186.How do you compensate for second order moment in engine

The vibration characteristic of engine is divided into 4 categories.

1.External unbalanced moments
   Happens due to inertia of reciprocating masses.
   There are two types
 
a) First order moment
    - Acting in both horizontal and vertical direction
    - Happens once in a cycle
    - Balanced by counter weights

b) Second order moment
    - Acting only in vertical direction
    - Happens twice in a cycle
    - Balanced by two wheels with balance weight fitted on the chain drive (Compensators) 

2.Guide force moments
   Cause due to gas force and inertia
   There are two types

a) H type
    - Engine less than 7 cylinders tend to rock top of the engine in transverse direction.
    - H moment = No of cylinders

b) X type
    - Engine more than 6 cylinders tend to twist engine in X shape
    - X moment = 1/2 No of cylinders

    To counteract top bracings is fitted.

3.Axial vibration
   - When crank throw is loaded by gas force through connecting rod the arms of crank throw deflects       in axial direction causing a vibration which through thrust bearing transferred to the hull.
   - Order = No of cylinders (For less than 7 units)
   - Order = 1/2 No of cylinders (More than 6 units)
   - To counteract axial vibration dampers are fitted.

4.Torsional vibration
   - Due to varying gas pressure
   - Cyclic in nature
   - Natural frequency of torsional vibration can be adjusted by shaft diameter.
   - If smaller shaft diameter is chosen then normal speed range can be placed above resonance , it is         called over critical running.
   - If larger shaft diameter is chosen then normal speed range can be placed below resonance , it is           called under critical running.