Reliability improvement measures-II.

1.       Problem - Mechanical Bonded Radiator Core leakage.

          Leakages are from mostly end-tubes. Both first and last row of inner tubes near both sides are found rubbing with corner plate which results in tube puncturing. 

Action to be taken:

firm should be advised to review the design to avoid rubbing of both first and last row of inner tubes with the corner plate.  Provision of Heat Resistant Rubber pads or any suitable material in between inner tubes and the corner plate should be provided to avoid rubbing and puncturing. Dummying of these tubes should be tried. Tightening of radiator header bolts during commissioning & yearly schedule to arrest water leakages should also be done.

2.       Problem – Leakage of Fuel Oil from Fuel Header Brazing Joint & Fuel Header Block.

          There have been cases of failures due to Fuel Oil Header brazing joint leakage and Fuel Oil Header Block was having leakage.

          Brazing and hydraulic testing of the detected area should be carried out. Shop should be requested to review the design of Fuel Header and ensure strict quality control for manufacturing of this header.

3.       Problem – A/C core discharge pipe line leak

There have been cases of leakage of A/C core discharge pipe line due to ’Y’ manifold flange welding crack.

Welding and hydraulic testing of the detected area should be carried out. 

         

4.       Problem – Suction Pipe line from tank to suction trap      

There have been cases of rubbing of Suction Pipe line from tank to suction trap in lube oil strainer.

Action to be taken:  Adequate gap should be maintained or flexible hose can be used. Cases of failure should be reported to Shop along with photograph & details.

5.       Problem – Fuel oil tank bottom crack

          There have been cases of premature welding crack in the bottom of the Fuel Oil Tank leading to fuel oil leakage and posing an unsafe condition.

Action to be taken:  shop should be requested to review the design and quality of fabrication of Fuel oil Tank as attention to Fuel Tank welding becomes a major work in Running Shed. Leg has been removed in some Sheds.

6.       Problem -     Drainage of Fuel oil through Fuel tank drain plug.

The drain valve provided in the Fuel tank for drainage of fuel oil is at slightly higher level due to which complete drainage of fuel oil contaminated with water becomes difficult.  Welding joint is also found at bottom of the tank.

Action to be taken: Welding of existing joint should be checked & one round re-building should be done. shop may be advised to review the drain valve location.  In addition, the design may be reviewed for re-location of welding joint from bottom to side.

7.       Problem – Fuel oil tank cleaning

Fuel oil tank is found rusted badly in number of locomotives.  

Action to be taken:  Shed must clean the tank before commissioning the loco. shop may be advised to provide bigger size man-hole & ensure through cleaning of tank before despatch.

         

8.       Problem – Ejector assembly adopter drop      

Ejector assembly adopter comes out as only few threads of adopter are found engaged with the Ejector assembly.  

Action to be taken:  Ejector assembly thread should be re-machined and refitted. shop should be advised to ensure proper fitment of the adopter with the Ejector assembly to improve the reliability.

9.       Problem – Oil pan to inlet pipe line flange leak       

There have been cases of failures due to leakage of lube oil from oil pan to inlet pipe line flange due to poor welding.

Action to be taken: shop should be advised to ensure quality workmanship in this area. Welding quality should be checked and these should be re-welded.

10.     Problem – Centre pivot bolt broken     

There have been cases of Centre Pivot bolt breakage.

Action to be taken:  shop should be advised to review the quality of Centre pivot bolts and also to use disc lock washer if warranted. Centre pivot bolt condition should be checked in 90 days & above schedule.

         

11.     Problem – Frequent breakage of Vertical Damper holding bracket bolt

There have been large number cases of breakage of Vertical damper holding bracket bolt. 

Action to be taken:  shop should be requested to review the design and fixing arrangement to improve the reliability. Anti-seizure compound should be used during fitment of bolts.

         

12.     Problem – Rubbing of Hand-brake chain with top gear case body

The hand-brake chain arrangement is found rubbing with top gear case body & may result in gear case oil leakage leading to traction motor seizure. 

Action to be taken:  Existing old design lever type hand-brake should be replaced with new design wheel type hand-brake arrangement and also ensure adequate clearance between hand-brake chain and the top gear case should be ensured.

13.     Problem –     Non-opening of ‘H’ type CBC (centre buffer coupler)

                              Provided in WDP_4B / WDP_4D

Transition rod is not aligned properly and due to this, it is not allowing lock lifter to lift the lock.  Especially when the CBC moves away from its central position and the Operator, it is not at all able to lift the lock.

