Tribology originated from word "Tribos" Greek word for rubbing.
In machinery friction is both wanted and unwanted. To elaborate it, friction is required for bolt to secure two components together, while still have capability to be dismantle in needs for maintenance.
In contrary, friction affect efficiency and losses in machinery performance, movement having friction, become heat as form of losses.
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| Tri similarly this 3 subject |
Friction cannot be zero except in vacuum condition or no air at all. Friction producing wear, it able to be minimize by good lubrication.
The surface smoothness also affects the friction coefficient. Which it is given condition of the component, how to minimize and control the losses is through lubrication, that is sometimes, when talk about tribology it is directly become lubrication subject.
LUBRICATION
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| Lubricant Classification |
1. OIL
1.1 MINERAL OIL
Paraffin based mineral oil have a good resistance but have high level of carbon and residual deposit. In contrary, Napthene is the opposite. The mix or blending between both may be implemented to reach win-win solution.
1.2 SYNTHETIC OIL
(*) PAO PolyAlphaOlefin or SHC Synthetic-HydroCarbon, the performance meets the paraffin-based lube and having better thermal stability, oxidation level, low residual or side product.
Synthetic oil and mineral oil shall never be mixed. I have firsthand experience on mixing these two types of oil in my motorcycle, the mixing resulting slippage in wet clutch and give high load to the engine performance due to thicken viscosity compared to if used mineral and synth oil individually.
Disadvantage oil syn oil as well known to all motorhead is higher cost compared to mineral based oil. The other downside is lower biodegradable capability, poor fire resistance, shrink seal and incompatible with several additive, although all this disadvantage is better in compared to mineral based oil.
(*) Diester or Ester based oil, a carboxylic acid and an alcohol react to form an ester and water. This lubrication is the top of the line, without any known disadvantage, it also being used to blend with PAO based oil to improve performance and reduce disadvantage of PAO.
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1.3 API Classification of Lubrication Group
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Ester have higher specific gravity, thus it will raise the weight of the lube by 30-50% different.
1.4 VISCOSITY
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Centistoke
cSt show how easy fluid able to flow, as comparison water at 20centigrade have 1cSt, as shown in table above, the lower the cSt define the easier it flows inside the engine crevice, tubing, hose, pipe. At high speed or high RPM, the lower cSt is on demand, to quickly flow and lubricate friction location.
Viscosity Comparison between well-known standards
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| viscosity chart |
The equivalency between ISO VG vs SAE J300 in term of viscosity able to determine using above chart
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The other important point to consider on lube oil selection is the ambient temperature where it will be use, since several oils only work on low temp, that have very low viscosity. Low viscosity tends to evaporate quick in high temperature. However, nowadays, oil have dual grade such as 0W-20, 10W-40, means W= winter, or cold, so in cold it follows low viscosity, while in hot condition the viscosity slightly higher to compensate the temperature load.
Viscosity is affected by overtemperature, it may change the viscosity value up to 20% increment.
1.5 ADDITIVE
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Basically, the additive improves given performance of the base lubricant itself, the lubricity, viscosity, pour point etc. The other additive tackle the durability of the lubricant, antioxidant, rust and anti-corrosion, anti-wear. In addition to that additive that helping compatibility of the blend between base lube and additive or another blend.
Typical compound use as additive are Zinc dithiophosphates, Silicone Polymer, Organic Polymer, Barium, Calcium, Magnesium, Molybdenum, etc.
OIL PROPERTIES
Common oil properties as shown in product oil datasheet shown below, taken from Shell Tellus ISO VG 46
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| Shell Tellus S2 V46 |
It gives general information of physical characteristics of the lube oil. However, equipment manufacturer usually required a lot more, such as:
- ASTM D130 Copper corrosion at 100centrigrade class (1b).
- ASTM D665 Rust Prevention Procedure Pass /not pass.
