Repair methods for basic machine elements

1. UNIT - 4
REPAIR METHODS FOR BASIC
MACHINE ELEMENTS

2. REPAIR METHODS OF MACHINE
BED
Most of the machine tool bed is made of grey cast iron,
owing to its ability to absorb / dampen 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 can be repaired by riveting
and hot clamping.
i.Scraping
ii.Grinding
iii.Machining
2

3. Repair by Scraping
3

4. 4

5. Repair by Grinding
5

6. Repair by Machining
When the average wear on the guide
surface is more than 0.3 mm it is advisable to go
for machining first and then scraping. The Non
linearity obtained by machining on a planer is
normally 0.03/1000 mm or above.
6

7. (i)Riveting:
Riveting is done with headless copper screws in a
definite order based on the size and length of the crack.
Initially a suitable diameter (d) is drilled at the extreme
ends of the crack, so that crack does not extend.
7

8. Riveting of Cracks
8

9. REPAIR METHODS OF SLIDE WAYS
OR GUIDEWAYS
The guideways are a part of machine tools which are used to
offer smooth sliding motion between the mating surfaces
(with minimum friction) and to withstand heavy load during
machining operation.
Proper design and manufacture of slideways helps to
maintain/achieve very good geometric dimensioning and
tolerancing of the job being produced.
• To have better and satisfactory performance of slideways
bearing, it is required to
possess adequate load bearing capacity
maintain alignment of guided parts
offer minimum friction particularly at low speeds
possess high stiffness
9

10. GEARS
Gears are kind of mechanical elements 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.
While designing gears due consideration on type of loading, range
of torque and operating speed, expected service life, duty cycle,
ambient temperature, size and weight and total system efficiency.
Gear drives are generally considered to, be packaged units
manufactured in accordance with the required specification, and to
be used for a wide range of power transmission applications.
10

11. Gear models
11

12. GOOD MAINTENANCE PRACTICE FOR GEARS
Satisfactory performance of gears/gear drives mainly depends on
oProper design and manufacture of drive
oSelection of proper type and size
oProper installation
oProper use of service
oProper maintenance of unit in its entire life
It is essential and desirable to have gears with a lengthy and
satisfactory life period.
In order to achieve this, it will be better to schedule an effective
maintenance programs.
12

13. American Gear Manufacturing Association (AGMA) describes the
wear of gears as follows: “It is the usual experience with a set of
gears in a gear unit. Assuming proper design, manufacture,
application, installation and operation that there will be an initial"
running-in" period during which ,if the gears are properly
lubricated and not over loaded ,the combined action of rolling
and sliding of the teeth may smooth out the manufactured
surface and give the working surface a high polish.”
Under continued proper conditions of operation, gear teeth will
the show little or no sign of wear.
13

14. Causes of tooth breakage
The common reasons for gear tooth breakage may be due to any of
the following reasons
Fatigue
Heavy wear
Overload
Cracking
The common types of breakages are:
Fatigue breakage: due to repeated bending stresses above the
endurance limit .
Heavy wear breakage: due to the consequence of severe pitting,
spalling or heavy abrasive wear.
Overload breakage: misalignment is the main cause.
Quenching cracks: This result from excessive internal stress
developed from heat treatment.
14

15. BEARINGS
Bearings are mechanical elements, which help to have frictionless
shaft rotation.
 The two basic categories of bearings are plain bearing and rolling
bearings.
Plain bearings are designed to support shafts which rotates
oscillate or reciprocate.
Even though it looks simple and least expensive of mechanical
components, sleeve bearings are highly engineered components.
They are commercially available in a wide range.
15

16. Basic Requirements
Surface action - also referred to as slipperiness and ability to resist
seizure
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.
16

17. oBearing Materials
• Most of the hydrodynamic bearings are metallic owing to their high thermal
conductivity. They may consist of one, two or three layers.
• (i) Monometals: Bearings made from a bar of solid bar of Aluminum or
Bronze alloys. They are used when load is not very high.
• (ii) Bimetals: It has steel back, to which bonded a liner of babbit, Copper,
Tin and Aluminium. They possess good embeddability, conformability or
surface action, but relatively low fatigue strength.
• Trimetals: They were developed for heavy duty applications. They possess
the properties of babbit bearings and strength of harder materials.
17

18. Plain Bearings
Factors influence the performance of sleeve bearings. The following
are the factors that affect the bearing performance
 Dirt
 Fatigue
 Hot Shot phenomenon
 Crush problem
Dirt - It is responsible for most of the bearing failures, than any other
mechanism.
Cause:
Improper cleaning of engine parts, road dirt, and wear of engine.
Corrective action:
(i) grinding and polishing of journal surfaces
18

19. Fatigue - When the load acting on bearings on time period of service
exceeds the capability of alloys, bearing fatigue occurs.
Other possible causes may be
Load concentration due to dirt
Poor shaft or bore geometry
Corrective action
Maintaining a dirt free environment
Shafts and bore produced to exact dimension and geometry
Types of Roller Bearings
Generally, bearings are classified into two categories such as
Radial Bearings
Ball bearings
Roller bearings
Thrust Bearings
19

20. Inspection of Bearings
It is necessary to inspect the bearings during operation to prevent
unnecessary failure. The following are inspected
oBearing Temperature
oNoise and vibration
oProperties of lubricant
Bearing Failures may be classified into
Flaking, Peeling, Spalling, Smearing, Stepped Wear, Speckles and
discolouration, Indentations, Chipping, Cracking, Rust and Corrosion, Seizing,
Fretting and Fretting Corrosion, Electrical Pitting, Retainer Damage, Creeping.
20