Action to be taken:  shop should be requested to ensure the alignment of the transition rod correctly and also ensure the easy opening of ‘H’ type coupler even when the CBC is at the extreme end from the central position.

14.     Problem – Chamber to crankcase gaskets given up

There have been cases of Loco Failures where the chamber to crank case gaskets have perished resulting in leakage of exhaust gases.

The gaskets received in the new locomotives turned-out from shop are of poor quality and they are failing frequently leading to exhaust gas leakage.

Action to be taken: Good quality of Gaskets (single integrated & compressed type) should be used to replace shop fitted gaskets.

         

15.     Problem – Premature failure of Tube Assembly water inlet pipe

          There have been cases of Loco failures where Tube assembly water inlet to power assembly had failed prematurely due to the circumferential crack, less thickness of tube and improper quality of manufacturing (notches, tool marks, etc.).

Action to be taken:  Proper alignment & lab testing of tube should be ensured before fitment. shop should be requested to review the design especially thickness of the tube.  Strict quality control from the Vendors is also extremely essential because this pipe gets weakened at the ends where brazing is done.

16.     Problem – Joint expansion assembly crack

          There have been cases of Joint expansion assembly crack. 

Action to be taken:  Only OEM approved joints should be fitted at vulnerable locations. shop should be requested to review the quality of indigenously manufactured Joint assembly expansion to improve the reliability.

17.     Problem – Water entering between power pack retainer to engine block

          There have been cases of water entering between power-pack retainer and engine block due to bad workmanship & Engine block crack. 

Action to be taken:  Suitable sealant should be applied at vulnerable locations. 

         

18.     Problem – MR pressure dropping

There have been cases of MR pressure dropping due to both inlet and discharge valves spring breakage.

Action to be taken:  Good quality spring should be fitted one round during 180 days & above schedules. 

19.     Problem – Compressor Dip stick flange bolt drop due to inadequate length.

Action to be taken: Diesel Shed should provide bolts of adequate length (20 mm long) with spring washer in all the locomotives. 

20.     Problem –     Defect in line assembly fuel oil manifold injector

Action to be taken:  shop  should be requested to review the specification of Cone material to avoid the damages as at present, the material is getting easily damaged. Drive during schedule should be started.

Reliability measures EMD Locomotives.

Following reliability measures may be incorporated in maintenance action plan for improving loco reliability.

1.    Incorporate all items of standard schedule form in Shed’s maintenance schedule form. It should also include Shed practices & lessons learnt. Action plan made section wise must be part of schedule form.

2.    Benchmarking of maintenance practices with other leading Sheds should be immediately started. It will help Shed in arresting problems which have already been eliminated by other Sheds.

3.    All drives must be completed in a time bound manner. Preferably within two months.

4.    Shed failures should automatically trigger launching of drives & review of maintenance practices in every section instead of waiting for line failures to happen.

5.    MMC (Mis-Management by Crew) list to be compiled and focused training should be arranged.

6.    All line and Shed failures must be reported to HQ & firms. Quarterly summarized reporting vendor wise should also be ensured.

7.    Power assembly testing, checking practice should immediately be audited.

8.    Time bound implementation of proven ALCO modifications in failure areas to be started.

9.    Welding should not be done in running schedules. Welding should be done during pre-commissioning & 3 yearly/6 yearly Schedules. Reason for frequent welding should be identified & same should be made part of major schedule work content.

10. Shed has identified vulnerable locations of water & lube oil leakages. Root cause of leakages from these identified locations should be ascertained. These root causes should be attended during major schedules to avoid recurrence of leakages.

However, joint drive to check lube & water leakages should continue to arrest line failure & to judge efficacy of corrective measures taken during major/medium schedule. All recently received loco (0-2)yrs. should be re-commissioned on the basis of root causes identified jointly with lab.

11.  Stablisation circuit should be provided to arrest large failures of Wood word governor coils by providing suitable diodes. Drive must be completed in next 2 months.

12. Proper layout and checking of all cable connections during commissioning of the locomotive should be organised. Checking of OFC cable and its alignment should also be organised.

13. Changing the position of the CPC coupler provided in ECC3 from outside to inside to avoid failure due to thermal load.  One clamp to be provided in compressor room.

