Shafting and Bearings Flashcards
What are the reasons for using this in propulsion drives?
Reduces vibration (Harmonic absorption)
Reduces noise
Handles slight misalignments
Allows for flexible mounting of the engine
Describe with the aid of a sketch, how alignment of a propeller transmission shaft may be checked?
State the indications of a high bearing when the shaft is running?
This is done by using feelers between the faces of adjoining flanges, wedging the lower half bearings
until faces are parallel with a 0.1 mm gap between.
A parallel block is used around the periphery of the flanges. The intermediate bearings are chocked
up by cast iron chocks about 50mm thick and bolted down. Couplings are continually rechecked. The
thrust block is now aligned coupling to- coupling and secured
Main engine alignment
Moving the engine mounts, got to remember that this will only be the case for one set of conditions
as the floating of the vessel and under load can cause hogging and sagging
Explain 3 advantages of hydraulically fitted shaft coupling bolts compared to parallel interference
bolts?
Interference fit and taking some of the load
Can be easily installed and removed without issue due to stretch/reduced diameter
Removes the chances of fretting and therefore early failure
RADIAL FITTED BOLT (Installation Guide)
1: Insert by hand (easy due to the reduced diameter of the main bolt)
#2: The hydraulic tensioner pulls the body of the bolt into the sleeve, creating a radial interference
fit
#3: Remove the hydraulic tensioner and mount the nuts
#4: Re-apply the hydraulic tensioner and stretch the assembly to the required limit
#5: Disconnect the hydraulic equipment and the bolt installation is complete
With reference to the main propulsion shaft hydraulic sleeve type couplings.
a) The removal procedure
A sealed nut is screwed onto the inner sleeve and oil is pressurised between the nut and the
outer sleeve. This drives the outer sleeve up the taper of the inner sleeve and clamps the
shafts together. Removal-The coupling is released by applying oil under pressure between the
outer and inner sleeves.
With reference to the main propulsion shaft hydraulic sleeve type couplings.
b) How it is determined, during assembly that the push fit is complete
Couplings security is checked by measuring the outer sleeve diameter before and after
tightening. The diameter should agree with the data stamped on the sleeve. Witness marks
also show if the coupling has moved in service
Whilst a single screw vessel is on passage it is noticed that the intermediate shaft bearing is running
hot.
Causes?
Procedure that should be followed in order to reach port for further investigation if there was
no obvious causes?
Mis-alignment, lubrication system gone, loss of lubricating substance
Overloading cooling system has failed, White bearing failure.
Reduce power immediately and ensure that oil levels and oil flow is sufficient. Oil sample
needs to be taken and sent away for analysis as wear particles constituents will give evidence
on what is happening. Oil changed and cooling applied. Increased monitoring.
Describe Hydrodynamic lubrication that occurs when the shaft is rotating?
To generate a lubricating film within a bearing, the opposed surfaces must be forced apart by
pressure generated within the fluid film. The simplest means is to introduce the fluid under sufficient
pressure at the point of maximum loading, but this hydrostatic method, although equally effective at
all speeds, needs considerable power and is avoided whenever there is a satisfactory alternative
Above a certain critical speed, which depends mainly on the size and loading of the bearing and the
viscosity of the lubricant, stable hydrodynamic forces are set up in a journal bearing that force .the
surfaces apart and permit full fluid-film lubrication
By suitable design, hydrodynamic lubrication can also be provided in certain other types of bearing.
As the shaft starts to revolve, friction between the journal and the
bearing bore causes the shaft to climb up the inside of the bearing
until torque, together with the increased thickness of lubricant film,
overcomes frictional resistance and the shaft starts to slip at the
point of contact The rotating shaft then takes up its equilibrium position, where it is
supported on a fluid film drawn beneath it by viscous friction
Explain the importance of lubricating oil temperature?
Viscosity changes can occur with changes in temperature, where its viscosity is only guaranteed
between a distinct temperature range. Drastic variations in temperature and therefore viscosity and
lubricating abilities will vary. The hotter the flued the thinner the fluid is, breakdown in the
hydrodynamic wedge resulting in excessive friction and melting of the journal material.
With reference to the comparing modern water lubricated stern tube bearings with those that are
oil lubricated?
3 Advantages
3 disadvantages and how to overcome?
a) 3 advantages
Removes the risk of oil leaking out into the environment
Simpler
Withstand edge and shock loading better with modern plastics such as thordon and railco
b) 3 disadvantages? Explaining how these may be overcome?
Relatively expensive - offset by the environmental advantages and might cause a reduction in
a vessels insurance premium due to less risk of pollution.
With reference to the comparing modern water lubricated stern tube bearings with those that are
oil lubricated?
Past Questions Page 32
a vessels insurance premium due to less risk of pollution.
Prone to damage due to dirt ingress - Can be overcome by a filtration system but its needs to
be designed that with a blockage occurring flow is still required when cleaning.
Sensitive to water temperatures - can be regulated from within the vessel with a three way
hydrostatic valve and a heat exchanger
Requires a constant supply of clean water.
With reference to stern tube bearings, state the advantages and disadvantages of using each of the
following?
white metal
Plastic
White metal
Long life is maintained
No affected by sea water temps
Initial lower costs
-pollution
-internal/external sealing systems
-failure can be catastrophic
b) Plastic
No pollution
Simpler, no external sealing arrangements
Non catastrophic failure
-high initial cost
-sensitive to dirt in the water
-constant supply of clean water
How some angular misalignment of the shaft is accommodated?
Spherical roller bearings, toroidal roller and spherical roller thrust bearings can accommodate
shaft deflections as well as the initial misalignment from machining or mounting errors.
How longitudinal movement of the shaft is accommodated?
Cylindrical roller bearings do not provide any axial restraint. The rollers in their cage a free to
slide axially along the outer race. This feature allows movement due to the expansion of the
shaft due to heating and also with the hog and sag of the vessel.
