U3 design of connecting rod-(i section, big&small eng, bolt, whipping stress)

Design of IC Engine Components

1. DESIGN OF IC ENGINE COMPONENTS DR.MAHALINGAM COLLEGE OF ENGINEERING AND TECHNOLOGY, POLLACHI. P.KARUPPUSAMY AP / AUTO R.VISHNURAMESHKUMAR AP / AUTO DEPARTMENT OF AUTOMOBILE ENGINEERING Reference: A text book of MACHINE DESIGN – R.S. KHURMI Design of MACHINE ELEMENTS – V.B. BHANDARI Year / Sem : III / VI AY : 2018 – 19

2. UNIT – III DESIGN OF CONNECTING ROD 2

3. ENGINE CONNECTING ROD The connecting rod is the intermediate member between the piston and the crankshaft. Function of a Piston: To transmit the push and pull from the piston pin to the crankpin To convert the reciprocating motion of the piston into the rotary motion of the crank.

4. Connecting Rod It consists of a long shank, a small end and a big end. The cross-section of the shank may be rectangular, circular, tubular, I -section or H -section. Circular section is used for low speed engines I -section is preferred for high speed engines .

5. The length of the connecting rod ( l ) depends upon the ratio of l / r, where r is the radius of crank. The smaller length will decrease the ratio l / r . This increases the angularity of the connecting rod which increases the side thrust of the piston against the cylinder liner which in turn increases the wear of the liner. The larger length will increase the ratio l / r . This decreases the angularity of the connecting rod and thus decreases the side thrust and the resulting wear of the cylinder . But the larger length of the connecting rod increases the overall height of the engine. Hence, a compromise is made and the ratio l / r is generally kept as 4 to 5. Ratio l / r

6. The small end of the connecting rod is usually made in the form of an eye and is provided with a bush of phosphor bronze . It is connected to the piston by means of a piston pin. The big end of the connecting rod is usually made split (in two halves) so that it can be mounted easily on the crankpin bearing shells. The bearing shells of the big end are made of steel, brass or bronze with a thin lining (about 0.75 mm) of white metal or babbit metal . The wear of the big end bearing is allowed for by inserting thin metallic strips (known as shims ) about 0.04 mm thick between the cap and the fixed half of the connecting rod. Small end and big end

7. Material Requirement should have adequate strength, stiffness and minimum weight . Materials The carbon steel having 0.35 % carbon has an ultimate tensile strength of about 650 MPa when properly heat treated A carbon steel with 0.45 % carbon has a ultimate tensile strength of 750 MPa. These steels are used for connecting rods of industrial engines . The alloy steels have an ultimate tensile strength of about 1050 MPa and are used for connecting rods of aero engines and automobile engines

8. Forces Acting on the Connecting Rod Force on the piston due to gas pressure and inertia of the reciprocating parts , Force due to inertia of the connecting rod or inertia bending forces , Force due to friction of the piston rings and of the piston , and Force due to friction of the piston pin bearing and the crankpin bearing .

9. Design of Connecting Rod Dimensions of cross-section of the connecting rod, Dimensions of the crankpin at the big end and the piston pin at the small end, Size of bolts for securing the big end cap, and Thickness of the big end cap .

10. Buckling of connecting rod A connecting rod is a machine member which is subjected to alternating direct compressive and tensile forces . Since the compressive forces are much higher than the tensile forces, therefore, the cross-section of the connecting rod is designed as a strut . A connecting rod, as shown in Fig., subjected to an axial load W may buckle with X-axis as neutral axis (i.e. in the plane of motion of the connecting rod) or Y-axis as neutral axis (i.e. in the plane perpendicular to the plane of motion). The connecting rod is considered like both ends hinged for buckling about X-axis and both ends fixed for buckling about Y-axis. A connecting rod should be equally strong in buckling about both the axes . 1. Dimensions of cross-section of the connecting rod