14. Application of silicon rubber sheet to prevent the damage of cable insulation in panel mounted modules like FCF, TLF, DVR, ASC, FCD; etc, in all locomotives.

15. Comprehensive instruction sheet for drivers based on last 3 years failures to be pasted in the drivers cab in the form of stickers.

16.  Application of GE 2000 RTV sealant on grid cover joints, ECC1 panel top, cyclonic filter joints, DB grid fans etc. during commissioning.

17.  Installing modified version of DVR software to reduce the battery charging voltage from 72 V to 69 V for improving the battery life. Latest version of EM 2000 software 23.08.00 as recommended by OEM should also be installed.

18. Cleaning of aspirator hole provided in the clean air chamber for draining the water to arrest problem of the grounding should be done.

19.  Sump cleaning and fuel tank cleaning during pre-commissioning to be done.

20. After cooler pipe should be realigned to arrest the water leakage. All such similar rubbing prone pipes to be realigned.

21.  During commissioning valve bridge should be removed and proper pressing of lash adjuster should be ensured to avoid damage of cylinder head seat.

22.  Use of anti seizure compound in all high torque areas, where removal of bolts is problem like top and bottom bracket mounting bolts of dampers etc.

23.  Provision of clamp in water inlet pipe of compressor to arrest water leakage.

24. Tightening of radiator header bolts during commissioning & yearly schedule to arrest water leakages.

25.  Welding of drain pipes to arrest dropping of drain pipes and subsequent water leakage.

Dear friends,
I will welcome the list of topics/issues, which you would like me to discuss for improving reliability of Diesel Locomotives.

Power Pack maintenance

Recent spate of failures of Power Pack components is a cause of major concern.

Power Pack maintenance practices should be reviewed by each Shed and measures suggested below should be incorporated with immediate effect.

1.   Ensure Proper lubrication

 Lubrication is heart of power pack functioning and availability of clean lube oil should be ensured by

·     Cleaning of Centrifuge: Centrifuge cleaning should be done in 30 days schedule. Sludge deposited should be weighed & functioning of Centrifuge should be checked by measuring RPM & quantity of sludge deposited.

·     Cleaning of sump, block and lube oil header:  Cleaning of lube oil pathway, sump &block during M₂₄ schedule should be scrupulously  done & lube oil passage should be wiped dry by lint free cloth. Lube oil headers should be cleaned, Lube oil passage should be checked & holes should be cleaned by rounded metal wire. Adequacy of lube oil flow should be checked in cylinder head, FIP support, cam gear by opening covers. Cam nozzles should be cleaned by wire & hole to be elongated to 3.2mm dia. Test filters should be used during major schedule. During M₂₄ /M₄₈ & POH pre-commissioning, lube oil should be changed after 100 hours of service & fresh oil should be topped up. Released lube oil can be used in inferior services.

·     Condition of filters: Released lube oil filters should be visually examined for filter condition sludge deposit, pressure drop & pleat condition.

·     Condition monitoring of lube oil should be done by blotter test, viscosity index & spectro report. Corrective measures should be taken during medium & above schedule based on above reports.

2.   Effective cooling

Temperature differential of incoming water & lube oil should be measured in engine block. A standard bench mark for temperature differential should be fixed & if temperature differential is falling then cooling needs to be improved.

Before stripping for M₂₄ schedule,  dilute HCL dose(less than 5% concn: as recommended by lab) should be used as descaling agent & loco should be run for six hours to dislodge the scale in engine block & pipe line. This de-scaling should be done before stripping only, to avoid damage of rubber components. Similarly in-situ recirculation should be done for cleaning radiators.

When heat exchangers are taken to section; oil, air & water path should be meticulously cleaned. Re-circulation with air stirring of hot solvent should be done. Effectiveness of cleaning should be judged by measuring temperature differential before & after M₂₄/M₄₈ schedule.

Water stabilization temperature should be measured at load box & should be kept below 75° for WDM_3A/₂ locos. The stabilization temperature should also be recorded during footplating and reading should be compared with load box reading. Water stabilization temperature reading should be made part of driver repair book format.

3.   Effective Air Supply

Good combustion depends upon availability of clean supercharged air in combustion chamber. This can be ensured by changing baggie air filter in quarterly schedule. Initial pre-filters should be attended/replaced during M₁₂ & above schedule.

