Module 7 (Maintenance Practices) Sub Module 7.2 (Workshop Practices).pdf

For Training Purpose Only ISO 9001:2008 Certified

PIA TRAINING CENTRE (PTC) Module 7 - MAINTENANCE PRACTICES

Category A/B1 Sub Module 7.2 - Workshop Practices

PTC/CM/B1.1 Basic/M7/01 Rev. 00 7.2 Mar 2014

MODULE 7

Sub Module 7.2

WORKSHOP PRACTICES

For Training Purpose Only ISO 9001:2008 Certified



PTC/CM/B1.1 Basic/M7/01 Rev. 00 7.2 Mar 2014

ISO 9001:2008 Certified For Training Purpose Only



PTC/CM/B1.1 Basic/M7/01 Rev. 00 7.2 - i Mar 2014

Contents

MAINTAINING TOOLS --------------------------------------------------- 1

TOOL CATEGORIES ----------------------------------------------------- 1

CARE OF TOOLS --------------------------------------------------------- 2

CONTROL OF TOOLS --------------------------------------------------- 5

USE OF WORKSHOP MATERIALS ---------------------------------- 8

DIMENSIONS ------------------------------------------------------------- 11

TOLERANCES AND ALLOWANCES -------------------------------- 15

STANDARDS OF WORKMANSHIP --------------------------------- 18

CALIBRATION OF TOOLS AND EQUIPMENT ------------------- 19

CALIBRATION STANDARDS ----------------------------------------- 28




PTC/CM/B1.1 Basic/M7/01 Rev. 00 7.2 - ii Mar 2014

Page Intentionally Left Blank




PTC/CM/B1.1 Basic/M7/01 Rev. 00 7.2 - 1 Mar 2014

MAINTAINING TOOLS All the tools used for aircraft maintenance have to be of the highest quality to ensure expert maintenance of aircraft to the level prescribed by the manufacturer. At the most fundamental level, woodshop tool maintenance simply means keeping your tools operating as well as they did when you took them out of the box. That's a minimum requirement for running a safe, successful shop. But a good tool maintenance regimen can take you even further. Taking a few extra steps in caring for work surfaces, cutting edges, alignment mechanisms and moving parts can work wonders for the performance of your tools. Add in a few affordable power tool upgrades and you can improve the performance of your woodworking machinery beyond like-new condition. Below, we'll show you how easy it can be to go beyond the basics in keeping the tools in your shop sharp, true, clean, and running smooth TOOL CATEGORIES A maintenance organization usually has two basic categories of tools in use. Personal Tools Tools that are issued to the maintenance personnel permanently on individual basis depending on the function they perform in the organization. These tools will form a personal toolkit comprised of tools that are of general nature and required by a considerable number of persons frequently. These tools are selected for their familiarity and personal

quality, their low cost and convenience.

Common Tools Tools that are required for performing specific tasks on specific aircraft and those tools that are too bulky or considered too expensive to be included in a personal toolkit are included in this category. Procedures should be in place to ensure that all tools in the inventory are available in serviceable condition. This can be achieved by implementing a tool maintenance program that encompasses the following.

Care of tools procedures for storing, cleaning, lubrication of tools and equipment regularly or as per usage.

Control of tools procedures to ensure tools remain

serviceable and available as per the requirements.

Calibration of tools Tools that are used for measuring and checking should be checked and adjusted regularly for continued accuracy of the measurements. Calibration of tools used for the determination of the serviceability of equipment or correct execution of a procedure is of absolute importance as the product quality of the organization is dependent on it.





CARE OF TOOLS Most high quality tools are manufactured to the highest standards and are designed to last a long time, provided that they are not abused and necessary servicing is carried out in due time. Most of the tools are manufactured from alloy steels and are susceptible to corrosion. To reduce this susceptibility to corrosion most tools are manufactured with a corrosion resistant exterior finish and mechanisms that are sealed against moisture. Therefore the corrosion resistance is dependent on the integrity of such protections provided. Despite efforts to reduce corrosion and associated degradation, corrosion and wear still takes its toll unless cleaning and re- protection such as lubrication is carried out on regular basis. Workshop fixtures and equipment supplied and installed by the appropriate department are maintained and repaired by that organization, and must not be interfered with by personnel, except for general cleaning, re-painting externally, etc. Other equipment and machinery must be systematically cleaned, lubricated and adjusted by competent workshop personnel; the following points have a general application.

