Bottom End Bolts, Particularly in Medium Speed Engines, are Prone to Failure under Normal Conditions
Connecting rod bottom end bolts keep the two halves of the bottom end bearing together. These bolts are subjected to complex nature of stress as follows:
Tensile Stress: Tightening of nuts stretches the bolts and subjects them to tensile stress, which helps keep the two components together under tension compression.
Fluctuating Tensile Stress: The tensile stress set in the bolt while tightening is of higher degree to reduce fluctuating stress in modern highly rated engines.
In 4-stroke engines, at the end of exhaust stroke the inertial load is acting upwards. As a result, the piston has a tendency to fly out. Therefore, momentary load reversal takes place in the bearings. At this period, the load is on the top-half of the main bearing, the bottom half of the bottom end bearing, and the top half of top end bearing- which is just opposite to normal load condition. This load reversal continues at every cycle.
This is the significant effect on bottom end bolts, because the whole connecting rod is under tension at the stage of load reversal. Therefore the bottom end bolt is subjected to additional tensile stress at the end of the exhaust stroke, resulting in severe stress fluctuations during the cycle.
Shear Stress: The two halves of the bottom end of the connecting rod have a tendency to pull apart. This results in shear stress in the bolt keeping the two parts together.
Bending Stress: Bending stress of a fluctuating nature also takes place while the bottom end is distorting. During the power stroke, the bolt bends outward and during the exhaust stroke the bolt tends to bend inward.
As these fluctuating stresses lead to the fatigue failure of bottom end bolts, and this is why the bottom end bolts are prone to failure under normal condition in medium speed engines.