After coolers must be clean & tubes should not be dummied. Air path must be sealed to ensure only filter air enters the combustion chamber. Released air filter should be examined by lab for filter quality, dust deposit & filtration efficiency. Pressure drop across air filters should be measured in M₄ & above schedules. Over aged filters should not be used as oil coating becomes dry.

4.   Efficient Combustion:

Cylinder head blow bye and compression test: These tests should be carried out in every quarterly and above schedule and power assembly should be replaced accordingly. Firing and compression readings should be taken during load box in medium schedules and above. Power assemblies having lower Kiene gauge readings should be replaced/overhauled.

Condition of nozzles should be checked 100% during M₁₂ schedules and  on sample basis during M₄ schedules as dribbling nozzles result in improper combustion, high soot formation and oil dilution.

5.   Replacement of cam gears and their Alignment 

Alignment and condition of camshaft, cam gears & split gear plays major role in power pack performance.

Cam shafts, cam gears, split gears & cam bushing should be replaced during M₂₄/₄₈ schedule. Before replacement of cam bush, condition of cam bush housing should be checked for ovality & welding integrity.

Cam gears must be checked during M₄ & above for wear and pitting by lab. Ovality of cam bushing, welding condition and wear should be checked in medium schedules; cam bushings should be replaced during M₂₄. L9/R9 locations are extremely prone to failures and must be checked for wear, ovality & welding in every schedule. Cam shaft run out should be checked during M₂₄ Schedule and should be kept below 4 thou.

Proper tools for extraction & fitment should be provided. Hydraulic puller/ pusher should be used for fitment and extraction of cam bushes, cylinder liners. Cam gears should not be cut/damaged during extraction. Anti-seizure compound should be used during fitment and recommended solvent should be sprayed for removal. Cam shaft flange, joining bolts should be properly tightened & thread locker should be used.

6.   Condition monitoring: Critical parameters like exhaust gas temperature, water stabilization temperature, eight notch rack, rack variation & zero error, centrifuge sludge deposit, temperature differential across block and heat exchangers, CDT, sudden notch down lube oil pressure differential and sudden notch up lube oil pressure build up and rpm differential must be recorded in every schedule.

Implementations of the above measures are essential for improving power pack reliability. Feedback on action taken should be reported through MCDO. Importance of these measures must be explained to concerned sectional staff & Supervisors.

Failure investigation report.
     
     Failure investigation & analysis is done by Sheds in case of line failures of diesel locomotives and investigation reports are prepared. Review of failure investigation reports indicates that most of the reports do not meet basic objective of whole exercise. Reports generally remain confined to symptoms and rarely penetrate root cause. Suggestive remedial measures remain flippant and only obliquely address key objectives.

It is essential to understand the basic objective of the whole exercise so that meaningful reports are prepared and corrective actions are taken. Reports should not leave a reader speculating the root cause and wondering further course of action. This write up is an attempt to address these issues and educate our supervisory staffs who have not been formally trained in this extremely important field.

Failure investigation report should

·        Tell us exactly why the component has failed

·        What we have learnt from this analysis

·        what has to be done to prevent a recurrence

Every failure leaves clues as to why it happened. In most of failure cases a trained person can use the basic techniques of failure analysis to diagnose the mechanical causes behind a failure, without having to resort to expensive and sophisticated analytical tools like electron microscopy. Then, knowing how a failure happened, the investigator can arrive at the root causes of why it happened.

      The most common reasons for failure of components include:

·  Service or operating conditions (use and misuse)

·  Improper maintenance (intentional or unintentional)

·  Improper testing or inspection

·  Assembly errors

·  Fabrication/manufacturing errors

·  Design errors (stress, materials selection, and assumed material condition or properties)

Failure analysis and prevention, Vol 11, ASM Handbook, ASM International 2002 recommends Nine Steps of a Failure Investigation - 

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1.     Understand and negotiate goals of the investigation

2.    Obtain clear understanding of the failure

3.    Objectively and clearly identify all possible root causes

4.    Objectively evaluate likelihood of each root cause

5.    Converge on the most likely root cause(s)

6.     Objectively and clearly identify all possible corrective actions

7.    Objectively evaluate each corrective action

8.    Select optimal corrective action(s)

9.    Evaluate effectiveness of selected corrective action(s)

  Failure Analysis Procedures

The principal task of a failure analyst during a physical-cause investigation is to identify the sequence of events involved in the failure. Like the basic process of the scientific method, failure analysis is an iterative process of narrowing down the possible explanations for failure by eliminating those explanations that do not fit the observations. The basic steps are:

1.    Collect data

2.    Identify damage modes present

3.    Identify possible damage mechanisms

4.    Test to identify actual mechanisms that occurred

5.    Identify which mechanism is primary and which is/are secondary

6.    Identify possible root causes

7.    Test to determine actual root cause

8.    Evaluate and implement corrective actions

     Generally, a failure analyst will start with a broad range of possible explanations but, over time, will narrow and refine the existing possibilities. The failure analyst must repeatedly ask the following questions as an investigation develops possible explanation(s) for actual events:

·  What characteristics are present in the failed/damaged component?

