Sources of Wear Metals in Lube Oil Analysis

Sources of Wear Metals in Lube Oil Analysis

Iron (Fe)

Iron is the most common of the wear metals, present in some form in virtually all equipment. Its widespread presence means that there are many sources of wear particles. Because of this knowledge, the metallurgy of the component allows the analyst to distinguish the source of the wear debris , e.g. cast Iron bolts Vs Stainless steel lube oil piping.

Equipment	Wear Metal Sources
Engines	Most common of the wear metals. Engines : cylinder liners, piston Ring, valve train, crankshaft ,rocker arms, spring gears , lock washers, nuts  , pins, connecting rods engine blocks oil pump.
Bearings	Rolling element Bearings; rollers (tungsten alloyed steel), raceways and cages, journal bearings journal shaft.
Gears	Bull gears, Pinions , case hardened teeth  locking pins
Transmission	Gears, bearings, brake bands, clutch, shift spools, pumps power take off
Hydraulic system	Pump, motor, vanes, pump housing ,cylinder bores and rods servo valves, piston
Compressors	Rotary Screw lobes, vanes connecting rods, rocker arm bearings, cylinders, housing, shaft, roller bearings.
Turbines	Reduction  gear, shaft bearings piping ,case.

COPPER

Widely used as an alloying element , copper is prized because of its material properties – very ductile, and excellent thermal and electrical conductivity. It is heavy used in bearing systems, as well as heat exchangers.

Equipment	Wear Metal Source
Engines	Valve train bushing , wrist pin bushings , cam bushing , oil cooler core, thrust washers, governor, connecting rods bearings, valve gear, train thrust buttons
Bearing	Rolling  element Bearings : alloyed element in cages, journal bearings Journal bearing pads, slinger rings , locking key
Gears	Bushing , thrust washers
Transmission	Clutches , steering discs, bearing
Hydraulic  system	Pump thrust plate, bushing , cylinder gland guides, pump pistons , oil coolers
Heat Exchangers	Cooler tubes , baffles , plates
Compressor	Bearings , cylinder guides , wear plates, thrust washers , bearings , oil pump , oil coolers thermostats , separator filters
Turbines	Bearing , Piping , cooler

TIN (Sn)

Tin is used an alloying element with copper and lead for sacrificial bearing liners.

Equipment	Wear metal Source
Engines	Valve train bushing , wrist pin bushing , cam bushing , oil cooler core, thrust washers , governor , connecting rods bearings , valve gear train thrust buttons
Bearing	Rolling element bearing : alloyed element in cages , Journal Bearings : Journal bearing pads
Gears	Bushing
Transmissions	Clutches, steering , disc, bearing
Hydraulic System	Pump thrust plates, bushings, can be residue from catalyst in some oils
Compressive	Bearings, Separator filter
Turbines	Bearing , Piping , coolers

Aluminium (Al)

Aluminium is valued in equipment because of it high strength to weight ration, and excellent corrosion resistance, alloyed with other elements improves its wear and temperature resistance. It is widely specified for equipment manufacturing now a day.

Equipment	Wear metal Sources
Engines	Engine blocks, pistons, blowers, oil pump bushings, Cabushing , Oil coolers
Bearing	Rolling element bearing : alloyed element in cages, locking key
Gears	Bushing , thrust washers, grease contamination
Transmission	Bushings, clutches
Hydraulic system	Cylinder glands pump, motor pistons , oil coolers, aluminium complex grease contaminant.
Heat exchanges	Cooler tubes, baffles, plate
Compressor	Housing ,bearing, cylinder  guides, wear plate, thrust washers, bearing , oil pump , oil coolers
Turbines	Bearing , piping coolers EHC systems, residue from synthetic media

Chrome (Cr)

Chrome is used as an engineering material for its great hardness and corrosion resistance. It is found in many systems operating under harsh conditions.

Equipment	Wear metal Sources
Engines	Rings, liners, exhaust valves , zinc chromate from cooling system inhibitor
Bearings	Rolling element bearing: alloyed /coated element in rollers, tappers
Gears	Bearing , shaft coatings, some special gears are chrome plate
Transmissions	Bearing, water treatment
Hydraulic system	Cylinder liners , rods , spools
Heat exchangers	Cooler tubes , baffles , plates
Compressors	Housing bearings, cylinder guides , wear plates , thrust washers, bearing , oil pump , oil coolers
Turbines	Shaft coating bearing

 Lead (Pb)

A soft metal used for sacrificial wear surfaces such as journal bearings .lead based babbits are widely used.

