The main concept behind this article is connecting rod bolt design feature, factors contribute to connecting rod bolt failure, explain the difference between connecting rod of large slow speed engines and medium speed trunk type engines.
Connecting Rod Bolt Design Features:
- High UTS alloy steel having higher fatigue strength.
- Long thin elastic bolts for higher percentage of elongation and fatigue strength.
- High degree of surface finish bolts is used to reduce stress concentration.
- Bolts are free with very little fitted portion, to reduce stress concentration.
- Shank of bolts is less in diameter than the core diameter at the bottom of the thread by 10 %, which increases extensibility.
- Well-formed fillet at place of change of section and between the shank and bolt head to prevent stress concentration.
- Bolt has smooth change of section.
- Bolt stiffness is less than component stiffness.
- Rolled thread and three or four additional thread and gradual transition.
- Dowel pins are fitted to reduce shear load on bolts and bolts are provided with interlocking arrangements.
Connecting Rod Bolts
Factors Contribute to Connecting Rod Bolt Failure:
The connecting rod bolts are subjected to high fluctuation of stress, which leads to fatigue failure. The following factors contribute to bolt failure.
- Stress concentration: Due to damage of surface finish and fillets.
- Over- tightening: Over-tightening or over stretching of bolts resulting in permanent damage (plastic deformation)
- Inadequate tightening: During running condition, inadequate tightening will cause high fluctuating of stress leading to fatigue failure.
- Uneven tightening: Uneven tightening results some of the bolts being overloaded.
- Improper seating: Improper seating of bolt head or nut will induce bending stress.
- Corrosion attack: Corrosive attack will damage surface finish and loss of strength of materials.
Difference Between the Connecting Rods of 2-stroke Large Slow Speed Engines and 4-stroke Medium Speed
The load on connecting rods is the result of gas loads on the pistons and inertia load from the piston, piston rod, cross head and skirt. The inertia forces from the reciprocating parts are connected to gas loads depending on their action.
- In 2- stroke engines: Gas load on the piston is unidirectional. In single acting 2-stroke cycle engine the load from the end thrust on the connecting rod is usually purely compressive.
- In 4-stroke engines: Load reversal takes place at the end of exhaust stroke. The end loading on the connecting rod may be “Push-pull, " with the connecting rod being subjected to high compressive load in the “Push" stage of power stroke and low tensile load in the “Pull" stage of exhaust stroke.
In service, swing movement of the connecting rod sets up an inertial load on the connecting rod in a transverse direction, and the inertial load depends on mass and velocity.
- In 2-stroke engines: The transverse inertia loading is not so much and the round section connecting rod of normal length is strong enough to sustain the connected gas loading in slow speed engines.
- In 4-stroke engines: The transverse inertia loading is of such magnitude that the weight of the connecting rod must be considered together with its strength to resist these loads. Rectangular and I-section connecting rods fulfill this requirement in the best manner, and this is the reason for using I-section connecting rods in spite of the higher manufacturing cost.