Two of the most common questions asked of analysts are: “What tests should I run?” and “How do I interpret the results?” The first question is easier to answer than the second.
Sample Classification
Incoming samples may be classified in a variety of general categories. Common test profiles include tests best suited to the component type. Common component types include the following:
- Engines
- Drivetrains (gear systems such as manual gearboxes, differentials and industrial gearboxes)
- Transmissions (automatic)
- Hydraulics
- Compressors and turbines
There are also other smaller special classes, such as aircraft engines and refrigeration compressors.
At Wearcheck, every sample gets four basic tests: ICP spectroscopy, particle quantification, viscosity at 40°C and water screening.
ICP Spectroscopy
There are approximately 30 different types of spectroscopy. One type, inductively coupled plasma (ICP) spectroscopy, measures light in the visible and ultraviolet regions of the spectrum. It is an atomic emission (AE) procedure whereby the diluted oil is passed through an argon gas plasma.
The plasma is maintained at a temperature of approximately 8,000°C. In the upper region of the plasma, acquired energy is released as a result of the electronic transitions, and characteristic light emissions occur.
Different elements produce different frequencies or colors. The intensity of the light emitted is directly proportional to the concentration of the element. ICP spectroscopy is used to measure the concentration of different elements in the oil.
The elements are divided into three broad categories on the reports:
- wear metals, such as iron from gears
- contaminants, such as lithium, which indicate the presence of grease
- oil additives, like phosphorus, which is found in extreme pressure and antiwear additives
Some elements can belong to more than one category. For example, silicon can be a component of wear debris (piston crown material), of the additive package (antifoaming agents), and of contaminants (dirt). Only by looking at a complete set of results is it possible to predict the source of the particular element.
Limitations
ICP spectroscopy is perhaps the most important and useful test in used-oil analysis, but is does have limitations. A key drawback is the size limit of the particles it can vaporize. It does not detect particles beyond the five- to eight-micron range.
While this limit does not affect the detection of most wear situations, there are times when it could be a problem. For instance, when a component fails due to fatigue, the wear particles generated tend to be larger than normal (this process is called spalling).
ICP does not detect these larger particles, so upon examining the trend, the iron level might seem to be dropping, even though the component is actually in trouble. Because of this limitation, other tests should be employed to provide an effective monitoring solution.
It is not always possible to use ICP analysis to measure the additive depletion of an oil. Take for example the detergent additive in an engine oil. This would reflect in the calcium value. If one measured the calcium level of both a new and a used oil, they would be similar, even though the detergent has been depleted in the used oil.
This is because the amount of actual calcium in the oil has not changed. What has changed is the form, or compound, in which the calcium exists. Before being “used”, the calcium was present in a compound with detergent properties. After being used, the calcium is still present, but now in an inactive form. Sometimes the depleted additive remnants settle out, and then ICP is useful, but apply judgment and experience when trending additive depletion.
There are exceptions to additive depletion-measuring limitations of ICP. Most notable is the case of borate-EP-containing oil contaminated with water. In this case, the extreme pressure additive containing the boron settles out of suspension and forms a sludge at the bottom of the sump.
If this precipitate is not captured in the sample, the boron level will read much lower than normal, indicating the oil is not fit for further use due to extreme pressure additive depletion. The converse, however, is still not necessarily true: if the boron level is correct, the oil may not necessarily still be fit for use.
| Test Class |
Silicon Limit
[ppm] |
| Engine |
25
|
| Drivetrain |
100
|
| Hydraulic / compressor / turbine |
35 to 45
|
| Automatic transmission |
35 to 45
|
In certain cases, Wearcheck uses limits for contaminants. In the case of dirt, the limits in Table 1 are typically observed. Silicon is found in dirt, as well as grease, oil additives and silicone sealant. It is possible to see engines and hydraulic systems with silicon readings in excess of 100 ppm, yet these are still considered normal.
