EASA Mod 7A Bk 3 Eng Pract

8/12/2019 EASA Mod 7A Bk 3 Eng Pract

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Book 3 Module 7A

CATEGORY B1 B2ENGINEERING PRACTICES

Licence By Post

For best examinationresults always use latest

issue number.

Licence By PostCopyright B EASA 66 7A.2 7A.3 7A.6 ISSUE 02 1103


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Licence By Post

No part of this study book may be re-produced or distributed in any form or byany means, or stored in a data base or retrieval system in whole or in part withoutprior written permission from Licence By Post.

Books in the LBP series are regularly up-dated/re-written to keep pace with the changingtechnology, changing examination requirements and changing legal requirements.


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AUTHORITY

It is IMPORTANT to note that the information in this book is for study/trainingpurposes only.

When carrying out a procedure/work on aircraft/aircraft equipment you MUSTalways refer to the relevant aircraft maintenance manual or equipmentmanufacturers handbook.

You should also follow the requirements of your national regulatory authority(the CAA in the UK) and laid down company policy as regards localprocedures, recording, report writing, documentation etc.

For health and safety in the workplace you should follow the regulations/guidelines as specified by the equipment manufacturer, your company,national safety authorities and national governments.


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ADDENDUM

Addendum action in response to student feedback after taking the CAAexaminations. EASA Part 66 module 7A LBP book 3 Maintenance Practices.

Copper sulphate is used to aid marking out on ferrous metals. It is painted onto the shiny surface of the metal, allowed to dry and a scriber is used to scribelines into the copper sulphate for marking-out purposes. The lines show upclearly against the reddish/brown of the copper sulphate. Remember,

aluminium and its alloys is notch sensitive (notches, nicks or scribed lines,with time, will instigate the propagation of cracks), so a soft pencil is alwaysused for marking out unless its a cut line.

***Oxygen bottles should not be discharged below a certain minimum pressure(CAAIPS state 200 to 300psi but check the AMM). This is to ensure that anymoisture in the O2 does not precipitate out to form water. This willcontaminate the oxygen and cause it to smell. Also applies to compressed airstorage bottles.

****


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CONTENTS

Page

Lubrication methods 1Care of tools 7Workshop materials 9Metals and composites 14Dimensions and measurement 19Calibration of tools and equipment 25Limit systems 28

Checking for wear 36


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HOW TO TACKLE THIS BOOK

This book brings together a collection of items, mostly unrelated to each other,but each too small to warrant a separate book.

As a B1 or B2 engineer you should be able to describe lubrication methodsand the care of tools in detail. It would be a good idea to have studied the bookin this series entitled Tools before studying this book.

It is difficult to say how much one should commit to memory regarding

workshop materials, but you should have a good idea of the range of materials,fluids, compounds and gasses, and their general uses.

The subject of dimensions and measurement brings together the practicalaspect of workshop practice, precision instruments and procedures likeaircraft rigging.

You should understand what equipment requires calibration and, in generalterms, how this is done.

Limit systems can be a dry subject to study but you should have a generalknowledge of how the systems work and how the subject can be appliedpractically.


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LUBRICATION METHODS

Lubrication of components and systems is carried out at the times specified inthe maintenance schedule and the method as stated in the AMM. Theequipment used is usually of local purchase though any special equipment

that might be required will be obtained (for a fee) from the aircraft/componentmanufacture.

The oils and greases used will have to meet the specification as stated in theAMM, and these are also obtained through local purchase. The specification ofthe oils and greases will be given in list format in chapter 20 of the AMM.

The list will include:

* Standard greases and oils for general use.* Alternative lubricants in case any of the above are not available.

* Mineral based and synthetic based lubricants.* High pressure greases.* Special lubricants for such things as threads on oxygen

connections. WARNING. An ordinary lubricant in contact withhigh pressure oxygen will cause an explosion.

* Anti-seize grease for threads normally a graphited compound.* Fuel resistant greases.* Lubricants for use with rubber sealing rings, O rings etc.

In general, a lubricate is designed to reduce the friction between moving parts

by interposing a low viscosity medium between them. This helps preventcontact between the moving surfaces and also helps disperse the heatgenerated. It also helps remove the products of friction (wear particles etc), andin such things as internal combustion engines helps remove debris from thecombustion process.

Some moving surfaces are notlubricated (check the AMM), others havepressure air supplied to them to keep the surfaces apart (air bearings), butmost are lubricated by the use of oil or grease.

Parts that are not lubricated are those that do not suffer from a lot of friction,

or would suffer from damage due to the lubricant getting dirty through dustetc (fairleads, control cable pressure bulkhead seals etc). Some parts are notlubricated where the materials used in the manufacture of the parts are selflubricating phosphor-bronze bearings, oil impregnated tufnol bearings, teflonbearings etc.

It is important to remember that bearings such as teflon will deterioraterapidly if lubricated so lubricate only those bearings that are specified in theAMM. The decision whether to lubricate a bearing or leave it dry is made bythe manufacturer, so always consult the AMM.

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Fig. 1 PART OF CHAPTER 20 COMPOUNDS LIST EXAMPLE A330

Some-times, threads (male and female) are lubricated check the AMM. Thishelps reduce friction between the threads and effectively increases the actualtorque between the parts. It is most important that the AMM is consulted tocheck whether threads are torqued wet (lubricated) or dry.

Serious accidents have happened in the past when bolts have failed after beingtorque loaded wet when they should have been torque loaded dry.

Remember, only the approved oil/grease must be used, and only thosebearings/lubrication points are to be lubricated. Do not over lubricate andalways clear up any additional oil/grease left after the operation.

Some oils and greases are used as preservative compounds to be used astemporary anti-corrosive treatments.

Remember oils and greases can be a skin irritant and some may cause

dermatitis. So always use a barrier cream and, if necessary, gloves.

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If any lubricants get on the hands wash off with soap and water and re-applythe barrier cream. If eye contamination occurs irrigate with copious amountsof water and seek medical advice.

In some manuals lubrication blocks or symbols are used. These will indicate

the method of application and the specification of the lubricant to be used.

Fig. 2 LUBRICATION SYMBOLS B767

Lubrication of sealed roller and ball bearings having lubrication provisionrequires care to prevent too much pressure being pumped in so as to causethe seal to blow-out. To help prevent this happening a flow restrictor can befitted to the charging gun (several flow restrictors are available commercially)that restrict the pressure to below 2500psi.

Fig. 3 LUBRICATION POINTS EXAMPLE B767 AMM TAILPLANE TRIM

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If pressure lubricating a sealed bearing, take careful note of any sealdeformation and/or grease seepage if either happens, stop immediately andallow the pressure to subside.

