A1. Lubricating grease is a mixture of three main components: lubricating fluid, performance enhancing additives, and thickener. The lubricating fluid can be petroleum-derived lubricating oil, any of various synthetic lubricating fluids, or vegetable-based oil. The lubricating fluid is usually the majority component in the grease formulation. The additives are typically present in relatively low concentrations, and are added to the grease to provide enhancement in one of multiple performance areas. The thickener is what sets grease apart from liquid lubricants. This component gives the grease the property of consistency, making the product semi-solid rather than liquid. Many different chemical compounds can be used to thicken grease. That subject is discussed separately.
A2. Lubricating grease is unique in that, in general, it stays where it is placed in the application, such as a bearing. It does not normally leak out of poorly sealed applications like liquid lubricants do. In addition, grease forms a seal to keep out contaminants like dust, dirt, water, and corrosive gases. It acts as a carrier for lubricating solids such as graphite, molybdenum disulfide, or PTFE. There is some evidence that grease provides extra film thickness in lubricated contacts beyond that which would be provided by a liquid lubricant of similar viscosity grade. Grease also provides for cost effective maintenance, since grease relubrication intervals can be quite long.
A3. Lubricating grease is used in any type of machinery with rolling or sliding contacts. This can include bearings, gears, couplings, slides, linkages, chains, pin and bushing points, fifth wheels, hinges, constant velocity joints, adjustment mechanisms, etc. Greases are used in components used in electric motors, machine tools, railroad and construction equipment, instruments, household appliances, etc. Greases are also commonly used in slow-speed sliding applications and small gearboxes. A large majority (as much as 90%) of all bearings are grease lubricated.
A4. Lubricating grease is made by dispersing the thickener (solid phase) into the lubricating fluid (liquid phase) to form a stable product. In most cases, the thickener is the reaction product of carboxylic acid(s) and alkaline earth metal hydroxide(s), forming an organic salt, commonly referred to as a soap. In those cases, specialized reaction vessels or processes are required to effectively carry out the reaction and produce a grease with the desired properties. In other cases, the thickener is a mineral such as clay or a pre-reacted material that requires only dispersion to produce the grease. Grease can be produced by both batch and continuous processes.
A5. Being a semi-solid material, lubricating grease has unique properties that are measured with unique tests. The two key properties are dropping point and consistency. Dropping point is the temperature at which liquid begins to separate from the grease. In some cases, the thickener melts, while in other cases liquid separates from the grease without the thickener melting. In practical applications, this is the temperature at which the grease no longer stays in place. Dropping point is measured using the laboratory apparatus and technique described in ASTM D2265 (or ISO 2176). Consistency of lubricating grease is measured by the cone penetration test, ASTM D217 (or ISO 2137). In that standard test, a cone of fixed dimensions and mass is allowed to drop into a sample of grease in a standard cup; the temperature and time period are defined in the test method. The test device measures the depth to which the cone penetrates the grease. That value is used to determine the NLGI consistency number of the grease.
A6. In general, for most soap thickened greases, the answer is no. Base fluid viscosity and grease consistency are independent properties of a lubricating grease. The viscosity of the base fluid is determined by the viscosity of the fluids used, as well as the effect of some additives. The consistency of a grease is determined by the type and concentration of the thickener in the product.
A7. Saponification is the reaction of a carboxylic acid (fatty acid) or ester with an alkali or an alkaline earth metal hydroxide to form an organic salt. The product of this reaction is commonly called a soap.
A8. Unopened lubricating grease containers should be stored indoors, out of direct sunlight, and in an upright position. Drums, kegs, and pails should be covered, if possible, to prevent the accumulation of dust, dirt, water, etc. on the lid. The lid should be blown off with compressed air (if available) and wiped free of contaminants before being removed. Drums and kegs of grease should never be stored horizontally. Smaller containers (cans or tubs) should be stored away from heat and out of direct sunlight. Tubes of grease should be stored upright, with the removable cap at the top, away from heat or direct sunlight.
