The build up of contamination in your lubrication systems is the number one cause of lube failure which in turn leads to equipment failure. Whether its the contaminant itself causing the issue (for example a particle gets trapped between two surfaces leading to a crack on the surface) or the contaminant causing the oil itself to prematurely degrade (water ingress can cause the oil to oxidize, which leads to oxidation, the formation of sludge's and varnish and can result in significant changes in the oils viscosity), its contamination that is the number 1 enemy to operating pieces of equipment.
If a machine uses oil for the transmission of power (ie Hydraulics), lubrication or combustion, it will be affected by the condition of the oil. Often blood in humans is used as the analogy, oil comes into contact with all the various components within the system having a direct impact on its performance and longevity. In fact there have been several studies that correlate the operating life extension factor that exists as particulate contamination is managed to tighter and tighter specification. There is a direct relationship between the level of contamination that exists within a system and how long it takes before the process fails. Below is just one failure mechanism (fatigue wear) showing how physical contaminants can lead to premature equipment failure. In this example, the size of the particle allowed to ingress into the process is slightly larger than the oil film thickness required for the application (various applications require various Film thicknesses, see chart below). As load is applied, the particle can initiate a crack in the machine surface, leading to further surface failure and more particulate introduced to the process.
Example of the failure mechanism when particulate is allowed to ingress.
Source: Noria Corporation
There's a reason they say that water and oil don't mix. Other than contributing to a great salad dressing, when water ingresses into your lubricating systems, a whole slew of chemical processes kick into play which impact the oils ability to protect the equipment. When water mixes with oil, it lowers the viscosity and load capacity of the emulsion. This can lead to metal on metal contact in some cases, or result in a condition called Micro-jetting which creates tiny explosions as the water collapses under pressure damaging the steel surface. Free hydrogen atoms released during this adiabatic compression can in turn cause the steel to em-brittle and make it more prone to crack. Water exposed to steel also causes corrosion and erosion leading to pitting on the steel surface. Water also acts as a catalyst for the itself to degrade speeding up the oils tendency to oxidize which can form sticky resins, sludge's and varnish. The build up of these oxidation by products can cause valves to stick and servo's to stop operating.
Water can be found in oil as either dissolved, emulsified or free. The state is determined by several factors, the base oil, the additive package, temperature, pressure, level of agitation etc. Some oils are designed to keep oils in suspension (e.g. Engine oils), while others are designed to separate oil and water (e.g. hydraulic Oils where its important to keep the water out of high pressure, thin film layer applications).
Other contaminants include:
Heat: When temperature goes outside of an acceptable operating range, the viscosity of the fluid changes which reduces the required film thickness necessary for the application and can lead to metal on metal contact, or lead to the fatigue wear described above as smaller particles get "caught" in the reduced film thickness. High temperatures also cause the oil itself to oxidize at a more rapid rate, leading to the formation of acids, tars and varnishes etc.
Other Oils: When the incorrect oils are added to a system, two significant impacts can result. If the viscosity of the incorrect oil is significantly different than the desired oil, the blend will change the viscosity of the system impacting the desired film thickness. Also if the oil is a completely different type of oil, the additive package could impact the systems ability to provide the type of protection required for the application. For example, if an oil that is designed to split water is mixed with an oil that is designed to emulsify oil, the combined fluid will impact lubrication performance.
Other Chemical Contaminants: As other chemicals or elements build up in the system, the composition or functional properties of the oil can change. For example, as copper builds up in the system due to the wear of a component, it can act as the catalyst which causes the oil to break down more quickly leading to the formation of acids, resins or varnish.