Pressures between 100psi and 200psi should be sufficient to get grease into a

sealed bearing, but at any rate should never exceed 2500psi.

Fig. 4 LUBRICATION NIPPLES

If a seal does blow, or the fitting/bearing becomes loose make sure thelubrication path is clear and change the bearing.

Non-sealed bearings are lubricated as required. Sealed bearings may belubricated by the manufacturer and sealed for life and are never lubricatedagain they are changed only. Other sealed bearings are lubricated regularly(see above) as stated in the AMM and maintenance schedule.

Nipples/Application Points

These may be of the push-on type or pressure clip-on or slide-on type. Theyare screwed into position and each one contains a small ball type non-returnvalve, which is pushed open when lubricant pressure is applied. The lubricantthen passes through a central drilling to the point of lubrication.

The nipple may be fitted with a dust cap, but at any rate it must be thoroughly

cleaned before fitting the grease gun. The grease gun adapter end must also becleaned prior to use. After the part has been lubricated excess lubricant isremoved with a soft cloth and any dust caps fitted.

Fitting the push-on type gun to the nipple is simply a matter of putting theadapter end on to the nipple, pressing firmly and pumping the grease gunhandle.

For the pressure slide-on type the adapter is slid onto the nipple so that thenipple flange engages with the engagement slots of the adapter it is then slidfully forward so that the lubrication hole on the adapter aligns with the nipple

non-return valve.

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Application Methods

Equipment listed below must be kept clean and end caps fitted when not inuse. Each oilcan/grease gun must be marked with the type of oil/greaseinside and it must not be used for any other specification lubricant. In other

words the existing lubricant must not be removed and a different lubricantsubstituted.

When purchasing equipment locally ensure (from the equipmentmanufacturer) that the equipment (seals etc) is compatible with the type oflubricant that is to go inside.

Hand. Smeared on by hand over the area to be covered. Oils and greases.

Brush. A brush or plastic spreader is used to apply the oil over the area. Maybe used with some of the thinner (those with a low viscosity) greases.

Fig. 5 PUMP ACTION GREASE GUN

Oilcan. A pump action (thumb or finger operated) oilcan to apply drops of oilinto an oil cup or nipple attached to the equipment or to apply a few drops tospindles, shafts, chains etc.

Pump Action Grease Gun. Used for pumping grease into an end nipple. Theend is placed over the nipple and the handle is pushed forward. This pushesthe piston inside the cylinder and pumps grease through the ball non-returnvalve of the push-on type nipple.

To refill with grease unscrew the end cap, remove the piston and handlecomplete, fill the cylinder with the correct specification grease, replace thepiston and end cap, tighten the end cap.

Hand Operated Grease Gun. Consists of a cylinder with a piston operated witha handle. Will apply grease, and oil, under pressure to the lubrication point.

The lubrication point normally being a slide-on nipple (for high pressure).

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The unit may come complete with a pressure gauge. The end connector is slidover the pressure type nipple so that it engages with the nipple flange. Theaction of sliding the adapter over the nipple will help unseat the ball springnon-return valve in the nipple.

Fig. 6 HAND OPERATED GREASE GUN

When the handle is pumped lubricant is forced under pressure through thenon-return valve, then through drillings to the bearing.

To refill, unscrew the base cap, pull out the internal floating piston and fill thecylinder with the correct specification lubricant. Replace the floating pistonand re-connect the base cap.

Pressure Operated Lubrication Equipment. Specialised equipment, airpressurised to supply oil through a trigger operated adapter.

Oil

Usually supplied in drums with the specification, manufacturers name etcprinted on the front. For application purposes, oilcans, hand operated pumpsetc are used. Always use oil as specified in the AMM and specified on the can.If in doubt use only oil from a sealed drum.

Grease

Supplied in tubes (similar to, but larger than, toothpaste tubes), and tins.Again, these must all have the manufacturers name and material specificationprinted clearly on the outside.

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CARE OF TOOLS

Tools can be divided into:

* Personal tools. Those tools that are purchased by the

tradesman/woman and kept in his/her tool box. They will includesuch item as, screwdrivers, spanners, socket sets etc. They mayalso include test instruments such as multi-meters.

* Special tools that are kept by the company in the stores and thesemay be dedicated to an aircraft type. May include specialistequipment too large to keep in tool boxes and expensiveequipment and test sets. Will usually include special to typeadapters, tools, test sets etc.

Routine Care of Tools

Each tool (spanner, socket, extension, test set, etc) should be marked with acode that identifies it and its normal location. Each trades-person should havehis/her own code etched into each item and that code recorded with the firmthat that person is working for.

Cleaning. All tools benefit from being kept clean. It also means less chance ofcontamination of the equipment being worked on (contamination of fluidsystems, air conditioning systems etc). Most metal tools are kept in good

condition by the application of a thin oil or an oil based solution such asWD40, which is then wiped clean using a clean lint free cloth.

Files are cleaned using a scratch card.

Test-sets are kept clean and free from liquids. Should they becomecontaminated then they are to be returned to the manufacturer fordismantling, cleaning, assembly and testing.

Storage. Most tools are simply stored in a secure box. Socket sets, spannersets etc are usually stored in the special box they are supplied in. This not

only keeps them all in one location (and easy to find), but also aids the processof checking all tools are present at cessation of work.

Delicate tools and instruments will be supplied and kept in boxes(micrometers, verniers, watchmakers screwdrivers, slip gauges etc).

Some large/special tools will be stored in a secure room. These may be placedon shelving or hung on shadow boards. These boards have clips or othersupports screwed to them onto which the tool is hung. Painted on the boardwhere the tool hangs is the silhouette of the tool. When the correct tool is

clipped into the correct place on the board then its shadow cannot be seen.

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It is useful when doing a tool check as one look at the board (which maycontain many dozens of tools) tells the observer immediately if any tool ismissing.

Files should be kept in their sleeves to protect the teeth, or in some sort of

hold-all. Sharp objects such as dividers, scribers etc should have their sharpends pushed into a piece of cork (a wine bottle cork) to protect them.

Maintenance. Some tools will need attention with a file from time to time totrim them to shape these include pin punches, taper punches, drifts, flatscrewdrivers etc.

Hammer heads will need checking for security to the shaft. If not secure tapthe head down with another hammer and drive the wedge/steg further home.

Chisels will need re-honing and drills will need re-sharpening (use a special jig)

though small drills are best replaced when they loose their edge.

Apart from the above most tools need no further attention, however, thosetools that are used for measurement will need checking/calibration. For thechecking of standard precision instruments see the book in this series entitledTools. For calibration of test sets etc see the chapter in this series entitledCalibration.

