Lubricant selection isn't subjective. The engineering rules are well defined. The consequences of failure to make the correct decision are also clearly understood. The decision inputs are measurable, and these inputs become the basis for the final selection of lubricant viscosity and additive structure.
Fortunately, whatever amount time that the lubricant manufacturers are willing to commit to helping clients improve their respective programs is dedicated to the act of lubricant selection. The remaining balance of potentially useful activities must be developed by the equipment owner.
Lubricant Technical Selection Practices
There are six distinct aspects of the technical selection in this section of benchmark practice. They are:
- Selection Criteria for Oil
- Selection Criteria for Grease
- Selection Criteria for High Performance Products(synthetic and mineral oil based)
- Selection Criteria for Applications Volumes
- Selection Criteria for Application Frequencies
- Selection Criteria for Application Methods
There are a variety of ways to measure the amount, frequency and quantity of lubricant that may be used for the common lubricated components, which are gears, bearings, hydraulic systems components, circulation systems, slideways, and chains. There isn't enough space here to explain the various engineering methods that could be used to select the correct product viscosity and type, frequency, volume and application methods, but those details are available in a number of published texts, such as the CRC Handbook on Machinery Lubrication, 2nd Edition.
Selection Criteria for Oil
Oil viscosity selection is perhaps the single most important factor in providing maximum lubricant effectiveness, followed closely by additive type. An error or compromise in either category can lead to shortened lubricant and component lifecycles.
Component manufacturers set a minimum acceptable limit for their respective components based on the component geometry, surface area, type of surface interaction (sliding or rolling), surface contact speeds, and expected load. Typically, the OEM doesn't try to allow for environmental factors since any given component could be placed into a wide range of operating environments. There are instances where the OEM, due to the nature of the product itself, might have a clear understanding of the likely operating environment, and adjust the recommendations accordingly. Caterpillar, for instance, is going to have a pretty good idea of the type of service their DC-9 will be exposed to on a routine basis.
The OEM Manual is always the first place to go to identify lubricant selection requirements. OEM manuals will be adequate 90% of the time. However, some OEM's know less about machinery lubrication than their customers, which leads to too much freedom for interpretation. Alternately, some OEM's specify lubricant products by brand, which is unnecessarily constraining for the equipment owner. It is common to have components from one OEM assembled by another OEM. In these instances, the best method to calculate the required viscosity will probably come from the component manufacturer.
For instance, bearing OEM's (SKF, FAG, NTN, etc..) provide excellent advice for their customers to follow for lube selection, but given the great range of applications for which any given bearing may be used, it is the bearing owner's job to verify that the machine assembly OEM has specified the correct lubricant for viscosity at the expected operating temperature, and with the correct additive type. There are several different bearing manufacturers specification charts that are useful to verify the assembly OEM specification. Regardless of the source, these recommendations derive from the physics of machine and surface dynamics, so any reputable company's guideline probably will suffice.
Once the decision has been made to use a given quantitative approach, the components in question should be reviewed, and the decision recorded as a permanent record which indicates why a specific lubricant was selected (generic lube designation, component type, operating environment, operating temperature, loading, etc..)
Creating a permanent record of the reasoning may seem overboard at first consideration, but without a record of the reason a given approach was taken, it won't be long before someone is looking to make a change for some apparently good reason.
Selection Criteria for Grease
The rules for product selection that were just reviewed for the oil-based lubricant selections apply directly to grease selection, with additional consideration for grease thickener type and consistency. Again, the component OEM's (bearing and gear components for the most part) are an excellent source of information from which to find a standardized, objective method for making the many selection decisions that must be made. Industry associations such as American Gear Manufacturing Association (AGMA) and National Lubricating Grease Institute (NLGI) also provide useful input to support the decision process. Lubricant manufactures can also be a rich source of information but keep in mind that the lubricant manufacturer's job is to move lubricants. Don't become overly dependent on a brand specific approach.
To reinforce the message of the previous section, the point is not to use a single specific approach, but moreso to have some approach that is objective, quantitative, and repeatable. These questions are intended to be answered with a Yes (1) or No (0). It should be obvious whether or not the lubricant specifications are based on good'ole Bert's most-heart-felt shade-tree-mechanic expertise, or from some objective standard.
Selection Criteria for Use of High Performance Products
There is a wide variety of lubricant products characterized as 'high-performance' lubricants. In some instances the nature of the performance edge is very clearly defined. For instance, lubricants used in outer space must be free of any gases. Since mineral and most synthetic base oils contain dissolved gasses (for mineral oils it is 10% air by volume at sea level) and since these gasses bleed out of the lubricant when exposed to a vacuum, obviously a specialized lubricant would be required for that environment (do you recall the first mirror in the Hubble Telescope? Gasses from materials used in satellite construction condensed on the mirror, which necessitated the replacement).
