To design connecting rod for the given specifications
Software: AUTOCAD 2000.
Hardware: 128 MB RAM, Keyboard, and Mouse.
MATERIAL OF CONNECTING ROD:
Medium carbon steel
C = 0.35 to 0.45 %
The function of the connecting rod is to convert the reciprocating motion of the piston in to the rotary motion of the crankshaft.
FORCES ACTING ON THE CONNECTING ROD:
A combination of axial and bending stresses acts on the rod in operations. The axial stresses are produced due to the cylinder gas pressure and the inertia force arising on account of reciprocating motion, where as bending stresses are caused due to the centrifugal force. To provide the maximum rigidity with minimum weight the cross section of connecting rod is made and I section.
The connecting rods are generally made by drop forging of steel or duralumin. However, with the process of technologies, the connecting rods these days are also cast from malleable or spheroidal graphite cast iron. In the later method, a closer weight tolerance can be maintained compared to the forging method. In general, forged connecting rods are compact and lighter in which an advantage from inertia view point, where as cast connecting rods are comparatively cheaper but on account of lesser strength their use is limited to small and medium size petrol engine
LENGTH OF CONNECTING RODS:
In determining the dimensions of the connecting rod the distance between the axis of the piston pin and the axis of the crank pin is referred to as the center-to-center distance the length is generally expressed in terms of stroke. Due to the higher combustion pressure in diesel engine, their connecting rod must be given larger cross section and their piston made larger than those of petrol engine of the same bore and stroke.
The connecting rod big end is made in two halves with a detachable cap held together by two bolts
A combination of axial and bending stresses acts on the connecting rod in operation. The axial stresses are produced due to cylinder pressure and inertia force arising on account of reciprocating motion, where as bending stresses are caused due to centrifugal effects. The cross section of the connecting rod is made as I section so that maximum rigidity is obtained with minimum weight.
Connecting rods are made by drop forging process, forged connecting rods are more advantages than cast connecting rods. Since it is more compact and lighter.
In some connecting rod, a hole is drilled between two ends for carrying lubricating oil from the big end to the small end for lubrication of piston and piston pin.
Small end of the connecting rod holds the piston pin and it has either a solid ‘eye’ or split ‘eye’.
FORCES ACTING ON THE CONNECTING ROD:
The combined effect of gas pressure on piston & inertia of reciprocating parts
Fa = (p / 4) × D² ×Pmax
F/A = (sy / n) x (1-((sy / 4p² x E) x (Lo / K) ²)
F = (p / 4) × D² × Pmax
n = FOS, Lo = L
A = 11 t²
K² = 318 t²
E = Young’s modulus
Buckling load = (Fc × A) / (1+a x (l / k) ²)
Fc = Ultimate crushing stress
A = 11t²
a = 1 / 1600 for C.I
Buckling load = Max gas load × FOS
= (p / 4) D² × Pmax × FOS
Permissible bending stress:
sb = sy / n
sb max = (g x a x l² x w² x r) / (9 √ 3g x Zxx )
g = Specific weight of material N/mm²
Zxx = (419 / 3) t³
Big end bearing:
L / D = 1.25 s
Gas force, Fa = LD ×Pb
Small end bearing:
L / D = 1.5
Gas force, Fa = L x D x Pb
Force on bolt = inertia force
= (wp / g) ´ w² ´ r
Force on bolt = No of bolt ´ (p / 4) ´ dc² ´ [s t] bolt
t = s y / n
Max B.M = w x l x b / 6
Width of cap, b = length of crank pin –2 [flange thickness of the bush]
Section modulus, Z = (b ´ tc²) / 6
Bending stress in cap = B x M max / Z
Thus the connecting rod is designed & drawn using AUTO CAD 2000