After reading this essay you will learn about:- 1. Meaning and Definition of Inspection 2. Objectives of Inspection 3. Organisation of the Inspection Department 4. Inspection Standards 5. Kinds 6. Methods 7. Tools 8. Practices for Controlling Product Quality 9. Problems.
Essay Contents:
- Essay on the Meaning and Definition of Inspection
- Essay on the Objectives of Inspection
- Essay on the Organisation of the Inspection Department
- Essay on the Inspection Standards
- Essay on the Kinds of Inspection
- Essay on the Methods of Inspection
- Essay on the Tools for Inspection
- Essay on the Inspection Practices for Controlling Product Quality
- Essay on the Problems of Inspection
Essay # 1. Meaning and Definition of Inspection:
A worker manufactures products according to his skill and ability. During manufacturing he inspects his assigned job whether he is doing it as per laid specifications or not. In mass and interchangeable production mostly semi-skilled workers are engaged to produce goods.
Hence separate inspecting staff is deputed to inspect jobs to ensure that manufactured products are according to established standards and to stop doing further work on spoiled products.
The reputation and success of any Industry largely depends upon quality of his products. Inspection does not create quality but it helps to control it. The aim is to prevent defects by finding and eliminating the causes of trouble. Inspection sorts out below standard materials and products and then stops their manufacturing.
Definition:
Inspection is the function to judge the quality of a product. According to Dr. W.R. Spriegel “Inspection is the process of measuring the quality of a product or service in terms of established standards.”
A broad definition given by Kimball, “Inspection is the art of comparing materials, products or performance with established standards.” Whenever products are manufactured, some will be within the limits of errors and some will be outside the allowances provided. With the help of inspection, those products are selected which will be satisfactory in all conditions of work.
Definition given by Alford and Beatly, “Inspection is the art of applying tests, preferably by the aid of measuring appliances to observe whether a given item or product is within the specified limit of variability.”
Inspection is an important aspect of production control. Products should be checked at various stages for the size, shape and quality. It stops the wrong production and thus wastage can be checked. Defective parts cannot reach the assembly points. To maintain the desired accuracy and quickness, special gauges and other measuring devices must be used. Therefore, inspection is a tool for controlling the quality of a product.
Inspection controls the quality of the product and is a must in a production shop, inspite of its adding to the expenditure. This statement can be justified, if we first study the objects of inspection.
Essay # 2. Objectives of Inspection:
The main objects are:
1. To collect information regarding the performance of the product with established standards for the use of engineering, production, purchasing and, quality control etc.
2. To sort out poor quality manufactured products and thus to maintain the standard.
3. To establish and increase the reputation by protecting consumers from receiving poor quality products.
For achieving these objectives there is inspection department in every organisation and its cost has to be borne by the employer but if there is no inspection, the products of poor quality may reach to the consumer and material wastage and spoilage will be more, machining time will be more. So products of poor quality and high cost will reduce the reputation of the concern and demand of their products will gradually go on reducing and the business may come to a standstill.
Hence with the little additional cost, inspection system can be organised to provide large benefits or we can say it is impossible to run any organisation economically and efficiently, whatever its size may be, without proper inspection.
Essay # 3. Organisation of the Inspection Department:
Inspection department should not be attached directly under those who are engaged in increasing the quantity of production, unless the quantity of product is of much importance.
If the production foreman will be the in charge of inspection work, then quality of product will receive less attention and he will simply try to increase the quantity of production.
If the quality is of average importance then inspection department may be kept as a staff department under his works superintendent. In this system production and inspection both work under the supervision of superintendent.
If quality is of maximum importance, the inspection should be a major manufacturing function directly under the control of works manager. In this way, inspection department holds a similar position as that of purchasing department and the engineering department.
Duties and Responsibilities of Chief Inspector:
Chief Inspector is the head of inspection department. The inspection is the responsibility of the Chief Inspector who organises this work and supervises the work of the inspectors.
To maintain the standard of quality, the inspection department prevents defective products from passing forward to next operations and should inform the way to prevent them.
Following are the main responsibilities of the Chief Inspector:
1. To organise and supervise the work of the inspecting staff.
2. To train the staffs carry out their duties effectively.
3. To reject the products which are below standard.
4. To send reports on inspected work and to suggest the ways to minimise faulty production.
5. To see that proper care of costly equipment’s in inspection department is being taken.
Functions of Inspection Department:
1. To Inspect Raw Materials:
This is done to see that raw materials received are physically and chemically in accordance with the specifications. This is done to eliminate the loss of labour and machining time on the defective materials.