There are many scenarios in the industrial world that warrant the use of specialized materials. The issues that may be addressed with a synthetic include:
- Temperature extremes (too much heat, too much cold)
- Pressure extremes (too much load, inadequate surface area, inadequate surface speed)
- Enviromental extremes (moisture, particulate, corrosive gases, high vacuum)
- Process issues (incompatibility with process chemicals, risk of process contamination)
- Installation and design issues (limited access, dangerous access)
The qualifier for the selection of a given high performance product would have to be the word 'extreme'. Zero Fahrenheit is not exactly extreme, but for a gearbox filled with an ISO 320 oil, this low temperature is likely to create start-up issues if a commodity type mineral oil product is selected, particularly if this is a common occurrence. For that scenario, a synthetic fluid could be justified.
Synthetics are often oversold, though, in an attempt to 'fix' problems that may not have a lubricant selection implication. For this reason, it is appropriate for a site to identify the routine parameters around which synthetics will be selected, and put those conditions in writing for all to understand.
The first criteria asks for specifically identified parameters that justify the use of a synthetic or high performance mineral oil product. Five conditions are identified a few paragraphs above. There could be many more, but for the sake of clarification, let's consider one of these five: Temperature extremes.
The second item in this section of the survey asks that the identified conditions used to justify synthetic applications be assigned a numerical value to guide the decision making process. Conventional mineral oils (API Group 1) begin to degrade rapidly around 180F, and loose their ease of flow around 10°F. A safety margin on each side of these temperature points would provide an adequate framework for a temperature-range profile for the use of mineral oils. Table X.1 gives an idea for what this chart might look like.
The remaining questions in this section seek to verify that each machine application (where a HP product is in use) follows accepted standards, that each application has a clearly defined base oil, viscosity and additive types, and each has appropriate supporting documentation. Again, vendors and industry organizations are each good sources of information from which to set objective standards. Additionally, the last item in this section looks to verify that the documentation includes a reason for the specific selection choice. This is level of detail in our machine history archive extends the benefit of the thought process to future observers.
Selection Criteria for Volumes and Frequencies
As was previously discussed the use of a particular 'objective, quantitative' approach is less important than whether there is some objective, quantitative approach. The FAG bearing lubrication manual provides, in detail, methods for estimating bearing re-supply volume, frequency, and even amount of adjustment that should accompany six different operational characteristics for a bearing application. SKF bearing lubrication guides provide slightly different methods, but the results are similar. Both represent quantitative methods that are appropriate for the majority of industrial applications.
Similarly, the AGMA standard 9005-EO2 provides very clear advice on the frequency and quantity of re-supply for gearing that requires lubricant replenishment (this pertains mostly to open gears, but may also pertain to high pitch line velocity oil lubricated gears).
The last item in both sections is about an identifier, a lubricant tag, that tells the worker attending to the machine what the calculated amount and frequency values are, and consequently, what should be done during each relubrication interval. It is becoming more common to use ultrasonic energy (UE) measurement devices to verify the need for more frequent lubricant and/or the correct volume per each scheduled interval. These tools can be useful or misleading depending on the integrity of the sample collection practice.
Selection Criteria for Lubricant Application Methods
The final aspect of this survey section is the question of lubricant application. This part of the routine relubrication practice (application methods) lacks the objective and quantitative bases that are useful for the other important decisions. Most plant personnel make the decision to proceed with manual practices, or automate, based on either extreme risk/difficulty accessing the lube points, or short relubrication intervals - measured in hours, or OEM advice. There are a few other items to consider when deciding whether to automate the process, including: reliability objectives, extreme operating environments, extreme operating conditions (heat, load, speed) long term staffing issues, and safety implications.
Some of these considerations can be boiled down into objective criteria, much like was done with criteria for selecting synthetic and high performance materials. It is useful to form a matrix of conditions that justify automation. Table X.2 shows 10 different general criteria that could be used to determine which approach to use. Table X.3 shows a set of production specific service factors that could be used to determine the approach. Both could be use together, or individually. There is some overlap, but not a great deal.
Product selection is predominantly determined by the machine components and operation factors, as was presented earlier in this section, but the select product must also work within the constraints imposed by the delivery system itself. There are ASTM methods, such as stiffness, ventability, and bleed tendency, which are used to test the relative friendliness of a lubricant to the delivery system. Limits should be established for automatic system lubricants to assure that delivery systems function properly once the lubricant supply system is selected and installed.
Lastly, and once again, there should be record for each machine that requires automatic application methods, a description of the type and operation of the system (single line progressive, single line parallel, dual line parallel, dual line parallel loop, etc...) and the rational for the decision to automate. If or when application method is debated again then all of the pertinent facts must be present to avoid a duplication of the initial analysis.
This third stage of the lubricant program benchmark process addresses the process used to select the lubricant, the process used to consider a high performance product (synthetic mineral oil), the process used to set the amount per application and frequency of application, and the process used to determine whether to automate delivery of the lubricant.
In each instance the surveyor should be looking for a systematic application of engineering principles and quantitative bases to support the decision process. The exact basis is not as important as the systematic application of some basis, providing that the method conforms to industry standards and practices.