A line diagram shows the shafting bearing (e.g. a line/stern tube bearing) with a hydraulic jack placed beneath the bearing housing (or a jacking pad under the shaft at the bearing). A dial gauge (dial test indicator) is mounted on a firm datum adjacent to the bearing so that its plunger bears on the bearing housing/shaft. The horizontal/vertical alignment direction is checked by jacking up and down.

The method: a small hydraulic jack is placed under the bearing pedestal/housing, and a dial gauge records the vertical/lateral deflection of the bearing (or of the shaft at that point) as the jack is progressively raised or released. The load on the bearing (the fraction of the shaft weight it supports) is found by jacking the bearing just clear (lifting it until the shaft no longer rests on it) or by observing lifted/heeling, and measuring the load required.

The graph plots the dial-gauge reading (jacking/shift, i.e. the jack movement/dial deflection, abscissa) against the bearing load carried (ordinate), obtained by jacking the bearing up and down between the fully-supported and just-lifted positions.

Procedure and characteristic obtained:

• As the jack lifts the bearing, the dial gauge shows the shift of the bearing/shaft; initially the shaft remains fully supported and the bearing load is zero; when the jack has lifted the bearing so the shaft is just lifted off the original bearing, the load registered is the weight that bearing portion carries.
• The graph is annotated with: the zero/initial position, the "break-free"/lift-off point (where the dial gauge reading starts to indicate the shaft has lifted = bearing light), the point of maximum bearing load, and the "settled" line (weight of the shaft portion).
• The characteristic bearing load is obtained from the point where the shaft is just lifted - the load at which the dial gauge reading changes slope - which corresponds to the actual load supported by that bearing in the "as-aligned" state. By comparing this with the design/expected bearing load, misalignment is detected: a bearing carrying much more or much less than the designed share indicates the shaft is not straight/level and must be re-aligned.

The graph therefore is used to set and check correct bearing load distribution, and to confirm that each bearing supports the correct fraction of the shaft, with the annotations showing lift-off, supported and over/under-load regions. The engineer adjusts by jacking/realigning until all bearings show the designed load.