For some materials visual examination may be sufficient while for others inspection may be necessary to test the electrical, mechanical, chemical properties etc.
2. Metallurgical and Metallographic Inspection:
To be sure to see that the metallic structure, hardness and other properties after the various processes of operations are according to specifications.
3. Purchased Parts Inspection:
The aim of such inspection is to see that all the purchased parts i.e. tools, spares, jigs and fixtures, general supplies etc. are according to laid specifications. This is to make sure that there is no loss in accepting inferior quality or short counts or damages.
4. Work in Process Inspection:
To see that product is correctly-formed and dimensioned in accordance with specifications during the process of their manufacture in various stages. This has the advantages of (i) preventing wastage of time and money on defective units and (ii) preventing delays in assemblies.
5. Tools Inspection:
To inspect different tools, which are purchased or made to see that whether they are capable of producing the parts in accordance with the specifications.
6. Periodic Gauge and Other Measuring Instrument’s Inspection:
To make sure that these equipment’s are capable of giving desired results, whenever used.
7. To correct errors in design or manufacturing methods that results in more assembly time or poor performance.
8. Finished Products Inspection:
To inspect finished products before they are marketed to see that product to poor quality may be either rejected or on certain industries may be sold out as ‘second’ at a low price.
9. Salvaging:
This is the function of inspection in which poor quality work is reworked to eliminate the defects or used for some other purpose or converted to scrap material etc.
10. Complaints Division:
It is the function of inspection which receives reports on defective materials or workmanship, design, shape etc., from the sales and service division. This then investigates and makes recommendation to the manufacturing department to prevent future occurrences.
Qualities of Inspector:
In order to carry out the full responsibilities, Inspector must possess certain special qualities.
Following qualities are necessary in any Inspector:
1. He should know his job thoroughly. For this purpose, technical knowledge and skill in the art of inspection is essential.
2. He should be intelligent, capable and of good grasping power.
3. He should understand his responsibility and be able to work with patience. Any negligence on his part may lead to serious loss to product quality.
4. He must know statistical quality control techniques programme well.
5. He should be cost conscious and therefore not set unnecessary strict and narrow limits.
6. He should be able to minimise or prevent wastage by using a substitute to the material already in use.
7. He should have the working knowledge of the general quality standards.
8. He should know the reasons for standard through an understanding of materials and processes.
Essay # 4. Inspection Standards:
To meet the requirement of quality control certain inspection standards are to be formed.
The most important may be:
(i) Inspection standards for raw materials.
(ii) Inspection standards for work in process.
(iii) Working inspection standards.
(iv) Inspection standards for finished product,
(v) Inspection standards of the completed mechanism.
(i) Inspection Standards for Raw Materials:
The inspection standards for raw materials are based upon purchase specifications.
(ii) Inspection Standards for Work in Process:
Inspection standards for work in process may be classified in 5 heads, as follows:
(a) Relating to physical condition or properties of materials. Inspection of the condition of the product after any given process may have reference to the result or to the fitness for the process or operation.
(b) Relating to shape and size. Interchangeable manufacture is based on such standards. To maintain control over quality, shape and size largely forms the basis for most of the machine shops, engineering and mechanical industries.
(c) Relating to degree of finish. Standards are to be fixed for smoothness, shade, polish, matt, tint, burnish etc. Formerly standards were not available but today with the help of profilometer, we can compare surface irregularities so that variations as fine as 0.0000025 mm are comparable.
(d) Chemical. In chemical industries such as food, soap, cosmetic, drugs etc., selected samples for inspection and products are tested in accordance with statistical quality control techniques.
(e) Functional or performance. This standard varies with the industry, for example in food industry, the product is tasted to see if it has the desired flavour or not. In the same way, in a diesel engine assembly, fuel pump and injectors are properly calibrated on specially developed testing equipment before delivery to the assembly department.
(iii) Working Inspection Standards:
After limits and tolerance have been determined, they are then given on working drawings. Precision in dimension is transferred to working drawings whose blue prints are given to operators and the work is carried out by them on specially designed machines of economical production. The work is simply inspected by use of “GO” and “NOT GO” gauge.