21. INTRODUCTION
The term “FAILURE” refers to malfunctioning, stoppage, crash, and
deterioration etc. of any equipment or system.
With regard to industrial scenario “FAILURE” is defined as:
 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
21

22. POSSIBLE SOURCES OF FAILURE
22

23. CAUSES OF FAILURE
Unexpected and unintentional damage - bearing seizure, gear
teeth breakage etc.
 Workmanship- unskilled, undevoted and not motivated e.g.
inaccurate measurement, mismatching, not considering safety
aspects etc.
 Design- Improper design which does not meet the require and
working conditions
Material- Manufacturing defects, mishandling and storage etc.
 Operation- incorrect usage of equipment etc. Failure Analysis is
the process by which information/data about failure occurring in
equipments / systems are collected and analysed to find the root
cause of failures.
23

24. FAILURE MODELS
Failures may be predictable or unpredictable sometimes.
So failures are classified as either predictable or unpredictable, in
order to select the best possible maintenance programs.
Failures follow anyone of the failure models or patterns.
24

25. PREDICTABLE FAILURE MODEL
Time dependent failures are called age dependent failures.
The figure depicts the fraction of items expected to have failed .at
any time (t) i.e. probability of failure F(t).
The figure presents the fraction of items surviving at running time t,
i.e. the survival probability P (t).
25

26. UNPREDICTABLE FAILURE MODEL
Sometimes the components may fail within a week time or a
month after installation.
In these cases, the probability of failure is constant and
independent of running time.
In spite of all the working conditions maintained at same level, the
cause of failure will be random in nature and cannot be assigned to
any particular mechanism of failure.
26

27. FAILURE ANALYSIS
In principle Defect or Fault Analysis are normally follow similar
approach in any industrial maintenance system.
Fault Tree Analysis (FTA) is another technique for reliability and
safety analysis.
Fault tree analysis is one of many symbolic "analytical logic
techniques" found in operations research and in system reliability.
Other techniques include Reliability Block Diagrams (RBDs).
 Fault trees are powerful design tools that can help ensure that
product performance objectives are met.
27

28. FAULT TREE DIAGRAM
Fault tree diagrams (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).
Like reliability block diagrams (RBDs), fault tree diagrams are also
a graphical design technique, and as such provide an alternative to
methodology to RBDs.
28

29.  Fault trees are built using gates and events (blocks).
 The two most commonly used gates in a fault tree are the AND and
OR gates.
 As an example, consider two events (or blocks) comprising a Top
Event (or a system).
 If occurrence of either event causes the top event to occur then
these events (blocks) are connected using an OR gate.
29

30. FAULT TREE STRUCTURE
30

31. EVENT TREE ANALYSIS
An event tree is a visual representation of all the events which
can occur in a system.
As the number of events increases, the picture fans out like
the branches of a tree.
Event trees can be 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.
The starting point (referred to as the initiating event) disrupts
normal system operation.
31

32. AN EXAMPLE OF EVENT TREE
32

33. SEQUENTIAL FAULT LOCATION METHODS
• Root Cause Analysis (RCA)
• Root Cause Failure Analysis (RCFA)
• Cause and Effect Analysis
• Failure Mode and Effect Analysis (FMEA)
• Failure Mode Effects and Criticality Analysis (FMECA)
33

34. ROOT CAUSE ANALYSIS
RCA is a step by step method that leads to the discovery of a
fault’s first 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.
 An RCA investigation from the end failure is back to the root
cause.
RCFA focuses on eliminating the risk of recurrence of the
failures by identifying the physical, human and latent system
roots that lead to the failure.
RCFA is simple but a well-disciplined to investigate, rectify
and eliminate equipment failure.
It is more effective when attempted with chronic
breakdowns.
34

35. ROOT CAUSE ANALYSIS PROCESS
•Define the problem
•Collect data
•Identify possible causal factors
•Identify the root causes
•Recommend and implement solutions
35

36. ROOT CAUSE FAILURE ANALYSIS (RCFA)
CAUSE AND EFFECT ANALYSIS
36

37. FMEA PROCEDURE
Failure Modes and Effects Analysis (FMEA) is methodology
for analyzing potential reliability problems early in the
development cycle where it is 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.
FMEA is a tool used to prevent problems from occurring.
The early and consistent use of FMEAs in the design process
allows the engineer to design out failures and produce
reliable, safe, and customer pleasing products.
FMEAs also capture historical information for use in future
product improvement.
37

38. oThere are several types of FMEAs; some are used much more
often than others.
oFMEAs should always be done whenever failures would
mean potential harm or injury to the user of the end item
being designed.
oThe types of FMEA are:
System - focuses on global system functions
Design - focuses on components and subsystems
Process - focuses on manufacturing and assembly processes
Service - focuses on service functions
Risk Priority Numbers (RPN) are calculated which is the
product of the numerical severity, occurrence and detection
ratings.
RPN= (S) x (O) x (D).
RPNs are calculated after three possible action opportunities
have occurred.
38

39. STEPS INVOLVED IN FMEA
39

40. 40

41. 41

42. Failure Mode Effects and Criticality
Analysis (FMECA)
Types of FMECA
Design FMECA
Process FMECA
System FMECA
Different approaches to FMECA
Bottom up approach
Top down approach
42