity when taken orally or applied to the skin in liquid form. It causes pain on contact with eye tissue, but animal studies and human experience indicate Skydrol fluid causes no permanent damage. First aid treatment for eye contact includes flushing the eyes immediately with large volumes of water and the application of any anesthetic eye solution. If pain persists, the individual should be referred to a physician.

In mist or fog form, Skydrol ® is quite irri­tating to nasal or respiratory passages and gen­erally produces coughing and sneezing. Such irritation does not persist following cessation of exposure. Silicone ointments, rubber gloves, and careful washing procedures should be utilized to avoid excessive repeated contact with Skydrol ® in order to avoid solvent effect on skin.

Hydraulic Fluid Contamination, - Experience has shown that trouble in a hydraulic system is inevitable whenever the liquid is allowed to become contaminated. The nature of the trouble, whether a simple malfunction or the complete de­struction of a component, depends to some extent on the type of contaminant.

Two general contaminants are:

(1)       Abrasives, including such particles as core sand, weld spatter, machining chips, and rust.

(2)       Non-abrasives, including those resulting from oil oxidation, and soft particles worn or shredded from seals and other organic components.

Contamination Check, - Whenever it is suspected that a hydraulic system has become contaminated, or the system has been operated at temperatures in excess of the specified maximum, a check of the system should be made. The filters in most hydraulic systems are designed to remove most foreign particles that are visible to the naked eye. Hydraulic liquid which appears clean to the naked eye may be contaminated to the point that it is unfit for use.

Thus, visual inspection of the hydraulic liquid does not determine the total amount of contamina­tion in the system. To determine which component is defective, liquid samples should be taken from the reservoir and various other locations in the system.

Contamination Control, - Filters provide adequate control of the contami­nation problem during all normal hydraulic

system operations. Control of the size and amount of con­tamination entering the system from any other source is the responsibility of the people who serv­ice and maintain the equipment.

Therefore, precau­tions should be taken to minimize contamination during maintenance, repair, and service operations. Should the system become contaminated, the filter element should be removed and cleaned or replaced. As an aid in controlling contamination, the fol­lowing maintenance and servicing procedures should be followed at all times:

(1)          Maintain all tools and the work area (workbenches and test equipment) in a clean, dirt-free condition.

(2)          A suitable container should always be pro­vided to receive the hydraulic liquid that is spilled during component removal or disassembly procedures.

(3)          Before disconnecting hydraulic lines or fit­tings, clean the affected area with dry cleaning solvent.

(4)          All hydraulic lines and fittings should be capped or plugged immediately after dis­connecting.

(5)          Before assembly of any hydraulic compo­nents, wash all parts in an approved dry cleaning solvent.

(6)          After cleaning the parts in the dry clean­ing solution, dry the parts thoroughly and lubricate them with the recommended pre­servative or hydraulic liquid before assem­bly. Use only clean, lint-free cloths to wipe or dry the component parts.

(7)          All seals and gaskets should be replaced during the re-assembly procedure. Use only those seals and gaskets recommended by the manufacturer.

(8)          All parts should be connected with care to avoid stripping metal slivers from threaded areas. All fittings and lines should be installed and torque in accord­ance with applicable technical instruc­tions.

(9)          All hydraulic servicing equipment should be kept clean and in good operating con­dition.

FILTERS, - is a screening or straining device used to clean the hydraulic fluid, thus preventing for­eign particles and contaminating

substances from remaining in the system. If such objectionable material is not removed, it may cause the entire hydraulic system of the aircraft to fail through the breakdown or malfunctioning of a single unit of the system.

The hydraulic fluid holds in suspension tiny par­ticles of metal that are deposited during the normal wear of selector valves, pumps, and other system components. Such minute particles of metal may injure the units and parts through which they pass if they are not removed by a filter.

Since toler­ances within the hydraulic system components are quite small, it is apparent that the reliability and efficiency of the entire system depends upon ade­quate filtering.

