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CHAPTER 3
METHODOLOGY
3.1 INTRODUCTION
The survey reported in Chapter 2 indicated the literature credibility
of chain performance especially on meshing dynamics and load on chains.
Not much work has been carried on chain elongation due to many variables
and uncontrollable factors present in manufacturing as well as in usage. In
this background, this chapter addresses the objective of this research work,
technical details of motorcycle chains and the method of manufacturing,
typical motorcycle specification. In addition to the above, stages of
investigation, factors that cause chain elongation and the method of
evaluation are also discussed.
3.2 OBJECTIVES
Based on problem definition and literature survey, wherein it is
observed that not much research work has been carried out on wear and
related aspects of chain elongation especially by analytical method due to
many variable factors, the following are made as objectives in the present
research work.
1. To estimate the chain elongation characteristics of 100 cc
motorcycle chains theoretically and compare with field
performance and to validate the theoretical evaluation. The
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reason for selecting 100cc motorcycle is that it is being widely
used on Indian roads, as discussed in section 1.3.
2. To modify the existing chain design and make a comparison
with the existing chain.
3. To investigate the possibility of increasing pin hardness by
chromizing process and to study the elongation characteristics.
4. To study the chain elongation characteristics by increasing
chain pin bearing area (bearing area= diameter length). Pin
bearing area in the existing chain cannot be increased due to
restriction in chain standards and possible reduction in fatigue
strength. Hence, the theoretical evaluation is carried out using
a higher pitch chain and compared with the existing chain.
5. To study the effect of improvement in grease lubrication due
to increased penetration of grease in the modified design chain
and to compare with the existing chain.
6. To study the elongation characteristics of existing chain
subjected to steady speed with maximum load and variable
speed with maximum load in different cycles of operation.
7. To study the fatigue strength capability of existing 100cc
motorcycle chains by using theoretical model and ANSYS
software.
It is observed from the literature that not much work has been
reported on the related areas mentioned in the objectives. Hence, this present
work makes an attempt to estimate the chain elongation performance in the
design stage itself, which may be useful to approximately estimate the life of
chain and to know the periodic intervals for relubrication of chains.
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3.3 TWO WHEELER TRANSMISSION CHAINS
In this study, two wheeler roller chains, which are used in final
drive, are considered. Two wheeler transmission roller chains are chains with
12.7 mm and 15.875 mm pitches. Motorcycle chains, which are being used in
100 cc motorcycles, are of 12.7 mm pitch with slight modification of pin
diameter and inner plate thickness compared with regular 12.7 mm standard
chain used in industrial and other applications. Construction is similar to that
of standard roller chains and the total number of pitches differs for each type
of motorcycle based on two wheeler manufacturers’ design (DID 2007).
Similar to other roller chains, motorcycle chains have links each
having two plates resembling eight in shape (outer plates) fitted on pins,
which is called outside links and two plates resembling eight in shape (inner
plates) fitted on bushings, which is called roller or inside links. Pins pass
through bushings of adjacent links to form swivel joints. Thus, pins and roller
links alternate. Mounted on bushings are rollers, which enter the tooth space
of sprocket with rolling friction. Pins are plain and riveted at both the ends. In
the case of 15.875 mm pitch chain, construction, materials and heat treatment
are similar to that of 12.7 mm pitch chains except dimensions of components.
The comparison of 12.7 mm pitch and 15.875mm pitch are shown in
Table 3.1 (DDB 2007).
As shown in the table, pin diameter of 15.875 mm pitch chain is
5.08 mm compared to 4.45 mm of 12.7 mm pitch standard chain. The pin
diameter of 12.7 mm pitch motorcycle chain is 4.51 mm max, which is
0.06 mm more than that of standard chain. Therefore, the corresponding
bearing areas for these chains are 67 mm2, 50 mm2 and 50.67 mm2
respectively. Even though 15.875 mm pitch chain is better based on bearing
area, its velocity variation due to polygonal effect is more compared to
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12.7 mm pitch chain. Also, its weight is more, causing more centrifugal force
on links at high speeds. This may lead to excessive noise and fatigue.
Table 3.1 Comparison of 12.7 mm and 15.875 mm pitch chains
Chain No.