A9. Oil naturally tends to separate from lubricating grease over time in storage. Storage conditions such as a warm environment can accelerate this separation. Some greases may have a greater tendency to separate oil that others. Contact your grease supplier for guidance on whether to mix separated oil back into grease or to pour it off the product, and if you have any questions about the suitability of a grease for service.
A10. Unused lubricating grease can “go bad” if it is stored for an excessively long period of time, or under poor storage conditions. Most manufacturers place a date of manufacture on their product labels and publish shelf-life information for their products. The grease manufacturer’s shelf life guidelines should be followed. Grease can have a shorter than stated shelf life if it is stored improperly. Grease containers that are stored outside can accumulate dirt, dust, and water on the lid, which can enter the container during the natural “breathing” that the container does upon heating and cooling. In addition, as oil tends to separate from grease over time, it can separate excessively if stored for a long time, to the point that the oil cannot be remixed into the grease. Solid additives can also separate from grease, resulting in a grease that is not suitable for use. Both the oil and solids separation phenomena can be aggravated by the improper storage of the grease container in direct sunlight or near a heat source.
A11. It is acceptable to leave unused grease in a grease gun. It is recommended to release the pressure from the grease by drawing the handle back and locking the spring in the retracted/compressed position. Otherwise, oil may separate from the grease over time in storage due to the increased pressure applied by the grease gun spring.
A12. The remaining unused lubricating grease can be stored for future use. The surface of the grease should be left smooth (without depressions) by smoothing it with a clean implement such as a spatula or putty knife. The container should then be gently banged on a hard surface to remove any entrained air bubbles and re-smoothed. This is to minimize the separation of oil from the grease during storage. The lid or cover should be placed on the container and secured as well as possible, and the container should be stored indoors out of direct sunlight and away from any heat sources.
A13. Unused lubricating grease should be disposed of in accordance with all federal, state and local environmental regulations. In some cases, used oil recyclers will accept unused grease. Some municipalities or industries hold household chemical disposal drives at which grease may be accepted.
A14. Used lubricating grease should be disposed of in accordance with all federal, state and local environmental regulations. In some cases, used oil recyclers will accept used grease. Some municipalities or industries hold household chemical disposal drives at which grease may be accepted. Depending on the service the grease has been in and contaminants to which it may have been exposed, used grease may be contaminated and considered to be a hazardous waste and may require special handling for disposal. Used grease should not be re-used.
A15. If it is a new container of the product, contact the supplier. If the package is being reopened, and a small amount of free water is present, then remove the water with a clean paper towel or similar absorbent material and dispose of it properly. Gross contamination with water may render the grease unsuitable for use. Small amounts of particulate matter (dirt/dust) can be removed from the grease surface with a clean spatula or putty knife and the removed material disposed of properly. It may be necessary to remove the entire top layer of the grease. For this reason, it is important to keep the lid securely on partially used containers of grease.
A16. It is best practice to never mix different lubricating greases. Greases should not be mixed because the thickeners, the lubricating fluids, and the additives in different greases may be incompatible. Some grease suppliers publish compatibility charts; those charts are at best a rough guide. If greases must be mixed, as during a product change-over in industrial equipment, compatibility testing, as described in ASTM D6185, should be carried out to determine to what degree the products may be compatible.
A17. It is best practice not to mix different lubricating greases. Since the grease in the bearing is unknown, the bearing should be purged with the replacement product until >90% of the old product has been displaced. In addition, the bearing should be monitored for any signs of grease incompatibility (grease softening excessively, bearing running hotter than normal, etc.). It may be necessary to shorten the relubrication interval to complete the displacement of the old grease from the bearing. Update maintenance records as appropriate.
A18. Most greases will burn, but they are generally not considered to be fire hazards. Most lubricating greases contain petroleum-derived mineral oil or hydrocarbon-based synthetic fluid as the lubricating fluid. Those materials are generally considered to be combustible (flash point at or above 38 °C (100 °F). In very few cases, the lubricating fluid in a grease would be considered to be flammable (flash point below 38 °C (100 °F). Consult the Safety Data Sheet (SDS) for firefighting information for any specific product.