| Element |
Symbol
|
Found in |
| Iron |
Fe
|
Gears, roller bearings, cylinder/liners, shafts |
| Chromium |
Cr
|
Roller Bearings, piston rings |
| Nickel |
Ni
|
Roller Bearings, camshafts and followers, thrust washers, valve stems, valve guides |
| Molybdenum |
Mo
|
Piston rings, additive, solid additive (Mo-di) |
| Aluminium |
Al
|
Piston, journal bearings, dirt |
| Copper |
Cu
|
Brass/bronze bushes, gears, thrust washers, oil cooler cores, internal collant leaks |
| Tin |
Sn
|
Bronze bushes, washers and gears |
| Lead |
Pb
|
Journal bearings, grease, petrol contamination |
| Silver |
Ag
|
Silver solder, journal bearings (seldom) |
| Silicon |
Si
|
Dirt, grease, additive |
| Sodium |
Na
|
Internal coolant leaks, additive, sea water contamination |
| Lithium |
Li
|
Grease |
| Magnesium |
Mg
|
Additive, sea water contamination |
| Zinc |
Zn
|
Additive (antiwear) |
| Phosphorus |
P
|
Additive (antiwear, extreme pressure) |
| Boron |
B
|
Additive, internal coolant leak, brake fluid contamination |
| Sulfer |
S
|
Lubricant base stock, additive |
Table 2 lists the most commonly occurring elements and their probable sources.
Knowing where the elements may be found is useful, but it is more important to be able to determine the actual source as accurately as possible. Table 3 shows a few cases of wear and contamination and how they typically appear.
At this stage the importance of submitting sample information, particularly service meter reading, overhaul/replacement information and period oil in use must be strongly emphasized. The service meter reading and overhaul/replacement information tells the diagnostician what sort of wear rates to expect.
A new component can be expected to wear faster than a component in the middle of its life span because it “seats” into other wear surfaces. A component with high hours of service can be watched for increased wear as fatigue sets in.
| Situation | Results |
| Dirt entry |
Si and Al present, usually between 2:1 and 10:1. Watch the increase in the trend. Often accompanied by associated wear when present over acceptable limits. |
| Piston torching |
Al and Siratio is 2:1. The Si originates from silicon carbide in the piston crown used to reduce the co-efficient of expansion. Seldom seen, as failure is usually rapid, and statistically there is little chance of getting a sample while occurring. |
| High Fe (alone) |
Because iron is the most used construction material, sources are often varied. Consider valve gear and oil pump wear. Rust formation also produces high Fe. |
| High Si (alone) |
Silicon by itself comes from a few main sources - anti-foaming agent additive, grease and silicon sealant. Usually seen in new/recently overhauled components. Usually can be ignored. |
|
Top-end-wear (engines) |
Characterized by increased levels of Fe (cylinder liner), Al (pistons), and Cr (rings). The presence of Ni usually indicates camshaft/cam follower wear. |
| Bottom end wear |
Characterized by increased levels of Fe (crankshaft) and Pb, Cu, Sn (white metal bearings and bronze bushes). This wear is often precipitated by reduced base number (BN) or over-cooling as bearings become subject to corrosion from combustion by-products (acids). Fuel dilution often causes this too, but effects may be masked as diesel dilutes the oil and the wear readings. |
|
Overheating (some cases) in engines |
Increased additive levels (Mg, Ca, Zn, P, S) and viscosity. When light ends in the oil vaporize, the oil level decreases. Topping up increases the additive concentrations, as the additives themselves do not evaporate. Oxidation often not evident, as topping up replenishes antioxidants and boosts the BN. Often accompanied by Pb, Sn, and Cu as bearing wear can result from this situation. |
| Bronze bushing wear | Increased Cu and Sn levels. Cu:Sn ration usually approximately 20:1. |
|
Bronze gear/thrust washer wear |
Increased Cu and Sn levels. Cu:Sn ration usually approximately 20:1. |
|
Internal coolant leaks |
Increased Na, B, Cu, Si, Al, and Fe. Not all elements may be present. Often accompanied by increased Pb, Cu, and Sn as white-metal bearing wear often accompanies this. Water usually not evident, as it tends to boil off at normal operating temperatures. |
| Roller-bearing wear |
Increased levels of Fe, Cr, and Ni, all components of race and roller materials. Increased Cu might result if brass/bronze cages are employed. |
| Hydraulic cylinder wear | Increased levels of Fe, Cr, and Ni. |
The hours of use on the oil strongly influences what can be considered normal. An engine with 100 ppm Fe at 250 hours is likely to be healthy. The same reading at 10 hours probably indicates a serious problem. The chances of inaccurate diagnosis, particularly in the latter situation, increase without this information.