Pitch mm
Roller dia
max mm
Width between
Inner Plates min mm
Pin Body
dia max mm
Plate Depth max mm
Trans-verse pitch mm
Overall over joint max, mm
Bearing Area mm2
Weight per
metre N
Breaking Load min
N ISO/ DIN BIS
08B-1 R1278 12.7 8.51 8 4.45 11.70 - 20.5 50 7.0 18200
10B-1 R1595 15.875 10.16 9.85 5.08 14.30 - 23.5 67 9.1 22700
As chain is elongated due to wear, it has a tendency to shift outward upon
the sprocket teeth profiles as shown in Figure 3.1 (Konyha et al 2007). For larger
number of teeth of sprockets, even a slight elongation of chain leads to larger shift of
the chain along sprocket tooth profile. The outward shift limits the number of teeth
of larger sprockets, which is limited to 100 to 120 (Reshetov 1978). Minimum
number of teeth on sprockets is limited by wear of chain joints, dynamic loads and
noise made by chain drive. The lesser the number of teeth, the greater the wear as
the articulation angle increases, which is given by 360/z (Shigley 2004). The range
of minimum number of teeth on sprockets in power drive with roller chains is 19 to
23 for high speeds, 17 to 19 for medium speeds and 13 to 15 for low speeds.
Figure 3.1 Chain slip over sprocket teeth
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The present study, focusing on holistic approach, has been carried
out in four distinct stages, as follows:
Stage I: Preliminary investigation on chain design.
Stage II: Analysis of factors that cause elongation of chain due to
wear.
Stage III: Performance evaluation of chain for elongation and
comparison.
Stage IV: Proposals for improvement.
3.4 PRELIMINARY INVESTIGATION
Chain failures are mostly due to wear of pins and bushes, which
cause elongation and fatigue that leads to breakage of roller or link plates.
Fatigue failure of chain is rarely reported in motorcycles due to proper
selection of chain in the design stage itself, applying factor of safety as high
as seven and above. The applied load is mostly within the safe operating load
that would withstand 1 million cycles without failure (RCN 2007).
The current research work initially has focused on preliminary
investigation to understand the prevailing conditions of the existing 100 cc
motorcycle transmission chain and substantiate the need for further
investigations according to wear principles. In this chapter, various activities
are carried out under preliminary investigation, namely data collection of the
existing chain and two wheeler specifications. In the present study,
motorcycle of a particular model of a typical brand is considered for
theoretical chain elongation evaluation and for elongations study in field. The
dimensions of the existing chain are taken from chain standards and the
critical dimensions, hardness, finish, lubrication and other relevant factors that
influence chain elongation are considered.
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3.5 CHAINS FOR 100 CC MOTORCYCLES
100 cc motorcycle chains of 12.7 mm pitch, unlike standard
12.7 mm pitch of ISO and ANSI series, are made with pins of diameter 4.51
mm (max) compared to standard chains whose pin diameter is 4.45 mm (max)
as discussed in section 3.3. The following is the dimensional specifications of
motorcycle chains stipulated in the chains standards ISO 606/IS 2403 as
follows: breaking load, 18600 N (min); pitch,12.7 mm; pin diameter (max),
4.51 mm; roller diameter, 8.5 mm; width between inner plates, 8.0 mm; width
over inner plates, 11.2 mm.
Apart from this, chains should satisfy the fatigue requirements.
Chains in most applications are typically loaded by cyclical tension. Fatigue
failure of the plates at the eyes is the main criterion for heavily loaded high-
speed roller chains. Fatigue limit will occur between 106 to 107 cycles (ISO
10190:1992.). When a chain is operating under load, outer surface of pins and
inner surface of bushings slide against each other due to articulation, thereby
causing wear. Due to continuous articulation movement, wear can occur in
every contact cycle causing cumulative wear and consequent chain
elongation. In general, for drive chains permissible elongation is 2-3% and for
industrial chains it is 3% of the initial length of chain (Berents et al 1989).
In these chains, plates and rollers are made of medium carbon steels
and pins and bushes are made of low carbon, low alloy steels. Plates are
stamped as shown in Figure 3.2 (Berents et al 1989) in mechanical press using
press tools, deburred and heat treated to hardness level 45±3 HRC and shot
peened. Shot peening improves fatigue strength of components (Zahavi et al
1996).