Personal tools Personal tools should be cleaned periodically or immediately after working in areas where the tools have been exposed to corrosive materials. General cleaning can be done with a stiff brush and a rag to remove dust/dirt. A cleaning fluid such as solvents may be used to remove grease, paints and such hard to remove material. After necessary cleaning has been carried out a suitable lubrication and/or re-protection material should be applied to prevent corrosion and to reduce wear. It is advisable to store measuring tools such as engineers scales, feeler gauges, etc., that are part of personal toolkits in a simple cover or case to offer them further protection from damage and degradation. Common tools Common Tools that are used frequently may also be cleaned and protected the same way as personal tools, but lubrication of internal mechanism should be carried out by authorized personnel only. Common tools that form a toolkit for a specific function and those tools that are not regularly used should be cleaned after every use and in addition, according to a schedule if such tools have not been used for some time. Lubrication and re-protection of





exposed areas should be carried out using recommended material so as to maintain protection against corrosion. In the following sections specific actions to be carried out while working on certain tools and machinery are briefly discussed. Such activities to be performed may also be stated when discussing about tools and their application in the following Modules. Care of tools general Benches Metal-covered and portable benches should have the bench surfaces cleaned with kerosene rag, while those of plain wooden benches should be cleaned with a stiff brush and by scraping if necessary. Some form of protection, such as a piece of hardwood or a lead block, should be laid on the bench top when carrying out punching or similar operations and care should be taken to prevent nails, pieces of metal etc., from becoming embedded in the bench surface. Portable benches should be given a thorough examination for security at regular intervals, during which all nuts should be checked to ensure that they are tight; the wheel bearings should also be lubricated as required. Vices Vices should be wiped over frequently with an oily rag. The moving jaw should be withdrawn to the limit of its movement to permit lubrication of the screw bearings and the thread, the jaw

insert screws and the bolts securing the vice to the bench should be tightened

periodically, and the vice handle should be kept rust-free so that it may slide freely when in use. Drilling machine The drilling machine should be cleaned and lubricated regularly; during this process the clamping screws of the drilling table should be slackened several turns to enable the threads and thrust faces to be lubricated. A piece of planed hardwood should be kept on the drilling table to protect the machined face when drilling sheet metals, etc. Grinder The grinding machine must be kept clean and as free from abrasive dust as possible. The bearings should be lubricated regularly, but care must be taken to prevent oil or grease coming in contact with the grinding wheels. The tool rests should be kept in adjustment at a position as near as possible to the grinding wheels. These wheels should be turned up, as required, by a wheel dresser; the resulting abrasive dust should be carefully removed after the operation has been completed, and the tool rests must then be reset. Motor drives Electric motors used for driving machine tools and portable apparatus must be kept clean and free from dust both internally and externally. Regular attention by authorized personnel is





essential for the effective maintenance of electrical equipment, and the work of these personnel is assisted if any defect, such as overheating or excessive sparking, is reported immediately. Workshop tool kits Special tool kits are supplied for servicing certain machines, assemblies, etc., and these kits must, of necessity, be available for general use. The fact that a kit is used by more than one person is not an excuse for neglect or maltreatment by the individual; such kits must be given the same care and attention that a good craftsman gives to his personal kit. Measuring instruments and appliances Equipment of this nature are normally kept in the workshop or tool store locker, and is issued on short-term loan as required. These items must be returned immediately after use; under no circumstances should they be left lying about on workbenches or stowed in personal toolboxes. In order to maintain the accuracy measuring instrument need proper handling Measuring instruments are usually issued with the storage box and other than during the time at which measurements are taken the instrument should be kept in the case.

Gauges and special tools These items should be kept in labeled boxes whenever practicable; the label should indicate the special purpose for which the gauge or tool may only be used. Drills and reamers Twist drills, when not in use, should be kept in a graded drill stand. Reamers should be kept in partitioned boxes or laid in grooved trays cut to receive each type of reamer.





CONTROL OF TOOLS The number and variety of tools in an aircraft maintenance organization can be in hundreds if not in thousands. Each tool may have to be maintained in a different way and be made available to certain group of persons frequently. In such an environment proper procedures have to be established to prevent the tool being misplaced or mishandled. Added to this certain tools require calibration and/or special servicing compounds the situation. Following procedures are generally adapted by most maintenance organizations to address such needs.

1 A person or persons are tasked with identifying the tool requirement of the organization and deciding tools that should form the personal toolkit and tools that should be categorized as common tools which should be maintained at a suitable accessible location to the required personnel.