·  What characteristics are present or expected in an undamaged component?

·  What are the possible explanations that would account for the differences between damaged and undamaged components?

· What test(s) can be performed to confirm or eliminate possible explanations and refine knowledge about the observed damage?

Synthesis of failure:

Before concluding the investigation, study all the facts and evidence of the failure, both positive and negative, in order to provide the answers to the typical questions for mechanical failures of components are given below:-

·        Was the part properly installed?

·        Was the part maintained properly?

·        How long was the part in service?

·        What was the nature of the stress at the time of failure?

·        Is there any vibration noted on the part before failure?

·        Was the part subjected to overload?

·        Was it subjected to service abuse?

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·        Where there are any changes in the environment before failure?

·        Was the part properly maintained during schedules?

·        Was the failure is due to ductility, brittleness, or a combination of both?

·        Did the crack or defect start recently or had it been growing for a long time?

·        Did the failure start at one point, or did it originate at several points?

·        Did the failure start at or below the surface?

The investigator must understand the potential ways a component could be damaged, the clues that would differentiate between these various scenarios, and the physical meaning each of these clues would have. Comparison of observations with characteristics of expected damage and mechanisms will enable the analyst to narrow down the possible failure explanations and understand the meaning of the observations made.

  Limiting conditions that refine the scope of explanations for observed damage can be defined by using the following two rules of thumb:

·  The Sherlock Holmes Rule: When you have eliminated the impossible, whatever remains, however improbable, must be the truth.

·  Occam's Razor: When two or more explanations exist for a sequence of events, the simple explanation will more likely be the correct one.

This combined with the theoretical analysis should indicate the problem that caused the failure.

Logic Tree: To interpret a failure accurately, one has to gather all pertinent facts and then decide what caused them. To be consistent, it is essential to develop and follow a logic path and start to build a "Logic Tree." that ensures a critical feature will not be over looked. A logic tree is a tool that uses deductive logic to guide thought processes used to draw correct conclusions. A logic tree is a disciplined methodology that prompts the user to answer questions that will eventually identify the root causes of a failure event.

The first step in building a logic tree is to properly define the failure event to ensure that the analyst is truly working on the problem and not the symptoms. To do this, one must identify the failure event in the top block, and the modes of the failure event on the second level of the tree.

Next step is clearly defining the failure and proceeding to analyze its root causes. Questioning to build the logic tree is simple and consistent. One must keep asking "How can the preceding event occur? One has to start out very broad getting more and more specific while vertically extending the tree.

As the investigator continues to each level and keeps asking the same question of "How Can?", he forces himself to look at all the "cause" possibilities instead of looking only at the most likely possibility. As these possibilities are explored it is necessary to verify whether they actually occurred or not. If they did indeed occur the analyst would go to the next level by asking "How Can" again. The process of hypothesizing and verifying continues until the various root causes are discovered.  Initiative logic to jump to a conclusion must be avoided even if it appears most obvious. (4)

Component Roots (or Physical Roots) are the tangible things that fail. These are the loco components that generally fail, and these are typically the roots that are most familiar to us. The Human Roots are the points of inappropriate human intervention. This is generally where a human did something wrong or forgot to do something. i.e. act of omission or commission. For this reason it is necessary to ask "why did the person decide to do what they did?" What was the rationale for the decision? People do not generally wake up in the morning and say, "I think I will go to work today and fail miserably!" The answer lies in why people do what they do. What about the systems in which we operate, allowed the person to do what they did? The answers to these questions are defined as the latent roots or the organizational system roots.

Road map for failure investigation is outlined below:

1.   Find out what happened. The most important step in failure analysis is to seek answers soon after it happened and talk to the people involved. Persons involved in maintenance and overhauling are able to throw lot of light from their direct and tacit knowledge on the issue. Try to understand exactly what happened and the sequence of events leading up to it. 