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Figure 3.2 Plate stamping
Pins are cut from wire in forging or pin cutting machine and ground
and heat treated to hardness level of 820 HV and above and polished to have a
good surface finish. Bushes are made by curling or forming as shown in
Figure 3.3 (Berents et al 1989) and heat treated to hardness level of 700 HV to
800 HV and polished.
Figure 3.3 Bush curling or forming
Rollers are made by forging or by cupping and deep drawing as
shown in Figure 3.4 (Berents et al 1989) and heat treated to hardness level of
450 – 550 HV and shot peened.
Figure 3.4 Roller forming
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All these components are assembled in automatic assembly
machines and preloaded (Berents et al 1989). After preloading, lengths are
checked randomly, so that chains fit properly in two wheelers without much
sag or looseness. Unlike standard chains used for other applications,
motorcycle chains are properly cleaned and lubricated with wax base grease.
The wax base grease is Servo chain compound having EP additives and melts
in the range of 1100 C to 1200 C and becomes solid upon cooling to room
temperature. Greasing is done by dipping chains in molten grease bath until
the gaps between pins and bushes are filled.
3.6 VEHICLE SPECIFICATION
Vehicle specifications of most popular brands used in INDIA are
given in Appendix 2. The parameters relevant to chain analysis are given in
Tables 3.2, 3.3 and 3.4, which are selected from Appendix 2.
In motorcycles, chains are fitted with drive and driven sprockets
and are enclosed in a cover to avoid dust and sand accumulation. There is no
auto tensioner or tensioning idler sprocket. The chain tension is adjusted,
when chain elongates, by adjusting the tension adjustment bolt provided in the
rear wheel. The chain alignment and tensioning adjustment should be
properly done; otherwise, excessive load will act on links causing twisting
and quicker elongation (Kidd et al, 1999). The upward movements of rear
wheel along with rear sprocket due to road undulation like bumps, potholes
may not cause appreciable load variation in chain due to slackness in the
driven side of chain. In the present study, the effect of vertical movement of
rear wheel is not considered.
58
Table 3.2 Brand A model 1 technical specification
Engine Air cooled, 4 stroke single cylinder OHC
Bore Stroke 50.0 49.5 mm
Displacement 97.2cc
Compression ratio 8.8:1
Maximum power 5.7 kW (7.7 bhp) @ 7500 rpm
Maximum torque 7.55 Nm @ 6000 rpm
Maximum Speed 80 kmph
Kerb weight 104 kg
Tyre size front 3.50 10 – 4 PR / 51 J
Tyre size rear 3.50 10 – 4PR / 51 J
Battery 12V- 5 Ah
Primary Reduction 3.722
1st Gear 3.181
2nd Gear 1.705
3rd Gear 1.238
Top Gear 0.958
Final Reduction 3.076
Chain 12.7 mm pitch roller chain (simplex)
Drive sprocket no of teeth 13
Driven Sprocket no of teeth 40
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Table 3.3 Brand A model 2 technical specification
Engine Air cooled, 4 stroke single cylinder OHC
Bore Stroke 50.0 52.0 mm
Displacement 102cc
Compression ratio 9.0:1
Maximum power 5.5 kW (7 bhp) @ 8000 rpm
Maximum torque 7.85 Nm @ 5000 rpm
Maximum Speed 77 kmph
Kerb weight 104 kg
Tyre size front 3.50 10 – 4 PR / 51 J
Tyre size rear 3.50 10 – 4PR / 51 J
Primary Reduction 3.722
1st Gear 3.273
2nd Gear 1.75
3rd Gear 1.238
Top Gear 0.958
Final Reduction 3.143
Chain 12.7 mm pitch roller chain (simplex)
Drive sprocket no of teeth 14
Driven Sprocket no of teeth 44
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Table 3.4 Brand B model 1 technical specification
Engine and Transmission
Type 4-stroke
CC 99.7
Max Power 5.50 kW (7.5 bhp) @ 7500 rpm
Max Torque 7.5 Nm @ 5000 rpm
Bore Stroke 51.0 48.8
Compression Ratio 9.15:1
Transmission 4-Speed Constant Mesh
Tyre (Front) 2.75 18
Tyre (Rear) 3.00 18
Vehicle Kerb Weight 104 kg
Primary Reduction 3.722
1st Gear 3.273
2nd Gear 1.75
3rd Gear 1.238
Top Gear 0.958
Final Reduction 3.076
Chain 12.7 mm pitch roller chain (simplex)
Drive sprocket no of teeth 13
Driven Sprocket no of teeth 40
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3.7 FACTORS THAT CAUSE CHAIN ELONGATION
The parameters that affect elongation of chain are relative hardness
of pins, bushes, load acting on pins, bearing area of pins, sliding distance of
pins during articulation, surface roughness, geometry and dimensional
tolerances, assembly dimensions, chain velocity, lubricant and method of
lubrication and chain sprocket misalignment and other related factors.
