This article will discuss industrial lubricants.
This article will give a better understanding of the topics below:
A lubricant is a substance that is applied on surfaces that have relative motion in between them. The lubricant reduces friction and wears between the surfaces. However, the lubricant can have other functions apart from these primary functions. Some of the other roles include serving as a:
The lubricants can be found in many forms, varying from liquid, semisolids, dry and gas lubricants, etc. The most common lubricants are oils and gases. Mechanical systems need to consider a balance between reducing friction and wear by a lubricant and its secondary functions. There are set recommendations from manufacturers which are critical to follow as they enhance optimum performance.
There are various lubricants, with the most common ones being liquid, solid, and grease lubricants.
Industrial grease is made up of a liquid lubricant combined with a thickener. This thickener is typically soap combined with additives that help with other characteristics such as tackiness and corrosion resistance. The grease usually liquefies at the temperature's dropping point, between 200 to 500°F. However, this can even be higher, depending on the thickening agent. For example, greases that are thickened lime or calcium soaps have lower dropping points while those that are clay thickened liquefy at higher temperatures.
According to the National Lubricating Grease Institute (NLGI), the grease consistency is rated from semifluid, i.e., 000 to very hard, i.e., 5, and block type, i.e., 6. This is based on the material penetration tests conducted in a worked state where standard objects are dropped into the grease at a given time and temperature. The depth at which the thing sinks is noted. As an example, grease-lubricated bearings have an NLGI 2 grade.
The significant advantage of industrial grease is that it can lubricate surfaces that are hard to reach or contain compared to oil. For industrial grease, the oil viscosity is not equivalent to the consistency rating. Thus, the rating is defined by the base lubricant's viscosity. As a result, greases can have identical NLGI ratings though their performance characteristics will be different. Manufacturers typically publish the respective data.
Industrial grease can also be modified using EP agents to protect against precision surface damage from severe loads, shock loading, frequent stops/starts, and static loading. However, EP agents should only be recommended when needed as they can make the bearing surfaces prone to wear, particularly at extreme temperatures.
The types of industrial grease applied across industries include:
These are mainly produced from synthetic and petroleum fluids. Petroleum-based oils are economical due to the abundance of petroleum availability. Synthetic oils are more expensive, but the benefit from improved performance outweigh the cost in some applications. The dominant factor among liquid lubricants’ characteristics is their viscosity. There are two main types which are dynamic and absolute viscosity. The typical units of measurement are lb-sec/ft2. Viscosity is defined as the velocity gradient between moving and stationary parts of a fluid. The kinematic viscosity is the dynamic viscosity divided by the lubricant density. Kinematic viscosity can be expressed as Saybolt Seconds Universal (SSU). The SSU is a number assigned to a lubricant after it runs under Newtonian flow conditions through a capillary-tube viscometer. The centipoise is the standard unit of dynamic velocity used in the cgs system. However, viscosity can be affected by shear, temperature, and high pressure.
The lubricant’s change in viscosity as temperature changes can be measured as a viscosity index (VI) with a number between 0 and 100 assigned. The higher the number, the less the change in viscosity as temperature changes.
The pour point is the temperature at which the lubricant (or oil) can flow. This is a critical consideration in gravity lubricators or cold starting engines. There are pour point depressants that can lower the pour point. On the other hand, the temperature at which the formulation’s wax starts to separate visibly is the cloud point. This is mostly above the solidification temperature and is critical as wax can clog filters.
The other attributes of lubricants include:
Extended pressure (EP) lubricants have been specially formulated to inhibit the metal-to-metal contact wear of highly loaded gears. However, this high pressure affects the viscosity as it increases when the pressure gets extreme. Thus, highly loaded machines are primarily designed with a recommendation to use relatively low viscosity fluids, which may not necessarily be suitable for lower pressure applications.
Synthetic oils are mostly used to increase the VI or thermal stability. This typically happens at the expense of another characteristic, e.g., pour point. Synthetic lubricants are much more expensive than mineral-based lubricants. They are thus used in industrial settings only, in which case the performance gains outweigh the extra expense. Examples of such industrial environments include instruments and heat transfer systems.
A variety of fluids can be used to make synthetics, and these can be phosphate esters for hydraulic fluids that are fire-resistant, polyglycol for brake fluid, the silicone used in plastic and rubber, etc. Oils such as that used in engines can have other functions like cooling, corrosion prevention, sealing, etc., besides lubrication. Manufacturers hybridize these products with various additives, including VI improvers, detergents, pour point depressors, EP enhancers, etc., to serve many purposes.
Solid lubricants are also known as dry film lubricants. They are natural graphite, synthetic, or molybdenum disulfide mixed with binders or applied to sliding surfaces. Solid lubricants are popular in applications where pressure or temperature extremes make liquid lubricants impractical. As an example, molybdenum disulfide is the preferred option in high-vacuum environments. This is contrary to graphite which would need water vapor to act as a lubricant under the said conditions. Molybdenum disulfide and graphite have low coefficients of friction plate-like laminar structure of their molecules. Therefore, their structure between plates is relatively weak.
