THE DEFINITIVE TRUCKING SITE

May 2007

Introduction

Bearing failure, piston ring sticking and excessive oil consumption are classic symptoms of oil-related engine failure. How do you avoid them? There are numerous ways, three of the most important being Scheduled Oil Sampling (S.O.S.), regular maintenance of the lubrication system, and the use of correct lubricants. 

Understanding Oil

Function

Engine oil performs several basic functions in order to provide adequate lubrication. It works to keep the engine clean and free from rust and corrosion. It acts as a coolant and sealant, and it provides an oil film cushion that keeps metal-to-metal contact to a minimum, thereby reducing friction and wear. 

The selection of a suitable lubricating oil should be based on the engine performance requirements as specified by the manufacturer, as well as the application and the quality of the available fuel. Diesel engines, for instance, normally operate at lower speeds but higher temperatures than petrol engines, making conditions exceptionally conducive to oil oxidation, deposit formation and corrosion of bearing metals. Under these conditions, the oil is expected to function in an expanded capacity. 

Base Stocks

Lubricating oil begins with base oil or base stock. Base stocks are of mineral (petroleum) or synthetic origin, although vegetable stocks may be used for specialised applications. The base stock provides the basic lubricating requirements of an engine. However, unless it is supported with additives, base oil will degrade and deteriorate very rapidly in some operating conditions.  

Mineral Oils

Mineral stocks are refined from petroleum crude oils. The crude oil source and the refining process will determine the base stock characteristics. Mineral base stocks are most prevalent for diesel engine oil formulation because they exhibit proven characteristics and are readily available at a reasonable cost. 

Additives

Additives strengthen or modify certain characteristics of the base oil. Ultimately, they enable the oil to meet requirements beyond the abilities of the base oil. The most common additives are detergents, oxidation inhibitors, dispersants, alkalinity agents, anti-wear agents, pour-point depressants and viscosity index improvers. Here is a brief description of what each additive does: 

Detergents help keep the engine clean by chemically reacting with oxidation products to stop the formation and deposit of insoluble compounds. Alkalinity agents help neutralise acids. Oxidation inhibitors help prevent increases in viscosity, the development of organic acids and the formation of carbonaceous matter. Depressants help prevent sludge formation by dispersing contaminants and keeping them in suspension. Anti-wear agents reduce friction by forming a film on metal surfaces and by protecting metal surfaces from corrosion. Pour-point depressants keep the oil fluid at low temperatures by preventing the growth and agglomeration of wax crystals. Viscosity Index improvers help prevent the oil from becoming too thin at high temperatures. 

NOTICE: In selecting an oil for any engine application, both the oil viscosity and oil performance category as specified by the engine manufacturer must be defined and satisfied. Using only one of these parameters will not sufficiently define an oil for an engine application. 

Prevent problems and reduce costs by knowing wear rates

Wear Rate Analysis is an integral part of an S.O.S. Services program that helps you maintain engine performance and maximise availability. Through regularly scheduled testing of oil samples from your engine, Wear Rate Analysis detects tiny metal particles caused by component wear. By monitoring trends in the type and quantity of particles, you can get early warning of problems before major damage occurs. 

Monitoring Your Components 

When S.O.S. analysis identifies an increase in the concentration of one or more metals, it can point to the wearing component most likely causing the increase and, often, the probable cause. For example, a sudden increase in silicon and iron in an oil sample would probably indicate a problem caused by either air system leaks or crankcase seal leaks.

Typical Examples of Oil-Related Failures

Most oil-related failures are caused either by contaminated or degraded oil flowing through the engine or by oil failing to flow to a given component. 

Bearings

Oil-related bearing failure is usually attributed to one of two sources: lack of lubrication or dirt in the oil. The first stage of this type of damage is smearing. This stage will show displacement of the lead-tin overlay, normally in the centre of the bearing. In the second stage of damage, scuffing, the aluminum in the centre of the bearing is displaced. The final stages of failure result in total seizure. 

Crankshafts

The oil that flows to the bearings forms an oil film between the crankshaft journal and bearing. Lack of lubrication, or oil "starvation", causes metal-to-metal contact, increased friction, and higher temperatures that lead to the bearing seizing to the shaft. 

Pistons, Rings and Liners

Oil-related piston failure is most commonly caused by the abrasive action of contaminated oil which results in wear of the piston skirt. Piston rings can show wear in the spring groove. Some spring groove wear is normal, but neglected oil changes will cause severe ring "lockup" that occurs when the spring catches in a worn groove and prevents full expansion Liner damage can be caused by lack of lubrication or by abrasives which can polish the bore (remove the crosshatch pattern) and leave a shiny surface. 

Turbochargers 

Oil-related turbocharger damage is caused by oil contamination or lack of oil that can be related to operating practices. The oil supplied to the turbocharger is required to provide bearing (both journal and thrust) lubrication and also for cooling the bearing, particularly on the turbine end. 

Valves

Most oil-related valve failures result from deposit formation or oil starvation. The usual cause of valve stem seizure is deposit collection between the valve stem and guide.