A19. There are many different types of fishing equipment for fresh and salt water fishing. Always consult the manufacturer of your equipment for their lubricant recommendation. Greases for fishing reels are specialty products, typically sold by the reel manufacturer. Those products are formulated specifically for that type of service. They are specially packaged to make application convenient. Fishing reel greases usually have very good resistance to water and protection against corrosion. Typical automotive or industrial greases are not recommended for the lubrication of fishing reels.
A20. Generally, a lubricating grease designed for automotive wheel bearings is sufficient. Look for the NLGI GC or GC-LB service mark on the product label. If the boat trailer is used in salt water, a specially formulated product with salt water corrosion protection is recommended. Such products are typically available from boating and marine specialty stores. Boat trailer wheel bearings may need more frequent relubrication due the immersion of the bearings to water. Follow the relubrication recommendations of the trailer manufacturer.
A21. In some locations, there are strict regulations to protect water from contamination. For those cases, there are specialty lubricants formulated with biodegradable base fluids and environmentally friendly additives. In other cases, greases that are designed for wheel bearing lubrication have been found to be suitable. Those products have reasonably good resistance to water, and typically do not readily dissolve in water.
A22. Lubricating grease is a specific type of product designed for the lubrication of industrial, automotive, and other mechanisms. It is formulated from lubricating oil (petroleum, vegetable, or synthetic), performance additives, and a thickener. It is distinguished from other materials that are commonly called grease, such as cooking oil, animal fat, and even hair care preparations.
A23. Several types of lubricating grease are needed to properly lubricate a heavy duty (Class 8 in the US) truck:
Wheel bearing grease – Some wheel bearings are “lubed for life” while others require regular relubrication. Unless the wheel bearings are “lubed for life,” they should be relubricated at the interval recommended by the OEM. The OEM should provide guidelines for the consistency and grease thickener type that was originally installed in the bearings. The grease used for most wheel bearings should meet the NLGI GC-LB requirements or the SAE J2695 requirements. Most wheel bearings require an NLGI 2 grade grease, while some sealed hub units require an NLGI 00 grade semi-fluid product. For those cases, the NLGI GC-LB or SAE J2695 consistency requirements do not apply. Each type is unique to the bearing design, and should not be used in the other type of bearing.
Chassis and universal joint grease – The grease used for the lubrication of chassis points and universal joints is often the same as the wheel bearing grease. However, if the wheel bearings require an NLGI 00 grade semi-fluid grease, the chassis and universal joint grease will be different. It should meet the NLGI GC-LB or SAE J2695 requirements.
Fifth wheel grease – Fifth wheels require a specific grease, different from the wheel bearings or chassis points. The grease for the fifth wheel is typically a water resistant product with a relatively high concentration of solid additives, such as molybdenum disulfide or graphite. This allows the metal-to-metal sliding contact to be properly lubricated.
A24. There are many different types of lubricating grease because there are many different types of applications where grease is used. Many greases are formulated to have specific properties unique to the application for which they are intended. Electric motors take one general type of grease, while flexible couplings require a very different type, and automotive wheel bearings require yet another type of grease. For more exotic applications, such as in aerospace, Arctic conditions, or high temperature bearings in the steel industry, there are specially formulated products that provide specific levels of performance. Those products are often formulated with synthetic base fluids and contain relatively high concentrations of additives, making them somewhat more expensive than general purpose greases. On a typical passenger car there may many different greases used.
A25. General purpose lubricating greases are sold in many retail outlets, such as auto parts stores, hardware stores, etc. They are typically packaged in a 14 oz. (400 g) tube for use in a grease gun or in a small reclosable container (can or tub). General purpose greases are used in household and automotive applications, such as automatic garage door opener mechanisms, farm and garden equipment, and user serviceable lubrication points on automobiles. For automotive applications, always look for the NLGI GC-LB service mark on the product label to be assured that the product is appropriate.
