RESONANCE AND CRITICAL SPEED
In a 2stroke engine, turning moment reaches its maximum for every 360 degrees of rotation. In a 4stroke, for every 720degrees. In a 4stroke, no. of peaks will therefore equal to number of cylinder firing in 720degrees.
If the peaks per 360 of crank rotation are multiplied by rpm, we can get frequency/min
If the frequency is similar to any of the natural frequencies of the engine shafting system, we have a resonant condition.
Speed of engine at which resonant cond occurs is referred to critical speed.
o The elastic shaft of multi-cylinder engine are subjected to varying periodic firing impulses from the different units which twist the shaft at varying torque. This causes the harmonic displacement of the masses in the plane of roatation due to inertia and elastic nature of the material which is termed as Torsional vibration.
o Application of torque to c/shaft causes it to twist within its elastic limits and den untwist on removal of stress at a frequency of 1/2pie (sq root of (R/J))
Where R= stiffness of the shaft
J= moment of inertia of disc
o The other causes are
– Power imbalance
– Bend crankshaft or bearing wear down
– Roatary imbalance due to damaged gearing, loose balanced weight.
– Fluctuation of flow around propeller
– Crash manoeuvring
o Torsional stresses leading to aggravating the minor surface defects
o Causes distortion and bending of the c/shaft which could increase the load on bearing.
o Tuning wheel or Vibration damper are used at the free end of the c/shaft to reduce the amplitude of torsional vibrations from peak.
o It makes use of concentric masses with sleeve springs and are filled by oil. The inertia lag of these masses will restrict the amplitude of the vibration to reach its critical range.
o Disadv of this is in the dashpot viscous damper, the elastic strain energy absorbed in the damping process heats up the oil and affects the damping efficiency.
o Further development led to resin/silicone damper.
o These dampers consist of a light weight rigidly fitted to the vibrating shaft. This loosely fitted floating mass is fitted within the flywheel rim casing, filled with silicone fluid.
o The inertia lag of this weight gives a lagging torque on the crankshaft.
o Vibration of the casing attached to the crankshaft will be opposed by inertia lag of floating mass and viscous drag effect of silicon with which both the surfaces have contact.
o METHOD USED IN DESIGNING – The system is modified to increase its stiffness by adding additional mass to the flywheel or using a larger diameter shaft or by adjusting the firing order. The resultant elastic curve will have 2nodes and the maximum angle of twist of shaft system is within acceptable limits.