(iv) Inspection Standards for Finished Products:
The standards for finished machine parts, sub-assemblies and assemblies allow:
(a) For components that will assemble with those which they must fit to make a subassembly,
(b) For sub-assembles that will group into a final assembly and
(c) For final assemblies that will enter a completed mechanism. Complex and elaborated gauges and fixtures are used for the inspection of such finished products.
The standards for other finished products will ensure that product will satisfy the customer.
(v) Inspection Standards of Completed Mechanism:
After the components of a mechanism have been assembled a final test or series of tests are conducted. For example, a life is tested to carry a specified overload without permanent set. A gun barrel will stand a heavy proof charge without bursting or bulging.
Essay # 5. Kinds of Inspection:
Because of the varieties of manufactured products and the wide differences in qualities, it reasonable to employ various kinds of inspection.
The most important of these are:
1. Tools Inspection (Trial-run Inspection).
2. First Piece Inspection.
3. Working Inspection.
4. Sample Inspection.
5. Operation Inspection.
6. Final Inspection.
7. Pilot Piece Inspection.
8. Key Operation Inspection
9. Functional Inspection.
10. Endurance Inspection.
The above kinds are based on method employed.
Following are concerned with the location of the work of inspection rather than with its function:
11. Floor or patrolling
12. Centralized Inspection.
1. Tool Inspection:
According to this method tools, fixtures, jigs and gauges are inspected in advance to the work of production. In this method it has been assumed that when tools are proper, the products manufactured by them will be satisfactory. This method can give good results in automatic production work. A trial run of a single piece is made to see that product is within tolerances.
2. First Piece Inspection:
In automatic machines, if first 3 or 4 products are inspected and are found to be satisfactory then it is assumed that product manufactured afterwards will also be satisfactory. It assures that work has begun correctly.
3. Working Inspection:
At this type of inspection, the products should be inspected while they are in process of production, to ensure that they are being produced according to specifications. Inspector should check products at definite intervals to make sure that they are being produced as per requirements.
4. Sample Inspection:
In this, certain percentage from a lot is inspected and the entire lot is judged on the basis of the result of these samples.
5. Operation Inspection:
It is the inspection at the completion of one operation and he- fore the work-in-process passes to another operation or machine or department.
6. Final Inspection:
In this type of inspection, articles are inspected when they are completely manufactured before sending them to the store.
7. Pilot Piece Inspection:
This is used in product layout. The product is passed through its entire sequence of operations on a series of machines installed for producing that product. After one piece is manufactured, each tool, each machine is tested so that all defective tools are replaced and all incorrect adjustments are made alright. When a good product starts coming, the production line is allowed for actual production.
8. Key Operation Inspection:
There are certain operations which are quite costly or difficult, i.e. operators usually do mistake at such places. Such operations are known as ‘key operations’. Inspection is done prior to and immediately after the completion of each of these operations.
9. Functional Inspection:
This inspection is carried out after the completion of assembly, to check the accuracy of assembly and to assure it will function as required.
10. Endurance Inspection:
The inspection is carried out to estimate as to how much time an assembly will withstand its use and to determine weakness for correction.
11. Floor or Patrolling Inspection:
According to this, inspection is done at or near the machine. In this inspection work ranges from general petrolling, supervision, i.e. keeping an eye on work at machines, to careful measurement of product at its place of production.
Advantages:
(1) Errors caught in time prevent spoilage of large amount of work.
(2) This inspection can help in removing the troubles or difficulties while doing the work.
(3) This requires less handling of job because work does not have to be moved from the place of production. Thus indirect labour cost will be less.
(4) Delays at inspection room and thus wastage of time is minimised.
(5) Products layout and mass production can well be achieved.
(6) The work of routing, scheduling and despatching can be eliminated.
(7) It reduces labour cost.
(8) Reduction of the work in process is possible.
Disadvantages:
(1) Inspector has to move from one area or another and thus his time is wasted.
(2) Advance machines for inspection purposes, cannot be used.
(3) Keeping a track of good or bad products is slightly difficult.
(4) Sometimes work may pile up at work stations. The worker has completed his work but awaiting for inspection.
(5) Presence of large quantities of work on the floor complicates to keep work moving.
(6) The Inspector may be influenced by some of the workers and start believing favaouritism.
(7) Highly skilled Inspector need to be engaged.
(8) There can be delays in deciding doubtful cases.