2.    Make a preliminary investigation. Examine the broken parts, looking for clues. Do not clean them yet because cleaning could wash away vital information. Document the conditions accurately and take photographs from a variety of angles of both the failed parts and the surroundings.

3.    Gather background data. Check the drawing, OEM manual, specification, schedule forms relevant to failure. Note down the current operating condition and relevant parameters; time, booster pressure, rack, temperatures, amperage, voltage, load, throttle condition, pressure, lubricants, materials, corrosives, vibration, etc. Compare the difference between actual operating conditions and design conditions. Look at everything that could have an effect on locomotive working. The first step in any failure investigation is to gain good understanding of the condition under which the part was operating. Collect all data regarding any repair done before the failure, any important work carried out prior to the failure, and the service period of the component, as well as any problems noticed during the operation of that particular component.

4.    Check the trend: look for trend of relevant data. See the graph of current, voltage, fuel oil pressure, lube oil pressure etc to see the slope of the graph. Is graph exhibiting positive or negative slope or it remains flat? Download the data in locos equipped with MBCS, MCBG, event recorder, TCC and CCB. Carefully see the trend and examine all fault messages logged. Check the recurrence, frequency & origin of failure. Is the problem endemic to one Shed only, then find out reason, what is done wrongly and what are not being done. Variation study in maintenance practices will reveal clues for cause of failures.

5.    Determine what failed. Look at the initial evidence and decide what failed first—the primary failure—and what secondary failures resulted from it. Sometimes these decisions are very difficult because of the size of analysis that is necessary. Find out what changed. Compare current operating conditions with those in the past.

6.    Examine and analyze the primary failure. Clean the component and look at it under low-power magnification, 5x to 50x. What does the failure face look like? There are often "chevron marks" on the face of a brittle fracture

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that show the progression of the failure across the piece. These chevrons or "arrows" always point to where the crack started from the failure face, determine the forces that were acting on the part. Important surfaces should

be photographed and preserved for reference. The most important point to understand when doing failure analysis on a fractured part is that the crack always grows perpendicular to the plane of maximum stress.

7.    Lab testing of  the failed piece and the support material. Laboratory studies in the investigation of metal failure include verification that the chemical composition of the material that failed is within the specified limits. The studies also include the checking of dimensions and physical properties of the failed component Perform hardness test, Non-destruction testing(NDT) like dye penetrant & ultrasonic examination, lubricant analysis, alloy analysis, Macroscopic examination, Chemical analysis etc.

8.    Incident Causation Scenario: Determine the failure type and the forces that caused it. Draw Incidence time line and logic diagram Review all the steps listed. Leaving any questions unasked or unanswered reduces the accuracy of the analysis. Alternative Incident Causation Scenarios should be explored and ruled out logically.

9.    Simulate the situation: recreate the situation where feasible and validitate your hypothesis wherever possible.

10. Determine the root causes.  Categorise root causes into technical, human and management/maintenance system and always ask, "Why did the failure happen in the first place?" this question usually leads to human factors and management systems. Each root cause may be required to be dealt with differently; people will have to recognize personal errors and to change the way they think and act.

After completion of failure investigation,

The completed failure analysis report should include the following sections:
a) Description of the failed component

b) Service condition at the time of failure

c) Prior service history

d) Manufacturing and processing history of component

e) Mechanical and metallurgical study of failure

f) Metallurgical evaluation of quality

g) Summary of failure causing mechanism

h) Recommendations for prevention of similar failures

When the cause of a failure has been determined, a corrective action plan should be developed, documented, and implemented to eliminate or reduce recurrences of the failure. To minimize the possibility of an unmanageable backlog of open failures, all open reports, analyses, and corrective action suspension dates should be reviewed to ensure closure. A failure report is closed out when the corrective action is implemented and verified or when the rationale is documented for any instances that are being closed without corrective action.

Recommendation should pin point specific action to be taken in improving product quality, method of inspection, knowledge and skill level of staff involved. Action plan should be both short term and long term. All line and Shed failure incidences should be investigated and investigation report should be prepared.

     It is suggested that Sr.DME should discuss this article with all their Shed officials including all supervisors. Power point presentation and interactive session will help in disseminating key concepts of failure investigation and report making. ACMT should be closely associated in failure analysis and report preparation.

major contributors of relay failure are interlock burnt /pitted and operating coil open circuited .

