Have you ever heard of silence being used as a performance standard? Consider lubrication. One result of a lubricant at work is the lack of noise as it minimizes friction between metal surfaces in motion. The full price of the silence, or lack of it, is often underestimated. Managers have a continuing concern with the economics of lubrication. As with all service functions, the costs of what is delivered must be balanced with the benefits provided. Seldom can all the costs be set down. When it comes to elements like poor service, delays and failures to act, they may not be measurable at all. But this does not make them less costly. Each manager has to decide what is important and then make his own evaluation.
The success of maintenance rests on lubrication. Poor lubrication guarantees production interruptions when critical equipment breaks down prematurely. It is regrettable that so often this remains unidentified as the real cause for the price paid. Fortunately, recognition is growing on the importance of good lubrication management, and the costs — measurable and immeasurable — are being better defined.
Lubrication is an integral part of the design of every piece of machinery with movable parts. It maintains an unbroken film of lubricant between the moving surfaces; it prevents excessive friction and the consequent heat. The lubricant, chosen to remain stable under severe conditions, carries away the heat normally generated. In general, the moving parts should be operated at the lowest possible temperature with a lubricant of lowest viscosity suitable to operating conditions.
Influences on the performance of the lubricant include surface speed, load or pressure, bearing design, lubrication method, temperature, clearance and such conditions as possible contamination by outside impurities. Continuous research and extensive testing within the oil industry have established a variety of lubricants that provide service lives considered impossible a few years ago.
Equipment manufacturers provide detailed instructions on the application of suitable, specified lubricants. During the last stages of construction, plants make use of surveys offered by major oil companies. These surveys are quite complete. Frequencies, types of lubricant and methods of application are spelled out. Sometimes additional services are available.
One company mails computer printouts of lubrication schedules along preselected routes for each upcoming month. Information on usage of a particular oil or grease throughout the plant is provided for inventory control. This strategy by the oil company probably hastens the commitment to their products. What’s more, as part of the maintenance system, it forces the timely establishment of the routes to be covered by the lubrication personnel.
A program for sampling and analysing used lubricants will reduce downtime and the cost of maintenance. During operation, many gallons of good lubricants are dumped because a system is drained regularly without knowing the condition of the lubricant. More seriously, many pieces of machinery have developed problems because the lubricant was not changed regularly.
It is impossible to know when the lubricant is due for a change without some method of determining its state. Guessing is not good enough. A test procedure is needed to determine whether the life span of the lubricant has expired, whether it has been contaminated by wear particles or external matter, and when these developments are critical. This is economically available from outside laboratories, which use elaborate and expensive instruments.
Forty years ago, lubrication engineers began to test the effects of wear rather than the cause — contamination. They theorized that wear particles generated in the oil-wetted parts of the machine would be carried away by the lubricant and held suspended. The number of these particles would indicate the equipment’s wear and an increasing trend would signal the severity. Thus the condition of the machinery as well as the oil would be revealed and catastrophic failure avoided. Today metal spectroscopy is an essential part of used-oil analysis. It helps to determine drain intervals by identifying whether the additive metals are in proper concentration or depleted, and by identifying wear on contaminant metals.
Used-oil analysis has become an important aid to preventive maintenance. Periodic evaluations are made of the oil viscosity, ph, and content of water, solids, contaminants and wear metals. The length of the sampling intervals varies with different types of equipment and operating conditions.
Sending an oil sample to an outside laboratory from every piece of equipment in an operation, following recommended frequencies of monitoring, is expensive. It is unnecessary. Only samples from critical equipment need to be sent routinely. The number of these can be reduced further by in-house screening of the condition of the oil. This can be achieved by relatively inexpensive tests such as viscosity measurement, solids-testing and water analysis.
In a grease, the fluid lubricant (usually a petroleum oil) represents 70% to 90%. Thickeners, which transform the oil into grease, determine many of its important properties and performance characteristics. They may be simple metal soaps, complex soaps, synthetic organic thickeners or inorganic gelling agents.
Grease selection depends on the viscosity of the mineral oil and type of soap used in its preparation, the consistency of the grease, and whether the extra protection of extreme pressure additives is needed. When the moving metal parts come in contact with grease, a small quantity of oil separates at the point of contact to provide lubrication. This oil is gradually broken down by oxidation or lost through evaporation or centrifugal force. To maintain lubrication, oil separation from the grease at the contact surfaces must continue at a controlled rate.
Lithium soap base greases, developed 50 years ago, were the first practical multi-purpose greases. They lubricate well over a broad range of conditions. They are water-resistant, emulsify with little water, have favorable high and low temperature characteristics and possess good mechanical and oxidation stability.
All greases will oxidize in time and the oil on the contact surfaces will be depleted. The soap component aids lubrication because of its adhesion, which holds the grease in place at the bearing, reducing leakage and providing a seal to exclude contaminants. The soap also provides an additional load-carrying capability.
Greases should not be mixed unless they have the same soap structure. If the grease type is changed, the bearing should be flushed out and cleaned thoroughly before new grease is applied.
The greased intervals depend on operating temperature, speed, contamination and the quality and type of the grease. The higher the speed and the larger the bearing, the more frequently grease should be added.
It is good practice to remove all of the grease about once a year in grease- lubricated bearings, because even greases of the highest quality have a tendency to oxidize and harden in service.
Effective lubrication of bearings, gears and other moving parts of machinery depends on a balance among several factors, all of which influence the choice and performance of the lubricant. Efficient lubrication depends on a knowledge of when the lubricant should be changed. At least once a year the manager should ask:
* Are we throwing away good oil?
* How do we know if our relube intervals are proper?
* When did we have our last lubrication survey by the oil company we are using?
* Are we using their most up-to- date products?
* Is our lubricant inventory consolidated?
This checklist provides the assurance that immeasurable costs are not accruing because lubrication is being denied the attention it deserves. Keith Bowley is a Toronto-based maintenance consultant.
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