2 Ensure that personal toolkits are complete and

maintained in an acceptable manner by carrying out audits periodically. Also non-approved tools, consumables and aircraft hardware should not be contained along with personal toolkits.

3 Tools that require specialized storage conditions should be identified and handled accordingly. Tools that require calibration and or servicing should be categorized and a procedure setup to ensure such activities are carried out in due time without affecting the tool availability requirements.

4 Maintaining an efficient issuing and tracking system to

ensure tools are issued to identify individuals by responsible persons who are well versed with the tools and procedures involved.

Common tools of a maintenance organization can be located and controlled in several ways. Tool Store or Tool crib This is a centrally located secure location for a large inventory of tools that are arranged in a precise manner with identified positions for tools. Such a location can also be equipped to handle tools with special storage requirements. Access into the location is strictly controlled and tools are issued to required personnel or authorized persons after the required information had been entered in a properly maintained issue register. Tool containers or toolbox kits Tools that have a specific application such as all specific tools required for an engine change on a specific aircraft may be located in a container that may be purpose-designed for easy





transportation and handling. These toolkits are also usually controlled by the central tool store and may be located at the main tool store or at another secure location. Some large organization may have several identical toolkits used for the same frequent function positioned at several line stations to reduce delays due to non-availability of such tools. Although these toolkits are located elsewhere, controlling of such toolkits will still be carried out by the centralized tool store or a dedicated section. Overseas or Line station tools Large aviation maintenance organizations usually maintain satellite-maintenance sections in addition to the main base, at other parts of the country or at overseas locations. These satellite-maintenance sections provide limited support for scheduled and unscheduled maintenance on aircraft depending on the requirement at these locations. For such organizations a separate section under inventory control department may exist, tasked with monitoring and controlling tools within the entire organization including the tools at such line stations. Local control and maintaining of tools at such location may be carried out by dedicated staff seconded from the main tool control section at main base or may be assigned to a member of the maintenance personnel of such locations if the inventory of tools involved is simple. Tools on loan Some maintenance activities may require tools to be acquired on loan basis from other organizations as the frequency of such activity or cost involved may not warrant the purchase and maintenance of such tools.

In organizations where a separate tool control is available, the task of acquiring tools on loan basis depending on the requirement, and also ensuring tool availability and serviceability will be the responsibility of such a section. Tool control procedures

Each tool in the inventory should be assigned a unique identification number and an entry should be maintained on each tool in a suitable register including relevant details about the tools.

In addition a complete record should be maintained on

each tool used for measuring and quality control that includes service history and calibration details.

An issue register will be maintained at the issue

counter of the tool store where the details pertaining to the individual receiving the tools are to be documented.

A new set of pages or section of the register is used for

every shift and issued items are tallied with received items at the end of the shift and handover to the next shift is carried out.

Issue and receipt is usually indicated on adjacent

columns so that outstanding entries can be identified at a glance. Those entries that have not been tallied are





transferred to another part of the register or separate documents for alternate action

If work is not spanned across shifts then outstanding entries are to be considered very serious and an investigation should be carried out to locate the tools as the possibility exists of such tools becoming the source of FOD (Foreign Object Damage).

When the user should carry out a cursory inspection of

the tool at book out and should bring to the notice of the stores personnel of any discrepancy immediately.

Upon completion of the work the user should make an

effort to return the tools to the stores as soon as it is convenient to enable another user to use the same tool if required and also to minimize the chances of misplacing the tools.

When returning the tools to the tool stores the staff

at the issue counter should check for the condition of the tool and properly mark the issue register for received status and position the tools in the assigned location. When documenting of a toolkit is done the number of tools issued and received are also mentioned

in the register.

It is the responsibility of the user to report of any

damaged or malfunctioning tool or equipment to the relevant person in charge of tools so that it can be repaired in time.

Unserviceable tools due to damage or malfunction is to be routed to the relevant sections or external repair organization for repair at the first available instance to prevent disruption due to unavailability. For tools that require frequent repairs an investigation should be done for possible mishandling or misuse.

Tools that require calibration will be tracked and

sent for necessary re-calibration prior to calibration due date or earlier if continued availability during a critical period is forecasted.





