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.