12. Centralised Inspection:
In this kind, inspection is carried out in the inspection rooms. Parts to be checked are moved to special rooms where precision measuring devices are located.
Main idea in this is to separate inspection from manufacturing.
Centralised inspection does not mean one inspection room but a number of such rooms or cribs may be installed, each located centrally in respect to a certain group of machines. The usual practice is to locate cribs/rooms parallel to the flow of work through the shop.
Advantages:
(1) Inspector having less skill can perform the job well.
(2) Better quality can be achieved because of no or little understanding between workers and Inspectors.
(3) With the help of fixed and automatic inspection devices mass inspection at lower cost is possible.
(4) Because of better working conditions and less interferences inspector can check the products with higher speed.
(5) Accurate inspection with delicate instruments can be done under controlled atmosphere.
(6) To avoid displacement or theft, work can be stored in self-counting trays.
(7) Less number of gauges, instruments etc., are required.
(8) Decision on doubtful cases can be taken at once by authority.
(9) The shops mostly remain neat and clean as there is no accumulation of finished parts. They are collected and sent to inspection room.
Disadvantages:
(1) Workers will come to know their weak points much after the completion of jobs.
(2) Material handling is more.
(3) Delay at inspection rooms causes wastage of time.
(4) Here routing, scheduling and despatching includes the inspection rooms, so the work of production control increases.
(5) If the inspection room is not built in the line of flow, there may be break in the plan of product layout for straight line manufacture.
(6) Due to non-detection of machining errors in time, there may be more spoilage of work.
13. Combined Method:
This is combination of the Floor and Centralised Inspecting systems. Large and heavy jobs usually require patrolling inspection. This is essential to keep operation running and clear of troubles on the spot resulting in the reduction of spoiled materials and machining costs.
The necessity for more accurate inspection of certain parts of sub-assemblies cannot be denied and they may require to move to the central inspection room.
Therefore, for introducing any kind of inspection, the situation must be studied thoroughly before taking the decision.
Essay # 6. Methods of Inspection:
To control the quality of manufactured products, following three methods of inspection are usually adopted in mass scale production.
In big industries, all these three methods together may be used:
1. Screening or 100% Inspection.
2. Lot by Lot Inspection.
3. Process Inspection.
Each method has been explained as under:
(i) Screening:
It is that method of Inspection in which each and every unit manufactured in inspected to meet the desired specifications. The aim is to catch all the defective products produce. But experience has shown that screening or 100% inspection does not guarantee a perfect product due to monotony. Monotony creates fatigue and lowers attention. If cent percent correct products are required then at least 200 per cent inspection is necessary.
Further there are many types of ‘destructive’ tests where 100 per cent inspection would result in 100 percent destruction of products, for example sharpness testing of razor blades, tensile strength tests of wire, chip test of enamel, filament test of electric bulb and firing tests of ammunition etc. For such products other methods of quality control are necessary.
Hundred percent inspection is necessary for:
(i) Important components upon which the functioning of the whole assembly depends,
(ii) Where a process normally yields a high percentage of defective products.
For example, production of high precision ball bearings in which generally 20% defective products may come.
(ii) Lot by Lot Inspection:
This method is also known as Sampling Inspection. This method was developed to eliminate the high cost of Screening Inspection. In this method, small number of samples is drawn from the lot and the inspector judges from them the acceptability of the whole lot. Here modern statistics or mathematical probability helps the inspector by furnishing him with readymade sampling plans.
(iii) Process Inspection:
The purpose of this method of inspection is to search out defective products where and when they occur, so that an immediate corrective action can be taken. This inspection deals with all causes of defective work, be it operator, operation, raw material, tool, machine etc. So, in process inspection inspector patrols in assigned areas and checks machines, methods of operations, and occasional pieces of product from raw material to finished products.
The limitation of this method is that inspectors cannot be stationed at all machines at all time. As a result large quantities of defective materials may slip from the knowledge of inspectors.
This method is used for inspecting precision machining, intricate casting, certain types of spot and resistance welding and in many chemical, mechanical and electrical measurements.
Some of the Disadvantages of this system are:
(1) Errors in time cannot be detected.
(2) Material handling is more.
(3) Delays at inspecting room causes wastages of time.
Essay # 7. Tools for Inspection:
Inspection is “the function of comparing or determining the conformance of product specifications.” The specifications provide necessary details as to what is required, then inspection is done to see what is actually produced. This work of inspection requires equipment’s. These equipment’s help in speed of inspection and precision.
