by Andre Verlinden, B.Sc.
wearcheck@innet.be
WearCheck Belgium

WBE-001

  Laboratories at Work: Used Oil Analysis at WearCheck, Belgium
BACKGROUND WearCheck laboratory was originally set up in 1974, under the name Spectrolab, and in 1987 became part of WearCheck , which is an international group of laboratories that concentrates on used oil analysis.

The throughput of the group in 1987 was 2,000 samples a year, which has grown to around 40,000 samples a year today. The Belgian company performs used oil analysis for Belgium, the Netherlands and Luxembourg, France, and other countries within an 8-hour transportation radius, so that results can be produced within a day.

WearCheck Belgium’s clients are mainly oil companies, as would be expected, who make up 64% of the work; the marine sector is 29%, and industry 7%. The distribution of lubricants and fluids that are analysed is: engine oils 62%, hydraulic oils 22%, transmission oils 12%, greases and emulsions 1%, and other oils 3%. The laboratory can handle over 90 different types and standards of test for the following materials: lubricating oils, water-emulsion and solutions, fuels, oils, and disel oil. Tests performed include property tests, such as aniline point, corrosiveness, and flashpoint, performance test, eg. Falex EP, and analytical determinations via spectrometry, microscopy, IR, and emissions ICP.

INTRODUCTION Used oil analysis is comparable to a medical analysis with a blood test. Like blood, lubricating oil contains a good deal of information about the envelope in which it circulates. Wear of metallic parts, for example, produces a lot of minute particles, which are carried by the lubricant. These small metal particles can give information about the machine elements that are wearing, and can be detected by various methods, for example, Atomic Emission Spectrometry. Determination of larger particles can be done using optical or electronic microscopy, or ferrography.

The acidity of an oil shows whether the oil is oxidised as a result of operation at high temperature, if there is a high percentage of moisture, or whether the oil has been in service for too long. The viscosity of the oil is a very important parameter and must be in conformity with the requirements of the machine builder. The alkalinity or the loss of alkalinity of the oil, proves that the oil is in contact with inorganic acids such as sulphuric acid or nitric acid.

CONTAMINATION OF OIL The causes of oil contamination are many, and can be classified according to source. Thus there is contamination coming from outside the system - dust (silica); liquids (mixture with other oils, water, other contaminated oil). The second is in open systems - chains, cables, gears in contact with dust, water, and so on. The third is in closed systems.

Impurities can also come from the settings and processes in which the lubricants work, e.g. manufacturing can produce welding debris, assembling involves dust, perhaps also silicones or polishing powder, while maintenance can introduce impurities via dirty rags or deteriorated joints, and lubrication systems may need or involve aspiration, or open tanks.

The lubricant itself can produce or contain contaminants - wear, sludge (deterioration of the oil), soot, acids (oxidation of the oil, sulphur from fuel), temperature changes or extremes, fuel, anti-freeze, deterioration of packings and seals (e.g. deteriorating through the action of synthetic oils or brake fluids).

The type of contamination can vary according to the source. Thus dust, for example, can arise within a shipyard as sand, i.e. Si, Al. In metallurgy, one can find the oxides or iron. On a ship, there are problems with salt water. Industrial and automotive settings are filled with potential contaminants, for example chemical products, or coal powder. Liquid contaminants can include water, acids, solvents, anti-freeze.

WEAR Wear means the loss of solid material due to the effects of friction of contacting surfaces. It is generally harmful, although in some cases it can also be beneficial, for instance during the running-in of an engine. The deterioration of the surfaces in an engine is generally due to isolated or simultaneous mechanisms, among which we can distinguish the following.

Adhesive wear
This occurs as a result of metal-to-metal contact, due to overheating or insufficient lubrication. This in turn causes the formation of microwelds, with often a subsequent deposition of soft metal onto heavy metal (e.g., aluminum onto iron, lead onto steel). Consequently, there is a shearing of the junctions and a transfer of metal particles.