A26. Most lubricating greases formulated for use in automotive wheel bearings are NLGI 2 grade and are designed to operate at temperatures as low as -40 °C (-40 °F). The NLGI GC-LB or SAE J310 specifications require the grease to operate at that temperature. For continuous operation at extremely low temperatures (below -40 °C/-40 °F), a product containing a synthetic base fluid may be considered. In addition, a softer consistency grease (NLGI 1 grade) may be needed.
A27. Most lubricating greases for automotive wheel bearings must meet the NLGI GC-LB or SAE J310 requirements. Both of those specifications require that the grease must operate over a temperature range of -40 °C (-40 °F) to 160 °C (320 °F). This covers the vast majority of driving conditions.
A28. Driving through water that is deep enough to cover a car’s wheel bearings can cause contamination of the grease with water, or even wash the grease out of bearings. After driving through deep water, the vehicle should be checked by a mechanic, and the wheel bearings should be relubricated if necessary.
A29. The color of unused lubricating grease can be natural (light tan to black, depending on the formulation), or colored with a dye. Black greases typically contain molybdenum disulfide or graphite, both of which impart a dark gray to black color to the grease. The natural color of most greases is light tan to medium brown. Manufacturers add dyes to some greases to give them distinctive colors, either for marketing purposes or to make them easy to distinguish for maintenance personnel.
A30. In-service lubricating greases can change color due to thermal degradation, oxidation, or contamination. The dyes used to color greases may lose color intensity when subjected to high temperatures. Highly oxidized greases may become very dark to black. Greases that are contaminated with water sometimes take on an emulsified or milky appearance. When different greases are mixed in service, the resulting color may be somewhere between the colors of the individual greases.
A31. A burnt odor may indicate overheating of the bearing. A burnt odor suggests that the grease may have been oxidized or thermally degraded. It is prudent to investigate the source of a burnt odor, and to determine its cause. If the source of the odor is a grease lubricated bearing, the equipment should be shut down and the problematic bearing checked.
A32. CBM stands for Condition Based Maintenance of in-service lubricants and equipment. CBM is a strategy for checking in-service lubricants and machinery in order to identify and possibly predict the likelihood of failure. CBM can reveal when a machine is operating properly, or when mechanical failure is just beginning. It is commonly used to monitor both lubricant and equipment condition.
A33. CBM is useful for any lubricated system. In-service lubricant samples may be collected from any equipment and the lubricant analyzed to check its suitability for continued service. This includes grease-lubricated equipment.
A34. The key to successful CBM for any in-service lubricant is to obtain a representative sample of the lubricant for analysis. In the case of in-service lubricating grease, it is imperative to obtain a sample of grease that has been in the contact zone of the bearing, not just in the grease cavity.
There are two general cases:
- When possible, collect a sample of in-service grease from the contact zone in a bearing, gear, etc.
- When bearings are totally enclosed (electric motors, etc.), collect in-service grease during relubrication as the old grease is purged from the bearing.
In either case, collect grease in a clean container to prevent contamination. The sample container must be impervious to the grease and not absorb oil from the grease.
A35. Key tests for in-service grease samples include:
- Appearance – The appearance of in-service grease can tell much about the condition of the equipment from which it was obtained. A small amount of the grease spread thin on a white ceramic tile will allow the analyst to examine it in detail. For example, visible metallic particles or free water may indicate a problem with machine wear or contamination.
- Fourier Transform Infrared (FTIR) Analysis – FTIR Analysis is a well-established and powerful technique for the analysis of in-service grease. It can be used to detect many conditions of in-service grease, such as contamination, oxidation, loss of base oil, etc.
- Metals – Trace amounts of metals in an in-service grease can indicate additive depletion and/or wear metal build-up. ICP, X-ray, and other instrumental techniques are used to measure traces of metals in grease.
