Govindaraju'H'K' Assistant Professor and Head, Department of Automobile Engg', S J M Institute of Te

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Govindaraju.H.K.Assistant Professor and
Head,Department of Automobile Engg.,S J M
Institute of Technology,Chitradurga 577
502.e-mail anchith_g_raj_at_yahoo.co.inSubject
Theory and Design of Automotive Engines Sub
Code - AU51 V Semester, Automobile Engineering
H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga
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Design Procedure for Connecting Rods
H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga
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Design of connecting rod involves

• Dimensions of c/s of the connecting rod
• Inertia force of Reciprocating Parts
• Total force on the connecting rod
• To find the thickness of the con rod flange and
  web
• Design of small end and Big end
• Design of Big end cap
• 8. Checking for strength

H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga
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Step 1. Dimensions of cross section of the
connecting rod
H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga
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Ixx
A (5t x 4t) (3t x 3t) 11 t2
H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga
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Step 2. Inertia force of Reciprocating Parts (F)
Wr mg Weight of reciprocating parts (N) r
Crank radius, ? Crank angle from the dead
center? 0 _at_TDC position. nI 4.33, g
Acceleration due to gravity 9.81Nm/s2 V
Crank velocity m/s
H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga
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Step 3 Total force on the connecting rod
Fc Force on the connecting rod Fp Force on
the piston Fi Inertia force of the
reciprocating parts.
Step 4 To find the thickness of the connecting
rod flange and web
Rankines Gordan Formula
Or
H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga
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Step 5 Design of small end
Fp Projected area x Bearing pressure
Fp Load on the piston pin or small end bearing
Where, Pbp Bearing pressure, 12.4 for gas
engines 15.0 for oil
engines. 15.7 for auto engines.
dp Diameter of piston pin, Assume Pbp
10 MPa lp length of piston pin 1.5 dp
H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga
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Step6 Design of Big end
Fp Projected Area x Bearing pressure
Fp Forces or load on the piston pin (N) dc
diameter of crankpin lc length of crankpin
1.25 dc Pbc 7.5 MPa Assume
H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga
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Step 7 Design of Big end Bolts
Fi F
dcb Core diameter of the bolts
Allowable tensile stress for the
material of bolts 12 MPa assume nb
Number of bolts usually 2 bolts are used
H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga
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Step 8 Design of Big end cap

• Load on big end
• The big end cap is designed as a beam freely
  supported at the cap bolt centers and loaded by
  the inertia force at the TDC on the exhaust
  stroke
• Load is assumed to act in between the UDL and
  the centrally concentrated load

H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga
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Stresses in the cap
sb Allowable bending stress for the
material of the cap
Maximum Bending moment as,
Fi Magnitude of Inertia force lo Distance
between bolt centers. Dia of crank pin or
Big end bearing Nominal dia of bolt (2
x thickness of bearing liner) Clearance
H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga
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dc db (2 x (0.05 dc 1)) 3
Section modulus for the cap,
Z
b width of the big end cap it is taken equal to
the length of the crankpin or Big end bearing
(lc) lc b h thickness of big end cap
H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga
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Step 9 Checking for strength
H.K. Govindaraju, Asst. Prof., SJM Institute of
Technology, Chitradurga