Tribological characteristics that affect wear are mapped as shown in
Figure 3.5 (Lésniewski and Krawiec 2008). In the present study, based on
Archard’s wear model, load acting on chains, bearing area, yield strength
based on hardness, sliding distance and lubrication are considered.
Figure 3.5 Factors that influence tribological characteristics
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3.8 FACTORS THAT ARE SELECTED FOR CHAIN
ELONGATION STUDY
Wear is due to articulation movement of pin and bush, when chain
rotates over drive and driven sprockets with load acting on chains. The wear
of chains can be reduced by proper selection of materials, heat treatment and
maintaining proper relative hardness between pin and bush. Also, surface
finish of pins and bushes can be increased to reduce wear. By taking
additional care in components manufacturing and chain assembly, wear due to
faulty or improper assembly can be reduced (Wright 2005).
In existing motorcycle chains, material for bush is low carbon
chromium steel and that for pin is nickel chromium molybdenum steel. Pins
and bushes are case hardened to improve the wear resistance; typical value for
pin is above 820 HV5 and for bush 700 to 780 HV5. Bushes are either curled
or formed and the inner bore finish is maintained as that obtained due to
rolling. Pins are polished to a finish around 0.2 Ra. Any further improvements
on the properties mentioned above are possible but productivity and cost are
the key factors that prevent from doing so. Hence, it may not be economical
to consider them due to low chain price. One of the key factors, which affect
chain elongation, is the lubricant and its method of application. Presently, wax
base grease with EP additives is used. Synthetic grease can be used, if the cost
of chain is not a matter to be considered. To avoid dust and dirt getting
entrapped on chains, ‘O’ rings can be fitted at the bush ends in between plates
(RCN 2007).
In the present investigation of theoretical chain elongation
evaluation, the effect of influence of following variables are studied by
considering one variable at a time assuming the rest of the variables constant.
63
Even though wear depends on relative hardness of pin and
bush, only wear of pin is considered and hence its hardness is
chosen as a variable. Wear of bush is relatively less and
assumed to be negligible and therefore its wear is not
considered.
Bearing area is considered as another variable and the change
in bearing area is considered in a chain of different pitch due
to restriction of changing pin dimensions and hence bearing
area in the same pitch as mentioned in section 3.2.
Other variables are steady speed, variable speed condition.
Design modification of bush is considered as an important
variable.
Chain elongation is divided into four stages, namely initial running
in wear % as stage one, (1.0 + initial wear) % as stage two, upto 2.0% as stage
three and finally the maximum elongation of 2.8% as stage four. It may be
noted that maximum permitted elongation is 3% as reported in section 3.5. In
the above four stages, Archard’s wear coefficient (Collins 2003) is assumed to
have four different values and maintained the same in all the theoretical
elongation comparisons.
3.9 CHAIN PERFORMANCE EVALUATION STRATEGY
Failure modes of chains are elongation due to wear, roller cracking
and fatigue apart from failure due to poor pin bush interference and assembly
error. Failure of chains due to elongation, roller cracking and fatigue can be
done in the following ways.
64
Theoretical method
Experimental method
Field survey method
Pin bush interference and assembly error can be studied during
manufacturing stage. This test is done to ensure that pin bush interference is
sufficient, so that pins and bushes neither rotate inside plate holes nor come
out of plates. The theoretical method of investigation for elongation, roller
cracking and fatigue are done in the present investigation. Experimental
method for chain elongation and roller cracking is generally done in four
square test rigs or in mechanical test rigs either brake drum type or eddy
current dynamometer type similar to that used for checking motor
performances. The reason for doing this test is to ensure that chain elongation
due to wear is within the normal level and to ensure stipulated quality. In the
case of fatigue failure, axial fatigue testing machine is used to ensure that
plates and pins do not fail within 1 -10 million cycles.