Polytetrafluoroethylene (PTFE) does not have a layered structure like molybdenum disulfide and graphite as a lubricant. It is thus used as an additive grease, oils, and other lubricants. In addition, a variety of machine parts can use the PTFE as an anti-friction film or coating and can be combined with aluminum to give a hard-coat anodizing.
Furthermore, solid lubricants can be mixed with inorganic and organic binders or applied as unbonded granules or powders to give curable coatings on the surfaces. Compression fittings can have molybdenum disulfide vapor-deposited to act as an anti-seize agent.
It is important to consider the fundamental issues associated with each lubrication option, such as oil or grease lubrication, and the impact on the process from viscosity, oil distribution, heat, etc. These considerations include:
Oil applications such as in-bearing lubrication should be synthetic oils or mineral oils of high quality. Factors such as bearing, load, speed, lubrication method, and operating temperature affect the selection of the oil type. Advantages and features of oil lubrication include:
There are various ways oil can be introduced into the bearing housing. The standard approaches are:
The rolling bearing elements can pass through the sump, which comes from the bearing housing design. Typically, the oil level should not exceed the lowest rolling element center point. Churning can be minimized by using lower oil levels where the speed is high. The proper oil level can be achieved and maintained by using elevation drains or gauges.
Typically, a pressurized circulating oil system has a pump, oil reservoir, filter, and piping. In some cases, a heat exchanger is used. The pressurized circulating oil system provides the following advantages:
An oil-mist lubrication system is used in continuous-operation, high-speed applications. The amount of lubrication reaching the bearings can be controlled. The system facilitates the oil to be metered, atomized and mixed with air, or the Venturi effect can be used to pick it up from the reservoir. Either way, the air is filtered and supplied to the bearings to ensure sufficient lubrication.
The operating temperatures need to be monitored to ensure control of the lubrication system. The entrance of the contaminants into the system is prevented by the use of labyrinth seals which give the oil and pressurized air a continuous passage.
Factors that enable such a system to be successful are:
The oil-mist system should be turned on a few minutes before the equipment starts to 'wet' the bearings and prevent possible damage from rolling elements and rings.
The lubricating oils are available in various forms across aircraft, automotive, industrial, etc. These oils can be synthetic type, i.e., made from chemical synthesis, or petroleum type, i.e., made from refined crude oil.
A variety of factors should be considered when selecting oil viscosity for bearing applications. These factors include speed, load, type of oil, bearing set, and environmental factors. The oil viscosity is typically inversely proportional to temperature; thus, viscosity values should be stated with their accompanying temperature values. High ambient temperature and low-speed applications use high-viscosity oils, while low ambient temperature and high-speed applications use common viscosity oils.
When comparing viscosity classes, synthetic oils can operate at extremely cold or hot temperatures and are less prone to oxidation. The different oil types have varying pressure-viscosity coefficients. Thus, caution is necessary when choosing between oils. polyalphaolefins (PAO) have pressure-viscosity coefficients and chemical structures similar to petroleum oil's hydrocarbon chemistry. Therefore, PAO oil is typically used in extreme hot and cold temperature environments on oil-lubricated bearings.
In comparison, ester, silicone, and polyglycol oils have structurally different oxygen-based chemistry from the PAO and petroleum oils. This difference affects the physical properties in that the pressure-viscosity coefficient is lower than that of PAO and mineral oils. Thus, such synthetic oils can yield a smaller and thinner elastohydrodynamic film than PAO or mineral oil with the same viscosity at operating temperature. The reduction in film thickness can lead to increased bearing wear and bearing fatigue life reduction.
This is a graphical representation of the relationship between friction and speed, load, and fluid viscosity. The stribeck curve illustrates the coefficient of friction when operated in different conditions. The x-axis shows how the lubrication parameter relates directly to the thickness of the fluid film, and is a function of viscosity, load, and speed. When viscosity increases, an increase in speed, or a load decrease yields a larger lubrication parameter and fluid film thickness.
In comparison, ester, silicone, and polyglycol oils have structurally different oxygen-based chemistry from the PAO and petroleum oils. This difference affects the physical properties in that the pressure-viscosity coefficient is lower than that of PAO and mineral oils. Thus, such synthetic oils can yield a smaller and thinner elastohydrodynamic film than PAO or mineral oil with the same viscosity at operating temperature. The reduction in film thickness can lead to increased bearing wear and bearing fatigue life reduction.
Low to moderate speed applications with grease temperature limits typically use grease lubrication. Each grease has its limiting characteristics and properties, and thus there is no universal bearing grease. These greases generally are made from base oil, additives, and a thickening agent. Polyurea is now being used as a thickener for lubricating fluids and has an excellent polyurea grease performance.
The commonly used grease thickeners are complex and simple soaps. The complex soaps are a result of a fatty acid with a long chain and a difunctional acid reacting with a shorter chain reacting with a single alkaline metal. The simple soaps are a result of a single fatty acid reacting with a single alkaline metal. Typically, hydroxides of calcium, lithium, and aluminum compounds are used as metal forms.