Figure 1 Abrasive wear

There are two types of adhesive wear - heavy, liberating relatively large metal particles (50 to 200 microns), called ‘scuffing’, and eventually leads to a failure of the engine; and moderate - the formation of very small metal oxides which is termed ‘soft’ or ‘normal adhesive’ wear (see Figure 1). Adhesive wear can be avoided by the use of an appropriate lubricant containing extreme pressure (EP) additives, and the choice of the correct viscosity oil.

Abrasive wear
This form of wear results from the grooving of a surface by hard asperities or by particles of rust or dust which have entered the oil. When these particles are very small, the phenomenon is known as ‘abrasive erosion’ (which is especially the case in hydraulic systems). Abrasive wear can be avoided by eliminating potentially abrasive particles through filtration.

Corrosive wear
This is chemical or galvanic attack, followed by the removal of the reaction products (chemical complexes) by mechanical action (friction). It can be avoided by the use of effective materials, also by the use of neutralizing additives in the oil. It may also be minimized by changing the oil in time.

Figure 2 Fretting

Wear by fatigue
This means the removal of spalled away particles by fatigue resulting from contact, aided by vibration, high pressure, high temperature and other aggressive conditions. This type of wear may be reduced by re-equilibration of the system.

Contact corrosion (fretting corrosion)
Corrosion due to contact means the removal of material between two surfaces which are in almost static contact but subject to mechanical vibration and oscillation. Consequently, there is oxidation of certain particles. Thus, for iron materials, there is an accumulation of ‘red powder’. An example of this is the bearings of cars transported by railway (see Figure 2).

Erosion by cavitation
The formation of cavities by entrainment of air of gas bubbles present in the fluid in movement is a destructive phenomenon, which can provoke the removal of material particles (see Figure 3).

Figure 3 Cavitation

Wear of electrical origin
This refers to the erosion by sparks, produced by inadequate electrical insulation in motors of alternators.

Consequences for the oil Wear particles
Some metal wear particles, such as iron and copper, play the role of catalysts in the process of oil oxidation.

Acids
Too high an acidity of the oil can cause corrosive wear. These acids can be neutralized by the presence of an alkaline additive in the oil. The presence of acids will be measured by the Total Base Number, TBN.

Water
This is the worst enemy of the oil and the machine. Too high a water content can influence the viscosity of the oil by formation of an emulsion, and/or by reacting with certain additives. Water can restrict the functioning of inhibitors, and can also hydrolze additives containing zinc.

Temperature
High temperature and the presence of oxygen or water accelerate the oxidation of oil by the formation of acids and increasing the oil viscosity, as well as by provoking metal corrosion. For most mineral oils, oxidation starts at about 80°C, and doubles with every increase of 10°C. To avoid this problem of oxidation, one can use lubricants containing anti-oxidant additives and/or detergent or dispersant additives, or synthetic oils, which have better oxidation resistance at high temperatures, e.g., polyalphaolefin oils.

Consequences for the engine Briefly, the implications for the engine of contamination of the lubricant are wear, and vibration, leading to a loss of efficiency, which in turn produces greater energy consumption, a loss of productivity, higher maintenance costs, a reduction in the machine’s life, a loss of oil and, ultimately, fundamental problems.
ROUTINE OIL ANALYSIS Several methods are used to analyze oil condition and contamination. These include spectromtery, viscosity analysis, the blotter test, dilution analysis, water detection, Total Acid Number assessment, Total Base Number assessment, particle counting, microscopy, and sediment analysis.
Spectrometry A spectrometer is an instrument with which one can measure the quantities and types of metallic elements in a sample of oil. The operating principle is as follows. A diluted oil sample is pulverised by an inert gas to form an aerosol, which is magnetically induced to form a plasma at a temperature of about 9000°C. As a result of this high temperature the metal ions take on energy, and release new energy in the form of photons. In this way, a spectrum with different wavelengths is created for each metallic element. The intensities of the emissions are measurable for each such element by virtue of its very specific wavelength, calculated in number of ppm (parts per million). An ICP spectrometer can detect the very small metal particles in suspension in the oil, i.e. with a size between 0 and 3 microns.