Following are the important methods of inspection:
(i) Inspection by Measurement:
Exact measurements are determined with the help of micrometer, vernier calliper, torque indicating spanners.
(ii) Limit Gauges:
These can be used even by an unskilled operator to know quickly whether the dimensions of the product are within the limits of the specifications.
(iii) Multiple Gauging:
These consist of electric contacts operating coloured signal lamps to indicate whether the dimension is correct, oversize or under-size.
(iv) Air Gauge:
This equipment is very useful for inspection in mass production work.
(v) Optical Comparators:
The object is magnified by 10 to 50 times and projected on a screen to enable to compare with known standards.
(vi) Non-Destructive Testing:
For detecting defects in material, these testing methods are employed. The test methods are visual, pressure and leak penetration, thermal, radiography, acoustic, magnetic, electrical and electromagnetic induction etc.
In order that parts produced are of consistent and of acceptable quality, the inspection gauges and test equipment are always kept in accurate condition. Adequate past inspection records should always be kept because it provides valuable information about recurring troubles and their remedies.
Essay # 8. Inspection Practices for Controlling Product Quality:
(i) Life Testing:
Life testing of products is the most essential requirement in certain industries specially those manufacturing Electric tubes, Razor blades etc. Life testing is performed so as to ensure whether the product is as per specifications or not. While manufacturing it is essential to test the components as well as operations.
Life test ensures the manufacturer that the product shall give good performance during his guaranteed life. Control of the quality of a product is performed by means of check applied at certain stages during the manufacturing line and at final stage. Similarly ‘life test’ must also be performed throughout the production period so that good life performance can be ensured.
However, life testing involves extra expenditure but the information obtained from these tests are very important so as to take guarantee of the life of product by the manufacturer. Thus, the life testing may be considered as an insurance policy.
Objectives of Life Tests:
Main Objects of life tests are as follows:
1. To make sure that user will get a reliable life of the product.
2. To show, if there is any manufacturing defect so that it can be rectified in future lots.
Classification of Life Tests:
Life tests are classified as follows:
(a) Quality Control Life Tests:
These tests are for checking the quality.
(b) Pilot-Run Life Tests:
The life tests are performed whenever any change in the production is proposed, that is, these are to check the proposed production, whenever any change in production occurs.
(c) Establishment Life Tests:
These life tests are performed whenever a new type of product is introduced.
(d) Application of Life Tests:
When conditions for the use of product Eire different than that for which it is generally used than these life tests are performed.
Life Testing Procedure:
Life testing is performed by putting the maximum stresses, load vibration, frequency or voltage for which it has been desired. If a product works satisfactory under its maximum loading and its performance is nice at extreme conditions than the life is said to have been satisfactory.
Main difficulty in life testing is that there is no reliable techniques of life testing and hence there is necessity of evolving a satisfactory method. However, the methods given above is being employed to give relative reliability?
Sample or Individual Life Testing:
Whether a product is to be tested for life test individually or by sampling depends upon their importance. For very important products such as Radar, Radio, transmitting tubes where high reliability, greater stability and long life is required, individual product must be tested for life tests. Products of less importance such as electric bulbs, fans, electric press, radio receiving tubes etc. are put to sample testing for life tests.
In sample testing percentage of products put on life tests are decided on the basis of their relative importance.
(ii) Testing Reliability:
Customer always wants that the products which he purchases must be reliable. Reliable means that these should meet requirements without failure for a considerable time. It is also the desire of the manufacturer that whatever he produces should be reliable. For this purpose manufacturers continually reassessing their products to rectify defects and discrepancies.
Today the word reliability has acquired a highly specialized technical meaning in relation to the control of quality manufactured product BAZCVSKY states the modern concept of reliability in popular language as follows:
“Reliability is the capability of an equipment to work well and works whenever called upon to do the job for which it was designed, such equipment is said to be reliable”.
The generally accepted definition of reliability is as follows:
“Reliability is the probability of a device performing its purpose adequately for the period of time intended under the operating conditions encountered”.
To clearly understand the above definition following points are highlighted.
Adequacy:
As per definition, system must work adequately. This concept of adequacy is different for different ages. The motor is required to stop, start and to run perhaps for years without failure, while missile system requires to be operated just once, but without fail.