Tool Checks

It is most important that tools (and anything else for that matter) are not lefton aircraft after completion of work. They are a serious hazard as they can jamcontrols, block system pipes etc. The term loose articles applies to anythingleft on an aircraft after maintenance this includes rags, split pins, lockingwire, and tools.

It is most important that a loose article check is carried out after every task onthe aircraft, engine, component, and as a double check a tool-check is alsocarried out.

Each person working on the aircraft should check his/her tool kit after eachtask and at the end of the working day, or before the next flight to ensure thatall tools are accounted for. Any special tools used from the tool-store should bereturned and the person in-charge of the store should do a tool check to seethat there are no tools missing.

If, during these checks, a tool is found missing then the aircraft should not beallowed to fly until it has been found. The search will include the work areas ofthe aircraft, all tools boxes, the tool store, hangar/workshop area, rubbishbins, scrap bins etc. A check will also be made of the signing in and out book

of the tool store to see if this record shows who has had what tool, and when.

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It is always a good idea to carry out a loose article check of the work area onthe aircraft after completion of the task in some organisations it is arequirement which must be signed for.

WORKSHOP MATERIALS

These can include:

* Barrier cream to be rubbed into the hands and any other partsof the body (arms etc) that might come into contact with anylubricants, fuels, compounds etc. Helps prevent skincontamination with these substances and possible dermatitis.Supplied in containers and wall mounted dispensers.

* Soaps and soap/cleansing solutions.

* Eye cleansing water. Sterilised water supplied in a dispenser forthe irrigation of the eye should a foreign body (piece of swarf, dustetc) get into the eye or if the eye is splashed with a harmful liquid.

* Oils. Supplied in sealed tins to an approved specification. Avoidcontact with oils. Use as specified in AMM chapter 20 or theappropriate component manual. Usually supplied locally to meetspecifications stated in the AMM/manual. These specifications

include: MIL specifications (American); AIR specifications (French);DAN (German); DTD (British) etc. May include any of the following:

Hydraulic Oils. DTD 585 or similar for some aircrafthydraulic systems and shock absorbers.

Lockheed 22 or similar on some olderaircraft hydraulic systems.

Phosphate ester based fluids such asSkydrol on many large aircraft hydraulic

systems.

Engine oil. For use on jet engines and APUs (AuxiliaryPower Units).

Lubricating oils. Different specifications available includinglow temperature oils, high temperature oils,synthetic based, mineral based etc.

Penetrating oil. For use when trying to undo very

tight/seized threaded items.

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* Special lubricants. Includes thread lubrication for oxygen systems.

* Greases. Supplied in cans or tubes. The same applies as to oils,for safety, use, and specifications. May include:

General greases. Different specifications available for differentapplications such as fuel and oil resistantgrease, O ring lubricant, general lubricant etc.

Graphite grease. Several uses including anti-seize grease forscrew-threads.

Sealing grease. Silicon based, used for metal to metal sealing.

* Fuels. Normally supplied in bulk tankers or supplied viaunderground pipes to pumping vehicles on large airfields. If kept

in small containers, should be clearly marked with a danger signindicating its contents and that it is a flammable fluid. Cansshould be sealed and stored in a non-combustible building awayfrom aircraft and hangars. As with oils and greases avoid contacton the skin and eyes.

* Fuel additives such as biocide additive.

* Refrigerants for refrigeration equipment on aircraft.

* Leak detectors. Specially formulated for the detection of leaks.Some are specific for use with oxygen systems, others for airsystems and some are coloured or fluorescent for use withvacuum testing.

* Other Compounds such as:

Thread locking compounds including permanent lockingcompounds (Loctite 270).

Zinc powder. When mixed with compounds such as Mastinox is

used to ensure electrical continuity between PCU components.

* Sealants. eg DTD 900, MIL-S-8802 for pressure cabin sealing andintegral fuel tank sealing.

* Cleaners such as Teepol 610, Turco Air Tec and Ardrox generalpurpose detergents and cleaners, and specialised cleaners.

Rain repellent cleaner for removal of rain repellent fromwindscreens (Coke should not be used).

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Plastic cleaners. May include plastic polishing compounds for useon transparent panels (passenger windows) and CRT screen/flatscreen cleaning materials.

* Metal polish. For polishing out small scratches in aluminium

alloys.

* Rust inhibitor. Such as Rocket WD40. Used to prevent corrosion,disperse moisture and lubricate.

* Adhesives. These include the following:

Structural and high temperature adhesives for metal to metal andmetal to honeycomb bonding. eg DOD-A-82720 (American) formetal to metal bonding.

Adhesive film for structural bonding.

Synthetic adhesives for general adhesive use.

Loctite 932 adhesive used for locking the backshells of electricalconnectors.

* Degreasants such as:

Dry cleaning solvent (Varsol/white spirit). Used for cleaning metal

parts.

Methyl-ethyl-ketone.

Trichlorethane.

Methyl Alcohol.

Safety solvent. Safer than those listed above. For general use andfor cleaning oxygen pipelines.

Paraffin/petrol. Metal parts should be thoroughly cleaned anddried after using paraffin to prevent any possibility of corrosion.

* Rust removers. Acid based removers formulated for specific taskssuch as, for use on Fe metals only, for use on Al alloys only etc.

* Lacquers. May be used as corrosion preventatives on metals.Special (epoxy) lacquers used as an electrical bonding lacquer.

* Anti corrosive solutions such as Alodine 1200 for the surface

conversion of aluminium alloys prior to painting.

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* Paints including: primer and top coat paints; polyurethane;synthetic, cellulose etc.

* Paint removers/strippers such as MIL-R-25134, Turco 800 etc

* Engineers Blue (marking ink). Used to indicate low and high spotson precision work using a surface table as an example.

* Jointing compounds such as polysulphide polymer and pigmentedjointing compound for use between metal contact surfaces.

* De-icing fluid. Type 1 used on aircraft when temperatures do not

go below minus 10C and holdover times are short otherwise usetype 2.

* Acids and alkalis. Used for battery electrolytes. Corrosive to skin

and metal.

* Tapes. Self adhesive and have special applications, eg for externalaerodynamic sealing (speed tape) of the airframe, internal dcorrepairs (decorfoil), erosion protective tapes, cargo liner repair tapesetc.

* Deodorants for toilets, disinfectants for drinking water systemsand sprays for passenger areas, antiseptic sprays for oxygenmasks, insecticides for passenger and cargo compartments.

* Desiccant. (Silica Gel crystals) Used to dehumidify the atmospherein a sealed container. Used for storage purposes and incomponents where a dry atmosphere is imperative. Turns pinkwhen life expired. Rejuvenated by being heating in an oven, whenit will regain its deep blue colour.

* Penetrants and removers. Special dyes, dye removers anddevelopers, often supplied in kit form, to be used when checkingfor surface cracks and flaws.