Furthermore, indicating a usage time value in months, especially for automotive components, is not particularly helpful - the vehicle could have been parked for that time or it could have had long daily commutes. For components without service meter readings, such as industrial gearboxes, an educated guess in months or years is better than nothing.
Particle Quantification Index (PQ or PQI)
In this test, each sample is passed over a sensor which measures the bulk magnetic content of the oil. Because iron is the major wear element in virtually all components, the PQI is really a measure of how much iron is present (ferrous density) in the sample, the amounts of other magnetic elements being negligible.
The PQI does not mention size - the bigger the number, the more iron. What the PQI is communicating could be interpreted as a concept of mass per capacity or, in metric terms, something like grams iron per liter of oil.
The PQI, unlike the ICP, does not have particle-size limitations. As such, it does not indicate the sizes of the particle. Remember the example of a ball bearing in a sample: a solid ball bearing and the same one ground to powder should give the same PQI.
Used in conjunction with the ICP iron reading, the PQI is invaluable in estimating the distribution of wear particle sizes. Table 4 shows this relationship. High, medium and low are relative concepts and should be interpreted in the context of other samples in the component's history.
|
Situation
|
Icp Iron (Fe)
[ppm] |
PQI
|
Inference | Wear profile |
|
1
|
Low
|
Low
|
Few wear particles | Normal wear profile |
|
2
|
High
|
Low
to medium |
Lots of small particles, few or no large onews |
Accelerated wear (type of operation) Wet-brake systems (normal or abnormal) Dirt entry (abnormal) |
|
3
|
Low
|
High
|
Few small particles, many large ones |
Fatigue |
|
4
|
High
|
High
|
Lots of particles of all different sizes |
Serious wear likely, catastrophic failure possible |
Situation 2 has various possible origins. It can be typical of a component experiencing accelerated but not abnormal wear; that is, the component is working harder than normal. This is illustrated by comparing the wear readings of differentials of identical trucks in different operations, for example, short and long-haul operations.
Differences in what can be considered normal wear for each situation can be up to two orders of magnitude. This situation is also typical of normal brake wear in immersed-brake systems (such as most front-end loaders). Dirt entry causing abnormal wear also generates this Fe-PQI relationship.
Viscosity
There are two types of viscosity: kinematic and dynamic (or absolute). Oil analysis concerns itself almost exclusively with the former. Kinematic viscosity is measured in centistokes (cSt) and is a measure of a fluid’s resistance to flow or, more simply, its thickness. It must always be quoted at a stated temperature because a fluid’s viscosity will change with temperature. At 40°C, a 200 cSt oil is thicker than a 100 cSt one.
Wearcheck carries out a viscosity measurement at 40°C on every sample. A viscosity measurement at 100°C can also be carried out on machines which operate at high temperatures, such as engines and some compressors.
The process is simple: a glass tube (the ends of which are kept open to the air) is immersed vertically in a bath at the required temperature; oil is introduced at the top and, as it flows down, it is brought up to the correct temperature. Its flow is then timed between two marks. The time measurement is converted to a viscosity.
