Types of lubrication
(i) fluid film thickness basically molecular 10-7cm
(ii) depends on molecular properties of lubricant and solid members, often chemical( E.P. additives actuated by high temperature generated by rubbing)
(iii) independent of shape and velocity in so far as these influence the temperature generated from rubbing.
(iv) obeys classical laws of friction
(v) Frictional coefficient = 0.05 to 0.1
-self acting (i) fluid thickness 1 mm to 0.01 mm
(ii) Depends on the viscosity of the lube oil and the shape and relative motion of the solid surfaces
(iii) Independent of properties of the solid members; so long as the elasticity does not deform the shape. and thermal properties do not effect the temperature of the lubricant
(iv) does not obey classical friction laws.
(v) Friction coefficient = 0.001
(vi) film may form by self action or by Hydrostatic pressure of the lube oil.
-externally pressurised as above except the separation of surfaces is caused by fluid being injected under pressure
This is the type of lubrication used with rolling element bearings. To clarify, the material of the running surface deforms under high pressure as the rolling element passes over it. The oil wedge forms in this deformation.
(i) Deformation and increased viscosity with pressure are involved
(ii) Frictional coefficient = 0.05
(iii) film thickness less than Hydrodynamic
How boundary lubrication got its name. This is remembering a while back so bear with me. A chap was hired to investigate the cause of failure or train carriage wheels. He run test on journal bearings using pressure gauges mounted around the circumference of the bearing. After repeatedly blowing these gauges due to the very high pressure created in the oil wedge, he eventually came up with the idea of dynamic lubrication and the oil wedge.
I believe the cause of failure of the carriage bearings was insufficient clearance preventing the journal from lifting and leading to rub.
Anyway, this chap gave a lecture to his peers. Someone in the audience said that he could understand his theory of dynamic lubrication, but asked to him explain how the bearing was lubricated during starting. To which our chap replied something along the lines of- " ....there are boundaries of lubrication beyond our knowledge...".
Oil distribution within a journal bearing
If the maximum radial clearance is Cr
then Cr = e + Hm
where e is the eccentricity between the shaft and bearing centreline and Hm is the minimum clearance (oil film thickness)
an eccentricity factor can be calculated from
n = e / Cr
Factors involved with the eccentricity factor n are; minimum oil film thickness, journal attitude angle, pressure distribution, peak pressure angle, friction, horsepower loss and oil flow through the loaded region. The latter three determine the temperature of the bearing which for high speed bearings can be a limiting factor.
Lube Oil requirements for Diesel Engines
The oil has to serve two purposes;
A good quality mineral oil will serve the purpose of reducing friction to an acceptable level depending upon the metals involved and other conditions such as temperature. All oils will oxidise and this reduces its effectiveness as a lubricant. Oxidation will also cause deposits which can block passage ways and coat working parts. The rate of oxidation will depend upon temperature, the higher the temperature the more rapid the rate. Anti oxidants are available which reduce the rate, also additional properties can be achieved by the use of additives.
Under high temperatures an oil is liable to thermal degradation which causes discoloration and changes the viscosity. Additives cannot change an oils susceptibility to this degradation. The refining process can remove compounds which effect the thermal stability of the oil and also those that lower oxidation resistance. Most of the chemicals found in an oil will react more or less with oxygen, The effects of this oxidation is always undesirable. Hence, a major objective of the refining process of a mineral oil is to remove those hydrocarbons i.e. the aromatics, the small amount of unsaturates together with molecules containing sulphur, oxygen and nitrogen.
Unfortunately these same molecules are those that improve the boundary lubrication performance. Hence, a careful balance must be struck. The use of anti-oxidants make a slightly better balance although there usefulness is limited.
Tin based whitemetal is susceptible to hardening as an oxide layers from on the surface.