* Fabric. Rags for general cleaning remember not to store wet ragsin a container as they may spontaneously combust. Specialistcleaning clothes. Upholstery fabric meeting current fire andsmoke regulations. Thermoweldable waterproof fabric for use whenprotecting components in storage.

Used in aircraft construction for the covering of some lightaircraft. Supplied in large rolls called bolts and may be:

Unbleached Irish linen

MadapollamPolyester

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Unbleached Irish linen and Madapollam are tautened by dopingwhilst polyester is tautened by the application of heat.

* Paper. In rolls or sheets for general use or as protection usinggrease proof paper or laminated paper.

* Cord/string/rope. May be used for general purposes, butspecialised cord/string/rope is supplied for such purposes asstringing up bag type fuel tanks, cleating/tying-up wire and cablelooms, stitching fabric to structure etc. May need to be waxedbefore use.

* Wood. Straight grained spruce used as structural members onsome very old aircraft along with high grade plywood, balsa woodetc. Lignum Vitae and similar very hard woods used in theconstruction of the earlier propellers. In the workshop/hangar

wood is likely to be used for such things as drilling supportmaterial; crating large components, and used as a block with ahammer when bending sheet metal.

* Wire. Locking wire soft iron steel stainless steel - copper brass depending on application.

* Gases nitrogen air oxygen. Stored pressurised up to 4500psi(31MPa) in colour (and words) identified gas cylinders.

* Small parts. Nuts, bolts, washers, rivets etc. Kept in their originalpackages to facilitate identification or kept in stores in a labelledbin system.

WARNING

Some of the liquids listed above are toxic and/or highly inflammable and/orcorrosive and/or harmful if in contact with the skin, or if their fumes areinhaled. Read the warning/safety notice on the container and the warningsgiven in the AMM/manual. In general always use barrier creams and avoid

unnecessary contact with any liquids, gasses, compounds etc.

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METALS & COMPOSITES

TABLE 1 - FERROUS METALS------------------------------------------------------------------------------------------------MATERIAL USES

------------------------------------------------------------------------------------------------Cast Iron Machine beds, frames, castings, journal bearings,

pistons, piston rings.------------------------------------------------------------------------------------------------Wrought Iron Cores of dynamos, lifting chains, crane hooks.------------------------------------------------------------------------------------------------Mild Steel Bolts and nuts. General workshop machined

components. Girders, forgings.------------------------------------------------------------------------------------------------Medium Carbon Leaf springs, wire ropes, general tools, axles,Steel crankshafts, hinge pins etc.

------------------------------------------------------------------------------------------------High Carbon Cutting tools. Coil springs. High strength attachment.Steel. Nuts and bolts.------------------------------------------------------------------------------------------------

TABLE 2 - ALLOY STEELS------------------------------------------------------------------------------------------------MATERIAL USES------------------------------------------------------------------------------------------------Nickel (Ni) Steel Case hardened parts. Stainless steels. Certain %

produces a non-magnetic steel. 36% Ni gives InvarSteel used for precision instruments.------------------------------------------------------------------------------------------------Chromium (Cr) Steel Ball and roller bearings. Nearly non-corrodible.------------------------------------------------------------------------------------------------Manganese (Mn) Steel Small percentage added to steels used for welding.

Higher percentage used in steel exposed to friction.------------------------------------------------------------------------------------------------Tungsten (W) Steel 14% tungsten used in high-speed steel drills, hacksaw

blades etc. Works at higher speeds andtemperatures.

------------------------------------------------------------------------------------------------Cobalt (Co) Steel Used with tungsten for drills and other cutting tools to

improve cutting performance at temperatures higherthan 600C. Used by it self in permanent magnets.

------------------------------------------------------------------------------------------------Vanadium (V) Steel Chrome-vanadium steels used for piston engine valves

and springs.------------------------------------------------------------------------------------------------Molybdenum (Mo) Similar effect to tungsten.Steel

------------------------------------------------------------------------------------------------continued

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TABLE 2 - CONTINUED------------------------------------------------------------------------------------------------Nickel Chromium Stainless Steel. Certain percentages produce a non-Steel magnetic steel, other percentages produce parts with

good strength and toughness. Used for gears,

crankshafts and engine and airframe parts.------------------------------------------------------------------------------------------------Invar Steel Precision Instruments and gauging systems. Has a

low co-efficient of linear expansion (0.9). (Mild steelhas a co-efficient of 15.0).

------------------------------------------------------------------------------------------------Stainless Steel Almost zero rate of corrosion. Structures - where heat

and corrosion resistance is required. Also small parts,pipe-lines, engine parts etc.

------------------------------------------------------------------------------------------------Austenitic Steels & There are several austenitic steels but most are based

Irons on 18:8 stainless steel. Besides the qualities ofstainless steel they are non magnetic. Same uses asabove.

------------------------------------------------------------------------------------------------Valve Steels For aero engine valves, usually contain 13%Ni, 13%

chromium and 3% tungsten.------------------------------------------------------------------------------------------------High Speed Steels Typically contain 18% tungsten, 4% chromium, 1%

vanadium. Drills and other cutting tools.------------------------------------------------------------------------------------------------

Permanent Magnet May contain up to 35% Cobalt. Various trade namesSteels are available eg, Columaxwhich contains 8% Al, 14%Ni, 23% Co and 3% Cu.

------------------------------------------------------------------------------------------------High Permeability Soft iron was used but metals such as Permalloy(78%Steels Ni) and Mumetal(75% Ni) are now more common.(These can bemagnetised andde-magnetised easily)------------------------------------------------------------------------------------------------

TABLE 3 - NON Fe METALS (For Al Alloys see Table 4)------------------------------------------------------------------------------------------------MATERIAL USES------------------------------------------------------------------------------------------------Titanium Alloys Used to replace steel with a saving in weight. Good

corrosion resistance. Used for jet engine compressorand fan blades. Fire proof bulkheads. Heat shields.

------------------------------------------------------------------------------------------------Nickel Anti corrosion treatments and withstands high

temperatures. Used in alloying.

------------------------------------------------------------------------------------------------continued

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TABLE 3 CONTINUED------------------------------------------------------------------------------------------------Nickel Alloys Gas turbine blades and hot end fittings.------------------------------------------------------------------------------------------------Magnesium Soft and poor corrosion resistance. Flares. Light

alloys.------------------------------------------------------------------------------------------------Magnesium Alloys Aircraft wheels, though most are made of aluminium

alloy these days. Airframe structures, enginecompressor casings.

------------------------------------------------------------------------------------------------Copper Tubing. Pipework. Electrical conductors. Used as a

base for brass and bronze.------------------------------------------------------------------------------------------------Brass Lightly stressed castings, pipe fittings, tubing, filter

elements, bushes, electrical contacts.

