Answer EKM Question 36

Q. Describe methods of static and dynamic balancing of an engine and describe what are first order, second order and higher order moments in engine.
Answer: Static Loads:
These are loads caused by the weights of the engine components and the bolt loads.

Dynamic Loads:
These are loads caused by the cylinder gas fluctuating pressure and inertia loads of the reciprocating and rotating masses. 

Static Balancing:
(a) It implies that the shaft is stationary or stops at a different position, if rotated when supported between centres.
(b) The sum of all moments taken about its centre of rotation should be zero at any angular position.
(c) It is done by placing counter weights to balance the moments so that their sum becomes zero.

Dynamic Balancing:
Although a shaft may be statically balanced; imbalance is caused while it is rotating, due to rotating and reciprocating masses producing inertia forces, couples and moments. Dynamic balancing is balancing of the unbalanced inertia forces together with their moments.
An inertia force is set up due to the translating (reciprocating) masses of the connecting rod-crank mechanism, and due to unbalanced gyrating (rotating) masses. Both forces cause foundation vibration.
The forces due to translating (reciprocating) masses of the connecting rod-crank mechanism tend to either tear the engine off the foundation or to press it against the foundation, depending on the direction of action.
The unbalanced gyrating (rotating) masses act along the crank web and are constant at any angle on the crank shaft at a given engine speed. They tend to shift the engine off the foundation or overturn it.

Moments caused by these two inertia forces:
(a) The gyrating (rotating) masses cause Moments to act in the vertical and horizontal planes.
(b) The translating (reciprocating) masses cause moments only in the vertical plane. 

 

(c) Primary and secondary forces are set up due to the inertia force caused by reciprocating masses.
(d) The variation in these forces are in the form of a sine waves of simple harmonic motion.
(e) Considering one revolution of 360 degrees, the variation of primary forces (Curve 1) and secondary forces (Curve 2) is shown.

Vibration:
(a) It is the oscillation caused due to a disturbing force.
(b) It can be longitudinal, axial, transverse or torsional.

Engine Vibration Causes:
(a) Constantly changing firing pressures.
(b) Unbalanced forces, couples and moments due to reciprocating and rotating masses.
(c) Pulsations due to gas forces including exhaust gases.
(d) Guide force moments.
(e) Axial forces due to in-plane bending of crank webs.
(f) Torsional vibration caused by varying torque and propeller thrust.

Amplitude: It is the maximum displacement of vibration from the point of equilibrium.

Node: It is the point in the vibrating system at which the amplitude of vibration is zero.

Order of Vibration: It is the number of vibration 'cycles' in one revolution of the engine.

Vibration Mode: It is designated by the number of nodes in a system.

Natural Vibration: It is the vibration caused by the elastic forces of the crankshaft material and the inertia of its masses in the absence of external forces.

Forced Vibration: It is the vibration of the crankshaft and the shafting coupled to it which is induced by a variable engine torque.

Resonance:
(a) It is the coincidence of the frequency of the natural vibration and the frequency of the forced vibration.
(b) It results in vibration, local over heating and overstressing of the shafting,
Vibrations During Starting:
(a) Balanced engines tend to vibrate during starting, and gradually the vibrations die out as more cylinders develop their own power.
(b) This is due to intermittent fuel delivery and misfiring of some cylinders giving rise to unbalanced inertia forces and moments. After a while, the combustion pressures in the cylinders level up and the imbalance is reduced.

Torsional Crankshaft vibration:
(a) The engine crankshaft, its flywheel gears and the different elements of the propeller shafting form an elastic system, incapable of being absolutely stiff.
(b) Application of a torque to the crankshaft causes it to 'twist' within elastic limits. Removal or reduction of the torque causes the crankshaft to twist or untwist in the opposite direction. This state will recur, for the crankshaft will be urged by the elastic forces of its material and the inertia forces of its masses to vibrate at a certain frequency.
(c) Torsional vibration is the relative vibration of the masses of the ' elastic system causing it to twist and untwist.
 
Critical Speed:
(a) It is the crankshaft speed at which resonance may occur.
(b) There may be more than one critical speed range for an engine.
(c) It manifests itself by a sharp increase in the amplitude of torsional shaft vibration.
(d) Critical speed can be measured by torsiograph, which automatically records the torsional vibration on a paper tape,

Barred Zone Range:
(a) It is a range of operational speed which is 'barred' i.e. overridden. This is a critical speed range which must be passed as soon as possible.
(b) Under Bridge control, the Bridge control unit program automatically increases the speed setting so that more fuel can enable the engine to cross over this speed range as fast a possible.
(c) It is specified for a given engine.
(d) The means of avoiding these resonant frequencies is to adjust the speed of the engine or the mass of the flywheel or the engine firing order.
(e) The most effective means of reducing the amplitude of torsional vibration is the sectionalizing of the shafting and interposing special couplings between the sections.
(f) Another method is to use vibration absorbers which are fitted to the crankshaft to dissipate the energy of vibration in a given range of engine speeds.

Reduction of Engine Vibration:
1. The, vibrations due to reciprocating and rotating masses can be countered by compensating masses rotating at the engine speed for first order frequency, and twice the engine speed for second order frequency. These compensators or balancers can be positioned in the chain drive.
2. Axial vibration due to in plane bending of crank webs can be countered by fitting an axial vibration damper at the free end of the crankshaft.
3. Torsional vibration due to varying torque and propeller thrusts is countered by detuning or damping.
4. Vibration due to guide force moments is countered by detuning, by using top bracing to increase the stiffness.