11. 1. Dimensions of cross-section of the connecting rod Connecting rod should be designed in such a way that it is equally resistant to buckling in either plane. I xx = 4 I yy I = moment of inertia of cross section (mm 4 ) I = Ak 2 Ak xx 2 = 4 Ak yy 2 k xx 2 = 4 k yy 2 k = radius of gyration of cross section k yy 2 = k xx 2 / 4 I xx / I yy = 4

12. 1. Dimensions of cross-section of the connecting rod

13. 1. Dimensions of cross-section of the connecting rod

14. This shows that the connecting rod is four times strong in buckling about Y-axis than about X-axis. If I xx > 4 I yy , buckling will occur about Y- axis If I xx < 4 I yy , buckling will occur about X-axis. In actual practice, I xx is kept slightly less than 4 I yy . It is usually taken between 3 and 3.5 and the connecting rod is designed for bucking about X-axis. The design will always be satisfactory for buckling about Y-axis. The most suitable section for the connecting rod is I-section with the proportions as shown.

15. 1. Dimensions of cross-section of the connecting rod Schematic diagram of crank and connecting rod mechanism is as shown in figure

16. Dimensions of cross section are calculated for buckling load in the plane of rotation or about XX axis

17. For a connecting rod made of mild steel and plain carbon steel In a plane of rotation, both ends are hinged and equivalent length is equal to the actual length of connecting rod. The constant ‘a’ for steel material is The critical buckling load, P cr = P c * ( fs )

18. Step by step procedure to find the dimensions of “I” section (i) Calculate the force acting on the connecting rod (ii) Calculate the critical buckling load by P cr = P c * ( fs ) (iii) Use Rankine’s formula, Calculate the value of “t”

19. (iv) Find the dimensions of “I” section using proportions. B = 4 * t & H = 5 * t (v) The width ‘B’ is kept constant throughout the length (vi) At middle section, H = 5 * t At small end, H 1 = 0.75 H to 0.9 H At big end , H 2 = 0.75 H to 0.9 H

20. Tutorial: Dimensions of “I” section Determine the dimensions of cross-section of the connecting rod for a diesel engine with the following data: Cylinder bore = 100 mm Length of the connecting rod = 350 mm Maximum gas pressure = 4 MPa Factor of safety = 6

21. 2. Dimensions of the crankpin at the big end and the piston pin at the small end Small End Big End

22. 2. Size of bolts for securing the big end cap The maximum force acting on the cap and two bolts consists only of inertia force at the top dead center on the exhaust stroke. The inertia force acting on the bolts or cap is given by, P i = inertia force on the cap or bolts (N) m r = mass of reciprocating parts (kg) ω = angular velocity of crank or angular speed of the engine (rad/s) r = crank radius (m) L = length of the connecting rod (m) n 1 = ratio of length of the connecting rod to crank radius = ( L/r ) θ = angle of inclination of crank from TDC

23. Mass of reciprocating parts m r = mass of piston assembly + (1/3) * mass of connecting rod Angular velocity of crank   N = crank speed (rpm) Crank radius r = l/2 The inertia force will be maximum at the top dead center position where ( θ = 0). When ( θ = 0), cos θ = 1 and cos 2 θ = 1

24. Forces on cap and bolt The cap, also called keep plate , is subjected to inertia force ( Pi) max , as shown in Fig.(b ). Treated as a beam freely supported at the bolt centres Loaded in a manner intermediate between uniformly distributed and centrally concentrated load in which case the bending moment is ( W*l/6 ).

25.   M b = bending moment acting on cap (N-mm) l = span length or distance between the bolt centers (mm) l = diameter of crank pin + 2 * [thickness of bush (3 mm )]+ nominal diameter of bolt (d )+ clearance (3 mm) The thickness of the Cap (c) is obtained by       b c = width of cap (mm); equal to length of crankpin Or big end bearing ( lc ) . t c = thickness of big end cap (mm)

26. The following data is given for the cap and bolts, of the big end of connecting rod: . Engine speed.= 1800 rpm Length of connecting rod = 350 mm Length of stroke = 175 mm Mass of reciprocating parts = 2.5 kg Length of crankpin = 76 mm Diameter of crankpin = 58 mm Thickness of bearing bush = 3 mm Permissible tensile stress for bolts = 60 N/mm 2 Permissible bending stress for cap = 80 N/mm 2 Calculate the nominal diameter of bolts and thickness of cap for the big end. Tutorial: Size of bolts for securing the big end cap