Those small particles are a good indication of general wear, except in cases of sudden metallic rupture, where there will be relatively more large particles liberated (50 microns and more). The human eye can detect particles of a size starting from 50 microns, which allows them to be visualized using more conventional means. Thus, complementary analysis of such larger particles can be done by spectrometry (after acid attack), by ferrography (or related systems) or by optical or electronic microscopy.

Viscosity Engine oils. In the early days of the IC engine there were only monograde oils (e.g., SAE 20, SAE 30, SAE 50). By putting an additive into these oils, called a VI improver, multigrade oils were created. The VI (viscosity index) improver is a flexible molecule, rolled up like a ball at low temperature and stretched out like a string at high temperatures. This allows the oil to remain viscous at high temperatures. One can recognize multigrade oils as being represented by two figures. The first figure, followed by the suffix ‘W’, stands for the viscosity calss at low temperature (W = winter). The second figure is the SAE class at working temperature. Thus, for example, ‘SAE 20W-50’ means that the viscosity of the oil at low temperature corresponds with a SAE 20W, and the oil viscosity at 100°C corresponds with a SAE 50. The table below gives some data on viscosities.

SAE viscosity grade crankcase oils

Viscosity Cp

3250

3500

3500

3500

4500

6000

-

-

-

-

at temp. °C, max.

-30

-25

-20

-15

-10

-5

-

-

-

-

Borderline pumping temp °C max.

-35

-30

-25

-20

-15

-10

-

-

-

-

Viscosity, cSt at 100°C

3.8

3.8

4.1

5.6

5.6

9.3

5.6

9.3

12.5

16.3

cP = centipoise
cSt = centistokes

                   

  The viscosity of used engine oil is mostly measured at 100°C, and can drop for reasons of fuel dilution, and/or shearing of the VI improver. Viscosity can increase as a result of heavy contamination of the oil by soots, and/or oxidation of the oil.

Industrial oils
The viscosity of industrial oils, by contrast, is mostly measured at 40°C, and must correspond with the ISO table below, i.e., the viscosity of an ISO class oil must be within the minimum and maximum for that class. (Moves are in hand to make the viscosity class statement contain more data, to reflect changes in the oil in use.)

  Min.
visc.
Max.
visc.
ISO 15 13.5 16.5
ISO 22 19.8 24.2
ISO 32 28.8 35.2
ISO 46 41.4 50.6
ISO 68 61.2 74.8
ISO 100 90.0 110.0
ISO 150 135.0 165.0

  The viscosity can be decreased by adding a more fluid oil, or as a result of high water content, of by shearing of the VI-improver. The viscosity can be increased by adding a more viscous oil, and by oil oxidation (e.g. as a result of overheating).
Blotter test This quick and cheap test, which consists in blotting a drop of used engine oil on a filter paper, gives good indications about the dispersancy of the oil, the soot content and the extent of oil oxidation. The spots can also be evaluated by photometer, where a camera scans the surface of the spot and calculates in a few seconds the opacity or the soot content and the percentage of dispersancy. For an average used engine oil, the soot content must be lower than 1 per cent, and the dispersancy higher than 80 per cent.
Dilution Dilution of a use engine oil can be measured precisely by gas chromatography (GC) or by Fourier Transform Infrared spectroscopy (FTIR). More common is the use of the SETA-FLASH tester, where the flash point of oil is tested by a certain temperature. When a flashpoint is detected, the dilution is heavy (more than 4%), when not, the dilution is acceptable (less than 4%).

It is evident that heavy dilution of the oil is unfavourable for the engine, since it involves a lower viscosity and reduces the resistance of the oil film. The principal causes of dilution are a defective fuel injection system, a defective air inlet (obstructed air filter), incomplete combustion due to too low a working temperature, and badly regulated valves, or insufficient compression.

Water detection The water-content of the oil is usually measured by the Aquatest or a Karl Fisher apparatus. The possible causes of water introduction include (a) condensation, due to too low a working temperature, defective crankcase ventilation, ‘stop and go’ in-service usage, and obstruction of the exhaust system; or (b) infiltration, due to leakage at the cylinder head gasket, or damage of the engine block.