These tin oxides are a grey-black in appearance and are extremely hard. There formation reduces the bearing clearance as the oxide layer is thicker than the original white metal material from which it formed. The oxide has a lower coefficient of friction than the original white metal but it will cause problems if it brakes up as fragments will become embedded edge on in the white metal and can score the pin.
Problems caused by water contamination,
May be heavy residual or light diesel/gas oil and can be sourced to faulty to cylinder combustion or faulty seals on fuel p/ps.
i.e. carbon from the cylinder combustion process, particularly of importance with trunk piston engines but also for crosshead engines with inefficient diaphragm. The carbon can lead to restrictions and blockages of oil ways causing bearing failure. Straight mineral oils hold 1% carbon in suspension, dispersant oils hold about 5%.
Certain bacteria will attack oil but water must be present. The bacteria may exist in a dormant state in the oil but water is required if they are to reproduce.. The bacteria digest the oil causing breakdown emulsions to be formed, acidity increases, dead bacteria block filters and corrosive films form on working surfaces.
In summary their must be three essential conditions for microbiological growth;
Two other factors which encourage the growth are a slight acidity in the water (pH 5 or 6) and a slightly raised temperature (20 to 40oC) which can lead to rapid growth.
Biocide additives are available but they are not always compatible with other desired additives and can lead to large organic blockages if treated in the machinery. The best solution is to avoid the presence of water. If mild attack takes place the oil may be heated in the renovating tank to above 90oC for 24hrs before being returned to the sump via the centrifugal separator. For a severe attack the only solution is complete replacement of the charge followed by sterilisation of the system. It may be noted that on replenishment the bacteria may be present in a dormant state in the new charge.
Test results of crankcase oils
Viscosity-Increases due to thermal degradation or hfo contamination, reduces with diesel oil contamination, corrective action needed if it increases by 25% from new oil.
Water content-Corrective action required at 1%
Insoluble Sediments-basically the result of wear and oxidation, corrective action at 1% by weight
Ash-a measure of incombustibles in the oil sample, corrective action at 0.13% by weight
TAN-Total acid number consists of the strong acids (mainly sulphuric acid) formed in the combustion process and weak acids resulting form oxidation of the lub oil.
SAN-Strong acid number, the oil should be renewed if any is detected
TBN-Total base number indicates the alkaline reserve particulary important for trunk piston engines
Closed flash point-highlightd fuel contamination, corrective action if reduces by 30oC from new
Cylinder lub oil
The type of cyl l.o. required will depend upon the cylinder conditions and the engine design e.g crosshead or trunk piston. However, the property requirements are basically the same but will vary in degree depending upon the fuel and operating conditions.
Normal properties required are;
All oils for all purposes can be designed to give particular properties through the careful use of additives to the base mineral oil stock.
Common additives are;
When running in, the cylinder lube oil injector pumps may be filled with a a straight mineral oil without anti-wear properties- typically the crankcase oil- once this small reserve of oil is exhauted, running in carries on with normal cylinder lube oil. The flow of oil is increased to carry away metallic particles.
Problems caused by stuffing box leakage oil entering crankcase
Low speed engines are particularly at risk from crankcase lubricant contamination caused by cylinder oil drainage past the piston rod gland and combustion products. This can lead to severe damage of engine crankcase components and reduction of life of oil which is normally expected to last the lifetime.
There has been a general increase in the viscosity and Base number of crankcase oils over recent years particularly for engines built since the early 1980's. Increased alkalinity, viscosity and insolubles, fuel derived elements such as vanadium and oil additive derived elements such as calcium, suggest that the contamination is from the cylinder oil drainage.
Deterioration of the crankcase oil has led to the expensive necessity of replacing up to 50% of the sump, this is particularly of concern as it is often only a temporary measure.