There are many models and styles of filters. Their position in the aircraft and design require­ments determine their shape and size. Most filters used in modern aircraft are of the inline type. The inline filter assembly is com­prised of three basic units: head assembly, bowl, and element. The head assembly is that part which is secured to the aircraft structure and con­necting lines. Within the head there is a bypass valve which routes the hydraulic fluid directly from the inlet to the outlet port if the filter element becomes clogged with foreign matter. The bowl is the housing which holds the element to the filter head and is that part which is removed when ele­ment removal is required.

The element may be either a micronic, porous metal, or magnetic type. The micronic element is made of a specially treated paper and is normally thrown away when removed. The porous metal and magnetic filter elements are designed to be cleaned by various methods and replaced in the system.

Maintenance of Filters, - Maintenance of filters is relatively easy. It mainly involves cleaning the filter and element or cleaning the filter and replacing the element. Filters using the micronic type element should have the element replaced periodically according to ap­plicable instructions. Since reservoir filters are of the micronic type, they must also be periodically changed or cleaned. Filter using other than the micronic-type element, cleaning the filter and element is usually all that is necessary. However, the element should be inspected very closely to insure that it is completely undamaged. The methods and materials used in cleaning all filters are too numerous to mention. Consult the manu­facturer's instructions for this information.

BASIC HYDRAULIC SYSTEM

A fluid power system in which the fluid in the system  remains  pressurized  from  the  pump  (or regulator) to the directional control valve while the pump is operating is referred to as a closed-center system. In this type of system, any number of subsystems may be incorporated, with a separate directional control valve for each subsystem. The directional control valves are arranged in parallel so that system pressure acts equally on all control valves.

Another type of system hat is sometimes used in hydraulically operated equipment is the open-center system.  An open-center system has fluid flow but no internal pressure when the actuating mechanisms are idle. The pump circulates the fluid from the reservoir, through the directional control valves, and back to the reservoir. Like the closed-center system, the open- center system may have any number of subsystems, with a directional control valve for each subsystem.

Unlike the closed-center system, the directional control valves of an open-center system are always connected in series with each other, an arrangement in which the system pressure line goes through each directional control valve. Fluid is always allowed free passage through each control valve and back to the reservoir until one of the control valves is positioned

to operate a mechanism.

The first of the basic components, the reservoir, stores the supply of hydraulic fluid for operation of the system. It replenishes the system fluid when needed, provides room for thermal expansion, and in some systems provides a means for bleeding air from the system.

A pump is necessary to create a flow of fluid. The pump shown is hand operated; however, aircraft systems are, in most instances equipped with engine-driven or electric motor­ driven pumps.

The selector valve is used to direct the flow of fluid. These valves are normally actuated by sole­noids or manually operated, either directly or in­directly through use of mechanical linkage. An actuating cylinder converts fluid pressure into use­ful work by linear or reciprocating mechanical motion, whereas a motor converts fluid pressure into useful work by rotary mechanical motion.

The flow of hydraulic fluid can be traced from the reservoir through the pump to the selector valve. With the selector valve in the position shown, the hydraulic fluid flows through the selector valve to the right-hand end of the actuating cylinder. Fluid pressure then forces the piston to the left, and at the same time the fluid which is on the left side of the piston is forced out, up through the selector valve, and back to the reservoir through the return line.

When the selector valve is moved to the opposite position, the fluid from the pump flows to the left side of the actuating cylinder, thus reversing the process. Movement of the piston can be stopped at any time by moving the selector valve to neutral. In this position, all four ports are closed and pres­sure is trapped in both working lines.

Power Driven Pump System, - a basic system with the addi­tion of a power-driven pump and filter, pressure regulator, accumulator, pressure gage, relief valve, and two check valves

The function of each of these components is described in the following paragraphs.

The filter removes foreign particles from the hydraulic fluid, preventing dust, grit, or other un­desirable matter from entering the system.

The pressure regulator unloads or relieves the power-driven pump when the desired pressure in the system is reached. Thus, it is often referred to as an unloading valve. When one of the actuat­ing units is being operated and pressure in the line between the pump and selector valve builds up to the desired point, a valve in the pressure regulator automatically opens and fluid is bypassed back to the reservoir. This bypass line is shown leading from the pressure regulator to the return line.