3.9.1 Limitations of Experimental Study
The experimental technique for chain elongation study will usually
have a two dimensional plot between percentage elongation and time in hours.
The elongation curve will provide the trend of elongation and it will not
match with chain elongation plot of motorcycles in field study and will not
provide chain life in terms of km run by motorcycles. However, the
experimental technique will be useful to compare chain performances of
different brands and different batches of the same brand. The problems
associated with experimental technique in normal test rigs, as shown in
Figure A9.1, is that it can be operated under steady load and steady speed
conditions only. To operate under cyclic load with various speeds,
sophisticated dynamometer is required as shown in Figure A9.2. Moreover, to
65
test chains till life, which is usually 15000 hrs (Barents et al 1989), it may
take at least 3 to 6 months duration for conducting one test.
Usually, chain manufacturers test chains for routine checking of
quality consistency in the manufacturing line. They also conduct test for
bench marking and for comparing competitor’s product in addition to test
newly developed samples. The entire tests are carried out for 100 to 500 hrs
and may not yield correct picture of total life of chains. To carry out this study
chains are required. In the case of developmental work, if the developed
samples are not satisfactory after testing, design or material change is to be
incorporated that might cause additional expenditure and incur loss due to
already developed toolings. Hence, longer lead time is required to supply
developed samples to motorcycle manufacturers. Moreover, it may not be
possible to study the influence of each factor that contributes to chain
elongation due to longer testing time. Typical elongation curve for chains
from one of the literature is shown in Figure 3.6 (Peeken and Coenen 1986).
Figure 3.6 Elongation curve by experimental method
66
It is observed from the Figure 3.6 that even after 500 hrs, the
elongation is 1.5 mm for 100 pitches of 12.7 mm chain, which accounts for
0.12% elongation, whereas permitted elongation is 3%. Hence, it shows the
limitations of experimental technique.
3.9.2 Features of Theoretical Study in this Research Work
In the present chain investigation work, theoretical elongation is
evaluated and validated with field result to find a correlation between field
and theoretical elongation plots. Based on the validation, the theoretical
estimate can be used to compare the influence of various factors considered in
this research work easily. Chain performance evaluation is mostly carried out
on comparison basis, which will eliminate the risk of assumptions made, since
whatever error that may occur will be same for both chains. In this aspect, the
present work will be very much useful to chain manufacturers to estimate the
life of chain in the design stage itself and any modification required can be
made at the early stage before prototype or sample development. This will
reduce or eliminate unnecessary expenses and delay in submitting samples to
customers. Also, this study helps to find the behaviour of chain till
permissible elongation and to note the change of slope of elongation curve.
Any appreciable change in the pattern may help to find the cause and for early
rectification. Moreover, this may facilitate for benchmarking of chain sample
and freeze the design quickly. Based on the above possible advantages,
theoretical evaluation of chain performance is carried out as shown in the
flow chart Figure 3.7.
67
Figure 3.7 Flow chart for chain elongation performance evaluation
From vehicle specification torque and chain load evaluation at maximum power condition
Design calculation of drive and driven sprockets and evaluating average pressure
angle
Chain link force calculation on drive and driven sprockets
Bush shrinkage, bush and pin deflection evaluation
Design modification of Bush and shrinkage
calculation. Modified bush and pin deflection
evaluation
Calculation of distance covered in the four stages mentioned above
Plotting distance covered in km along X axis and percentage elongation along Y axis
and curve fitting
Chain elongation calculation in four stages 1).initial-up to shrunk depth in %
2). Initial+1% 3). upto 2% 4).upto 2.83%
Plotting field performance with distance covered in km along X axis and percentage
elongation along Y axis and comparing
Comparing the plots
68
3.10 EVALUATION OF CHAIN ELONGATION BY
COMPARISON
From the different models of motorcycles, which are most widely
used on Indian roads, motorcycle characteristics of brand B - model-1 shown
in Table 3.4 is considered for theoretical chain wear evaluation. Also, speed
of motorcycle in top gear is assumed to be constant under steady state at the
maximum engine power condition. Even though this condition can never
prevail on roads, theoretical chain performance evaluation is done based on
the above assumption on comparison basis. Therefore, error in the assumption
will not affect the relative performance of chains. Chain torque load is
calculated from the engine power and the drive sprocket speed using standard
formula. Based on PCD of sprocket, the chain force on tight span is
calculated.