- Crackle test for water – The crackle test is a quick and simple way to check for the presence of water in grease. Other techniques such as FTIR Analysis can detect water in grease.
Analysts often compare test data for in-service grease against baseline values for the unused grease. Trends in the test data make it possible to assess the condition of both the lubricant and the machinery being lubricated. Results for trends can be used to make an informed decision about the need for maintenance.
A36. Certain commercial laboratories specialize in the testing of in-service lubricants. Several of these laboratories analyze in-service lubricating grease. These labs can be found through on-line searching. Samples can be shipped via US Mail or other delivery services if packaged properly, or may even be delivered in person. Contact a commercial lubricant testing laboratory for more information.
A37. Samples of in-service lubricating greases should be tested promptly to obtain current status of lubricants and machinery. If there is a delay in shipping samples, they should not be exposed to direct sunlight, elevated temperature, or moisture. Laboratories typically dispose of grease samples after they test them. If in-service grease has been collected during relubrication, but is not to be tested, then it should be disposed of promptly according to all applicable regulations.
A38. The preferred containers are open mouth jars made of unbreakable high density polyethylene or other materials that are impervious to grease. Although glass containers are impervious to oil and grease, they may break. Some commercial testing laboratories supply sample kits that include a sample container, a label, and a pre-addressed box for shipping.
A39. The following information should be included on the label of a sample of in-service lubricating grease:
- Customer or company name and contact information
- The date that the sample was collected
- The designation of the equipment from which the sample was collected
- The name of the product that was in service
- The operating duration of the lubricant (hours or miles/km in service)
- Any additional information about the conditions of service that will aid the analyst in determining the condition of the grease
A40. Waterproof lubricating grease is a grease that is formulated to resist water and its effects. This type of grease repels water, forms an excellent seal against water, and provides protection against rust and corrosion.
A41. Lubricating grease that is formulated for use in food processing plants is generally food grade. Food grade greases are registered as H1 for use in applications where lubricant may have incidental contact with foodstuffs.
A42. The term “EP” refers to extreme pressure, which means the grease is formulated with additives that increase its load carrying capacity. EP greases are capable of withstanding conditions under which ordinary grease may not provide sufficient performance: heavy applied loads, shock loads, high loads coupled with higher speeds, and vibration. EP grease is needed for many applications of heavy-duty construction equipment, automotive wheel bearings, mining machines, industrial machines operating under high loads, etc.
A43. Anti-seize compounds are formulated to prevent mating surfaces from seizing under high loads. For example, anti-seize compounds are used on threaded connections and static joints so that they can be disconnected easily.
A44. Lubricating paste typically contains fine particles of a lubricating solid such as graphite or molybdenum disulfide and a small amount of oil. The consistency of a paste depends on the amounts of particles and oil.
A45. It is important that any seal in contact with a lubricant is compatible with that lubricant. The base oil in a grease may soften, harden, shrink, or swell elastomers used to make seals. It is advisable to check the compatibility of the lubricant with the seal material before putting a grease in service. Many grease manufacturers and seal suppliers have data for compatibility of standard elastomers with common greases, oils, and other fluids. In some cases, it is preferred to run compatibility tests between the grease and the seal or seal material at the operating conditions.
A46. If the seal has been in service for an extended period of time, it may simply be worn out. Seals are sacrificial components in most machinery designs, and they are intended to be replaced periodically. If the seal is in new equipment or has been replaced recently, the seal may have been installed improperly, or there may be a compatibility issue between the seal material and the grease. For example, if lubricating fluid (oil) in grease shrinks a seal, then it may leak.
A47. Lubricating grease typically has natural sealing properties. The solid thickener component of grease can form a barrier at the edge of a seal that helps to limit or prevent leakage of fluid through the seal. Many seals are designed to be used with grease for this reason. Elastomers can be formulated to slightly swell and soften when in contact with lubricants for use in seals. This behavior generally improves their sealing capability.
