Transcript Fault Finding Techniques

Fault location and Diagnosis
Preparation for fault diagnosis
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Access: have all obstructions been removed? Inspection covers?, Clutter? Signs
barriers are in place
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Visibility: has dust, grease and dirt been cleaned to reveal as much as possible of
the job in hand?
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Symptoms: do you understand the nature of the problem? What is that the
equipment has failed to do? How is the performance abnormal?
Now eliminate
the obvious
Method
1.
Collect the evidence
Approach with an open mind
Before disturbing or changing anything- Stop, Look, Listen and Learn
Collect only relevant information, if in doubt about its validity include it and discard
later if unrelated
Question the user
Check the system running if possible
Use sources of information, manuals, maintenance records etc
Use test equipment to verify calibration and settings
2.
Analyse the evidence
separate the various symptoms and work out the importance of each one
Check if the evidence leads to a previously recorded fault
3.
Locate the Fault
Use all resources available- test equipment etc
If possible locate fault to component level
Never assume- always remove and check suspect components
4.
Determine and Remove the Root cause
The root cause of the fault must be located and removed otherwise the fault will
eventually reoccur
5.
Rectify the Fault
Any adjustment, repair or replacement should only be carried after identifying and
removing the root cause of the problem
6.
Test the System
Ensure the machine, equipment or system is functioning correctly before being put
back into service- fine tune if required
Identify the cause
Symptom - signs or indications that there is a deviation from the normal operating mode
Fault – the particular defect or combination of defects that cause the symptom to occur
Cause – the reason why a fault has occurred
Symptom (overheating)
Fault (worn bearing)
Cause (poor lubrication)
Sensory (visual)
fractures, surface cracks, corrosion, leaks, damage, contaminated oils, Loose
fastenings, excessive wear, metallic particles, broken chains and belts, missing
parts
Sensory (smell)
Smell- hot oil, burnt out motor, burnt out belts, overheating
Sensory (Touch)
Touch- vibration, wear, play, belt or chain tension, out of balance,
excessive clearance
Sensory (Noise)
Noise- unfamiliar noises , out of balance,
vibration, worn bearings, air leaks
Six Point Technique - Used on complex
faults with a history of data
Observe machine running
(if safe to do so)
Refer to manufacturers manual
(look for fault finding aids)
Use test equipment
(built in or otherwise)
Use sensory
techniques
1. Collect the Evidence
Question the operator
(use good social skills)
Consider machine
history records
Consider
2. Analyse the evidence
(diagnosis)
Packaging
materials
Raw materials
Services (air water
etc)
3. Locate the Fault (based
on analysis of evidence)
4. Determine and remove cause
(to prevent reoccurrence)
5. Rectify the fault
6. Check the machine or
system (before handing
over to production)
Emergent Problem Sequence
In any equipment failure there are factors that have caused a deviation from the normal
mode of operation (changed factors)
Sequence
• Analysis: determine the real problem or problems
• Prioritise: seriousness, urgency, the expected outcome
• Statement: write down the problem or variance
• Describe the problem
What- object or
defect
When- time
or life cycle
Where –
geographically or on
the object
Size- how big-how
much-how many
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Set Boundaries - use the description to narrow down the field of investigation
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Differences- look for factors that are unique to the problem
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Identify – changed factors
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Diagnose – list and categorise each changed factor in terms of which it explains the
facts
Ask yourself
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Is there a deviation from the standard or norm?
Is the cause unknown?
Do I need to know the cause?
Do I need to be involved?
If all points are answered ‘Yes’ then use the six point technique
Half Split Technique
This is used to reduce diagnosis time in circumstances where a series of components are
working together. It is based on the process of iteration. This simply means narrowing
down and revisiting
fault
1st
check
signal
ok
3rd check
signal ok
2nd
check
no
signal
Function Testing
There is normally a specification for the operation of an electrical component,
equipment or system. By ensuring the outputs are connected correctly and the
correct input is applied, the operation or function of the electrical equipment can be
measured against the specification. Does it do what it designed to do within the
given parameters? If not then it is faulty
Input (240v) ok
Output (heater) not heating
up – heater is at fault
Input to Output Technique
If the required function is known, and the input and output can be measured, then the
location of the fault is narrowed down or found
Inputs
A
Output x
B
C
Output y
Unit Substitution
If the input and output from a functional component can not be measured, a method of
fault location is to replace each component in turn, useful where components are easily
substituted with a known functioning unit
Diagnostic Aids
Equipment manuals
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Valuable source of information
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Most manuals will include a section that is
dedicated to the tracing of faults. It will usually be in
the format that describes the symptoms, the
possible faults that have caused the symptoms and
the appropriate remedial actions
Algorithms
Action
Question
Direction
Stop
A step-by-step problem-solving procedure, for solving a problem in a finite number of
steps.
Self Diagnostics
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Many Modern Machines especially production and processing equipment, are
equipped with self-diagnosis panels. They come in a variety of forms but they usually
have a visual representation of the system.
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The elements of the system that may fail have a sensor that checks for correct
operation
Maintenance/Equipment
Records/CMMS
It is imperative that any information relating to the equipment or system is recorded so
as aid future maintenance and continual improvement
Records can be in:
Technical Files or Maintenance logs – written or electronic
Recent trends have seen the introduction of software packages that are designed to hold
and access equipment history (Computer Maintenance Management Systems)
All Records should include:
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Breakdowns (fault, cause and remedy)
Planned Maintenance (work done)
Condition monitoring results
Computer Maintenance Management Systems
Using software packages will:
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Reduce maintenance cost
Optimise inventory and purchasing
Implement standards and compliance
Store and retrieve records
Improve maintenance management program reliability