Moreover, it is found that relative movement between chain pins
and bushes, in tight span of links between the entry to drive and exit from
driven sprockets, is very less and hence do not contribute to wear of pins.
Similarly, there is less relative movement on the slack span of chain.
However, portions of links, which are in contact with sprockets, are subjected
to articulation movement due to rotation of sprockets. This articulation
movement, with load acting on links causes wear and the cumulative wear
leads to extension of chain, which is called elongation.
The motion of chain links entering drive sprocket is shown in
Figure 3.8. Pins, in pin link always contact at outer ends of bushes in roller
links. Figure 3.9 shows pin and bush contact under no load (Peeken and
Coenen 1986).
69
Figure 3.8 Motion of chain links entering drive sprocket
Theoretically there is clearance between pin and bush and there is
no contact between pin and bush as shown in Figure 3.9 when chain is placed
on a table under no load. When the chain is loaded, pin-bush contact would
take place in the outer end of roller link, which is marked black in the
Figure 3.9.
Figure 3.9 Pin bush contact
Forces acting on chain links vary depending on the angular position
of drive and driven sprockets as shown in Figure 3.10 (Eldiwany and Marshek
1984). The load depends on pressure angle φ and articulation angle α and its
value is calculated using Equation (3.1) (Eldiwany and Marshek 1984).
Contact area marked black
70
0sin
sin( )
n
nt t
(3.1)
where t0 is the load acting on link just before entering drive sprocket i.e. load
acting on tight span. t1 is the load acting on link1 and tn is the load acting on
nth link. Load acting on the links are evaluated for this angle of rotation of
sprockets and the average load is taken for drive and driven side.
Wear is evaluated separately for drive and driven side of links and
the total wear is found by adding these wears for one cycle of chain link
movements. In all the theoretical evaluations, as already discussed in
section 3.8, bush wear is not considered and only wear of pins is considered.
Based on this assumption, from the total wear of each pitch or link, the critical
angle of articulation is calculated. The angle of rotation of sprockets, which
cause the critical angle of articulation, is evaluated.
a) Link 2 b) Link 1
c) Sprocket and link
Figure 3.10 Load acting on links and drive sprocket teeth
71
The load distribution follows geometric progression and variation
of load is shown in Figure 3.11 (Eldiwany and Marshek 1984). It is seen from
the figure that load decreases rapidly from first tooth to second tooth and
decreases progressively when in mesh with chain.
High torque Low torque
Figure 3.11 Load distribution in chain links
3.10.1 Study of Existing Motorcycle Chain Elongation Under Steady
Speed
In this study, 100 links/ pitches is chosen for theoretically
evaluating the chain elongation performance. Using Archard’s wear model
(Collins 2003) wear of one link/pitch of chain is estimated choosing proper
values for the variables given in the model and multiplied by 100 to give total
wear for 100 pitch length chains. As already reported, in section 3.7 the
variables considered in Archard’s wear model are load on chains, bearing area
of pins, hardness or yield strength of pins and sliding distance. Other
parameters are not considered. However, the effect of lubricant during the
total life cycle of chain is considered by selecting proper Archard’s wear
coefficients. The elongation value is calculated in percentage for 100 pitches.
72
The percentage of elongation is given by (length of chain after wear –initial
length)/initial length 100 (RCN 2005).
Also, the number of revolution of rear wheel is calculated based on
overall gear ratio from the engine speed and using the effective radius of tyre
the distance covered by vehicle is calculated. The results are plotted taking
percentage elongation on Y axis and distance travelled in km along X axis.
The results are compared with the field result and curve of best fit with
mathematical equation is arrived.