There is another property of an oil related to its viscosity. This is the viscosity index (VI). It is known that as the temperature of an oil increases, its viscosity decreases. The VI of an oil indicates how much it is going to thin out.
A monograde oil has a lower VI than a multigrade, meaning the monograde tends to thin out more than the multigrade with increasing temperature. For example, a typical SAE 30 monograde and a typical SAE 15W40 multigrade can both have a viscosity at 40°C of 100 cSt. But at 100°C they have respective viscosities of 10 and 15.
To determine the VI of an oil, measure its viscosity at both 40°C and 100°C.
Table 5 shows some of the causes of changes in viscosity. It is important to note that concurrent conditions can mask the effects of changes of viscosity. Fuel dilution accompanied by overheating could leave the viscosity reading looking normal.
| Component |
Viscosity change
|
Cause |
|
Engine
|
Increase
|
Overheating (may or may not be accompanied by oxidation) |
| Sludging (poor combustion or overextended use) | ||
| Fuel dilution (marine engines fired with heavy fuel oil) | ||
| Severe water contamination | ||
|
Decrease
|
Fuel dilution | |
|
Breakdown of VI improver additive in multigrade oils with extended use |
||
| Overheating | ||
|
Other components
|
Increase
|
Grease contamination |
| Severe water contamination | ||
| General breakdown of the oil | ||
| Mixture of oils | ||
|
Decrease
|
Contamination by a volatile substance | |
|
Breakdown of VI improver additive (particularly noticeable in transmissions filled with a multigrade) |
||
| General breakdown of the oil |
Once again, the importance of submitting accurate information should be emphasized. Perfectly good oil may be recommended for change-out due to discrepancies between the oil grade in the machine and the oil grade identified in the paperwork.
Furthermore, an engine described as having an SAE 30 or a SAE 15W40, but actually running an SAE 40 or SAE 20W50, may go unchecked for fuel dilution, because the decreased viscosity resulting from fuel dilution may compare favorably with the normal viscosity of the described oil.
Water
Water is one of the more common contaminants. If it can be introduced via internal coolant leaks, high-pressure hose cleaning procedures or condensation. Water has several negative effects on the performance of oil, including:
- Formation of rust, which in turn contaminates the oil.
- Increased wear rate from decreased lost load-bearing capacity.
- Creation of weak and strong acids from chemical reactions between additives and base oils.
- Biological formation and growth in low-temperature applications.
- Loss of critical additives and additive function.
It is important that water contamination be kept to the absolute minimum. Seals and breathers should be regularly inspected and maintained. Pressurized cooling systems need to be pressure-tested regularly to confirm their integrity.
Engine samples are screened for water using Fourier transform infrared (FTIR) analysis and every other sample is screened for water using the crackle test. This test involves putting a drop of oil onto a steel surface which is maintained between the boiling points of water and oil.
If the oil drop contains water it spits and crackles, hence its name. The crackle test can detect water contamination of less than 0.1 percent, or 1,000 ppm. If a sample fails the crackle test, the actual water content is measured. Once again, tentative limits for water contamination are used (Table 6), although these will vary in situations of abnormal or unusual usage.
|
Component
|
Limit [%]
|
|
Engine
|
0.0
|
|
Drivetrain
|
1.0
|
|
Transmission
|
0.5
|
|
Hydraulics
|
0.5
|
|
Compressors
|
Variable according to type
|
Water should not be relied upon as an indication of an internal coolant leak, particularly in engines. It tends to evaporate at normal operating temperatures.
Read more on oil analysis best practices:
How to Select the Right Oil Analysis Lab
Statistical Techniques to Simplify Oil Analysis Data
How to Interpret Oil Analysis Reports
Editor’s Note:
This article was authored by Ashley Mayer but was originally published as Technical Bulletin Issue 19 by Wearcheck Africa, a member of the Set Point group.