Four causes are put forward,
From above the suggestion is the most likely cause for contamination is leakage past the piston rod. It is seen that maintenance of the stuffing box is of the utmost importance. Tell tales and drainage lines should be proved free and use of oil drained from the uppermost drain should not be allowed even after purification due to the high level of contamination which can destroy the properties of the oil in the sump
I know of a case where due to the increased viscosity of the oil a 50% charge of hydraulic oil was added to the sump of a very large slow speed engine under advice from the manufacturer
Properties of Lubricating Oil
Composition of lubricating oils
Lubricating oil fractions extracted from crude oil are a widely varying mixture of straight and branched chain paraffinic, napthenic aromatic hydrocarbons having boiling points ranging from about 302o to 593oC. Some specialty lubricants may have boiling point extremes of 177 and 815oC. The choice of grade of lubricating oil base is determined by the expected use.
General capabilities expected from an engine lubricant
Properties ideal for bearings
Properties ideal for gear case
Compromise between above two requirements
Improvements in lubricating oil over the last twenty years have come about almost entirely from the use of additives.
These are added for three main reasons;
to protect the lubricant in service by limiting the chemical change and deterioration
To protect the mechanism from harmful combustion products and malfunctioning lubricating oil
To improve existing physical properties and to create new beneficial characteristics in the oil
Typical additives are; Barium, calcium, phosphorus, Sulphur, chlorine, zinc, oxidation inhibitor-increases oil and machinery life, decreases sludge and varnish on metal parts
Corrosion inhibitor-protects against chemical attack of alloy bearings and metal surfaces.
Antiwear improvers-protects rubbing surfaces operating with this film boundary lubrication. One such antiwear ( and oxidation inhibitor) chemical is Zinc dithiophosphate or ZDDP
Detergent-tend to neutralise the deposits before formation under high temperature and pressure conditions, or as a result of using a fuel with high sulphur content. The principle detergents are soaps and alkaline metals, usually calcium ( often referred to as 'matallo-organic compounds'). They are usually ash forming and spent additive will contribute to the insolubles loading of a used oil. It should be noted that additives which do not burn cleanly without ash tend to be avoided for use with Cylinder Lubricating Oils.
Dispersant-used to disperse or suspend the deposits forming contaminants. Typical dispersants, such as polyesters and benzlamides, are usually clean burning. The molecules have a polar charge at one end which attracts and holds the deposits
Alkaline agents-neutralises acids, htese form the TBN of the oil and includes additives such as the above dispersants and detergents. An excess of acid neutralising alkalis are present in the oil and these help to keep parts clean. Failure to keep an oil alkaline can lead to damage to bearings due to acidic attack as well as increased liner wear.
Pour point depressants-improves low temperature viscosity
Oiliness agent-reduces friction seizure point and wear rates
EP additives-increases film strength and load carrying capability
Antifoam agents-prevents stable bubble formation
Viscosity Improvers-an additive that improves the viscosity index of the oil. I.e. reduces the effect of temerpature of=n the viscosity of the oil. Shear stability property is measured indicating the effect of high rates of shar on the VI improver as the improver molecules are broken down into smaller molecules
Metal deactivators-prevent catalytic effects of metal
Oxidation degrades the lube oil producing sludges, varnishes and resins. Presence of moisture, and some metals particularly copper tend to act as a catalyst. Once oxidation starts, deterioration of the properties of the oil is rapid.
When recharging no more than 10 % of the working charge should be topped up due to heavy sludgeing that can occur due to the heavy precipitation of the sludge.
EP additive oils
Can assist in healing of damaged gear surfaces but should be used as a temporary measure only due to risk of side effects
This occurs due to water contamination; also, contamination with grease, fatty oils, varnish, paint and rust preventers containing fatty products can also promote emulsification.
The presence of an emulsion can be detected by a general cloudiness of the sample. Salt water emulsifies very easily and should be avoided.
Water entrained in the oil supplied to a journal bearing can lead to loss of oil wedge, rub and failure.
Fresh water contamination whilst not in itself dangerous can lead to rusting. The iron oxides catalyses the oil to form sludge's. The additives in the oil can leach out to change the water into an electrolyte.