Many hydraulic systems do not use a pressure regulator, but have other means of unloading the pump and maintaining the desired pressure in the system.

 The accumulator serves a twofold purpose:

(1)    It acts as a cushion or shock absorber by maintaining an even pressure in the system, and

(2)    It stores enough fluid under pressure to pro­vide for emergency operation of certain actuating units. Accumulators are designed with a com­pressed air chamber which is separated from the fluid by a flexible diaphragm or movable piston.

The pressure gage indicates the amount of hydraulic pressure in the system. The relief valve is a safety valve installed in the system to bypass fluid through the valve back to the reservoir in case excessive pressure is built up in the system.

The check valves allow the flow of fluid in one direction only. Check valves are installed at various points in the lines of all aircraft hydraulic systems. In figure 8-4, one check valve prevents power-pump pressure from entering the hand-pump line; the other prevents hand-pump pressure from being directed to the accumulator.

The units of a typical hydraulic system used most commonly are discussed in detail in the fol­lowing paragraphs. Not all models or types are included, but examples of typical components are used in all cases.

RESERVOIRS, - there are two types of reservoirs and they are:

(1)  In-Line-this type has its own housing, is complete within it and is connected with other components in a system by tub­ing or hose.

(2)  Integral-this type has no housing of its own but is merely a space set aside within some major component to hold a supply of operational fluid. A familiar example of this type is the reserve fluid space found within most automobile brake master cyl­inders.

In an in-line reservoir, a space is provided in the reservoir, above the normal level of the fluid, for fluid expansion and the escape of entrapped air. Reservoirs are never intentionally filled to the top with fluid. Most reservoirs are designed so the rim of the filler neck is somewhat below the top of the reservoir to prevent over filling during servicing. Most reservoirs are equipped with a dipstick or a glass sight gage by which fluid level can be conveniently and accurately checked.

Reservoirs are either vented to the atmosphere or closed to the atmosphere and pressurized. In vented reservoirs, atmospheric pressure and gravity are the forces which cause fluid to flow from the reservoir into the pump intake. On many aircraft, atmos­pheric pressure is the principal force causing fluid to flow to the pump intake. However, for some aircraft, atmospheric pressure becomes too low to supply the pump with adequate fluid, and the reser­voirs must be pressurized.

There are several methods of pressurizing a res­ervoir. Some systems use air pressure directly from the aircraft cabin pressurization system; or from the engine compressor in' the case of turbine-pow­ered aircraft. Another method used is an aspirator or venture tee.

In other systems an additional hy­draulic pump is installed in the supply line at the reservoir outlet to supply fluid under pressure to the main hydraulic pump.

Pressurizing with air is accomplished by forcing air into the reservoir above the level of the fluid. In most cases, the initial source of the air pressure is the aircraft engine from which it is bled. Usually, air coming directly from the engine is at a pressure of approximately 100 Psi. This pressure is reduced to between 5 and 15 PSI, depending upon the type of hydraulic system, by using an air pressure regu­lator.

Reservoir Components, - baffles and/or fins are incorporated in most res­ervoirs to keep the fluid within the reservoir from having random movement such as vortexing (swirl­ing) and surging. These conditions can cause fluid to foam and air to enter the pump along with the fluid.

Some aircraft have emergency hydraulic systems that take over if main systems fail. In many such systems, the pumps of both systems obtain fluid from a single reservoir. Under such circumstances a supply of fluid for the emergency pump is ensured by drawing the hydraulic fluid from the bottom of the reservoir. The main system draws its fluid through a standpipe located at a higher level. With this arrangement, adequate fluid is left for opera­tion of the emergency system should the main sys­tem's fluid supply become depleted.

Double Action Hand Pumps, - is used in some older aircraft as a backup unit. Double-action hand pumps pro­duce fluid flow and pressure on each stroke of the handle.

Power-Driven Pumps, - Many of the power-driven hydraulic pumps of current aircraft are of variable-delivery, compensa­tor-controlled type. There are some constant­ delivery pumps in use. Principles of operation are the same for both types of pumps. Because of its relative simplicity and ease of understanding, the constant-delivery pump is used to describe the prin­ciples of operation of power-driven pumps.

