Repair methods for beds, slide ways, spindles, gears, lead screws and bearings – Failure analysis –Failures and their development – Logical fault location methods – Sequential fault location

1. RAMCO INSTITUTE OF TECHNOLOGY
Mr.M.LAKSHMANAN
Assistant Professor (Senior Grade)
Department of Mechanical Engineering

2. UNIT IV
REPAIR METHODS FOR BASIC
MACHINE ELEMENTS

3. SYLLABUS
Repair methods for beds, slide ways, spindles,
gears, lead screws and bearings – Failure
analysis –Failures and their development –
Logical fault location methods – Sequential
fault location.

4. FAILURES
Failure refers to malfunctioning, stoppage, crash
and deterioration , etc.
• In industry scenario of failures,
 Any loss that interrupts the continuity of
production
 A loss of asset availability
 The unavailability of equipment
 A deviation from the status quo
 Not meeting target expectations
 Any secondary defect

5. Possible causes of failure
Major
sources
of defects
Unintentional
damage
Unskilled
workmanship
Improper
Design
Defective
Material
Operational
Problems

6. Failure Models
The causes of failure may be predictable or
unpredictable.
1. Predictable Failure Model:
Time dependent failures are called age
dependent failures.
2. Unpredictable Failure Model:
Sometimes the components may fail within a
week time or a month after installation.

7. Predictable Failure Model
(Age-Dependent Failure)

11. Unpredictable Failure Model
(Purely Random Failure)

13. Failure Analysis
Failure Analysis is an investigation carried out
to determine the cause of failure of a certain
product or equivalently the mistake in the
continuous process of engineering design-
manufacturing-performance in order to
prevent its recurrence in the future.
• Quality of the material
• Quality of the fabrication process
• Possible abuse during service

14. Failure Analysis Design to:
1. Identify the Failures Modes
-the way the product failed
2. Identify the Failure Site
-where in the product failure occurred
3. Identify the Failure Mechanism
-the physical phenomena involved in the
Failure
4. Determine the Root Cause
-the trigger point which led to failure
5. Recommend Failure Prevention Methods
-corrective action or improvement

16. Fault Tree Analysis
Fault Tree Analysis (FTA) is one of many
symbolic “Analytical logic Techniques” found in
operations research and in system reliability.
Objectives:
• Identify the cause of a failure.
• Monitor and control safety performance of a
complex system.
• To identify the effects of human errors .
• Minimize and optimize resources.

17. History
• Fault Tree Analysis was originally developed in
1962 at Bell Laboratories by H.A. Watson.
• FTA is a deductive analysis approach for
resolving an undesired event into its causes.
• Logic diagrams and Boolean Algebra are used
to identify the cause of the top event.

18. • A logic diagram called Fault tree is
constructed to show the event relationship.
• Probability of occurrence values are
assigned to the lowest events in the tree in
order to obtain the probability of
occurrence of the top event.

19. Fault Tree Diagram (FTD)
FTD or Negative analytical trees are logic
block diagrams that display the state of a
system (Top Event) in terms of the states of
its components (basic events).
The pathways interconnect contributory
events and conditions, using standard logic
symbols (AND,OR etc).

20. Basic Fault Tree Structure

24. Fault Tree Structure

25. Fault Tree Structure

26. • Examples: Over pressure in vessel, Motor
fails to start, Reactor high temperature
safety function fails etc.,

27. Specifications:
• Undesired top event : Motor does not start
when switch is closed.
• Boundary of the FT : The circuit containing
the motor, battery, and switch.
• Resolution of the FT: The basic
components in the circuit excluding the
wiring.
• Initial State of System: Switch open,
normal operating conditions.

30. Applications
• Used in the field of safety engineering and
Reliability engineering to determine the
probability of a safety accident or a
particular system level failure.
• Aerospace Engineering.

31. Event Tree Analysis (ETA)
• It’s a visual representation of all the events
which can occur in a system.
• Its used to analyze systems in which all
components of the system are continuously
working or for systems in which some or all
of the components are in standby mode –
those that involve sequential operational
logic and switching.

33. ETA Example:

34. Root Cause Analysis (RCA)
• RCA is a step by step method that leads to
the discovery of a faults or root cause.
• Every equipment failure happens for a
number of reasons. There is a definite
progression of actions and consequences
that lead to a failure.
• RCA investigation from the end failure is
back to the root cause.

37. Root Cause Failure Analysis (RCFA)
• RCFA is simple but a well disciplined to
investigate, rectify and eliminate equipment
failure.
• Its more effective when attempted with
chronic breakdowns.
• The methodology is similar to that of RCA.

