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After oxygen, silicon is the most abundant element in the earth’s crust. Silicon does not occur naturally in elemental form but rather combined with oxygen in a compound called silica (silicon dioxide).
Silica occurs in a free form (quartz, sand, etc.) or combined with a variety of metallic oxides, in which case it is called a silicate (Feldspar). Another class of silicon compounds that should not be confused with silica and silicates is silicones.
Silicones are man-made organic compounds that find extensive application in the polish, paint and lubrication industries.
Figure 1. Effects of Particle Size and
Film Thickness on Wear Rates
Silica and silicates make up a large proportion of the earth’s crust, and are present at high concentrations in natural soils and dusts. It is for this reason that silicon is used as the main indicator of dust entry into a component.
Several studies have been conducted on the causes of premature wear in components. The figures vary from study to study but it is clear that external contamination of lube oil by silicon, or dust, is a major cause of accelerated wear.
Particles of airborne sand and dust vary in size, shape and abrasive properties. In an engine, the ingress of atmospheric dust takes place primarily through the air intake.
Efficient air filters remove 99 percent of the dust that an engine ingests. The remaining one percent consists of small dust particles that pass through the air filter. These vary between submicron particles to particles greater than 10 microns.
This dust will pass between the piston, rings and cylinder and eventually become suspended in the lubricating oil. Particles similar in size to the oil film clearance do the most damage.
A particle smaller than the clearance will pass straight through causing little harm; a particle larger than the clearance will be unable to enter to do any damage. In an engine, the clearance between the piston ring and liner bore is extremely small; therefore, the small dust particles are the biggest threat when a leak occurs in an induction system.
Once the dust particle has entered an oil film, it forms a direct link between the two surfaces, nullifying the effect of the oil film (Figure 1).
The first and immediate effect is a “scratching” of the surface as the particle is dragged and rolled across the surfaces. The second and potentially more serious problem is that once the dust particle is introduced in between the two surfaces, it changes the loading of the surface from an even distribution to a load concentrated on the particle with a tremendous increase in pressure at this point.
The increase in pressure causes a deflection of the surface, which will eventually result in metal fatigue and the breaking up of the surface. Among other problems, the increased wear will increase the oil consumption rate (Figure 2). The solution is to keep the dust out, and to achieve this, design engineers use air cleaners, breathers and seals at any point that dust may enter.
Figure 2. Dirt Causes Increased Wear and Oil Consumption
Before the use of oil analysis, a dust entry problem would go undetected until a routine teardown or failure occurred. Even then, the wear would often be attributed to lubricant breakdown or normal wear and tear. The picture changes with the use of oil analysis.
As soon as a dust entry problem occurs, there is an increase in the silicon level of the oil and an acceleration of the wear pattern. As long as the oil samples are taken at regular intervals in the correct manner, the dust entry will be detected at an early stage.
If corrective action is taken, the life of the component will be significantly increased, reducing maintenance costs.
It is beyond the scope of this article to discuss every possibility of dust entry in all machines and components therefore, engines will be used as the example application.
Engines are at high risk for dust entry as large volumes of air are taken into the system and the close tolerances make it susceptible to even the smallest dust particle.
When an engine has a dust entry problem, the type of wear that occurs is related to the manner in which the dust enters. Therefore, by examining the type of wear taking place, it is possible to discover how the dust is entering the system.
An oil analysis report can indicate four possible wear patterns: normal wear, increased top-end wear, increased bottom-end wear and all wear rates increased. Table 1 illustrates data as it might appear on an oil analysis report.
Table 1. Samples of High Silicon. All readings in PPM.
It is unlikely that there will be a dust entry problem without an increase in wear rates. Normal wear patterns combined with high silicon readings suggest that a silicon-rich contaminant increase has occurred. There are two distinct possibilities: a silicone sealant, grease or silicon-rich oil additive is in use, and/or accidental contamination of the sample from or during the collection process.
Check if an additive, grease or sealant has been used recently on the engine. If not, make sure the correct sampling technique was used. If an additive, grease or sealant has been used, call the oil analysis lab to find out if the substance could cause a high silicon reading. If in doubt about what caused the high reading, take a confirmation sample.
Was the oil changed when the first sample was taken? If not, send a confirmation sample and, as a precaution, change the oil. If the oil was changed, send a confirmation sample only after the engine has run for 50 hours or 1,000 km (621 miles).
Increased top-end wear, such as iron, chromium and aluminum, is caused by airborne dust that has been drawn into the combustion chamber being forced down between the ring, piston and cylinder. This is caused by a defective air cleaner or a damaged induction system.
Inspect the air filter element thoroughly, and check its seals and support frame for damage and distortion. Check the pleats for damage. If there is any doubt about a filter element, it should be changed.
Check the induction hosing for damage, cracks etc., and ensure that all hose clamps are secure. The breather and compressor are often connected to the induction system but are frequently overlooked. Verify that both are functioning normally and their hoses are sound and secure.
Check the inlet manifold for cracks and verify that the gaskets are sound and secure. Was a leak found? If not, run the engine at idle and block off the air intake. The engine should stall within three seconds. If the engine does not stall, listen carefully to the joint for air being sucked in. Take a confirmation sample after 50 hours or 1,000 km (621 miles).
If a leak is found, determine the condition of the engine, check compression and blowby and repair the leak. If all of these parameters are found to be normal, monitor the oil consumption of the engine as a safeguard. If any of these parameters are abnormal, schedule to have the piston rings replaced and the piston and liners examined.
Wear on substances such as iron, lead, tin, copper and aluminum indicates that dirt is entering the lube oil directly and not getting past the pistons and rings. The likely sources are leaking seals, a defective breather, damaged seal on oil filler cap or dipstick, or dirty storage containers and/or top-up containers, or dirty lubricant provided by the supplier.
Action to be taken: Any dust that is in the oil will be pumped through the oil filter before entering the bearings. Therefore, the first step is to examine the oil filter for dust contamination and bearing material. If excessive dust is found, check all seals and breathers, etc. Inspect the oil storage containers and top-up containers for the source of contamination.
If excessive dust is not found, check the sampling technique and examine the oil filter at the next service. If bearing material was found, drop the sump and inspect all bearings. Replace as necessary. If bearing material was not found, monitor the oil pressure and inspect the oil filter at the next service.
This is the worst case. Carry out all checks for top and bottom wear. Several other checks must also be performed:
Was any repair work done on the engine? It is possible that the increased wear rates are due to a “seating” process and silicon comes from contamination while the engine was open for repairs.
Piston torching is a rare occurrence. If a piston is torched by a misdirected or badly timed spray from an injector nozzle, the piston releases silicon, and all the wear rates will increase (silicon is alloyed to aluminum to reduce its rate of expansion).