USE OF WORKSHOP MATERIALS Many of the wide variety of materials, used in workshops, require some form of control in their handling. This control can involve: Safety: relating to such topics as the toxicity, corrosiveness

or other health risks associated with the use of certain materials

Management: referring to the storage, use and correct

handling of all materials whether they are solid, liquid, or, in some instances, gaseous

Economy: involving such matters as to the using of the

correct dosage or proportions when mixing compounds, using only as much material as required for a specific task and to the keeping in stock of only sufficient materials and thus avoiding lifed items reaching their expiry dates before being used.

Abrasive papers, solder and brazing materials, wire wool, tyre

powder, oil spill powder and so on, all

require control of issue and use, though they may not, normally, require stringent safety precautions. A huge range of liquids can be used in the workshop situation, some of which are harmless and some of which are extremely toxic. It is vital that the work-force make themselves aware of the risks involved when dealing with ANY materials, and especially when working within enclosed areas. Some materials are flammable and must, therefore, be stored outdoors. These include oils, greases, some adhesives, sealing and glazing compounds in addition to many paints, enamels and epoxy surface finishes, which are stored in metal cabinets and, usually, located (in the Northern hemisphere) on the North side of a workshop or hangar. This ensures that the cabinet remains in the shade of the building and does not get exposed to the suns hot rays during the day. It is also important that only the minimum amount of these materials is taken indoors for the work which is being done. When handling materials that give off fumes, it may be necessary to have the area well ventilated and/or have the operator wearing a mask or some form of remote breathing apparatus. The finished work may also give off fumes for some time afterwards, so care must be taken to keep it ventilated if necessary. Obviously all liquids must only be used for the purpose for which they





are designed and never mixed together, unless the two materials are designed to be mixed, such as with two part epoxy adhesives and sealants. Many liquids used in workshops and in the hangar have (as mentioned earlier) a fixed life. This date is printed on the container and must be checked before use, because many materials are unsafe if used beyond their expiry date.





The disposal of liquids is a critical operation, and must only be carried out in accordance with company (and, often, national or international) regulations. Liquids must never be disposed of by pouring them into spare or unidentified containers and they must not be allowed to enter the domestic drains systems. The working with, and the use of, high pressure gas containers and oxygen systems, was adequately discussed in the Safety Precautions topic.





DIMENSIONS Information is communicated from one person to another primarily through spoken and written word. Such communication requires the use of previously defined, basic characters, the complete set of which is commonly known as an alphabet. Likewise, the scientific community has, in effect, established an alphabet of its own. The elements, or most basic parts of this communication system, are known as dimensions. Common dimensions The following are a few common dimensions and their definitions: The length of an object is the distance between its ends, its linear extent as measured from end to end.It is usually represented by the capital letter L. Mass (M) is the amount of matter in an object. Every object has a mass that does not change as the object is moved from one place to another. A force (F) has the capacity to change the motion of a body or cause stress in a body. It can also be described as a push or pull that can cause an object with mass to change its velocity (which includes to begin moving from a state of rest), i.e., to accelerate, or which can cause a flexible object to deform. Force has both magnitude and direction, making it a vector quantity.

Time (T) can be defined as a period or interval between two events. It is a component of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify the motions of objects. Temperature is physical property of a system that measures degree of hotness or coldness of object, ambience, etc. The temperature of a substance is a measure of the internal energy of the molecules (i.e., energy caused by movement of its molecules). Temperature is measured with thermometers that may be calibrated to a variety of temperature scales. The Celsius scale is used for most temperature measuring purposes..Many engineering fields also use the Kelvin and degrees Celsius scales. Other engineering fields also rely upon the Rankine scale and Fahrenheit scale





Fundamental and derived dimensions After a few dimensions are defined, it should be obvious that other dimensions can be obtained by combining one or more of them. This observation leads to the need to differentiate between the original dimensions and the combined dimensions, and thus the terms fundamental and derived dimensions were born. Fundamental dimensions The most elementary dimensions, like length (L), mass (M), and time (T), are known as fundamental dimensions. Fundamental units

Quantity Standard Unit Symbol

1 Length meter m

2 Mass kilogram kg

3 Time second s

4 Electric Current ampere A

5 Temperature Kelvin K

6 Luminous Intensity

Candela Cd

7 Matter mole mol

8 Plane Angle Radian rad

9 Solid Angle Steradian sr

Derived dimensions Dimensions obtained by combining one or more fundamental dimensions are called derived dimensions.

Area (L2) and volume (L3) are examples of derived dimensions obtained by combining the same dimension (i.e., L).

Velocity (LT-1), acceleration (LT-2), and pressure (ML-

1T-2), on the other hand, are examples of derived dimensions obtained by combining different fundamental dimensions (i.e., M, L, and T).