------------------------------------------------------------------------------------------------Bronze Bearing bushes. Small castings.------------------------------------------------------------------------------------------------Phosphor Bronze Bearing bushes.------------------------------------------------------------------------------------------------Tungum Pipe lines. Radiator matrix.------------------------------------------------------------------------------------------------Lead Counter balance and mass balance weights. Alloyed to

make solder.------------------------------------------------------------------------------------------------

Tin Used for tin plating. Alloyed to lead to make solder.------------------------------------------------------------------------------------------------Solder Alloy of tin and lead. Low melting point. Soft.------------------------------------------------------------------------------------------------Zinc Corrosion protection of steel parts.------------------------------------------------------------------------------------------------Depleted Mass balance weights. Though being phased out dueUranium to health hazard.------------------------------------------------------------------------------------------------Gold Used for plating of some electrical contactors. Reduces

contact resistance. Good corrosion resistance

------------------------------------------------------------------------------------------------Monel Metal 70% Ni, and 30% Cu. Resistant to corrosion. Some

structural uses and tucker pop rivets.------------------------------------------------------------------------------------------------Cadmium Corrosion resistant. Anti corrosive plating.------------------------------------------------------------------------------------------------

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Aluminium Alloys

These are supplied in the wrought or cast forms and may be heat treatable ornon heat treatable. The British Standards cover:

BS 1470 to 75 - Wrought.BS 1490 - CastBSL Series - Aircraft Al AlloysDTD Specs. - Aircraft Al Alloys

(DTD = Directorate of Technical Development - UK).

TABLE 4 - ALUMINIUM & ITS ALLOYS------------------------------------------------------------------------------------------------MATERIAL USES

------------------------------------------------------------------------------------------------Aluminium Used in light alloys as a base material, and used for anti-

corrosive cladding on aluminium alloys.------------------------------------------------------------------------------------------------Duralumin Al alloy used for airframe structural parts. Sheets, rivets,

tubes.------------------------------------------------------------------------------------------------

Alclad Duralumin Sheet with coating of aluminium. Same uses as(wrought) above.------------------------------------------------------------------------------------------------

Alpax Al alloy. Intricate castings. Airframe and engine parts.-------------------------------------------------- ---------------------------------------------Y Alloy Al alloy. Aero engine pistons and cylinder heads.------------------------------------------------------------------------------------------------Hiduminium Airframe structural parts. Rivets, pistons and cylinder

heads.------------------------------------------------------------------------------------------------2000 Series Used in critical airframe structural areas.Al Alloys------------------------------------------------------------------------------------------------7000 Series Similar to above.

------------------------------------------------------------------------------------------------Lithium Being developed for newer aircraft to replace both the 2000Based Al alloys and 7000 series alloys.------------------------------------------------------------------------------------------------

Plastics

Man made plastics (polymers) can be divided into two main groups -Thermoplastics and Thermosetting Plastics.

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* Thermoplastics includes such material as Perspex(polymethyl methacrylate) (transparent panels) and Nylon(ropes etc).

* Thermosetting Plastics such as Bakelite (phenol

formaldehyde) (electrical insulators), Formica, Ebonite andEpoxy resins (adhesives).

Rubber

A naturally occurring thermosetting plastic. Natural rubber comes from treesand is normally vulcanised with sulphur to produce a tough elastic material.

Used in anti vibration mountings; drive belts; shock absorbers (simple bungeecord types) and of-course, tyres. It can be made electrically conductive by

adding carbon.

FIBRE REINFORCED COMPOSITES

Panels and parts made up of fibre, adhesives/resins and cores to produce alight weight high strength material/structure.

* Glass Fibre. Made of fine glass fibres lubricated to improvehandling and may have other coatings to improve bonding

etc. Supplied in different forms: A glass; C glass etc. Eglass is currently the most popular. Used for theconstruction of radomes and dielectric panels

* Aramid Fibre. Supplied as Kevlar (Du Pont). Kevlar 29 usedfor cordage and ropes. Kevlar 49 supplied for plasticsreinforcement.

* Carbon Fibre. Carbon fibres are strong in tension and areoften coated to improve handling and bond strength. Usedfor structural parts of the airframe. Some structures are

almost all carbon fibre, eg the Boeing 787 Dreamlinerfuselage.

* Hybrid Fibres - made up in different forms and can includea mix of carbon and glass fibres.

* Resins. Many types are available to include the following:

Unsaturated Polyester Resin for use with glass fibre toproduce glass reinforced plastics (GRP). Used in conditions

where temperatures no not exceed 150C.

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Vinyl Ester Resins. Similar to above.

Phenolic Resins. Used on composites used for cabininteriors because of its low smoke emission properties.

Epoxy Resins. Thermosetting, versatile, and used widely inengineering. Usually supplied as a two part mix.

Polyamide Resins. Supplied powders, films, varnishes andlaminates. Used where higher temperatures are to beencountered (up to 300C).

* Cores. Used as low density (low weight) material betweentwo outer layers that take most of the stress (compressiveand tensile). May be made of honeycomb, foam, or someother low density material while the outer fibres are made of

metal, fibre composites etc. Core material may be:

Balsa Wood. Not used much these days but is used on someplywood structures.

Honeycomb. Used extensively as core material in aircraftfloors, structures, control surfaces, helicopter blades, etc.Can be made of aluminium or composite material.

Foam (Polyvinyl chloride). PVC is used as the core of some

composite structures.

Microballoons. Used as a low density infill medium whenrepairing honeycomb and similar structures. Some specialfillers also available such as Araldite AW106.

DIMENSIONS & MEASUREMENT

Dimensions

May be linear or angular and will be specified in words or symbols indocuments such as the AMM, SRM, drawings, manuals, etc.

LINEAR MEASUREMENTS

The imperial unit of length is the yard (defined in 1878 and kept in alaboratory in the UK as the standard yard) divided into feet (3) and inches (36).

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Fig. 7 SURFACE TO SURFACE LINEAR MEASUREMENTS

* Vernier Callipers.

These are not so easy to read as the micrometer.

The English vernier calliper has an accuracy of 0.001 and themetric vernier calliper has an accuracy of 0.02mm. Again, theycan be made so that internal and external face to facemeasurements can be taken as well as depth gauging etc.

For more information on micrometers and verniers see the book in this seriesentitled Tools.

ANGULAR MEASUREMENT

The universal unit for measuring angles is the degree (). There are 360degrees in a circle. The degree is split into 60 equal divisions called minutes ()and each minute is further divided into 60 equal divisions called seconds ().

For scientific work the radian is used of which there are 2radians in a circle

(2 x 3.142 = 6.28 radians). This means that there are 360 6.28 degrees in aradian (about 57.29 degrees).