Salt water contamination is very serious as it causes tin oxide corrosion, and also leads to electrochemical attack on the tin matrix in the white metal. The sea water act as then electrolyte.
A major problem of water within a lub oil is where the mix enters a bearing, here it is possible for the water to be adiabatically heated causing it to flash off collapsing the oil wedge.
Stresses on Lube oil
The main stresses experienced by Lube oils in diesel engines operating on heavy fuel oils are expressed as follows
Acid Stress- Caused by sulphuric and oxidation acids. This leads to increased corrosive wear, deposits, reduced Base Number and shorter oil life.Rapid depletion of the BN is the clearest sign of oil stress
Thermal/Oxidative stress-This caused by elevated temperatures leading to increased rates of thermal/oxidative breakdown of lubricant and fuel. This leads to increased levels of deposits, sludges, corrosive wear of bearing material, oil thickening and reduced oil life. In addition deposits on the under crown side of the piston can lead to increased hot corrosion on the piston.
Asphaltene Stress-This caused by fuel contamination of the lube oil and can lead to increased levels of deposits, sludges, lacquers, oil thickening and reduced oil life. In addition deposits on the under crown side of the piston can lead to increased hot corrosion on the piston
Regular testing of crankcase lub oil is important to ensure that deterioration has not taken place. The results of in service deterioration could be a reduction in engine protection or actual attack on working points by corrosive deposits. Oil samples are generally tested every 3 to 4 months depending on the system and experience. Shipboard testing is taking a rising prominence to allow monitoring of oil condition between testing.
To ensure good representation, care should be taken where the sample is drawn
Samples should be drawn over a period of several minutes
The viscosity is the most important property of the oil. Oil of correct viscosity will provide optimum film strength with minimum friction losses and leakage.
The viscosity of a L.O. may fall due to fuel dilution if running on gas oil, and rise if running on heavy f.o. Viscosity may also increase due to heavy soot loading if purifiers and filters not operating efficiently. Oil ageing caused by oxidation and thermal degradation increases viscosity.
A simple shipboard test is the Mobil flow stick where drops of new and used oil are placed in separate channels on an inclined 'stick'. The rate the oil flows down the stick is proportional to its viscosity.
Initially determined by 'crackle' test. The presence of Na and Mg in a 4:1 ratio indicates salt water contamination.
Limits are laid down by the manufacturer, but as a rule of thumb a limit of 0.2% should cause investigation into source and remedial action at 0.5%
Gross contamination can be remedied by placing the charge in a separate tank and heating to 70oC and circulating through purifier.
Indicates the presence of metal element composition and identifies additive and contaminant levels.
Zinc(Zn),Phosphorus(P)- are components of many oils such as diesel engine oils, hydraulic oils and gear oils, to enhance antiwear and over properties of the oil
Calcium(Ca)- primarily a component of engine oils, provides detergency,alkalinity and resistance to oxidation. Residual fuel engine oils have higher Ca levels
Nickel(Ni)- Bearings, Valves, gear plating, fuel derivative
Barium(Ba)- Multi purpose additive, declining importance
Magnessium(Mg)- as for Ca, may also be due to sea water contamination if found in Ratio of 1:4 of Na
Chromium(Cr)- Piston rings, hydraulic actuator cylinders
Manganese(Mn)- Cylinder wear
Aluminium(Al)- generally comes from wearing piston skirts, levels rise where new piston fitted to old engine. Typically 10ppm, but rises during bedding in. May also indicate the presence of catylytic fines in residual fuels.
Iron(Fe), Molybdenum(Mo), Chromium(Cr)- metals alloyed for piston ring etc, a rise in level may indicate ring pack/liner wear.
Copper(Cu), Lead(Pb) , Tin(Sn), Silver(Ag) - soft metals used in the overlay of shell bearings, and phosphor bronze gears.Note that high copper content can also occur when samples are drawn from copper pipes which have not been flushed as well as gear wear.