39. History of FMEA
• First used in the 1960’s in the Aerospace
industry.
• In 1974, the Navy developed MIL-STD-1629
regarding the use of FMEA
• In the late 1970’s, the automotive industry
was driven by liability costs to use FMEA
• Later, the automotive industry saw the
advantages of using this tool to reduce risks
related to poor quality.

40. Failure Modes and Effects Analysis
(FMEA)
 FMEA is methodology for analyzing potential reliability
problems early in the development cycle where its easier
to take actions to overcome these issues, thereby
enhancing reliability through design.
 FMEA is used to identify potential failure modes,
determine their effect on the operation of the product,
and identify actions to minimize the failures.
 Its also capture historical information for use in future
product improvement.

41. Types of FMEA
• System: focuses on global system functions
• Design: focuses on components and
subsystems
• Process: focuses on manufacturing and
assembly processes
• Service: focuses on service functions

43. FMEA Flow Diagram

44. Risk Priority Number (RPN)
Severity Occurrence Detection RPNX X =

45. FMEA Form

46. Benefits of FMEA
 Improve product/process reliability and quality
 Increase customer satisfaction
 Early identification and elimination of potential
product/process failure modes
 Emphasizes problem prevention
 Documents risk and actions taken to reduce risk
 Provide focus for improved testing and development
 Minimizes late changes and associated cost
 Catalyst for teamwork and idea exchange between functions

47. Examples of potential failure modes:
• Corrosion
• Electrical Short or Open
• Torque Fatigue
• Deformation
• Cracking

48. Examples of failure effects
• Injury to the user
• Inoperability of the product or process
• Improper appearance of the product or
process
• Degraded performance
• Noise

49. Examples of potential causes
• Improper torque applied
• Improper operating conditions
• Contamination
• Improper alignment
• Excessive loading
• Excessive voltage

50. Sequential Fault Location
F1 F2 F3 F4 F5 F6 F7
T1 0 1 1 0 0 0 0
T2 1 0 0 1 0 0 0
T3 1 1 0 1 0 1 0
T4 0 1 0 0 1 0 0
T5 0 0 1 0 1 1 0
T6 0 0 1 0 0 1 1
Diagnostic tree:

51. Gears
Gears are a kind of mechanical element which are widely
used where changes of speed, torque, shaft direction or
direction of rotation are required between a primary
mover and the driven machinery.
Design considerations:
• Type of loading
• Range of Torque
• Operating speed
• Expected service life
• Ambient temperature
• Size
• Weight
• Total system efficiency

52. Types of gears

53. Causes of gear teeth surface deterioration
Gear teeth
deterioration
Plastic
flow
Scoring
Surface
Fatigue
Wear

54. Plastic Flow:
• Surface deformation due to soft gear
materials or heavy loads.
Maintenance procedure:
• Wherever possible even distribution load on
tooth surface to be ensured.
• Backlash should be avoided to reduce
impact loading.

55. Scoring:
• Its sudden removal of metal from tooth
surface caused by tearing out of tiny
contacting particles that have welded
together.
• Its caused by rupture of oil film due to load
concentration at contact areas.

56. Maintenance procedure:
• Use of extreme pressure lubricants
• Polishing of surfaces
• Metallurgical hardening

57. Surface Fatigue or Pitting:
• This type of failure is caused as a result of
repeated surface stresses which exceed the
endurance limit.
Maintenance procedure:
• For small cavities grinding or polishing
tooth bearing surface.
• Metallurgical hardening
• Use of extreme pressure lubricant

59. Wear:
• Wear is a kind of tooth damage where in
layers of metal are removed more or less
uniformly from the surface. It is nothing but
progressive removal of metal from the
surface. Consequently tooth thins down and
gets weakened.
• Three most common causes of gear tooth
wear are metal-to-metal contact due to lack
of oil film, ingress of abrasive particles in
the oil and chemical wear due to the
composition of oil and its additives.

60. Adhesive Wear Abrasive Wear
Corrosive Wear

61. Gear inspection
• Pitch Error
• Axial
• Radial Runout
• Tooth Profile

62. • https://www.youtube.com/watch?v=qWekQ3
mzWNo

63. GUIDEWAYS
It’s a part of machine tools which are used to offer
smooth motion between the mating surfaces
(Minimum Friction) and to withstand heavy load
during machining operation.
Types:
• V- Shape
– Apex upwards
– Apex downwards
• Flat
• Dovetail
• Cylinder

64. Apex upwards Apex downwards Flat
Dovetail
Cylinder

65. Wear occurs on the guideways due to:
• Misalignment of mating surfaces
• Improper and ineffective lubrication
• Presence of foreign particles in lubricants
• Overload acting on the contacting surfaces
• Dusting of guiding surfaces
• Improper maintenance and inspection

66. Methods of Repairing
• Scraping:
To remove an unwanted covering or a top
layer from guideways, especially using a
sharp edge.