A constant-delivery pump, regardless of pump r.p.m. forces a fixed or unvarying quantity of fluid through the outlet port during each revolution of the pump. Constant-delivery pumps are sometimes called constant-volume or fixed-delivery pumps. When a constant-delivery pump is used in a hydraulic system in which the pressure must be kept at a constant

value, a pressure regulator is required.

A variable-delivery pump has a fluid output that is varied to meet the pressure demands of the sys­tem by varying its fluid output. The pump output is changed automatically by a pump compensator within the pump. Various types of pumping mechanisms are used in hydraulic pumps, such as gears, gear-rotors, vanes, and pistons. The piston-type mechanism is com­monly used in power-driven pumps because of its durability and capability to develop high pressure. In 3.000 PSI hydraulic systems, piston-type pumps are nearly always used.

 

Hydraulic pressure must be regulated in order to use it to perform the desired tasks. Pressure regulating systems will always use three elemental devices; a pressure relief valve, a pressure regulator and a pressure gage. A pressure relief valve is used to limit

the amount of pressure being exerted on a confined liquid. This is necessary to prevent failure of components or rupture of hydraulic lines under excessive pres­sures. The pressure relief valve is, in effect, a sys­tem safety valve.

The design of pressure relief valves incorporates adjustable spring-loaded valves. They are, installed in such a manner as to discharge fluid from the pressure line into a reservoir return line when the pressure exceeds the predetermined maximum for which the valve is adjusted, Various makes and designs of pressure relief valves are in use, but, in general, they all employ a spring-loaded valuing device operated by hydraulic pressure and spring tension.

Pressure relief valves are adjusted by in­creasing or decreasing the tension on the spring to determine the pressure required to opens the valve.

Pressure relief valves cannot be used as pressure regulators in large hydraulic systems that depend upon engine-driven pumps for the primary source of pressure because the pump is constantly under load, and the energy expended in holding the pres. sure relief valve off its seat is changed into heat. This beat is transferred to the fluid and in turn to the packing rings causing them to deteriorate rap­idly. Pressure relief valves, however, may be used as pressure regulators in small, low-pressure sys­tems or when the pump is electrically driven and is used intermittently.

Pressure relief valves may be used as:

• System relief valve. The most common use of the pressure relief valve is as a safety device against the possible failure of a pump compensator or other pressure regu­lating device. All hydraulic systems which have hydraulic pumps incorporate pres­sure relief valves as safety devices.
• Thermal relief valve. The pressure relief valve is used to relieve excessive pressures that may exist due to thermal expansion of the fluid.

Pressure Regulators, - the term "pressure regulator" is applied to a device used in hydraulic systems that are pressur­ized by constant-delivery type pumps. One purpose of the pressure regulator is to manage the output of the

pump to maintain system operating pressure within a predetermined range. The other purpose is to permit the pump to turn without resistance (termed unloading the pump) at times when pres­sure in the system is within normal operating range.

The pressure regulator is so located in the system that pump output can get into the system pressure circuit only by passing through the regulator. The combination of a constant-delivery type pump and the pressure regulator is virtually the equivalent of a compensator-controlled, variable-delivery type PUMP.

Pressure Gage, - the purpose of this gage is to measure the pres­sure, in the hydraulic system, used to operate hydraulic units on the aircraft. The gage uses a Bourdon tube and a mechanical arrangement to transmit the tube expansion to the indicator on the face of the gage. A vent in the bottom of the case maintains atmospheric pressure around the Bourdon tube.

Accumulator, - is a steel sphere divided into two chambers by a synthetic rubber diaphragm. The upper chamber contains fluid at system pres­sure, while the lower chamber is charged with air.

The function of an accumulator is to:

1. Dampen pressure surges in the hydraulic system caused by actuation of a unit and the effort of the pump to maintain pressure at a preset level.
2. Aid or supplement the power pump when several units are operating at once by sup­plying extra power from its "accumulated" or stored power.
3. Store power for the limited operation of a hydraulic unit when the pump is not oper­ating.
4. Supply fluid under pressure to compensate for small internal or external (not desired) leaks which would cause the system to cycle continuously by action of the pressure switches continually "kicking in."