Examples of other grease thickeners include calcium sulfonate, polyurea, fumed silica, and PTFE.
Grease specifications are important in ensuring consistent performance when rightly selected. The specifications usually include physical properties dependent on the manufacturing process, including mechanical stability, low temperature flow, water resistance, consistency, and oil separation. The thickener concentration in the grease affects each of the properties. Thus, it is also affected by the variables of the manufacturing process. Better control of these variables in the process means the grease specifications are maintained, providing consistent performance.
Due to the resistance to initial movement, which can be excessive depending on the application, the starting torque at low temperatures is critical for a grease-lubricated bearing. Some machines struggle to start when very cold and need greases with low-temperature oils with a wide operating temperature range. Where water ingress is a challenge, Aluminum and Calcium-based grease are used due to their water resistance. Lithium-based greases, multipurpose, are also used in various wheel bearings and industrial applications.
As established before, synthetic base oils have a higher maximum temperature when compared to petroleum-based greases. These synthetic base oils like organic esters, esters, and silicones are typically used with conventional additives and thickeners. Synthetic greases can operate at temperatures from -100°F to 550°F.
However, the starting torque where lubricating grease is applied is not proportional to the grease's channel properties or the consistency. It is instead a function of the grease's rheological properties.
For lubricating greases, the high-temperature limit is related to the effectiveness of oxidation inhibitors and the fluid's oxidation stability. The composition of the base oil and the grease thickener's dropping point define the temperature range of the grease. As a general rule, the grease life is halved whenever the operating temperature increases by 50°F.
Grease selection thus has to consider the temperature limitations, oxidation resistance, and thermal stability for high-temperature applications. In applications that are non-lubricated at temperatures above 250°F, chemically stable synthetic fluids or highly refined mineral oils are used as the grease's oil component.
Bearing damage can be propagated by moisture and water. A measure can be given to lubricating greases to account for this contamination. As established before, Aluminium and calcium complexes have good water resistance. Conversely, Sodium soap greases should not be used in exposure to water as they are water-soluble.
Suspended or dissolved water in lubricating oils can affect the bearing fatigue life negatively as the water can cause etching of the bearing. As much as it is unknown how water lowers fatigue life, it permeates through bearing rings' microcracks caused by the stress cycles. This causes hydrogen embrittlement and corrosion in the microcracks, thereby quickening these cracks' propagation.
Some water-based fluids like inverted emulsions and water-glycol also reduce bearing fatigue. Such water is different from contamination; however, they still contribute as water-contaminated lubricants.
Grease is commonly supplied in 35 lb kegs, and oil is typically provided in 5-gallon pails and 55-gallon drums. Additives in the lubricants are generally used to determine lubricants' shelf life. It is advised to use the oldest stock first using the concept of first-in, first-out (FIFO). Dry, clean, and less prone to temperature swings environments provide conditions for storage that maximize shelf life. If drums have to be stored outdoors, they should be positioned on their sides, and shelters and tarps should be used for drum protection.
They can be rolled on their sides when handling the drums but must not be dropped. Forklift blades are not the right tool to grasp the drum sides, but rather drum handling jaws should be used on forklifts as they can surround drum perimeters.
The oil cleanliness affects the equipment life. Particles per millimeter based on size and number of particles have been used by the International Standards Organization (ISO) to rate oil cleanliness. However, new oils can also have high particle counts and may not be recommended for other machines and thus need filtering before use in order not to shorten equipment life.
Diligence in handling lubricants is essential as this helps not introduce contaminants and prevents different formulations from being mixed. Therefore, this should be a critical part of the lubrication program. By environmental safety practices, used lubricants should be disposed of or recovered.
Purchase costs and disposal costs can be saved by maintaining the condition of the lubricant for a long period. The service life of the oils can be extended by adding solubility enhancers to the in-service oil. The solubility enhancers are synthetic base oils that have high solubility. They act as supplements that inhibit particles from degradation to exit the solution thereby fouling the filters prematurely. The supplements also prevent varnish.
Manufacturers of filters are making their filter casings to be more environmentally friendly as they can be recycled or reused thereby cutting down on waste and disposal costs. Inserts made of fiber or paper can be easily removed and then disposed of. Thus, the wire meshes, end caps, etc can be used and reused cutting down on waste by 80-90%.
Oil intended for waste can be reclaimed especially in bigger manufacturing operations. This is because oil leaks are unavoidable in such operations and even the better designed systems have their leaked oil collected by drainage sumps. Therefore, each lubricant type is separately collected, and then dried and filtered. It is then reused. The leaked oil that is reclaimed reduces environmental impact and can save the lubrication costs of a company through the reduction in disposal costs and waste.
A lubricant can be used as a substance applied on surfaces with relative motion in between them. The lubricant would aim to reduce friction and wear between the surfaces. However, the lubricant can have other functions apart from these primary functions. These additional functions include serving as a sealing agent, heat transfer agent, corrosion preventative agent, and an agent for trapping and expelling mechanical systems contaminants. Regardless of the system is automated or manual, the objective is to apply the right lubricant type at the right time in the right amount.
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