(a) Meaning of Fair Curve / Rational Alignment

Fair curve alignment refers to the method of shaft alignment where the bearings are not arranged in a single straight line, but are deliberately set with calculated vertical offsets so that the shaft follows a smooth curve.

Explanation:

• For small-diameter shafts, bearings can often be kept in a straight line without issues.
• For large-diameter or high-power shafts, straight-line alignment causes:
  • Uneven bearing loading
  • High bending stress in the shaft
  • Excessive wear and vibration
• In modern ships, fair curve alignment is preferred because:
  • Bearing heights are adjusted individually
  • Shaft load is distributed uniformly
  • Bending stresses are minimized, preventing fatigue and vibration

  Advantages of Fair Curve Alignment:

  1. Uniform bearing load distribution, reducing localized stress.
  2. Lower shaft bending stress, enhancing structural integrity.
  3. Reduced vibration, ensuring smoother operation.
  4. Longer bearing life, lowering maintenance costs.

  (b) Shaft Alignment Check Using Hydraulic Jacks

  The hydraulic jacking method is commonly used to verify shaft alignment by measuring the bearing loads when the shaft is lifted and plotting a graph of jack load vs. vertical displacement.

  Procedure:

  1. Place a hydraulic jack near the bearing to be checked.
  2. Fix a dial gauge to measure vertical movement of the shaft.
  3. Slowly lift and lower the shaft using the jack.
  4. Record jack load and shaft displacement readings.
  5. Plot a graph of load versus displacement.

  Graph Indications:

  • (i) Static Load
    • The load acting on the bearing at zero lift.
    • Represents the actual operational load on the bearing when the shaft is at rest.
  • (ii) Hysteresis
    • The difference between the lifting and lowering curves.
    • Caused by:
      • Friction between shaft and bearing
      • Oil film resistance
      • Elastic deformation of the bearing
    • Hysteresis indicates energy loss and affects measurement accuracy.
  • (iii) Influence Number
    • Represents the change in load per unit vertical movement of a bearing (N/mm).
    • Shows the effect of raising one bearing on the load of other bearings.
    • Used in fair curve alignment calculations to adjust bearing heights accurately.

    (c) Limitations of Hydraulic Jacking Method

    1. Measures Only Vertical Loads
       • Does not accurately measure horizontal bearing reactions.
       • Less effective for resiliently mounted reduction gears.
    2. Time-Consuming
       • Requires many readings for multiple bearings.
       • Labour-intensive and difficult in restricted engine room spaces.
    3. Accuracy Issues
       • Misalignment of the jack or dial gauge introduces errors.
       • Shaft centerline mismatch reduces precision.
       • Can produce wide hysteresis, complicating interpretation.
    4. Requires Skilled Interpretation
       • Jacking curves vary depending on bearing type.
       • Only trained personnel can correctly analyze the results.
    5. Hysteresis Effects
       • Friction and oil film can cause non-linear readings.
       • Lack of a load cell amplifies measurement errors.