Lube oil analysis is used extensively to help companies maintain their equipment. In order to take full advantage of the test data from oil samples, it is important to understand the basic properties of a lubricant. Equally important is the understanding of how these properties affect the ability of the lubricant to function. Lastly, knowledge of the common test techniques and instrumentation used to analyze oil can aid in data interpretation and lead to more productive corrective action.
To effectively monitor how well a lubricant is working, the functions of the lubricant must be understood. The primary function of a lubricant is quite obviously to lubricate (that is, to reduce friction). By reducing friction, wear is reduced, as is the amount of energy required to perform the work.
When the proper lubricant is used, and the proper load is applied, the asperities are not in contact and in theory no wear will occur. When inadequate lubrication is present, or the load is increased, the oil film will not be thick enough to fully separate the asperities. Mixed lubrication, a cross between boundary and hydrodynamic lubrication, occurs when the oil film thickness is equal to the average asperity height. The largest asperities will come in contact, resulting in increased wear.
Many oils are fortified with anti‐wear additives to combat wear under these circumstances. As load continues to increase, or lubrication degrades, boundary lubrication occurs and the oil film thickness cannot separate the friction surfaces. This can result in metal‐to‐metal contact. At this point, asperities are adhering to one another, causing severe machine wear. This can also manifest itself during periods of shock loading, startup or shutdown. Extreme pressure additives are used in oils that frequently encounter these types of situations.
Lubricants also control the temperature of the equipment. Oil absorbs heat generated at the friction surface and carries it away to be dispersed. Many systems incorporate heat exchangers or radiators to aid in removing heat from the system. Along with heat, lubricants transport dirt and other debris away from the friction surface.
Particulate contamination leads to increased wear through abrasion and reduced oil flow. Some oil additive packages contain agents that break up contaminants and hold them in suspension to be filtered out. This prevents harmful deposits and varnish from forming within the equipment. Alkaline additives also protect the components by neutralizing acid and preventing corrosion.
Hydraulic oils have the added function of transmitting power. To function properly, hydraulic oil must be clean and free of contaminants. Many contaminants will cause oil to foam and entrain air or water. Entrained air causes the oil to compress under pressure, resulting in a loss of power. Particulate contaminants can cause the control valves to malfunction and restrict the oil flow.
To learn more about lubrication, check our TestOil’s webinar calendar for dates offering “The Basics of Lubrication.”