Cooling water contains most often an anti-freeze based on glycol. Therefore a glycol test should be performed when water infiltration is suspected. The inhibitor in the anti-freeze agent is usually a sodium borate type.

TAN (Total Acid Number) The acidity of the oil is measured by titration through a base, and expressed in mg KOH/g. The figure below shows this graphically, showing the evolution of TAN as a function of time.

TBN (Total Base Number) The alkalinity of an oil is measured by titration through an acid, and expressed in mg KOH/g. The comparison between the TBN volume of the fresh oil and that of the used oil allows the determination to be made of whether the used oil is still capable of neutralizing acid residues. These acids are produced by combustion (sulphur in fuel) and oxidation of the oil and oil additives. When the oil is in service too long, the TBN will drop significantly.

Too low a TBN volume can be due to: heavy oxidation of the oil, when the oil has been in service for too long, of the oil level was insufficient, or due to a defective cooling system, producing overheating; use of a fuel containing a high sulphur content; use of an inappropriate lubricant; or contamination of the oil by fuel or water.

Particle couting This is an especially useful test for a hydraulic system with high sensitivity (e.g., servo-valves). Insuch a text, a certain quantity of hydraulic oil flows through a sensor, where all the insoluble material in the oil is detected and counted using the principle of light absorption. The particles counted are classified cumulatively:

>5µ;>15µ;>25µ;>50µ;>100µ.

Or differentially:

>5-15µ;>15-25µ;>25-50µ;>50-100µ;>100µ.

The results of particle counting can be expressed according to either ISO 4406 or NAS 1638. According to ISO 4406, the results are expressed cumulatively, and the ISO classification is deduced from the two first classes, >5µ and >15µ, following the ISO table.

ISO 4406

Particles/100 ml

ISO number

from

to

 

8M

16M

24

4M

8M

23

2M

4M

22

     

1M

2M

21

500k

1M

20

250k

500k

19

     

130k

250k

18

64k

130k

17

32k

64k

16

     

from

to

 

16k

32k

15

8k

16k

14

4k

8k

13

     

2k

4k

12

1k

2k

11

500

1k

10

     

250

500

9

130

250

8

64

130

7

     

32

64

6

16

32

5

8

16

4

     

4

8

3

2

4

2

1

2

1

According to NAS 1638, the results are expressed differentially, in five classes. In each class one can get a NAS quotation, and the NAS code is the figure given to the first class.

NAS 1638

size
range,
microns

Classes

 

00

0

1

2

3

4

5

6

7

8

9

10

11

12

5-14

125

250

500

1,000

2,000

4,000

8,000

16,000

32,000

64,000

128,000

256,000

512,000

1,024,000

15-24

22

44

89

178

256

712

1,425

2,850

5,700

11,400

22,800

45,600

91,200

182,400

25-50

4

8

16

32

63

126

253

506

1,012

2,025

4,050

8,100

16,200

32,400

50-100

1

2

3

6

11

22

45

90

180

360

720

1,440

2,880

5,760

100+

 

-

1

1

2

4

8

16

32

64

128

256

512

1,024

  For example, after a particle count produces the following figures
   
> 5 µ 450,000
> 15 µ 55,000
> 25 µ 2,400
> 50 µ 300
> 100 µ 15

this is differentially written as

5-15 µ 395,000
15-25 µ 52,600
25-50 µ 2,100
50-100 µ 285
> 100 µ 15

According to ISO 4406, the classification is in this case 19/16. According to NAS 1638 the NAS class is 11.