These failures warrant  review of the current maintenance practices and plan for additional value input beyond existing maintenance paradigm. 

Quality of workmanship and checking must be improved along with proper record keeping of testing and checking dovetailing  Predictive maintenance and trend monitoring relay parameters.

The action plan is given below-

1)  Relay interlock burning and contact surface pitting-

  reasons of contact burning or contact surface damage are –

A)     Timing of drop out and pick up –

When the contacts are closing, the metal surfaces will collide and hit against each other several times (bouncing), causing elastic deformation of the contact area and mechanical destruction of the thin layers. 

Again successful "breaking" of a DC load requires that the relay contacts move to open with a reasonably high speed. A typical relay will have an accelerating motion of its armature toward the un energized rest position during drop-out. The velocity of the armature at the instant of contact opening will play a significant role in the relay's ability to avoid "tack welding" by providing adequate force to break any light welds made during the "make" of a high current resistive load (or one with a high in-rush current).The more drop out time means more heat generation resulting extensive damage to contact surface and failure.

The pickup and drop out time may vary with the following factors-

a)    Defective armature spring (EMD part no 8373528) due to loss of property ,corrosion etc .

b)    Restricted movement of armature or contact carrier assly

c)    Loss of property of core material causes delay in drop out time.

d)    Increase or decrease of resistance value of operating coil.

 Suggestion-

·        Armature spring tension should be checked with digital gravimeter during overhauling . Spring length (Free length 1.421” and stretch length under 1-1/2 Lbs is 1.646”)should be measured.

·        Armature spring should be checked for any deformity and crack with illuminated magnifying glass.

·        Armature or contact carrier assly should be checked for wear and restricted movement. Lubrication should be done if needed,

·        Sometimes core material loses its property and start to behave like permanent magnet. It should be checked.

·        The coil resistance value should be remained as per MI of manufacturers and temperature correction chart should be used. If the value achieved is beyond the limit as per MI it should be discarded.

·        Measurement of contact temperature rise during drop out of the relay should be done .

B)     Contact failure due to high contact resistance –

The contacts are the most important element of relay construction. Contact performance is influenced by contact material,  voltage  and  current  values  applied  to  the  contacts,  the  type  of  load,  frequency  of  switching,  ambient  atmosphere, form of contact, contact switching speed and of bounce. Contact resistance is one of major factor for failure of relays.

     Contact resistance mainly depends upon-

·                  Contact pressure.

·                  Cross sectional area of Contact surface ( Contact matching)

·                 Formation of non conductive film of oxides, sulphides and other    compounds due to flash over at the time of contact breaking.

Checking and adjusting relay contact pressure-

    Check the pressure required to open all normally closed contacts with a    

    digital gram gauge (5 to 150 gm range).

v  Connect the normally closed contacts in series with a simple low voltage (6 volt) lamp circuit.  The reading should be taken at the position the lamp is de-energized.

v  Place the probe of the gauge up close to the movable contact. A minimum reading of 40 grams is acceptable before contact opening .With a DC voltage of approximately 2 to 3 volts above the rated pickup voltage, energize the relay coil.

v  Check the pressure required to open all contacts which close when coil is energized. A minimum reading of 40 grams should be obtained on this test.

Ø   If the minimum reading of 40 grams is not obtained, the contact brush assembly will   have to be adjusted. Using a bending tool make gradual adjustments along the length of the contact brush assembly.

Note- Do not make any sharp creases or bends in the assembly. When making a correction for a pressure reading of the normally closed contacts, this will affect the pressure reading of the contacts that are closed when the coil is energized. Therefore, all contacts should be rechecked whenever an adjustment is made.

Ø  Check the air gap between all normally open contacts and between open contacts when the relay is energized. This air gap should be 0.045” minimum.

Ø  Check the travel gap from the centre of the relay core to the carrier assembly. This travel gap should be 0.038” minimum.

Typical relay adjusting tool

c) Contact surface mismatching-

As we know the resistive value of any conductor is inversely proportional to cross sectional area of the conductor ,so the contact surface must be kept as wide as possible to allow rated current of the circuit to flow though the contact surface without generating heat. If contact surface reduces ,the resistive characteristic of the contacts will be come into play and heat will be generated on contact point resulting pitting or burning of contact surface.

·        Contact surface to be checked minutely for any deformation or pitting mark.

·        Contact surface matching to be checked by using Prussian blue washable color.