Equipment	Wear Metal Sources
Engines	Main bearings, connecting rod bearings. Lead can be present as a contaminant from gasoline improver , anti - knock
Bearings	Rolling element bearing : alloyed element in cages, journal bearing : major alloying element in babbit bearings , alloying element
Gears	Bearing , can also be red lead paint flakes from gear case walls
Hydraulic System	Bearing
Compressor	Bearings
Turbines	Bearing

Silicon (Si)

Silicon is the most common contaminant found in lube oil analysis. Abundant in all areas send is a very hard crystalline material, and very abrasive to metal components.

Equipment	Wear metal source
Engines	Engines blocks , ingested dirt from breathers, external source , can also be from defoamant additive in lubricant.
Bearings	Rolling element bearings: alloyed element with aluminium an cages
Gears	Bushings, thrust washer, silicone sealant , defoamant  additive
Transmissions	Brake shoes , clutch plates ingested dirt
Hydraulic system	Elastomeric seals pump , motor pistons, oil coolers
Heat exchanger	Coolers tubes, baffles , plates
Compressors	Ingested dirt, silicon sealant , bearings , cooler
Turbines	Ingested dirt, silicone sealant  defoamant additive

Silver (Ag)

Silver has exceptional thermal conductivity, and is an excellent bearing plate material , providing minimum friction . It is susceptive to corrosive attack by zinc – based additives,and so railroads ensure that they receive zinc free oil if they run EMD locomotives, Silver is used more outside of the US in general industrial equipment

Equipment	Wear metal source
Engines	Valves, valve liner , cylinder liner bearings
Bearings	Rolling element bearings : alloyed element in rollersa , raes
Gears	Alloying element for tool steel gears
Hydraulic system	Bearings, servo valve plating pumps
Compressor	Bearings
Turbines	Bearings, shaft , reduction gears

Metallic additives

Several other elements are detected with oil analysis. The major elements found are listed here:

Element	Possible Sources
Sodium	Corrosion, inhibitor additive, also indicates coolant leak into oil, can be road salt, sea water, ingested dirt.
Boron	Corrosion inhibitor additive, anti wear/antioxidant additive ,can indicate coolant leak , grease contamination
Magnesium	Detergent/dispersant additive can also be alloying element in steel
Calcium	Detergent/dispersant additive, alkaline , reserve additive, for high sulphur fuelled engines , can be grease contamination
Molybdenum	Solid /liquid anti wear additive, alloy in bearing and piston
Barium	Corrosion inhibitors , detergents ,rust inhibitor
Zinc	Anti wear, corrosion inhibitors , anti – oxidants , alloying element for bearings , thrust washer galvanized cases
Phosphorus	Anti wear , corrosion inhibitors, anti – oxidants additive , EP additive

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 

RDE Atomic Emission Spectrometer for Lube oil Analysis

Rotrode type Atomic Emission Spectrometer for lube Oil Analysis:

Introduction

For the past forty years or so, spectrometric analysis of used oil samples has been applied as a machine health monitoring technique (the terms condition monitoring and predictive maintenance are more commonly used today). Spectrometric analysis determines the elemental concentration of various wear metals, contaminants, and additives present in a used oil sample. Results are usually reported in ppm (parts per million). Commercial oil analysis laboratories report on as many as 32 different elements.Atomic emission spectroscopy was relied on to provide an insight into abnormal wear rates, even though the technique was known to have decreasing sensitivity as particle size increased.

Particle Size Limitation of Spectrometers

What is not widely appreciated is that spectroscopy is more-or-less blind to the larger particles in an oil sample, precisely those particles which are more indicative of an abnormal wear mode. Most severe wear modes such as spalling, severe sliding wear, and cutting wear generate large particles which go undetected by spectroscopy. Large contaminant particles are also missed by spectroscopy. The particle size at which spectrometers begin to lose their detection ability depends on a number of factors including spectrometer make and type, but it is generally agreed that spectrometers lose their ability to detect particles in the 1 to 10 micrometer range. For purposes of this note, particles larger than 10 micrometers will be called large.