67. • Grinding
To make something into small pieces or a powder by
pressing between hard surfaces.

68. Grinding of different profiles

69. • Machining:
When the wear of the grinding surface
exceeds 0.3mm, its preferred to employ
machining operation.

70. Checking of Guideways
• Straightness
• Flatness
• Parallelism both on horizontal and vertical
surfaces.

71. BEARING

72. Basic Requirements of Bearing:
• Embeddability: Ability to absorb foreign particles.
• Comfortability: Must be soft enough to creep or flow
slightly to compensate minor geometrical irregularities.
• Fatigue strength: Ability to withstand load without
cracking.
• Temperature strength: Property of material to carry
load at elevated temperature.
• Thermal conductivity: Ability to dissipate the heat
generated.
• Corrosion Resistance: Must be resistive to corrosive
effects.
• Slipperiness: Ability to resist seizure.

73. Bearing Materials
• Monometals: Solid bar of aluminum or Bronze
alloys. They are used when load is not very
high.
• Bimetals: It has steel back, to which bonded a
liner of Copper, Tin and Aluminum. They are
posses good embeddability, comfortability but
relatively low fatigue strength.
• Trimetals: They were developed for heavy
duty applications. They posses the properties
of bearings and strength of harden materials.

74. Plain Bearings
Factors influencing the performance of sleeve
bearings:
• Dirt
• Fatigue
• Hot shot phenomenon
• Crush problem

75. Dirt
Improper cleaning of engine parts, road dirt
and wear of engine parts causes small
fragments to enter the oil supply.
Corrective action:
• Grinding and polishing of journal surfaces
• Periodical change of oil and filters
• Installation of new bearings

76. Fatigue
When load acting on bearings on time period
of service exceeds the capability of alloys,
bearing fatigue occurs.
Corrective action:
• Shaft and bore produced to exact dimension
and geometry
• Proper assembly and mounting of bearings.

77. Hot shot phenomenon
This type is characterized by the removal of
large area of lining from steel back.
Causes:
• Occurs when bearing temperature exceed
the melting point of its lowest melting point
metal.
• Insufficient flow of lubricant

78. Corrective action:
• Checking the blockage of oil passages
• Ensure oil pumps and pressure relief valves
operating properly.

79. Crush
The bearings will be elastically deformed
(crushed) during assembly.

80. Causes:
• Filing of parting lines
• Dirt or burrs on the contact surfaces
• Insufficient bolt torque
• Oversized housing bore
Corrective action:
• Bearing should not be altered
• Mating surface should be free from burr
• Using correct size of bore
• Proper torque applied

81. Cavitation
This is induced by rapid fluctuation in oil film
pressure.
Precautionary measures:
• Ensuring no air or water entrapment in oil
• Use of high viscous oil
• Increased oil pressure

82. Inspection of Bearings
• Bearing Temperature
• Noise and vibration
• Properties of lubricant

83. Bearing Temperature
The temperature of bearings will be 10 to
40˚C higher than room temperature.
• Extremely insufficient lubricant
• Poor installation of bearings
• Small bearing clearance and heavy load
• Improper lubricant selection

84. Noise and vibration
Continuous monitoring of vibration bearings
will help to study the performance of
machine. If any damage occurs, the degree
of damage will be reveled by the amplitude
of vibration signals.

85. Properties of lubricant
The purpose of lubricants are to cover the
rolling contact surfaces and sliding contact
surfaces with a thin film of oil to prevent
direct contact between them.
Effective Lubricants will have the following:
• Reduces the friction and abrasion
• Prolongs the life period
• Prevent corrosion
• Protects the surfaces from contamination

86. Bearing Failures

87. MACHINE BED
Its ability to absorb vibration that may arise
during the functioning of machine tool. The
most probable problem that occurs in
machine bed may be the cracks.
Repair of Cracks by,
• Riveting
• Hot clamping

88. Riveting
Riveting is done with headless copper screws
in a definite order based on the size and
length of the crack.

89. Hot clamping
Over the location of the cracks on cast iron
bed, two pieces of plates are placed and
bolted with special type and tightened till
the portion shear off.
The welding is done at the end faces of the
two plates. While the welding is cooling
down, the shrinkage of the plates keep the
cracks closed up.