- ASTM D892 Foam limits in mm max, (50/0)
- ASTM D943 Oxidation Resistance hours to 2.0 Neutralization number, min (2000hours)
- ASTM D1401 Water Separability (Emulsion) (40-40-0 [30])
- ASTM D4628 Zinc Weight percent max (0.005 wt %)
- ASTM D4172 Wear Preventive character, scar dia 75degC, 1200RPM, 40kg 1hr, mm, max (0.9mm)
- ASTM D5182 FZG Visual Method, Failure Load Stage, min (7)
- ASTM D3427 Gas Bubble Separation 50degC minutes max (6)
- ASTM D2272 Rotating Pressure Vessel Oxidation Test (RPVOT)
- ASTM D4308 or D1169 Electrical conductivity pS/m at 0deg C or resistivity MOhmAm at 20degC
- ASTM D664 D974 Total Acid Number max (0.1 mg KOH/g)
- ASTM D6304 water content max (200ppmw, 0.1 wt %)
CASE SAMPLE
Gear oil application usually demands Extreme Pressure additive that suit the application, since gear tooth experiencing very high-pressure during contact.
Hydraulic oil HLP (High Viscosity Index) vs HVLP (High Viscosity Index and Low Temperature), although on paper HVLP have better temperature range (able to go until 120°C[HLP 90°C] and able to go as low as -35°C[HLP -20°C)) and claimed to be more stable on temperature change, some known experience when shear forces applied the viscosity may loss 20-30%. HVLP also have higher viscosity in compared to HLP.
The other specific concern shall be address on compatibility of hydraulic oil or oil in flexible hose application, since in several condition, it may soften and swollen a hydraulic hose, that the hose may made of natural rubber or equivalent material.
Water content may go up up to 10times (200 into 2000ppm) compared to new oil condition, it may still consider as acceptable. However, it indicates the water separation system on the filter may have problem, it shall be kept as low as reasonably possible.
OIL CLEANLINESS LEVEL
ISO 4406 specification for oil cleanliness based on 4/6/14micron size, why we care, imagine for equipment that have thousand liter of lubrication oil, it is not easy task to replace the engine oil, every maintenance team will try to push the lubrication life as much as possible to match the equipment downtime interval.
NAS 1638 method also commonly used to determine oil cleanliness that a legacy system of aerospace.
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| ISO vs NAS |
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| ISO vs NAS |
OIL SAMPLING
Oil sampling is common method to determine oil quality and condition, so additive may be added to address the oil quality, since after duration of running, taking all the heat and friction of the equipment, the lube getting contaminated either by grams or wear particle or oxidation from ambient if it is not enclosed enough.
There are onsite oil sampling and laboratory level oil sampling, which online give quick result but limited on the capability, while laboratory level able to do through test.
It is a good practice also to respect equipment principal recommendation about lube oil condition and quality. Some equipment even has design to do oil filter change during running without the need to switch off the equipment by utilizing dual (duplex [not material]) train oil filter system. Example for SOLAR Turbine common ISO 4406 oil cleanliness acceptance level is 16/14/12, as per table above, the 4micron particle only allowed to have 320-640 particle/ml, the 6micron 80-160particle/ml, the 14micron 20-40particle/ml. We will need calibrated particle counter tools to measure this particle on the lube oil that flowing.
If particle counter gives value above the acceptance, it may indication there are excessive rubbing, we shall do check in all possible location, such as bearing points, contact points, etc.
2. GREASE
Grease use where a lubricant oil cannot apply, the grease able to stick on component that oil not able to (vertical gravity position), use to seal component, or very high temperature. Grease extensively use in ball bearing or roller bearing application.
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| Composition of Grease |
Grease consists of base around 70% and variation of Thickener and Additive as needed, with additive max 10% and thickener max 20%.
2.1 ADDITIVES
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| Grease Additive |
Lithium additive is to most extensive use as the capability for general purpose and good in high temperature application, Calcium is prominently use for water resistant, Sodium is for rush inhibition, Barium is for water proofing. While non soap, majority is to boost capability of much higher temperature.
Do not mix, grease with different additive added, since the effect will be varied and produce unwanted effect to component.
2.2 NLGI
The rating of grease based on NLGI (National Lubrication Grease Institute)
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| NLGI Rating on grease |
The common NLGI rating is 1 or 2, use for ball bearing lubrication and act as seal or water repellant also anti rust on the bearing ball and bearing race.
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| Common selection of grease based on ambient temperature |
reference
Handbook of Lubrication and Tribology, 2nd ed, Robert W. Bruce, CRC press.
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