A48. The thickener in a lubricating grease is the component that sets grease apart from fluid lubricants. Thickeners are molecules, polymers, or particles that are partially soluble in lubricating fluid; they arrange themselves in such a way that they impart a semi-solid consistency to the grease. Many different types of chemical compounds can be used to thicken grease.
Simple soaps are the most common grease thickeners. A simple soap is the reaction product of an organic acid (long-chain or fatty carboxylic acid) and an alkali metal to form an organic salt. Thus, simple soap is an acid-base reaction product. This reaction has a special name: saponification. Simple soaps are most commonly based on salts of lithium and calcium, and less commonly on salts of sodium, aluminum, and barium. Examples of simple soap thickeners include lithium 12-hydroxystearate and calcium stearate.
Complex soaps are also used widely as grease thickeners. The term “complex” refers to the combination of a simple soap and a complexing agent. For example, a lithium complex thickener typically contains lithium 12-hydroxystearate (simple soap) and a salt of a shorter chain difunctional carboxylic acid, boric acid, or an aromatic acid (complexing agent). Complex thickeners are usually based on lithium, calcium, or aluminum compounds. In some cases, dissimilar thickener types are combined in a grease. This type of thickener system can be referred to as a hybrid thickener or in some cases as a complex thickener.
Grease can also be thickened with non-soap materials. Common non-soap thickeners include polyurea, organophilic clay, fumed silica, fluoropolymers, and others.
Polyurea is a generic term that includes include diurea, tetraurea, urea-urethane, and many related chemistries. A typical polyurea thickener is the reaction product of a di-isocyanate with mono and/or diamines. The ratios of the ingredients determine the characteristics of the thickener. It should be noted that because polyurea thickeners do not contain any metallic elements, they are ashless and tend to be oxidatively stable.
Organophilic clay thickeners include the minerals bentonite and hectorite. These minerals are purified to remove any non-clay material, ground to the desired particle size distribution, and then chemically treated to make the particles organophilic (more compatible with organic chemicals). Clay particles are then dispersed in a fluid lubricant to form grease. Clay particles must be activated with a polar material to stabilize the thickener structure. No chemical reaction takes place in the production of clay thickened greases. Clay thickeners have no defined melting point, so they have been used historically in high-temperature greases.
Fumed silica powder is used in relatively few grease products. Like clay, silica particles are dispersed in lubricating fluid. These greases also have no defined melting point and can be used in high-temperature applications. Fumed silica is used to thicken only a limited number of specialty grease products.
Fluoropolymer powders such as PTFE (polytetrafluoroethylene) can be used to thicken lubricating fluids to form grease. These greases are premium niche products with very good resistance to chemicals, oxygen, and water. They are formulated to withstand wide temperature ranges and often provide an extended service life in demanding applications.
A49. Yes, concern for the environment, energy efficiency, and new applications are motivating the development of new grease technologies.
In the environmental arena, there are new regulations for lubricants used aboard ships. The United States Environmental Protection Agency (EPA) requires lubricant manufacturers to assure that Vessel General Permit (VGP)-compliant lubricants meet EPA EAL (Environmentally Acceptable Lubricant) specifications.
There is widespread interest in reducing electricity consumption in manufacturing plants. Energy efficient lubricants are formulated for this purpose. These formulations are based on certain synthetic fluids and additives that reduce friction. A manufacturing plant may have hundreds of electric motors in service; a small reduction in energy demand per motor can translate into significant energy and cost savings.
The deployment of wind turbines has encouraged the development of specialized greases for bearings. These greases are designed to perform under the challenging conditions of wind turbines, which include low speeds, high loads, both high and low temperatures, and oscillating conditions. These greases are used in both on-shore and off-shore wind turbines in a wide variety of weather conditions.
A50. The NLGI Lubricating Grease Guide is an excellent resource for learning about lubricating grease. It covers topics such as grease formulation, production, properties, and applications. The Guide is available for purchase on the NLGI website. The Society of Automotive Engineers (SAE) and the Society of Tribologists and Lubricating Engineers (STLE) are good sources of information on lubricants including greases.