10.1.1 The Surveyor
is to examine the stern seal and witness the tailshaft clearance gauge
readings being measured and note these for comparison with the previous
readings and the original measurements recorded immediately after
the shaft was fitted.
10.1.2 Seals for
oil lubricated stern bearings are designed to protect the stern bearing
against the ingress of sea-water and to retain the lubricating oil
within the bearing whilst being flexible enough to accommodate movement
of the shaft under varying loads and water pressures. The effectiveness
of a seal is dependent on the quality of the contact area between
the sealing ring and the liner. All signs of oil leakage should be
investigated.
10.1.3 The wear-down
gauge is used to check the clearance of bearings. This relative measurement
provides an indication of wear quantified by a change of clearance
when compared with the original value. On a new ship the first (datum)
measurements are taken immediately the shaft is fitted. Since it is
important for all measurements that the weardown gauge is always used
in the same position, the line mark of the wear-down gauge is already
punchmarked on the casing of the aft seal during the first measurement.
For all measurements, care must be taken that the ‘O’
mark is always in the same position, which is punched into the circumference
of the flange of the liner. After each disassembly of the seal, new
measured values are determined. The gauge readings, when compared
with the original or datum readings, give an indication of wear on
the lower bearing surface and the shaft or liner when fitted. If the
lower gauge reading decreases, this may indicate wear of the bearing.
If the upper gauge reading increases and the increase is the same
as the decrease on the lower, this indicates wear on the bearing.
If the increase is greater than the lower gauge decrease, then this
indicates wear on the shaft or liner. These changes are considered
together with the Owner’s representative and a course of action
decided.
10.1.4 A forward
seal is fitted at the inboard end of the stern bearing and this should
also be examined.
Figure 7.10.1 Typical seal arrangements - I
Figure 7.10.2 Typical seal arrangements - II