Fig. 8 THE RADIAN

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The accuracy of measuring angles depends on the distance from the fulcrumwhere the measurement is taken. Most drawing protractors have graduationsof half a degree.

For measuring angles between surfaces a bevel protractor can be used. This

has an accuracy of 5 minutes of arc (5). For more details on the bevelprotractor see the book in this series entitled Tools.

For measuring angles from the horizontal an inclinometer can be used.Figure 9 shows the Hilgar & Watts clinometer which measures degrees on thedegree scale and minutes on the minute scale and has an accuracy of 1 of arcto the horizontal.

The instrument is first checked on a wooden or metal straight-edge held in avice (protected by a cloth). Both the degree scale and minute scale are zeroedby turning the minute knob and the unit placed on the straight edge. The

straight edge is moved until the spirit level reads zero. The clinometer is thenturned around and placed back on the straight edge at the same position. Ifthe clinometer is serviceable then the bubble will still read zero.

For checking for specific angles the instrument will have to be sent to astandards room.

Fig. 9 CHECKING THE CLINOMETER FOR ZERO READING

The instrument is then placed on the surface to be measure with the arrowpointing up the slope.

For quick adjustment the minute scale can be disconnected from the degreescale by pushing the quick release minute knob down, this allows the degreewheel housing the spirit level to rotate freely.

Releasing the pressure on the minute knob allows it to re-engage with thedegree scale gear and final adjustments can be made on the minute knob tobring the bubble in the spirit level into the middle position.

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The degrees can be read from the degree scale in the window and the minutesfrom the minute scale. It has a range of 90but others may have a range of

only 10.

An electronic clinometer is available. It is placed on the surface to be

measured and the start button pressed. It will automatically give a digitalreadout of the angle and the direction of the slope.

STANDARDS OF ACCURACY

To allow for variations in manufacture which will not jeopardise theperformance of the part being manufactured allowances are given on any sizequoted. This allows parts to be manufactured economically but meet theirperformance specifications.

For example, when manufacturing an aircraft toilet seat it would be possible tospecify its size to say 18in diameter plus or minus 0.0001in. It would be veryexpensive to manufacturer to this specification and it would work well.

It could be made considerable cheaper, however, if its size was 18in plus orminus 0.1in diameter andit would work just as well.

On the other hand if a close tolerance bolt was manufactured to say 1indiameter plus or minus 0.1in then it would be cheap but would not do its job,so its tolerance would be something like plus or minus 0.0001in.

Allowance

When a dimension is quoted (say 30mm) then an allowance is also quoted, forexample plus or minus () 0.01mm. This means that the part could be30.01mm (for a male part like a bolt sometimes called Maximum MetalCondition) or 29.99mm (Minimum Metal Condition) and still be acceptable.29.99 is called the lower limit and 30.01 is called the upper limit.

Tolerance

This is the difference between the upper limit and the lower limit. In theexample above this would be 30.01 29.99 = 0.2mm.

Where the tolerance is allowed on both sides of the nominal dimension (30mmin this case) it is said to be a Bi-lateral tolerance. Where the tolerance is all toone side of the nominal dimension it is said to be a Unilateral tolerance eg:

Bi-lateral tolerance 30mm 0.01

Uni-lateral tolerance 30mm + nil 0.2

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The tolerance for both is the same (0.2mm). For the first example it is evenlysplit either side of the nominal size, for the second example it is all on thelower side of the nominal size. It could equally be 30mm + 0.2 nil and still bea uni-lateral tolerance.

Size specifications will be stated on drawings, modification documents, aircraftmanuals etc, together with allowances and tolerances. For example, manyrepair drawings state cutting sizes for sheet metal repairs and give allowances

of 1/64thin (0.4mm).

It is most important that, when manufacturing items (metal repairs, crimpedcable joints, wire locking, pipelines etc) that the dimensions and anytolerances are carefully followed.

This also applies when assembling/inspecting/checking components.Checking for play, clearances, end float, wear, dimensions etc.

Drawings

Drawings, modification leaflets, manuals etc will specify the nominal size of apart and state the allowance. This may be repeated at each item in thedrawing, eg:

34.0mm 0.2

Some drawings may not specify the nominal size but just give the upper andlower limits of size, eg:

34.2mm33.8mm

If the allowances are the same for all the components on the drawing thenthere might be a box on the drawing stating the tolerances for all thedimensions given (this might include the size nomenclature mm, inches etc).

For example:

ALL DIMENSIONS IN mm AND ALL TOLERANCES ARE 0.01mm

This would mean that dimensions would be then given as (for example):

33.10

104.0 etc without nomenclature stated or tolerances given.

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CALIBRATION OF TOOLS/EQUIPMENT

All instruments and tools that are used for measuring must be calibrated atperiods stated by the manufacturer. These items include:

* Electrical test instruments/test sets such as multi meters,bonding testers, meggers, Flukes etc.

* Torque wrenches and torque wrench testers such as theAcratork test rig.

* Pressure gauges used for systems charging such as theTurner Adapter and gauges attached to charging equipment(pressure guns for charging shock absorbers, O2 and aircharging equipment etc).

* Charging equipment for batteries.

* Pitot static test sets.

* Theodolites (used when airframe rigging on some aircraft)spirit levels, clinometers etc.

* Dial Test Indicators (DTIs) or Clock Gauges.

* Micrometers, verniers, bevel protractors etc

All of the above will have pre-use checks carried out on them prior to use and,at the specified dates they will have to be sent away for calibration or at anytime their accuracy is suspect. Calibration facilities may be available at theuser unit or the equipment may have to be sent to a manufacturer or specialistestablishment.

Each item of equipment once re-calibrated should be labelled and returned (insuitable packing to prevent transit damage) together with supportingdocumentation to the user unit.

The label should state:

The calibration companys name.The companies approval number.Calibration date.Next calibration due date.

The item should be returned from the calibration room with documentationrecording:

* Firms name and approval number.

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* Equipment name, part number and serial number.* Inspectors stamp and signature.* Tolerances on readings if applicable.* Calibration printout if available.

The calibration room should keep records of all equipments tested dates nomenclature part numbers serial numbers the procedure used and whatcompany the item came from.

STANDARDS OR CALIBRATION ROOMS

These have the specialist equipment and manuals to carry out the testsspecified by the manufacturer. For some rooms the environment is not of greatimportance testing torque wrench equipment, testing multimeters forexample. But for some other items the environment is critical.

Many standards rooms have a controlled environment in that the temperature

is kept strictly at 21C and airborne particles (dust etc) are kept to aminimum.These conditions are needed when dealing with instruments that aretemperature sensitive. These include micrometers and verniers.