Named units derived from SI base units

Name Symbol Quantity Expression in terms of other

units Expression in terms of SI base

units

hertz Hz frequency 1/s s-1

Newton N force, weight mkg/s2 mkgs2

Pascal Pa pressure, stress N/m2 m1kgs2

joule J energy, work, heat Nm = CV = Ws m2kgs2

watt W power, radiant flux J/s = VA m2kgs3

coulomb C electric charge or electric flux sA sA

volt V voltage, electrical potential difference, electromotive force

W/A = J/C m2kgs3A1

farad F electric capacitance C/V m2kg1s4A2

ohm electric resistance, impedance, reactance

V/A m2kgs3A2

Siemens S electrical conductance 1/ m2kg1s3 A2

Weber Wb magnetic flux J/A m2kgs2 A1

tesla T magnetic field strength, magnetic flux density

Vs/m2 = Wb/m2 = N/(Am)

kgs2 A1

Henry H inductance Vs/A = Wb/A m2kgs2 A2





Although dimensions are necessary to describe an object or an event, they are not sufficient. That is, it could be correctly stated that both a football field and a matchstick possess the fundamental dimension of length, but if one were interested in knowing their relative sizes, additional information would obviously have to be provided about the dimension of length. This additional information is provided in the form of the units associated with each dimension. A unit is the standard of measurement applicable to a given dimension. For example, inches, feet, meters, furlongs, and fathoms all are units associated with the dimension of length. Similarly, cubic inches, liters, cubic meters, and gallons are units associated with the dimension of volume. Throughout history, different units have been adopted for quantifying the various dimensions, as illustrated for length and volume. Therefore, we may often need to convert numbers from one set of units into another (e.g., feet to meters, yards to centimeters).





TOLERANCES AND ALLOWANCES An impossible task to do in engineering manufacture is to make a part to exact dimensions called for by a design document. Dimensions may seems to match if measured using a measuring instrument with less accuracy, but if the measurements are taken using an instrument with higher accuracy, a dimensional discrepancy will exist between the stipulated and manufactured. When production to exact dimensions is not achievable during manufacture, the next possible scenario is to achieve the closest possible dimensions to what is required. Method of dimensioning and tolerance wherein the tolerance is taken as plus or minus from an explicitly stated dimension; the dimension represents the size or location which is nearest the critical condition (that is maximum material condition), and the tolerance is applied either in a plus or minus direction, but not in both directions, in such a way that the permissible variation in size or location is away from the critical condition.

The following terms are used generally when indicating dimensions. Nominal size The dimension of an object when variations in size are disregarded; the actual size of a part will be approximately the same as the nominal size but need not be exactly the same; for example, a rod may be referred to as inch, although the actual dimension on the drawing is 0.2495 inch, and in this case inch is the nominal size. Approximate or rough cut dimension by which a material is generally called or sold in trade, but which differs from the actual dimension. In lumber trade, for example, a finished (dressed) 'two by four' piece is less than 2 inches thick and less than 4 inches wide. Also called nominal size. Basic Size The basic size is that size from which the limits of size are derived by the application of allowances and tolerances. Limits - The stated maximum and minimum allowable dimensions when variation on the basic size is taken into consideration. Here, the largest allowable dimension is called the upper limit and the least allowable dimension is called the lower limit.





Tolerance The difference between the upper limit and the lower limit of a dimension. The amount that the size of a machine part is allowed to vary above or below a basic dimension; for example, 3.650 0.003 centimeters indicates a tolerance of 0.003 centimeter. Bilateral Tolerance When variation is allowable in both directions from the basic size. Here the actual dimensions of the object may be larger or smaller than the basic size by an allowable margin. Unilateral Tolerance When the variation is allowed only in one direction from the basic size. Here the actual dimensions of the object must comply with either of the following conditions but not both. Actual size can be larger than the basic size but the minimum allowable size should be that of the basic size and not less. OR Actual size can be smaller than the basic size but maximum allowable size should be that of the basic size and not more.

Allowance An allowance is a planned deviation between an actual dimension and a nominal or theoretical dimension, or between an intermediate-stage dimension and an intended final dimension. The unifying abstract concept is that a certain amount of difference allows for some known factor of compensation or interference. For example, an area of excess metal may be left because it is needed to complete subsequent machining.