·        Ensure more than 90% of contact surface matching.

·        Millivolt drop test should be conducted to check contact resistance on board in running loco and in test room after overhauling.

D)    Formation of non conductive film over contact surface-

The  contacts  are  practically  not  clean  because  the  surfaces  are  covered  by  thin  layers  of  low  conductivity, semiconductor properties or even isolating characteristics.  These layers of oxides, sulphides and other compounds will be formed on the surface of metals by absorption of gas molecules from the ambient atmosphere within a very short time. The growth of these layers will be slowed down and eventually stopped as the layer itself prevents further  chemical reaction. The resistance of these layers increases with their thickness.  To get a reliable electrical contact these layers have to be destroyed. 

·        In general relays are designed to wipe out thin surface of non conductive layers at the time of making contact.

·        If the mv drop test value during test on running locos is found more than the normal value ,the relay to be removed and dismantled for physical check up of contact surface.

·        The of movable contact shunt should also be checked for non integrity and bad soldering.

E)   In-rush current-

When an electro-mechanical relay is de-energized rapidly, the collapsing magnetic field produces a substantial voltage transient in its effort to disperse the stored energy and oppose the sudden change of current flow. This relatively large voltage transient has created EMI, breakdown, and contact tips wear problems. The type of the load and its inrush current characteristics together with the switching frequency are important factors that cause contact welding or burning.

The study of transient voltage and in-rush current reveals following fact with respect to various loads -

·        Incandescent Lamp Load ( Flasher lamp) -I/I_R = 10 to 15 times for 1/3 Sec approx

·        Motor Load (Fuel pump motor and Turbo lube pump motor)-I/I_R = 5 to 10 times for 0.2 to 0.5 Sec approx

·        Solenoid Load- I/I_R = 10 to 20 times for 0.1 Sec approx

·        Capacitive Load- I/I_R= 20 to 40 times for 1/30 Sec approx

                     (I= Normal load current and I_R=In-Rush current )

Suggestion-

·          Use of Transient suppression component or snubber circuit in parallel to load –

For suppression of transient effect based upon the impact on armature motion and optimizing for normally open contacts, the best suppression method is to use a silicon transient suppressor diode. This suppressor will have the least effect on relay dropout dynamics since the relay transient will be allowed to go to a predetermined voltage level and then permit current to flow with a low impedance. This results in the stored energy being quickly dissipated by the suppressor avoiding damage to the relay contact. This technique is already used in ALCO locos where two freewheeling diodes are used in reverse direction as arc suppression rectifier across radiator fan contactors R1 and R2 coil to protect temperature switch contact tips.

2.Operating Coil open circuit failures-

Cause of Operating coil failure are mainly due to open or short circuit of coil. Failures of relay occurred due to failure of soldering joint and melting of coil metal due to excessive heating or insulation damage.

Coil heating-

A negative effect of power consumption is the heating of the coil and, in turn, the entire relay. The coil temperature is a result of:

·        Ambient temperature

·        Self heating (Due to coil Power consumption= V*I)

·        Induced heating (Due to heat generated by contact system)

·        Magnetization losses (Due to eddy currents)

·        Other sources (Due to components in the vicinity of the relay)

Due to coil heating coil resistance increases. Resistance at elevated temperature is expressed by R_t = R₀ [(1+ α(T-23)]

Where  R₀ is the resistance at ambient temperature (23º C), T is the elevated temperature and α is the temperature coefficient on winding wire (Copper).

 The pick-up coil resistance of copper wire increases or decreases by 0.4% per degree C. Due to the increase in coil temperature the coil resistance increases as per the ratio mentioned above.  Hence the pick-up voltage for a hot coil should be higher to

generate required pick-up current.

For example if a copper coil having coil resistance of 400Ω at 20ºC , the resistance value increases to 432 Ω at 40ºC and pick up voltage must be increased to retain the same pick up current.

·        So, measurement of coil resistance at ambient temperature Vs resistance correcting chart to be used for fruitful results.

·        Dry or bad soldering to be checked for if coil resistance value differs from the value mentioned in MI.

·        Temperature rise during continuous operation and drop out to me measured and monitored for and deteriorating trend.

·        Color of the coil to be checked for any sign of overheating.

·        Insulation resistance of the coil to be checked with 500V meggar for deteriorating value. 

·        To obtain initial performance throughout life,  avoid dropping or hitting the relay