Diaphragm type accumulators consist of two hollow half-ball metal sections fastened together at the centerline. One of these halves has a fitting for attaching the unit to the system; the other half is equipped with an air valve for charging the unit with compressed air. Mounted between the two halves is a synthetic rubber diaphragm which di­vides

the tank into two compartments. A screen covers the outlet on the fluid side of the accumu­lator. This prevents a part of the diaphragm from being pushed up into the system pressure port and being damaged.

 

Piston-Type Accumulators

 

The piston-type accumulator also serves the same purpose and operates much like the diaphragm and bladder accumulators. This unit is a cylinder (B) and piston assembly (E) with openings on each end. System fluid pressure enters the top' port (A), and forces the piston down against the air charge in the bottom chamber (D). A high-pressure air valve (C) is located at the bottom of the cylinder for servicing the unit. There are two rubber seals (represented by the black dot s) which prevent leakage between the two chambers (D and G). A passage (F) is drilled from the fluid side of the piston to the space between the seals. This provides lubrication between the cylinder walls and the piston.

Maintenance of Accumulators, - consists of inspections, minor re­pairs, replacement of component parts, and testing. There is an element of danger in maintaining accumulators. Therefore, proper precautions must be strictly observed to

prevent injury and damage.

BEFORE DISASSEMBLING ANY ACCUMU­LATOR, MAKE SURE THAT ALL PRELOAD AIR (OR NITROGEN) PRESSURE HAS BEEN DISCHARGED.

Failure to release the air could result in serious injury to the mechanic. (Before making this check, however, be certain you know the type of high-pressure air valve used.) When you know that all air pressure has been removed, go ahead and take the unit apart. Be sure, though, that you follow manufacturer's instructions for the specific unit you have.

Check Valves, - For hydraulic components and systems to operate as intended, the flow of fluid must be rigidly con­trolled. Fluid must be made to flow according to definite plans. Many kinds of valve units are used for exercising such control. One of the simplest and most commonly used is the check valve which allows free flow of fluid in one direction, but no flow or a restricted flow in the opposite direction.

Check valves are made in two general designs to serve two different needs. In one, the check valve is complete within itself. It is inter-connected with other components, with which it operates, by means of tubing or hose. Check valves of this design are commonly called in-line check valves. There are two types of in-line check valves, the simple-type in-line check valve and the orifice-type in-line valve.

 

Simple in-line check valve, - (often called check valve) is used when a full flow of fluid is desired in only one direction Fluid enters the inlet port of the check valve forc­ing the valve off its seat against the restraint of the spring. This permits fluid to flow through the passageway thus opened. The instant fluid stops moving in this direction, the spring returns the valve to its seat.

Orifice-type in-line check valve, - is used to allow normal operating speed of a mechanism by providing free flow of fluid in one direction, while allowing limited operating speed through restricted flow of fluid in the oppo­site direction. The operation of the orifice-type in-line check valve is the same as the simple-type in-line check valve, except for the restricted flow allowed when closed.

Line-Disconnect or Quick-Disconnect Valves, - these valves are installed in hydraulic lines to prevent loss of fluid when units are removed valves are installed in the pressure and suction lines of the system just in front of and immediately behind the power pump. These, valve units consist of two intercon­necting sections coupled together by a nut when installed in the system. Each valve section has a piston and poppet assembly. These are spring loaded to the CLOSED position when the unit is disconnected.

 

ACTUATING CYLINDERS - an actuating cylinder transforms energy in the form of fluid pressure into mechanical force, or action, to perform work. It is used to impart powered linear motion to some movable object or mechanism.

A typical actuating cylinder consists funda­mentally of cylinder housing, one or more pistons and piston rods, and some

seals.

Seals are used to prevent leakage between the piston and the cylinder bore, and between the piston rod and the end of the cylinder. Both the cylinder housing and the piston rod have provisions for mounting and' for attach­ment to an object or mechanism which is to be moved by the actuating cylinder.

Actuating cylinders are of two major types:

(1)          Single-action and

(2)          Double-action.