Microscopy After filtration of a certain amount of oil through a cellulose filter (of 0.8µ), the filter is examined under an optical microscope (magnitude 100x, 200x), and one is able to distinguish:
  • white or brilliant metal particles (demonstrating recent wear).
  • black metal particles (already oxidized)
  • rust particles
  • silt (i.e., very small particles below 5µ, responsible for erosive wear)
  • silica (sand, dust)
  • polymers (from oil additives)
  • welds
  • paint flakes
  • other impurities (fibre, plastics, and so on)
Sediment A sample of oil is filtered through a 0.8µ filter. After drying and weighing the filter, the total amount of insolubles in the oil greater than 0.8µ is computed and expressed in mg/L oil.
CONCLUSION Oil analysis has proven to be very helpful to maintenance engineers. Besides analyzing the condition of the lubricant itself, oil analysis also tells the engineer a lot about the condition of the equipment, and so allows preventative maintenance. Based on the premise that it is better to prevent that to cure, oil condition monitoring should become a valuable aspect of modern engineering and maintenance.

APPENDIX:
INTERPRETATION OF AN OIL ANALYSIS REPORT

  Each test method has limits, within which the oil should be. The following rubrics give some examples of these.
Engine oils Spectrometry
It is not possible to determine to which API classification an engine oil belongs, only to look at its additive level.

Wear metals
The determination of the wear level is related to several parameters, e.g.

  • make and type of engine
  • running hours or kilometrage of the oil
  • running hours or kilometrage of the engine
  • oil addition (top-up)
  • volume of the sump
  • oil type
  • individual particularities: running-in period, mechanical intervention
  • place where the sample has been taken
 

Wear elements, ppm max.

  Si Fe Cr Mo Al CU Pb Sn
Gasoline engines 20 100 10 15 10 20 - 10
Diesel engines 20 100 10 15 10 20 40 20

  Viscosity at 100°C
The viscosity is estimated as a function of the SAE limits.

SAE 30 SAE 40 SAE 50
9.3-12.5 12.5-16.3 16.3-20.5

Water content
Limit = 0.2%

TBN
50% of decrease is tolerated (in comparison with the TBN of the fresh oil).

Dilution
Diesel fuel (a) on the road - 0 to 4% = normal, >4% = problematic (b) stop-go service >6% = problematic

Gasoline (a) on the road - 0 to 4% = normal, >4% = problematic (b) stop-go service >8% = dangerous

Industrial oils Spectrometry
The following table is a guide to the number of wear elements per type of metal found in certain types of industrial equipment.
 

Wear elements, ppm max.

  Si Fe Cr Mo Al Cu Pb Sn
Compressors                
air 15 20 10 - 10 20 10 10
cooled, screw 10 20 10 - 10 20 10 10
cooled, piston 10 20 10 - 10 20 10 10
Automatic gearbox 15 20 10 - 20 20 10 10
Mech. transmission 30 100 10 - 10 300 50 10
Turbine 10 10 10 - 10 10 10 10
Hydraulic systems                
normal 15 20 10 - 10 20 10 10
HVI 15 20 10 - 10 20 10 10
Heat transfer system 15 50 10 - 10 10 10 10
Reduction units, gearboxes                
industrial 100 10 - 10 20 20 10  
automotive differential 100 500 10 - 10 150 1000 10

  Viscosity at 40°C
If possible, the viscosity at 40°C is estimated as a function of the ISO table given above.

TAN
1 mg KOH/g for a non-additivated oil, 2 mg KOH/g for an additivated oil.

Water content

System Limit
Transmission 0.2%
Hydraulic 0.04%
Heat transfer oil 0.1%

Particle counting
The limits for particle counting are generally given by the manufacturers of the equipment. The following limits can apply.

 

Normal hydraulic systems

Servo valves

 

up to 70 bar

71-210 Bar

>210 Bar

 

ISO Code

17/14

16/13

15/11

14/10

  Sediment
Equipment Filtration
mg/L max.
   
Compressors:  

cooled, screw

500

cooled, piston

500
   
Autom. Gearbox 250
Mech. transmission 250
   
Turbine 100
   
Hydraulic systems  

normal

250

HVI

250
   
Heat transfer system 500
   
Reduction, gear-boxes  
industrial 1000
automotive differential 1000

[ Top ]

Copyright ©1997 WearCheck Canada Inc. All rights reserved. Terms of use.