3.10.2 Study of Existing Motorcycle Chain Elongation Under Variable
Speed
In the above study, only steady state condition viz. the chain is
subjected to constant torque load under maximum power condition and speed
is considered. However, in practice this is impossible since load and speed
vary due to traffic, road surface, behaviour of drivers and vehicle conditions.
Considering the influence of all these parameters for theoretical evaluation is
difficult. However, in order to understand the performance of chain due to
fluctuating speeds, loads, traffic, road conditions and driving habits of drivers,
different drive cycles based on the report by Alessandrini and Orecchini
(2003) are considered for theoretical evaluation and compared.
3.10.3 Study of Chain Elongation with Increased Bearing Area
In motorcycle chains, the pin diameter is 4.51 mm max. The
possibility of increasing pin diameter further is ruled out, due to possible
reduction in breaking strength and fatigue strength. The reason being, larger
punched hole is required in pin link plates, which will reduce the plate
effective area of cross section to withstand breaking as well as fatigue loads.
73
Hence, it is not advisable to increase the pin diameter. However, in order to
study the effect of chain elongation due to increased bearing area of pins,
chain with higher pitch viz. 15.875 mm is chosen. Here, the pin diameter is
5.08 mm and the contact length is 13.7 mm. The mass of this chain is 1 kg/m
which, is higher than the existing motorcycle chain’s mass of 0.8 kg/m and
hence, more centrifugal force will act on this chain than the existing
motorcycle chain, in addition to force developed due to torque (Reshtov
1978). Hence, the centrifugal force effect is considered for both 15.875 mm
pitch and 12.7 mm pitch chains and the elongation characteristics of these
chains are evaluated using the Archard’s wear model.
3.10.4 Study of Chain Elongation with Increased Pin Hardness
The effect of increasing the pin hardness from 850 HV to 1200 HV
by chromizing is studied. Chromizing treatment is given to timing chains to
increase the wear resistance (Davies et al 1983). But force acting on timing
chain pin is much lower than transmission chain. Hence, before attempting to
check the possibility of using chromized pins, a study of Hertz contact stress
is made for the existing transmission chain and compared with that of timing
chain.
3.10.5 Study of Chain Elongation in Field
In the case of field evaluation, wear of each pin-bush cannot be
measured without dismantling chain links. However, total wear, which
contribute to chain elongation can be measured by measuring the chain length
after removing connecting link and using a proper scale. One more method
recommended by chain manufacturers is to measure a fixed, say 10 pitches
using vernier at different locations and to take the average value. From this
percentage elongation is evaluated using initial length as 127 mm for standard
74
10 pitches. While measuring the elongation of chain, the corresponding
distance covered by motorcycle has to be noted from the odometer reading
which, is usually integrated with speedometer.
3.10.6 Evaluation of Improved Model Chain by Elongation
Comparison
In the existing chain, shrinkage of bush at the ends due to
interference with inner link plates are calculated and found that the pin
contact with bush are mostly at the ends during initial running in period. To
reduce/avoid pin contact at the bush shrunk portion, the factors that influence
this problem are considered. A new design is made to reduce this problem.
Using wear model, elongation performance of improved chain design is
evaluated and compared with the existing chain. In both the cases, all
variables listed above are considered to be same except design change in bush
profile. Thus, the comparison result obtained using wear model gives only the
percentage improvement and the error in the assumption of values for
variables in wear model is nullified.
3.11 EVALUATION OF CHAIN FATIGUE
The modal analysis of existing chain is carried out and the
harmonic analysis is also carried for the existing chain. The failure analysis
due to fatigue of the existing chain is done by theoretical evaluation using
models and verified using ANSYS software (ANSYS Release 9.0). This is
done to find the stress pattern and level of stress acting on chains, which will
help for design modification for wear performance in this research work. The
details of fatigue evaluation are discussed in section 4.9.
75
3.12 SUMMARY
In this chapter, the objectives of the present research work are set.
The research methodology to be adopted in this study has been made. The
methods of estimation of chain life based on elongation and fatigue have been
presented. Based on literature survey, the methods adopted for study on chain
elongation by industries and research scholars are mostly post operative,
whereas the present study is intended to evaluate the chain elongation in the
design stage itself. It has a lot of advantages as reported in section 3.9.1. The
details of performance evaluation on elongation and fatigue are discussed in
chapter 4.