The single-action (single port) actuating cylinder is capable of producing powered movement in one direction only. The double-action (two ports) actuating cylinder is capable of producing powered movement in two directions.

A single-action actuating cylinder, - fluid under pressure enters the port at the left and pushes against the face of the piston, forcing the piston to the right. As the piston moves, air is forced out of the spring cham­ber through the vent hole, compressing the spring. When pressure on the fluid is released to the point that it exerts less force than is present in the com­pressed spring, the spring pushes the piston toward the left. As the piston moves to the left, fluid is forced out of the fluid port. At the same time, the moving piston pulls air into the spring chamber through the vent hole. A three-way control valve is normally used for controlling the operation of a single-action actuating cylinder.

Double-Action Actuating Cylinder, - (two-port) actuating cylinder is usually con­trolled by a four-way selector valve. Figure 8-26 shows an actuating cylinder interconnected with a selector valve. Operation of the selector valve and actuating cylinder is discussed below.

Placing the selector valve in the "on" position admits fluid pressure to the left-­hand chamber of the actuating cylinder.

This results in the piston being forced toward the right.

SELECTOR VALVES, - are used to control the direction of movement of an actuating unit. A selector valve provides a pathway for the simultaneous flow of hydraulic fluid into and out of a connected actuating unit. A selector valve also provides a means of immediately and conveniently switching the directions in which the fluid flows through the actuator, reversing the direction of movement.

One port of the typical selector valve is connected with a system pressure, line for the input of fluid pressure. A second port of the valve is connected to a system return line for the return of fluid to the reservoir. The ports of an actuating unit through which fluid enters and leaves the actuating unit are connected by lines to other ports of the selector valve.

Selector valves having four ports are the most commonly used, the term four-way is often used instead of four-port in referring to selector valves.

 

“A” illustrates a four-way, closed-center selector valve in the "off" position. All of the valve ports are blocked, and fluid can not flow into or out of the valve.

“B” the selector valve is placed in one of its "on" positions. The PRESS port and CYL I port become interconnected within

the valve. As a result, fluid flows from the pump into the selector valve PRESS port, out of the selector valve CYL I port, and into port A of the motor. This flow of fluid causes the motor to turn in a clockwise direction. Simultaneously, return fluid is forced out of port B of the motor and enters the selector valve CYL 2 port. Fluid then proceeds through the pas­sage in the valve rotor and leaves the valve through the RET port.

PRESSURE GENERATION

The hydraulic power drive used to drive and control such equipments consists of the following:

• Main Hydraulic Systems, controls by engine driven pumps (EDP). An EDP supplies about 22 gpm at 3000 Psi through a variable displacement pump mounted on each side of the engine. The EDP is driven through a splined shaft by the engine accessory drive gearbox.
• Electric Motor-Driven Pump (EMDP), an EMDP supplies 6.0 gpm at 2700 Psi. Each pump assembly is composed of an oil-cooled three phase 115 volt ac motor, a centrifugal pump and a single stage, variable displacement, pressure-compensated hydraulic pump.
• Some of aircraft hydraulic power system generates with a motor ram air turbine.
• Ground service cart or with the manual fill pump installed at the servicing station.

General, - separate hydraulic systems provide fluid at 3000 Psi to operate the airplane systems. The standby hydraulic system provides reserve power for critical systems. The indicating systems provide information for crew monitoring of the operating conditions of each hydraulic system.

The pressure source for each engine driven pump (EDP) is directly coupled to the engine accessory gearbox and runs all the time that the engine is running.

Electric motor driven pump (EMDP), when an ELEC pump switch is ON, the respective EMDP runs all the time.

Hydraulic system components are located on each engine and within the main gear wheel well section.

 

A typical ram air unit, this type of emergency system is intended for use only when normal hydraulic pumps are completely inoperative. The ram air turbine provides a means for emergency hydraulic and electrical power when the normal aircraft hydraulic system has failed. The turbine-driven hydraulic pump supplies fluid under pressure as well as to an emergency hydraulically driven alternator. Consists of a dropout governor-controlled turbine, a hydraulic

 

The indicating system consists of warning lights and gages, Fluid pressure, temperature (overheat) and reservoir quantity are monitored in the cockpit while reservoir quantity and pressure are indicated in the

wheel well.

