(a) Cause and Effect of Torsional Vibration
Causes of Torsional Vibration
Torsional vibration in a multicylinder engine arises from the dynamic behaviour of the elastic shaft system when acted upon by periodic varying torque. This vibratory motion is caused by the following:
- The fluctuating torque imposed by the gas pressure in the cylinders and the inertia forces of reciprocating masses contributes significantly to torsional vibration.
- Unequal power output from the cylinders leads to greater torque variation during each cycle, exacerbating torsional vibration.
- Operating the engine close to its critical speed, where the natural frequency of the system matches the excitation frequency, results in resonance, amplifying torsional vibration.
- Faults in the gearing system can introduce rotational imbalances that are transmitted to the shaft, further driving torsional vibrations.
Effects of Torsional Vibration
- The cyclic nature of torsional vibration adds shear stress to the crankshaft, raising the overall stress level during operation and increasing the risk of failure.
- High-stress areas, such as fillets or oil holes, are prone to crack initiation and propagation under torsional vibration.
- The increased wear rate on the contact faces of gear teeth can lead to misalignment and gear damage.
- Excessive torsional vibration distorts the shafting, causing misalignment and increasing the load on bearings.
- Prolonged torsional vibration may result in catastrophic crankshaft failure, compromising the transmission and propulsion systems.
(b) Critical Speed
Critical speed is the range of speed at which resonance in vibration may occur, which is experienced when the engine’s operating frequency coincides with the natural frequency of the hull, which results in vibrations of amplitudes higher than normal, which is very dangerous. The critical speed range should be passed as soon as possible. In this condition, the torsional vibration of the shaft increases greatly and will impose very high shear stress on the crankshaft. These levels of stress could cause crankshaft failure.
In crankshafts, fatigue cracks develop due to:
- Cyclic Stresses from Combustion Pressure: High combustion pressure results in repeated loading and unloading, causing microcracks that can propagate over time.
- Bending Stresses from Bearing Wear: Worn main bearings cause the crankshaft to bend, inducing additional bending stresses that accelerate fatigue crack formation.
(d) Torsional Vibration Damper
A torsional vibration damper functions by absorbing and dissipating the vibratory energy generated by the crankshaft during operation. One common and effective type of damper used in marine engines is the viscous damper.
The viscous damper is composed of the following components:
- A light metal casing, fixed securely to the front end of the crankshaft
- A heavy annular mass (inertia ring) mounted inside the casing and supported by a centring bearing
- A high-viscosity silicon fluid filling the narrow clearance between the casing and the annular mass
As the crankshaft rotates, the entire damper assembly rotates with it. Under normal operating conditions with minor angular accelerations, the viscous drag of the silicon fluid ensures that the internal heavy mass rotates in unison with the casing.
However, during torsional vibrations, the crankshaft experiences rapid angular accelerations and decelerations. Under these conditions, the heavy internal mass begins to slip relative to the casing, due to the inability of the viscous fluid to maintain complete coupling. This relative motion between the casing and the mass causes shearing of the viscous fluid, thereby dissipating the vibratory energy as heat.
This process:
- Reduces the amplitude of torsional vibrations
- Minimizes stress on the crankshaft
- Prevents fatigue failure of the crankshaft and other drivetrain components