Bilateral Tolerance

Unilateral Tolerance





STANDARDS OF WORKMANSHIP Whilst the standards of workmanship, during the hand-working of metals and other materials, is controlled by the craftsperson, once machinery is used in the manufacturing process, then the standards of finish and workmanship depend upon the allowances set by the designer and on the type of machinery being used. With hand tools, there are standards of finish, but these depend upon the skill of the craftsperson and, again, on the tools being used. For example, when filing metal, different grades of files are used, to obtain a comparatively smooth surface finish while other methods, such as abrasive papers, pastes and polishes, are then used, to provide the final finish. When sawing, the same procedures apply in that blades with finer teeth will give a better finish to the sawn edges, which may then be further smoothed, using an appropriate selection of files. When drilling a hole, the conventional twist drill will only produce a finish of a certain standard. If a finer finish, to the inside of the hole, is required, then a reamer would be used, to smooth the material inside the hole, so that, if a tight fitting pin is to be fitted through the hole, there will be better surface contact.

There are a variety of machines that can generate a smooth surface on a piece of metal, the selection between them being decided by the quality of finish. A lathe can produce an exceptionally smooth surface on a bar or some other rotated shape. If a large area is required to have a smooth finish, then perhaps, after initial casting or forging, the choice may be of employing either a grinding machine or a milling machine, to provide the desired result. In summary, the quality of the finished article is dependent both on the skill of the craftsperson and the equipment available to complete the task. It does not matter whether the tools in use are files and emery cloth or an expensive milling machine; the standard of workmanship of the craftsperson can make a great deal of difference to the finished article.





CALIBRATION OF TOOLS AND EQUIPMENT Instrument calibration is one of the primary processes used to maintain instrument accuracy. Calibration is the process of configuring an instrument to provide a result for a sample within an acceptable range. Eliminating or minimizing factors that cause inaccurate measurements is a fundamental aspect of instrumentation design. their accuracy. In any industry, measurements related to product quality are an essential part of quality control systems. In the aviation maintenance industry such measurements play a more important role, as decisions that have a direct impact on safety may be based on them. Measurements affect the product quality directly or indirectly.

Measurements affect the product directly when they take the form of dimensional measurements that determines the quality of the product. E.g. Diameter of a roller when checking for wear.

Measurements affect product quality indirectly when

they take the form of monitoring and control measurements of a process. E.g. Temperature maintained during heat treatment of material.

In trying to maintain and improve on product quality and level of safety, a fundamental requirement is the use of instruments that will provide measurements that are accurate to a high degree of the actual property being measured. Before dealing with calibration it is important to know the characteristics of measuring instruments and what factors affect their accuracy.





Instrument classification, characteristics Knowledge of the possible error level in measurements is essential, and a necessary pre-requisite for this is a proper understanding of the operational characteristics of instruments and an examination of the way in which instrument performance is specified. A convenient way to achieve this knowledge is to classify instruments into different types and then to study the characteristics of each of these various instrument sub-groups. Instruments consist of one or more separate components, which together serve to give an output reading, which is some function of a measured physical quantity. The primary component in an instrument is a transducer, which translates the measured physical quantity into another form. Further possible components within the instrument are an amplifier, an amplifier-analyzer and an output display system. The term 'instrument' is used somewhat loose, throughout this text, as is fairly common practice, to describe any or all of these components.

Instrument classification Instruments can be sub-divided into separate classes according to several criteria. These sub-classifications are useful in broadly establishing several attributes of particular instruments such as accuracy, cost, and general applicability to different applications. Active/ passive instruments Instruments are divided into active or passive ones according to whether the instrument output is entirely produced by the quantity being measured or whether the quantity being measured simply modulates the magnitude of some external power source. This might be more easily understood if it were illustrated by an example. An example of a passive instrument is the pressure-measuring device. The pressure of the fluid is translated into movement of a pointer against a scale. The energy expended in moving the pointer is derived entirely from the change in pressure measured; there are no other energy inputs to the system. An example of an active instrument is a petrol-tank-level indicator, as sketched in Figure 2.2. Here, the change in petrol level moves a potentiometer arm, and the output signal consists of a proportion of the external voltage source applied across the two ends of the potentiometer. The energy in the output signal comes from the external power source; the primary transducer float