27. Determine the dimensions of cross-section of the connecting rod for a diesel engine with the following data: Cylinder bore = 100 mm Length of connecting rod = 320 mm Maximum gas pressure = 2.45 MPa Factor of safety against buckling failure = 5 Tutorial:

28. Tutorial: Determine the dimensions of small and big end bearings of the connecting rod for a diesel engine with the following data: Cylinder bore = 100 mm Maximum gas pressure = 2.45 MPa ( l/d ) ratio for piston pin bearing = 1.5 ( l/d ) ratio for crank pin bearing = 1.4 Allowable bearing pressure for piston pin bearing = 15 MPa Allowable bearing pressure for crank pin bearing = 10 MPa

29. Tutorial: The following data is given for the cap and bolts of the big end of the connecting rod : Engine speed = 1500 rpm Length of connecting rod = 320 mm Length of stroke = 140 mm Mass of reciprocating parts = 1.75 kg Length of crank pin = 54 mm Diameter of crank pin = 38 mm Permissible tensile stress for bolts = 120 N/mm 2 Permissible bending stress for cap = 120 N/mm 2 Calculate the nominal diameter of bolts and thickness of cap for the big end.

30. The small end of the connecting rod is subjected to pure translation motion while the big end is subjected to pure rotary motion . The intermediate points on the connecting rod move in elliptical orbits . The lateral oscillations of the connecting rod induce inertia forces that act all along the length of the connecting rod causing bending. This type of action is called whipping . The bending stress due to inertia force is called “ whipping stress ”. 3. WHIPPING STRESS

31. The mass of the connecting rod

33. The following data is given for a connecting rod Engine speed =1800 rpm, Length of connecting rod = 350 mm Length of stroke = 175 mm Density of material = 7800 kg/m 3 Thickness of web or flanges = 8 mm. Assume suitable cross-section. Calculate whipping stress in connecting rod Tutorial: Whipping Stress

34. The following data is given for a connecting rod: Engine speed = 1500 rpm Length of connecting rod = 320 mm Length of stroke = 140 mm Density of material = 7830 kg/m 3 Thickness of web or flanges = 6 mm Assume the cross-section. Calculate whipping stress in connecting rod Tutorial: Whipping Stress

35. Tutorial: Design of Connecting Rod The following data is given for a connecting rod Cylinder bore = 85 mm Length of stroke = 350 mm Maximum gas pressure = 3 MPa Factor of safety against buckling failure = 5 ( l /d ) ratio for piston pin bearing = 1.5; ( l /d) ratio for crank pin bearing = 1.25 Allowable bearing pressure for piston pin bearing = 13 MPa Allowable bearing pressure for crank pin bearing = 11 MPa Length of stroke = 140 mm Mass of reciprocating parts = 1.5 kg

36. Tutorial: Design of Connecting Rod ( contd. ) The following data is given for a connecting rod Engine speed = 2000 rpm Thickness of bearing bush = 3 mm Material of cap = steel 40C8 Yield strength of cap material = 380 N/mm 2 Factor of safety for cap = 4 Material of bolts = chromium molybdenum steel Yield strength of bolt material = 450 N/mm 2 Factor of safety for bolts = 5 Density of connecting rod = 7800 kg/m3

37. Calculate : dimensions or cross-section of connecting rock dimensions of small and big end bearings ; nominal diameter of bolts for the cap thickness of cap; and magnitude of whipping stress Tutorial: Design of Connecting Rod ( contd. )

38. Design a connecting rod for a high-speed IC engine using the following data : Cylinder bore = 125 mm Length of connecting rod = 300 mm Maximum gas pressure = 3.5 MPa Length of stroke = 125 mm Mass of reciprocating parts = 1.6 kg Engine speed = 2200 rpm Assume suitable data and state the assumptions you make. Tutorial: Design of Connecting Rod