Period of time intended:
One cannot expect a device or a system to work forever and that too in same condition. The device or system wears out with time and hence the capability of the system to perform its task adequately depends upon the age of the system. In other words, we can say that reliability is the study of failure rate.
Under the Operating Conditions Encountered:
The performance of an equipment is also affected by the environment in which it was manufactured and operated. Environmental conditions like temperature, humidity, vibration and shock are contributory to failures.
Failure Characteristics:
Experience has shown that failure characteristics of most of the systems, follow a definite pattern, as indicated in Fig. 58.1.
To understand it we can divide the life of a device or system in following three-periods:
(a) Burn-In Period:
When system is given a trail run during initial period there may be rapid failures, known as initial failures. These failures are concerned with malfunctioning due to poor design, poor workmanship and poor quality. The sub-standard components are selected and then replaced. This will improve the reliability.
(b) Useful Life Period:
After the burn-in period, actual useful life of normal use starts. During this period failure rate is low but occurs unexpectedly and at random intervals such failures are known as ‘Normal failures’ or ‘Chance failures’.
(c) Wear-Out Period:
Beyond its useful life period, failure rate increases very much. This is due to excessive wear after the expected useful design life of the device or system has been exceeded. The failures during this period are known as ‘Aging failure’ or ‘Wear out failures’
The curve shown in Fig. 58.1 is sometimes referred as the ‘Bath Tub’ characteristics.
Reliability Centred Maintenance (RCM):
Reliability Centred Maintenance is a process used to determine the maintenance requirement of any physical asset in its operating context.
Objectives of RCM:
i. Maintenance strategy optimisation.
ii. Extension of equipment life span.
iii. Elimination of chronic machine problem.
iv. Root cause analysis
v. Justified preventive maintenance tasks.
What RCM Achieves:
i. Greater safety and environmental integrity.
ii. Improved operation performance (Output, product quality, customer service).
iii. Greater Maintenance cost effectiveness
iv. Longer useful life of expensive items.
v. A comprehensive database
vi. Greater motivation of individuals.
Reliability and Quality:
Quality and reliability must be distinguished from one another. Quality is defined as the degree to which a device meets various standards specified for it. Quality standards include efficiency, accuracy, aesthetic standards in addition to the reliability. Whereas reliability means ‘Able to the trusted’ or ‘Not to breakdown in operation’. This shows that reliability factor is an important factor amongst the quality factors.
Some Terms:
1. Mean Time Between Failures (M.T.B.F.):
It is the average value of time intervals between successive failures of equipment.
2. Failure Rate:
It is the number of failures per minute.
3. Probability of Failures:
It is defined as one minus the reliability of a system.
4. Mean Time to Restore (M.T.T.R.):
It is time required to locate a failure and to repair it.
5. Reliability Improvement Factor:
It is ratio of probability of failure of present system to that of improved system.
6. Availability:
It is the ratio of satisfactory operating time to the total operating time of the system, i.e.
Example 1:
An electronic system has a M.T.B.F. of 1000 hours and a M.T.T.R. of 40 hours, what is availability?
Solution:
Example 2:
System is composed of1000 elements each having a M.T.B.F. of 100,000 hours. What is the probability of failure of the system, if its cumulative operating time is 10 hours?
Solution:
Since, probability of failures = 1 Reliability
Example 3:
If the reliability of a system is increased from 0.9 to 0.95, find out the Reliability Improvement Factor.
Solution:
Probability of failure in Case = 1 – r1 = 1 – 0.9 = 0.1
and Probability if failure in II case = 1 – r2 = 1 – 0.95 = 0.05
Conclusion:
Thus by increasing the reliability of the system from 0.9 to 0.95, probability of failure has been reduced to half hence system is twice as reliable as first system.
Example 4:
(a) An electronic device can be designed utilising either vacuum tubes or transistors. If a device having 5 tubes of M.T.B.F. of 10,000 hours each is transistorised with 20 transistors having M.T.B.F. of 80,000 hours. Does it pay to transistorise from the reliability point of view?
(b) If 40 transistors are employed in above example, what conclusion can you reach from the results?
Solution:
(a) Probability of failure in I case (i.e. with 5 tubes)
Thus we conclude that by increasing the number of elements reliability decreases. Hence to get maximum reliability, number of elements must be kept minimum.