system is merely modulating the value of the voltage from this external power source. In active instruments, the external power source is usually in electrical form, but in some cases it can be other forms of energy, such as pneumatic or hydraulic. One very important difference between active and passive instruments is the level of measurement resolution, which can be obtained. With the simple pressure gauge shown, the amount of movement made by the pointer for a particular pressure change is closely define by the nature of instrument. While it is possible to increase measurement resolution by making the pointer longer, such that the pointer tip moves through a longer arc, the scope for such improvement is clearly bounded by the practical limit on how long the pointer can conveniently be. In an active instrument, however, adjustment of the magnitude of the external energy input allows much greater control over measurement resolution. While the scope for improving measurement resolution is much greater but it is not infinite because of limitations placed on the magnitude of the external energy input, in consideration of heating effects and for safety reasons. In terms of cost, passive instruments are normally of a simpler construction than are active ones, and are therefore cheaper to

manufacture. Choice between active and passive instruments for a particular

application thus involves balancing the measurement-resolution requirements carefully against cost.

Fig. 2.1 Passive Pressure Gauge





Fig. 2.2 Petrol Tank Level Indicator (Active) Null/ deflection-type instruments In deflection-type device, the measured quantity produces some physical effect that engenders a similar but opposing effect in same part of the instrument. The opposing effect increases until balance is achieved, at which point deflection is measured and the value of measured quantity inferred. The last pressure gauge is a good example of a deflection type of instrument, where the value of the quantity being measured is displayed in terms of the amount of movement of a pointer. A null-type device attempts to maintain deflection at zero by suitable application an effect opposing that generated by the measured quantity but not suitable for dynamic measurement

(fluctuating).An alternative type of

pressure gauge is the dead-weight gauge shown in Figure 2.3, which is a null-type instrument. Here, weights are put on top of the piston until the downward force balances the fluid pressure. Weights are added until the piston reaches a datum level, known as the null point. Pressure measurement is made in terms of the value of the weights needed to reach this null position. The accuracy of these two instruments depends on different things. For the former, it depends on the linearity and calibration of the spring, while for the latter; it relies on the calibration of the weights. As calibration of weights is much easier than the careful choice and calibration of a linear-characteristic spring, it follows that the second type of instrument will normally be the more accurate. This is in agreement with the general rule that null-type instruments are more accurate than deflection types. In terms of usage, the deflection-type instrument is clearly more convenient. It is far simpler to read off the position of a pointer against a scale than to add and subtract weights until a null point is reached. A deflection-type instrument is therefore the one that would normally be used in the workplace. For calibration purposes, however, the null-type instrument is preferable because of its superior accuracy. The extra effort required to use such an instrument is perfectly acceptable because of the infrequent nature of calibration operations.





Fig. 2.3





Fig. 2.4

Monitoring/ control instruments

An important distinction between different instruments is made according to whether they are suitable only for monitoring functions or whether their output is in a form that can be directly introduced as an input into an automatic control system. Instruments, which only give an audio or visual indication of the magnitude of the physical quantity measured, such as a liquid-in-glass thermometer, are only suitable for monitoring purposes. This class normally includes all null-type instruments and mostly passive transducers. For an instrument to be suitable for inclusion in an automatic control system, its output must be in a suitable form for direct input into the controller. This usually means that an instrument with an electrical output is required, although other forms of output such as optical or pneumatic signals are used in some systems.





Fig. 2.5 Dead Weight Pressure Gauge (Null Type)

Analogue/ digital instruments

Instruments which use a needle or a hand moving around a dial to provide information are called analogue instruments while digital Instruments provide a numerical display of information An analogue instrument gives an output, which varies continuously as the quantity being measured changes. The output can have an infinite number of values within the range that the instrument is designed to measure. The deflection type of pressure gauge described earlier in this chapter is a good example of an analogue instrument. As the input value changes, the pointer moves with a smooth continuous motion. Though the pointer can therefore be in an infinite number of positions within its range of movement, the number of different positions, which the eye can discriminate between, is strictly limited, this discrimination being dependent upon how large the scale is and how finely it is divided. A digital instrument has an output, which varies in discrete steps and so can only have a finite number of values. The rev counter sketched in Figure 2.6 is an example of a digital instrument. In this, a cam is attached to the revolving body whose motion is being measured, and on each revolution the camp opens and closes a switch. The switching operations are counted by an electronic counter. This system can only count whole revolutions and therefore cannot register any motion, which is less than a full revolution.