The pressure indicators get their signals electrically from the pressure transmitters. An overheat warning system is provided to monitor the fluid operating temperatures of each system placed in case drain line is connected to a respective amber light on the control panel. The hydraulic fluid quantity indicating system shows the quantity of fluid in the reservoirs. A low pressure warning system is provided for each hydraulic pump. The switches are connected to amber low pressure lights on the control panel. Activation of a low hydraulic pressure warning circuit will cause the Master Caution and hydraulic annunciator lights to illuminate.

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SISTEM HIDRAULIK PADA PESAWAT BOEING 737 SERIES

Diagram dibawah adalah gambar sistem hidraulik pada pesawat Boeing 737 series.

Sistim hidraulik basic pada pesawat menggunakan tekanan accumulator sebagai tekanan back up untuk system pembalik gaya dorong/thrust reversal.

B-737 diperlengkapi dengan 3 sistem hidraulik yang masing-masing berdiri sendiri dan setiap system memiliki reservoir fluida. Reservoir system “A” diberi tekanan dari bleed air engine untuk memastikan aliran dari fluida. Pipa penghubung yang seimbang menyambung ketiga reservoir dari masing-masing system hidraulik.

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Tekanan pada system “A” diperoleh dari pompa yang digerakkan oleh engine (Engine driven pump) pada setiap engine. Pompa ini normalnya bekerja selama penerbangan.

Dua pompa listrik menghasilkan  tekanan pada system “B”. Pompa listrik ini dioperasikan dengan system kelistrikan pada pesawat.

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Sistem Standby menerima tekanan dari pompa hidraulik yang digerakkan oleh motor listrik. Pada operasi normal pompa ini tidak bekerja, dan dimaksudkan sebagai sumber pendukung tekanan hidraulik ke rudder dan peralatan leading edge.Pompa untuk system Standby diaktifkan oleh pilot ketika kondisi abnormal terjadi pada kedua system hidraulik lainnya.

Sistem “A” dan “B” secara normal selalu bekerja. Bila salah satu system baik”A” atau “B” tidak berfungsi, system Standby dapat digunakan untuk menggerakkan Rudder dan peralatan leading edge.

Sistem “A” diperlengkapi dengan pengukur Jumlah fluida(Quantity gauge). Pada kokpit terdapat pengukur tekanan untuk mengetahui tekanan output pada system “A” dan “B”.

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Lampu peringatan TEKANAN RENDAH dipasang pada setiap system. Lampu JUMAH RENDAH juga dipasang pada system “B” dan Sisten Standby.

Pada Sistem Standby tidak terdapat pengukur Jumlah atau pengukur tekanan.

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Lampu peringatan TEKANAN RENDAH/LOW PRESSURE juga dipasang pada setiap system. Lampu tanda LOW QUANTITY juga dipasang untuk system “B” dan Sistem “STANDBY”

Setiap system mempunyai reservoir. Sistem “A” diberi tekanan oleh pompa yang digerakkan engine,

Sistem “B” tekanan diberikan oleh pompa listrik.

Apabila system “A” dan “B” tidak berfungsi, Sistem “STANDBY” akan memberikan tekanan fluida .

Semua pompa Hidraulik secara normal menyalurkan tekanan fluida sebesar 3000 psi.

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Tekanan pada Reservoir selalu dijaga agar aliran fluida dari reservoir ke pompa hidraulik selalu constant. Tekanan reservoir didapatkan dari Bleed Air.

Pada reservoir system”A” tekanan didapatkan dari Bleed Air Engine.

Pipa-pipa penghubung yang seimbang menghubungkan semua reservoir. Pipa-pipa balance ini dimaksudkan agar tekanan pada semua reservoir sama dan mengalirkan fluida kesemua reservoir.

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Rudder digunakan sebagai control arah pada sumbu vertical pesawat. Rudder pada pesawat Boeing 737 series dioperasikan/digerakkan menggunakan system hidraulik. Secara normal Rudder menerima tekanan hidraulik dari system “A”: dan Sistem “B” Bila salah satu system tekanannya menjadi rendah, digunakan Sistem Hidraulik “STANDBY” sebagai cadangan.

