Question

With reference to main propulsion shaft systems:

(a) Describe a method of hydraulic jacking to check bearing loads. (8)

(b) Sketch the Bearing Load versus Shift Lift Dial Gauge Reading graph obtained by the method described in part (a), annotating the graph and how the characteristic of bearing load is obtained. (8)

Answer 1

Part (a)

Hydraulic jack is placed below the shaft to lift it just clear of the bearing. The dial gauge fixed to the bearing indicates the lift. Hydraulic pressure is applied to the jack and the load applied to the jack shows the load on the bearing.

Part (b)

As the hydraulic jack pressures are raised from zero, the concentrated loading initially causes deformation of the shaft. Only after the journal section has been bowed up out of shape to some degree, and the bearing material resumes its relaxed primary shape, does the sagging centre part of the journal lift clear and out of contact with the bearing. The plot shows that the dial gauges register upward movement as soon as the shaft is pushed out of shape by increasing hydraulic pressure. The curve takes a different shape as the shaft lifts clear.

If the jacking is taken too far, then adjacent bearings gradually become unloaded and the plot is affected by a change in the elastic system. To guard against this, dial gauges are fixed on adjacent bearings to ensure that the lift is limited to the bearing that is being checked.

The jack-up curves indicate a higher jack load than the jack-down curves. This hysteresis is caused by friction. It is compensated by taking the average value of the jack loads obtained from the lifting and the lowering curve.

✓ Verified Model Answer (Text Solution)

Hydraulic jack is placed below the shaft to lift it just clear of the bearing. The dial gauge fixed to the bearing indicates the lift. Hydraulic pressure is applied to the jack and the load applied to the jack shows the load on the bearing.