The distinction between analogue and digital instruments has become particularly important with the rapid growth in the application of microcomputers to automatic control systems. Any digital computer system, of which the microcomputer is but one example, performs its computations in digital form. An instrument whose output is in digital form is therefore particularly advantageous in such applications, as it can be interfaced directly to the control computer. Analogue instruments must be interfaced to the microcomputer by an analogue-to-digital (A/D) converter, which converts the analogue output signal from the instrument into an equivalent digital quantity, which can be read into the computer. This conversion has several disadvantages. Firstly, the A/D converter adds a significant cost to the system. Secondly, a finite time is involved in the process of converting an analogue signal to a digital quantity, and this time can be critical in the control of fast processes where the accuracy of control depends on the speed of the controlling computer. Degrading the speed of operation of the control

computer by imposing a requirement for A/D conversion thus degrades the accuracy by which the process is controlled.





Fig. 2.6 Revolution Counter (Digital)

Static instrument characteristics Instrument Performance Characteristics are of two types: Static having nonlinear or statistical effects Dynamic described by linear differential equations





Static calibration All inputs (desired, interfering and modifying) except one are kept at some constant values. Then the input under study is varied over some range of constant values. The input-output relationship is valid under the stated constant conditions of all the other inputs. Measurement method: ideal situation all other inputs are held constant Measurement process: physical realization of the measurement method Steps in Static Calibration

1. Examine the construction of the instrument and identify and list all the possible inputs.

2. Decide which of the inputs will be significant in the application for which the instrument is to be calibrated.

3. Procure apparatus that will allow you to vary all the significant inputs over the ranges considered necessary. Procure standards to measure each input.

4. By holding some inputs constant, varying others, and recording the output(s), develops the desired static input-output relations.

The various static characteristics are defined in the following paragraphs. CALIBRATION STANDARDS History





In United Kingdom, the appropriate procedures for attaining quality assurance are defined in document BS 5750 (Parts 0-4). This was first published by British Standards Institution in 1979, since then it has been adopted in a wide range of industries. This first version has been modified in collaboration with the International Standards Organization in the light of user A revised version was published in 1987 by both the British standards Institution and the International Standards Organization as two separate but identically worded documents. ISO versions are numbered ISO 9000ISO 9004. At the end of 1987, the procedures were also adopted by the European Committee for Standardization and published as identically worded documents numbered EN 29000-EN 29004.3. Prior to 1987, a separate document, B8 5781, existed that detailed the necessary measurement and calibration procedures associated with quality assurance systems, but this became obsolete when these procedures were subsumed within BS 5750 in 1987.

Requirement

As specified in BS 5750, The supplier shall provide, control, calibrate and maintain inspection, measuring and test equipment suitable to demonstrate the performance of the product to the specified requirements. Equipment shall be used in a manner, which ensures that measurement uncertainty is known. STANDARD PROCEDURE BS 5750 lays down procedures to be followed when selecting, using, calibrating, controlling and maintaining measurement standards and measuring equipment. A summary of the requirements is given below:

1 The supplier shall establish and maintain an effective system for the control and calibration of measurement standards and measuring equipment.

2 All personnel performing calibration functions shall have

adequate training.

3 The calibration system shall be periodically and systematically reviewed to ensure its continued effectiveness.

4 All measurements, whether for purposes of calibration or product assessment, shall take into account all the errors and uncertainties in the measurement process.





5 Calibration procedures shall be documented.

6 Objective evidence that the measurement system is effective shall be readily available to customers.

7 Calibration shall be performed by equipment traceable to

national standards.

8 A separate calibration record shall be kept for each measuring instrument. These records must demonstrate that all measuring-instruments used are capable of performing measurements within the designated limits. The record for instrument shall contain as minimum:

a description of the instrument and a unique

identifier;

the calibration date;

the calibration results; The calibration interval (plus date when next calibration due). Some or all of the following information is also required in the calibration record, according to the type of instrument involved:

the calibration procedure; the permissible error limits;

a statement of the cumulative effects of

uncertainties in calibration data;

the environmental conditions required for calibration;

the source of calibration used to establish

traceability;

details of any repairs or modifications which might affect the calibration status;

Any use limitations of the instrument.

9 All equipment shall be labeled to show its calibration status and any usage limitations (if practicable).

10 Any instrument, which has failed or is suspected (or

known) to be out of calibration shall be withdrawn from use and clabelled conspicuously to prevent accidental use.

11 Adjustable devices shall be sealed to prevent tampering.

STANDARD PROCEDURE

Module 7 (Maintenance Practices) Sub Module 7.2 (Workshop Practices).pdf

Documents

Transcript of Module 7 (Maintenance Practices) Sub Module 7.2 (Workshop Practices).pdf