Silicon(Si)- Indicates poor air filtration, possible fuel derivative
Sulphur(S)- May indicate the presence of clay based (bentonite) greases
Sodium(Na)- With Mg indicates the presence of sw contamination, possible coolant system and fuel derivative
Vanadium(V)- Usually indicates the presence of fuel oil
Alkalinity and acidity
TBN-TOTAL BASE NUMBER- measure of alkaline additives available for the neutralisation of acids from combustion products and oxidation. Level governed by type of fuel.
For crosshead engines the TBN will tend to rise due to contamination by liner lubrication, it should not be allowed to raise more than twice that of the new charge.
As a guide, the TBN of fresh oil should be at least:
A simple shipboard go,no-go test is available for measuring the TBN, it involves the addition of an indicator and acid reagent to a 30ml sample. The quantify of acid reagent added is determined by the required level of TBN, for TBN2.5 0.5ml are added, for TBN20 4ml is added. After three minutes the colour is checked against a chart
TAN-TOTAL ACID NUMBER-measure of organic acid and strong acid content of oil. Where SAN is nil, the TAN represents the acidity in the oil due to both the acids in the additives and the oxidation of the hydrocarbons in the oil. The TAN of fresh oils varies with oil type, and tends to climb with age. A high TAN may indicate that an oil should be changed or freshened by top up. A high TAN may be accompanied with increased viscosity.
SAN-STRONG ACID NUMBER-indicates the prescience of strong, highly corrosive (inorganic) acids, usually formed from combustion products. If SAN is non zero the oil should be changed immediately
IC-INDEX OF COMBUSTION-measures soot loading of oil
MD-MERIT OF DISPERSANCY-Ability of an oil to disperse contaminants, such as soot, wear debris and water and thereby carry them away from the critical areas. Measured by oil blot test and should not be allowed to fall below 50
DP-DEMERIT POINTS- combination of IC and MD: the lower the value, the healthier is the condition of the oil
Shipboard water content test
Oil whirl is a problem associated with sleeve type bearings. This vibration occurs only in machines equipped with pressure lubricated sleeve bearings and operateing at relatively highspeed- normally above the second critical speed of the rotor. Oil whirl vibration is often quite severe, but easily recognised because the frequency is slightly less (5 - 8%) than one half the rpm of the shaft.
The mechanism of oil whirl can be explained by referring to the diagram below.
Under normal operation, the shaft of the machine will rise up the side of the bearing slightly. How far the shaft will rise depends on shaft RPM, rotor weight and oil pressure. The shaft, operating at an eccentric position from the bearing centre, draws oil into a wedge to produce a pressurised load bearing film. If the eccentricity of the shaft within the bearing is increased from this normal operating position, say be external shock or load transient, additional oil will immediately be pumped in to fill the space vacated by the shaft thus increaseing the oil film supporting the shaft. This oil film may drive the shaft in a whirling motion around the bearing. If damping in the design is sufficient then the system will return to its original position otherwise the whirling motion will continue.
Alternately, a lightly loaded bearing may rise under normal conditions reducing the
clearance above the bearing to a point where an oil wedge forms forcing the shaft
back down. In doing so the clearance is restored at the top of the bearing and the
oil wedge fails removing the downward pressure.
Steam Turbine bearings are susceptible to oil whirl as they tend to have larger than normal clearances to allow for high oil flows for cooling
Normally associated with poor bearing design, . Other problems are
Temporary remedies include
Improved Bearing Design
Shorter bearings increase the bearing load which can help prevent oil whirl
Lemon Pip Design
This is achieved by machining the two shells whilst shims are fitted between the faces
Tilting Pad-mitchell type
A thermocouple is fitted to the lowest pad
Loss of bearing material means reduction in load carrying capacity
Formed by boring non-concentric circular bearing surfaces in a bearing allowing the formation of three wedges whilst maintaining the correct bearing clearance