SPECTROMETERS MEASURE ONLY VERY SMALL PARTICLES AND DISSOLVED MATERIAL IN OIL

Consequently, spectrometer readings increase steadily between oil changes because, unlike large particles, small particles and dissolved material are not captured by filters, nor do they settle out easily in piping or tanks.

Traditional Methods of Determining Large Particles

Acid Digestion Method

Acid digestion methods have been developed whereby the particles in an oil sample are filtered and then dissolved using such acids as hydrochloric, nitric, and hydrofluoric. The resultant sample is then processed very carefully in a specially prepared spectrometer. This expensive and hazardous method has proven too costly for regular use in predictive maintenance.

Microwave Digestion Method

Microwave digestion methods are in use developed whereby the particles in an oil sample are collected and then dissolved with microwaves in a specially designed autoclave. The resultant sample is then processed very carefully in a specially prepared spectrometer. This method does provide a total concentration of elements in the sample, but it is expensive and time consuming. Like the acid digestion method, you do not know the ratio of large and small particles in the original sample.

Rotrode Filter Spectroscopy

RFS (Rotrode Filter Spectroscopy) was developed to provide an improved spectroscopic method for analysis of used oils for condition monitoring/predictive maintenance without the particle size or metal-type limitations of previous combined spectrochemical and DR ferrographic techniques.

This patented method uses a rotating disc electrode spectometer, known as an RDE spectometer, already in use in many military and commercial laboratories which perform spectrographic oil analysis

Method

·         In the RDE spectrometer, a graphite carbon disc is pressed onto the end of a shaft which rotates causing the disc to rotate.

·         In the normal use of this spectrometer, a quantity of oil is poured from the sample bottle into the sample bottle cap and positioned so that the bottom of the rotating disc passes through the oil.

·         A spark gap is formed between the top of the rotating carbon disc and the tip of the carbon rod electrode.

·         An electric discharge across the gap vaporizes the oil which has adhered to the rotating disc. The light emitted contains wavelengths characteristic of the elements in the oil sample.

·         The spectrometer optics and electronics quantify these wavelengths and report in ppm of up to 20 elements in 30 or 40 seconds. The carbon discs are known as rotrodes.

  

Preparation Station

The rotrodes are porous/Graphite carbon discs and this characteristic makes them ideal filters. A fixture has been designed to to clamp the discs so that oil can be drawn through the outer circumference of the discs when a vacuum is applied to the inside (hub) of the discs. The particles in the oil are captured by the surface of the rotrode. The oil is then washed away with solvent, the disc is allowed to dry, and the particles are left adhered to the rim of the rotrode in just the right position to be vaporized and detected when the rotrode is "zapped" in the RDE spectrometer. The sensitivity of the method is excellent. A multi-station fixture is used so a number of samples can be filtered at once. The procedure is fast and economical to perform. It is an ideal screening test for analytical ferrography.

Advantages of Technique

·         The technique has several advantages which make it a powerful predictor of equipment failure. Most abnormal wear modes cause a significant increase in concentration and size of wear particles. Using porous graphite rotrodes as a filtering media, large particles are captured and subjected to RDE (Rotating Disk Emission) Spectroscopy to obtain a multi-elemental analysis. These captured coarse particles are measured essentially independently of fine and dissolved particle contaminants in the sample. Removal of the used oil from the spectroscopic analysis reduces the energy required to vaporize the sample. This, in effect presents a more concentrated particle sample to the plasma produced during the RDE spectroscopy, and lead to greater sensitivity of the instrument.

·         By combining such large particle results with conventional RDE analysis of the dissolved and fine particles in the oil sample, a complete wear analysis picture for a machine of interest can be obtained. After seven years of applying this new technology in our lab, we and our customers are convinced of its powerful diagnostic capability.

·         Rotrode Filter Spectroscopy is fast and efficient, and is used as a standard screening test for every oil sample entering the lab.

·         The large particle data will flag abnormal wear modes at a very early stage, indicating when an analytical ferrogram should be made, and be of great benefit in defining the metallurgy of the wear particles.

·         The data is also excellent for contamination analysis as it will give elemental composition of large contaminant particles( such as silicon). The nice feature of this is that you may determine whether the element is sourced from an addtive package (such as a silicone polymer for defoaming) or a contaminant(sand/dirt particles)