Combined Failure Rates:
Example 5:
A 20 component system with each component, having reliability of 0.99. Find out the reliability of the system.
Solution:
Since Reliability of system = R1. R2….. Ri……… Rn = (R)n
where n = 20
Reliability = (0.99)20 = 0.82 Ans.
(iii) Redundancy:
Redundancy technique utilise two parallel elements to reduce the probability of failure.
Reliability = 1 – (pf)2 = 1 – F2
Group Redundancy:
In practical approach, the purpose of group redundancy is to provide each channel of elements, which perform a function, with another duplicate channel. Thus, we have a normal channel or a redundant (stand by) channel.
The probability of failure of a system containing a total number of N elements, n elements in each group and having separate redundant groups, is given by pf = N. nt/T.
Example 6:
Two units of system, A and B have reliabilities of 0.9 and 0.50 respectively.
Determine the reliability for the following configurations:
(a) A and B are cascaded
(b) A and B are cascaded and is redundant
(c) A and B are cascaded and only B as redundant
(d) A and B are cascaded and the C cascaded combination has group redundancy.
Solution:
(a) Reliability of configuration
= Ra x Rb = 0.90 x 0.50 = 0.45 Ans.
(6) Reliability of redundant A = 1 – F2
1 – 0.1 x 0.1 = 1- 0.1 = 0.99
Similarly, reliability of redundant B = 1 – F2 = 1 – 0.5 x 0.5 = 1 – 0.25 = 0.75
... Reliability of configuration = 0.99 x 0.75 = 0.74 Ans.
(c) Reliability of A = 0.9
Reliability of redundant B = 1 – F2 = 1 – 0.5 x 0.5 = 0.75
... Reliability of cascaded combination = 0.09 x 0.75 = 0.675 Ans.
... Reliability of group redundancy = 1 – F2 =1 – (1 – 0.45)2
= 1 – 0.55 x 0.55 = 1 x 0.28 = 0.72 Ans.
Example 7:
A system consists of1000 elements each having a M.T.B.F. of 100,000 hours. Its cumulative operating time is one hour.
(a) What is the probability of failure?
(b) What is the probability of failure if it is designed in a group redundant manner each group consisting of 10 elements?
(c) What is the Reliability Improvement Factor?
Solution:
Essay # 9. Problems of Inspection:
Following are the problems of inspection, which should be decided before the inspection starts:
(1) Where to inspect?
(2) When to inspect?
(3) How to inspect?
(4) How much to inspect?
1. Where to Inspect?
Inspection may be done either at the machines or in a central inspection room separated from actual production. Floor inspection saves the cost of transportation of the material from the place of production to the inspection room. Floor inspection provides quicker inspection service.
Floor inspection is very useful specially when the product is very bulky and when it becomes very difficult to carry the products from production site to inspect in room.
In continuous industries where subsequent operations are linked together by moving belts or other mechanical handling devices, then it is out of question to carry the product off into separate inspection room.
On the other hand, a separate inspection room provides accurate and rapid inspection than that provides at production site. At central inspection room, inspection condition, will be better, and proper precision instruments may be provided.
In the case of central inspection room, there are less chances of the inspector being influenced. Therefore, one of the above mentioned way is selected after considering all the factors.
2. When to Inspect?
This problem of when to inspect’ depends upon the nature of products and the procedure employed for manufacture. No general rule can be given about whether inspection should be done after each operation in the process or only at the final stage of completion.
Usually a product is inspected when it is transferred from one production department to another, so that responsibility may be fixed for any defective work. Inspection is necessary after that operation in which there are more chances of defects, so that main aim of inspection to control the quality with a minimum possible cost can be achieved.
3. How to Inspect?
One of them should be adopted after considering all the factors like type of products, operations involved etc.
Now-a-days, GO and NOT GO gauges are being used for inspection because this system requires very less time and even a semi-skilled worker can perform the inspection work. These gauges have replaced the micrometers which require more time and a skilled worker. Therefore, the use of GO and NOT GO gauges is economical and simple hence being used widely.
4. How Much to Inspect?
This factor is decided in the light of the product and accuracy desired. In some cases sample inspection is employed. In this, defective products may reach to the customers. Therefore, this system is not advisable where accuracy is required. Although this system is cheaper but now-a-days this system is adopted only for quite ordinary articles and where automatic machines are used.
The products, in which accuracy is important consideration, should be inspected through a 100 percent inspection.