The feasibility of bio-degradable lubricants in marine stern tubes and their tribological performance

Bio-lubricants are an important part of modern tribology. Their potential
to replace toxic mineral oil lubricants is being realised in applications from
metal working fluids and hydraulics, to forestry machinery and power
generation and more recently in the propellor shaft bearings of marine
vessels (stern tube bearings). These bio-lubricants are often referred to as
EALs (environmentally acceptable lubricants). This thesis was completed
because research into marine bio-lubricants was lacking given the relative
size of the market compared to other applications and the environmental
impact which these can lubricants have.
The aim was to assess the feasibility of bio-lubricants in marine stern
tube applications in terms of their tribological properties. Four EAL
candidates were assessed and compared to a mineral oil equivalent stern
tube lubricant using two bespoke journal bearing test rigs. One of these
test rigs, DiMITRI, was designed specifically for this work. The design
and development of DiMITRI is described in this thesis.
Initially two hypothesis were tested. The first was that EAL performance
at least matches that of mineral oil based lubricants in terms of their frictional
properties and film thickness and the second was that there would
be a trade-off between EAL stability and lubricity. These were tested
using experiments on the Baxter journal bearing rig which compared the
film thickness and friction generated with each lubricant under realistic
stern tube shear rates. The initial experiments revealed that for the first
hypothesis, that EAL performance would at least match that of mineral oil
based lubricants in terms of their frictional properties and film thickness,
was answered in the hydrodynamic lubrication regime and highlighted
limitations with the test rig in the mixed lubrication regime. The second,
that there would be a trade-off between EAL stability and lubricity also
required significant modifications to the test rig. Hence a novel test rig,
DiMITRI was developed.
The development of DiMITRI aimed to remove limitations found in the
initial experiments. It was designed to match the Sommerfeld number
of a real marine stern tube bearing. This took into account load, speed,
temperature and bearing dimensions into a single non-dimensional number.
DiMITRI was designed so that it could operate at low Sommerfeld
numbers in the range 0.004-0.01 (lower than all other journal bearing
test rigs) which would replicate the most extreme conditions found in a
real stern tube bearing. A low lubricant volume is required (50ml) to
allow experimental lubricant samples produced on a laboratory scale to
be tested. Stribeck curves which spanned across the mixed and hydrodynamic
regimes were generated for the lubricants and the point at which the
transition occurred could be identified and compared between lubricants.
The use of the Sommerfeld number enabled the results obtained on DiMITRI
to be put into context of a scaled up marine stern tube bearing in a
novel way and predictions about EAL performance in the field were made.
It was found that the EALs had equivalent performance to the mineral
oil under operating conditions in real vessels.
The effect on the tribology of the lubricants with ageing through oxidation
has been investigated and assessed using DiMITRI. The lubricants were
oxidised in the presence of a copper catalyst and with a novel method using
a tin catalyst. Both metals can be found in stern tube bearings. It was
found that the aged sample performance was inferior to the fresh samples
(increased Sommerfeld number at the inflection point) for all lubricants
and generally displayed higher friction in the mixed lubrication regime of
up to a 69% increase in the mixed lubrication regime.
Comparison between the mineral oil and EALs showed no notable difference
in either the friction or the transition between the mixed and
hydrodynamic lubrication regimes. The presence of tin as an oxidation
catalyst suggested that the EALs may be more sensitive to this metal as
a catalyst than mineral oil which would need further investigation.
Overall it was found that EALs have equivalent performance in both fresh
and oxidised forms with their mineral oil counterparts in terms of their
friction performance and ability to form a lubricant film for a Sommerfeld
number range of 0.004-0.01. Given this and their superior biodegradability
and lower toxicity to mineral oils they should be more widely adopted.
This thesis finds that bio-lubricants are indeed feasible for use in marine
vessel stern tubes.