Instruments that are temperature sensitive should be brought into thestandards room 24 hours before the test to allow them to temperaturestabilise.

Most instruments that are temperature sensitive will have the calibrationtemperature marked on them.

Testing

The item is inspected for damage, contamination etc and any parts foundunserviceable are changed. The tests carried out, briefly, are as follows:

Electrical Test Sets. Multimeters, bonding testers etc. Tested against standard

known resisters, standard known voltages etc and against masterinstruments.

Torque Wrenches. These are tested on a standard test rig such as the Acratorktest rig. (The test rigs are tested using a moment arm fitted horizontally to thesquare boss and accurate masses attached to the free end of the arm. Themoment and hence the torque is calculated as mass times the distance fromthe mass attachment and the centre line of the boss (m x d = torque).

Pressure Gauges. Tested on rigs such as the Dead Weight Tester using known

masses on a piston producing calculated pressures (pressure = force per unitarea = F/A).

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Electric Charging Equipment. Used for charging batteries. Checking voltage,current and frequency against master instruments.

Pitot Static Test Sets. For Bourdon tube type and other pressure operatedcapsules, equipment such as the Dead Weight Tester are used.

Calibration rooms may also make use of vacuum chambers. (Heavy steelconstruction with a toughened glass front about the size of a microwave oven.It is sealed shut after the instrument to be tested is placed inside.

Various connections pressure, vacuum, electrical are connected to theinstrument prior to placement and pass-out side via leak proof connections inthe wall of the chamber. Pressure or vacuum is then applied to the chamber asper the manufacturers instructions and the performance of the instrumentnoted and recorded).

Angle Measuring Instruments. Accurate angles may be calculated usingJohannson Blocks or Slip Gauges and a sine bar. The slip gauges are wrungtogether to create a specific height to a considerable degree of accuracy andusing rollers and a sine bar the angle can be calculated accurately.

Fig. 10 TESTING A CLINOMETER

Sine bar, rollers and slip gauges are machined and polished to a high degree ofdimensional accuracy and should be kept in their boxes in a standards room

at the specified temperature (usually 21C). They must always be dismantledimmediately after use as they will bond to each other if left together for toolong. Assemble immediately prior to use so as to keep their assembled timeshort.

Angle measuring instruments are available that are electronic. These have tobe checked in a similar way to the non-electronic variety. If they rely on a

horizontal then a perfectly horizontal surface table is also required.

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Dial Test Indicators. These can be checked using slip gauges and feelergauges. The DTI is fixed to a rigid fixture on a surface table and slipgauges/feeler gauges are used to check the amount of pointer movement.

Precision Measuring Instruments. These may be electronic or non electronic

They are checked against standard test pieces supplied by the instrumentmanufacturer or against slip gauges all in a temperature controlledstandards room of course.

LIMIT SYSTEMS

The correct functioning of component parts of a machine depends on thedifference in size and finish of the surfaces in contact. Clearance must existbetween the parts to permit lubrication, but if the shaft is either too large ortoo small the efficiency of the machine is impaired.

It is impossible to make a part to an exact dimension due to unavoidableimperfections in manufacture, workmanship and accuracy of measurement.In practice the interchangeability of mass produced parts can be achieved iftheir dimensions are within certain limits.

To allow parts to be machined within certain limits (between the high and lowlimit) Limit Systems were devised. These systems formulate the sizes of holesand shafts and the various limits that may be applied. This means thatinstead of actually specifying the upper and lower limits (tolerance) for a shaft

or hole a code can be found in a table that corresponds to the values required.This code can be used in drawings, manuals and related documents andwould be understood by all those working to this particular standard.

History

Years ago each manufacturer would have its own standard of Limits and Fits,but using another firms products such as nuts or bolts could be difficult, if notimpossible. To overcome this problem national systems were developed.

In the UK two parallel systems were developed; the Newall System and theBritish Standard System. These were both national systems. Since then thingshave moved on a little. Standards have become more international particularly within Europe, though there are still many different standardsworld-wide.

British Standard 4500

British Standard 4500 ISO is more international than the previous limit

systems. It contains data and information on the fit of shafts and holes.

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This information can equally be applied to dimensioning in general and doesnot have to be applied to shafts/holes exclusively.

The standard contains a list of the terms used and their definitions. It alsocontains, in tabulated form, the upper and lower deviations for a vast number

of sizes of shafts and holes.

The standard allows for a hole and shaft based system. (See Terms Usedbelow).

Fig. 11 TERMS USED IN BS 4500

Definitions (Figure 11)

Clearance Fit. The shaft is smaller than the hole irrespective of thedimensional tolerances. Used where ease of fit and/or movement is requiredeg, rivet hole and rivet, moving piston in a cylinder, etc. Usually a clearance fitmay be assembled by hand with no undo force.

Transition Fit. Nominally the shaft and hole are the same size. It is notpossible to machine a hole and shaft to the same size so in reality the shaftmay be very slightly larger or very slightly smaller than the hole depending on

variations due to tolerancing.

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A small force such as a press may be required to assemble the parts. It is usedfor parts which need to be a close fit and are not often dismantled and there isno movement between the male and female parts.

Technically a Transition Fit will be either a (small) clearance fit or a (small)

interference fit.

The following drawing shows a transition fit.

It can be seen from the drawing that if the shaft size is machined to itsmaximum metal condition (upper deviation limit) and the hole is machined toits maximum metal condition (lower deviation limit) then the fit will be aninterference fit.

Fig. 12 TRANSITION FIT

If the shaft is machined to its lower deviation limit then there will be aclearance fit. If the hole is also machined to its upper deviation limit then theclearance will be even greater.

Remember that if this is the specification then the two parts will be machinedto within the given tolerances so that any male/female fit nominally classed asa Transition Fit could be either a clearance fit or an interference fit.

Interference Fit. Under normal conditions the shaft is larger than the hole. Itwill require force such as a hydraulic press to assemble or may be assembledusing temperature (eg, keeping the shaft in a refrigerator before assemblywhich will cause it to shrink in size).

Used for parts which are not normally secured together in any other way (valveguides in piston engines for example) and will not normally be dismantled. Theinterference fit will be interference irrespective of the tolerances given.

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Fig. 13 INTERFERENCE FIT

Tolerance. This is the difference between the maximum and minimum size of apart ie, the difference between the upper and lower deviation.

Basic Size. This is the size of the part about which the upper and lowerdeviation is quoted. The upper and lower deviation may be quoted either sideof the Basic Size or both on the same side.

Hole Basis. Fits may be obtained by producing the hole to a nominal size andchanging the size of the shafts.

Shaft Basis. Opposite to the Hole Basis in that the shaft is produced to a

nominal size and different fits obtained by varying the size of the holes.BS4500 allows for either system to be used.