Flight Control yang digerakkan dengan system Hidraulik dapat dilihat pada gambar dibawah :

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Pada gambar dibawah yang berwarna biru, normalnya menerima tekanan hidraulik dari Sistem “A” dan system “B”

Sistem STANDBY Rudder Hidraulic dapat digunakan untuk menggerakkan Rudder, bila dipilih “ON” , setelah terjadi kegagalan pada kedua Sistem hidraulik lainnya

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Tingkat kebocoran cairan system HIDRAULIK:

A.EXTERNAL

    Mur- mur  B ; tidak diperbolehkan, Suatu kebocoran pada Mur-mur B, diambil kemungkinan benda- 

    benda ini rusak.

B.INTERNAL  - Kondisi umum dari system ditentukan dengan memperbandingkan pengukuran

                            Tingkat kebocoran dalam dengn batasan kebocoran perawatan yang dianjurkan yang 

                Telah disertakan untuk bermacam-macam system. Ada 2 cara untuk mengukur

                            Tingkat aliran : (a) Ammeter methode, (b) Flow meter methode.

                            Ammeter methode, mengevaluasi perubahan pada aliran keluar dari pompa 

                           “B”.Perubahan dibandingkan dengan grafik, untuk menentukan tingkat aliran. Dua

                           Orang dapat meleksanakan system check ini sekitar 30 menit.

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                           Flowmeter methode menggunakan sebuah hydraulic flow meter yang dipasang pada

                           Pipa bertekanan dari kendaraan service hidraulik yang terhubung dengan power

                           Module Dua orang dapat melakukan check ini dalam waktu dua jam.

Pemeriksaan cairan hidraulik..

Pengecheckkan periodic pada cairan hidraulik pada system pesawat terbang harus dilakukan untuk menguji agar cairan tidak kehilangan sifat-sifatnya. Pemeriksaan ini melibatkan analisis kimia.

FLUIDA HIDRAULIK

Fluida hidraulik pada pesawat terbang harus memenuhi beberapa persyaratan pokok :

1. Tidak dapat dikompressi
2. Memiliki sifat pelumas yang baik
3. Tidak mudah menguap
4. Tidak menimbulkan karat
5. Tidak menimbulkan buih
6. Memiliki viskositas yang stabil, pada temperature 80º C
7. Memiliki titik nyala diatas 100º

Pada umumnya ada 3 jenis fluida Hidraulik :

Fluida yang terbuat dari tumbuh-tumbuhan, dikenal dengan kode Mil-H7644, berwarna kebiru-biruan, bahan dasarnya adalah minyak kastor dan alcohol. Paking gasket dari karet alam

Fluida mineral- Mil-H-5606.Bahan dasarnya adalah minyak mineral, berwarna merah. Sifat viskositasnya stabil pada variasi macam-macam suhu. Paking-gasket dari karet sintetis

Fluida sintetis, Mil-H-8446 atau Skydrol – 500A.Terbuat dari bahan mineral, tidak tahan terhadap jilatan api. Bila bertekanan tinggi dan terjadi kebocoran , mudah terbakar. Untuk mengatasinya dibuat fluida sintetis yang tahan terhadap tekanan dan suhu yang tinggi yang merupakan persyaratan pokok untuk sistim hidrolis pada pesawat jet modern. Berwarna hijau atau ungu. Paking-gasket dari karet sintetis.

Penambahan atau penggantian cairan hidrolis selalu berpedoman pada Buku Manual pesawat bersangkutan.

PELUMASAN DAN SISTEM PENDINGINAN

SAE =  Society of Automotive Engineers yang membagi oli menjadi 7 kelompok besar yaitu SAE 10 sampai SAE 70 berdasarkan viscositasnya pada suhu 130º atau 210º F

Copmercial                      Comercial                       Army and Navy

Aviation No                       SAE No.                       Specification No

      65                                     30                                    1065

      80                                     40                                    1080

     100                                    50                                    1100

     120                                    60                                    1120

     140                                    70