Tolerance Grades

BS 4500 has 18 grades of tolerance, 01, 0, 1, 2 ....................16

The larger the number the greater the tolerance. Figure 14 shows some of thetolerance grades for sizes 3 to 6mm diameter. BS4500 contains tables for a

range of sizes.

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Fig. 14 SOME SELECTED TOLERANCE GRADES (3 to 6mm)

Fundamental Deviation

The position of the tolerance zone will decide on the type of fit obtained thus

BS4500 gives 27 fundamental deviations for holes and 27 for shafts. Holes arelettered A to ZC with H used on the hole based system. Shafts are lettered a tozc with h used on the shaft based system.

The following figures illustrate the tolerance zones for some typical fits for:

1. A hole based system.2. A shaft based system.

blank

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Fig. 15 SOME SELECTED HOLE BASED FITS (3 to 6 mm)

Fig. 16 SOME SELECTED SHAFT BASED FITS (3 to 6 mm)

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EXAMPLES

Hole Based System

Interference Fit H7 - s6

Transition Fit H7 - n6Clearance Fit H7 - h6

Shaft Based System

Interference Fit h6 - P6Transition Fit h6 - K6Clearance Fit h6 - F7

The letter and number designations are those used on drawings and relateddocuments.

If a close tolerance hole is to be manufactured at user unit level then this isdone by hand using a reamer. A drill is selected 0.005 (0.13mm) smaller thanthe reamer and the reamer size is that of the finished hole. See the book inthis series entitled Tools.

Clearance and Wear

When a shaft has to rotate or slide within a hole the fit has to be a clearance

fit. This clearance allows a gap (however small) which prevents too muchcontact between the parts and allows a lubricant layer. However, wear willoccur. If the two materials are the same then the wear is likely to be the sameon both the male and female. If one part is made of a softer material (lessresistant to wear and abrasion) then its wear rate will be higher.

In many applications one part is deliberately made of a softer metal this partbeing easier or cheaper to replace than the other.

In relation to a shaft and a hole, in general wear will reduce the size of theshaft and increase the size of the hole, which may make both items

unserviceable if each is worn beyond the permissible limit (sometimes calledmaximum worn dimension).

On the other hand, when the parts wear they could increase the clearance sothat it exceeds the maximum permissible clearance butindividually each partcould still be within its size limitations. This means that each part separatelyis serviceable but put together make up an unserviceable item or component.

In component maintenance bays this problem may be overcome by SelectiveAssembly.

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Selective Assembly

By selecting a shaft on its high limit of size and mating it with a worn hole themaximum permissible clearance may not be exceeded. The same could applyby selecting a hole on its low limit of size and mating it with a worn shaft.

If this procedure is allowed it will be stated in the Schedule of Fits andClearances for the component concerned.

DRG Parts and Dimension Permissible Clearance PermissibleREF description new worn new worn RemarksNO dimension clearance

9 Shaft collar Transfer plate 0.750 0.756 Shaft collar andin transfer bore 0.752 end bush areplate 0.002 0.008 selected for width

Shaft collar 0.746 0.742 0.006 iaw Repair Scheduleoutside dia 0.748

10 End bush Rear cover 0.750 0.754in rear bore 0.752cover 0.002 0.006

End bush 0.746 0.744 0.006outside dia 0.748

11

Fig. 17 PART OF A SCHEDULE OF FITS & CLEARANCES

Schedule of Fits and Clearances

Issued by the manufacturer for each component on the aircraft whoseperformance is dimensionally dependant (example page is shown figure 18).

DRG REF NO refers to the item number in a drawing often an exploded view.

Parts and Description gives a brief description and title of the male and femalemating parts.

Dimension New gives the upper and lower limit of size as delivered from themanufacturer. Outside these the part must be rejected.

Permissible Worn Dimension gives the size of the part that it is allowed to wearto. Beyond this limit the part is unserviceable and must not be used.

Clearance New gives the upper and lower limit for the new assembled parts.Outside these limits the assembly must be rejected though selectiveassembly may meet the clearance requirements.

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Permissible Worn Clearance states the maximum permissible clearanceallowed. If permissible wear sizes are exceeded the part must not be used andthe manufacturers manuals/literature must be consulted. It may be possibleto re-condition the item or it may have to be scrapped.

When taper or ovality is found in a shaft the minimum size anywhere along thelength is taken and used in calculations unless the schedule statesotherwise. Ovality and taper limits may be specified in the schedule.

For shafts limits of bow and twist will also be stated.

CHECKING FOR WEAR

The following paragraphs deal with procedures that require the use ofprecision instruments. Details of these will be found in the book entitled Tools.

On shafts wear may be uniform reducing the diameter equally around thecircumference and along the length. This may be checked using a micrometeror vernier and taking many readings around the circumference and along thelength. If all the readings are the same then the wear has been uniform. If thereadings get smaller (or larger) as they are taken along the length then theshaft has worn more at one end than the other and it has become tapered.

If the readings are different taken at one place along the length but rotatedaround the circumference then ovality has occurred. Ovality (similar to the UK

50p coin) may be checked by placing the shaft on a set of V blocks and using aDTI. Rotate the shaft slowly and note any fluctuations of the DTI pointer.

Bow may be checked by using a three point trammel and feeler gauges. It mayalso be checked by rotating the shaft in a set of V blocks and using a DTI. Withthe DTI positioned in the middle of the shaft and the shaft supported in the Vblocks at its ends the shaft is rotated. Any bow will result in a rise and fall ofthe pointer once per revolution of the shaft. Be careful of this interpretation asit could also indicate an usual wear pattern so check with the three pointtrammel.

Holes and bores may also be checked for taper, wear and ovality. Internalvernier callipers, internal micrometers and bore gauges may be used, butunless the hole can be removed, ie a single journal bearing, rotation andchecking with a DTI is not possible.

Bearings may be checked for play by clamping one race (say the outer race),suitably protected and moving the inner race as far as possible by hand andmeasuring the distance moved. A DTI can be used for this and two movementsshould be checked radial and axial several times with the inner race atdifferent positions each time.

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Angular movement might also be checked by twisting the inner race as far as itwill go and using a linear measurement (DTI) or an angular measurement(bevel protractor, clinometer etc). Note some races will rotate completely out oftheir outer shell so this test does not apply to those.

Wear will be indicated if the axial and radial movements are high and damageand brinnelling to the balls/rollers/races will show up a rough running whenthe bearing is spun by hand.

The fingers of one hand are placed inside the inner race to make a tight fit. Theouter race is spun using the other hand and any rough running is felt throughthe fingers. Inner and outer diameters may be checked as for shafts.

EASA Mod 7A Bk 3 Eng Pract

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