Do you know the Influence of Lubricant Selection in the Design Process on the Lifetime Lubrication of Rolling Bearings?

1. Introduction

Lubrication plays an important role in the performance and lifespan of rolling bearings, but its importance is often underestimated. The most important task of lubricants is to separate relative moving parts (balls or rollers and raceways) from each other to minimize friction and prevent wear. Lubricants designed for specific operating conditions can provide a load-bearing protective film. The ideal scenario is for the protective film to separate the friction surface. In addition to providing load-bearing protective film, lubricants should also be able to dissipate the heat generated by friction, preventing bearing overheating and lubricant deterioration. The correct use of lubricants can also prevent corrosion, moisture, and pollutants from entering the bearings.

The lubricant used in rolling bearings should have the following characteristics:

·Maintain stable viscosity over a wide temperature range;

·Good oil film strength that can withstand loads;

·Stable structure that can provide long service life;

·Non corrosive, compatible with adjacent components;

·Provide a barrier to prevent pollutants from entering the bearings and moisture from leaking from the bearings.

Figure 1 Schematic diagram of lubricating grease composition (oil, thickener, additive)

2. Types of lubricants

·Oil: Both petroleum based and synthetic oils are available. Examples of synthetic oils include silicone, diesters, PAO, and fluorinated compounds. Bearings lubricated with oil have lower starting and operating torque, and have higher rotational speed capabilities. However, due to the evaporation loss of oil, their service life in bearings is shorter than that of lubricating grease. During the service life of bearings, micro instrument bearings are usually lubricated only once, so the selection of lubricants is crucial. As part of the mechanical maintenance cycle, larger bearings require re lubrication. These bearings are lubricated through oil recirculation systems designed in machinery or equipment. The key characteristics to consider when selecting lubricants include temperature range, viscosity, and evaporation rate.

·Lubricating grease: Lubricating grease includes base oil and added thickener. These thickeners are mainly metal soaps (lithium, sodium, aluminum, and calcium), organic compounds (urea), or inorganic substances. However, these thickeners greatly affect the characteristics of lubricating grease, and the lubricating performance of lubricating grease is determined by its base oil. In addition, lubricating grease may contain additives that improve its performance. The types of additives include antioxidants, anti-corrosion, wear-resistant, fillers, enhancers, and extreme pressure enhancers. When selecting lubricating grease, key characteristics such as temperature range, base oil viscosity, and stiffness or penetration level should be taken into account. Most of the lubricating grease used for rolling bearings is NLGI grade 2 grease.

·Solid oil film: This is a non fluid coating applied to friction surfaces to prevent wear. This oil film is used in harsh environments such as extreme temperatures, vacuum, or radiation. In this environment, oil or grease cannot survive, and solid lubrication is usually the last resort or choice. Non fluid coatings include graphite, molybdenum disulfide, silver, gold, or polytetrafluoroethylene. Solid state membranes are designed for specific applications. The selection and dosage of lubricating oil also affect the maximum working speed and torque during startup and operation. In micro bearings, lubricants can affect noise levels. It is recommended to use filtered lubricating grease and oil for micro or instrument bearings.

Figure 2 Schematic diagram of lubrication mechanism

3. Factors to consider when selecting lubricants

Lubrication is one of the most important factors that designers consider. When selecting lubricants, it is necessary to examine factors including temperature, load, speed, environment, and expected lifespan. In addition, many characteristics of lubricating grease and oil need to be taken into account, such as oil separation, evaporation loss, droplet point, oxidation stability, crossflow ability/stiffness, etc.

So far, lubricating grease is the most commonly used lubricant for radial ball bearings in electric motors and gearboxes. Lubricants have lower torque characteristics, but are prone to evaporation loss and migration, and are not always suitable for lifelong lubrication.

3.1 Characteristics of lubricating grease

As mentioned earlier, lubricating grease and oil are composed of base oil, mineral oil or synthetic oil, thickeners, and other additives. The properties of standard lubricating grease are determined by these components, and it is up to the grease manufacturer to properly handle, store raw materials, and implement good process control.

Figure 3 Schematic diagram of Stribeck curve

Figure 4 Schematic diagram of the "boundary" Stribeck curve

3.1.1 Types of base oil

When evaluating potential lubricating greases, the viscosity of the base oil is the primary consideration. Viscosity is an indicator of "fluidity" and refers to the flow resistance caused by internal friction between lubricant molecules. This characteristic determines the load capacity, film thickness, and operating temperature. The higher the viscosity, the higher the strength of the film. The viscosity changes with temperature. The higher the temperature, the lower the viscosity. Therefore, it is very important to choose lubricants based on the temperature range during operation. Special high-temperature lubricating grease, special low-temperature lubricating grease, and lubricating grease with a very wide temperature range can be used to meet specific temperature requirements.

3.1.2 Rigidity

Lubricating grease is classified according to its consistency or hardness. The American Society for Testing and Materials (ASTM) has developed a testing method by placing a specified weight and size cone into a lubricating grease sample to determine the hardness of the grease. After 5 seconds, remove the cone and measure the penetration depth in tenths of a millimeter. The larger the value, the deeper the penetration, and the softer the lubricating grease. Then place the lubricating grease sample into a machine that will knock it (such as using a mixer or egg beater for baking) to simulate operating conditions. Then retest it. This result is called work penetration and serves as the basis for classification. The following table lists the classifications of the National Lubricating Greases Association (NLGI).

The lower the NLGI value, the softer the lubricating grease. The lower the ASTM value, the harder the lubricating grease.

Figure 5 Schematic diagram of the "mixed" Stribeck curve

3.1.3 Thickener

Lubricating grease is composed of solid soap, such as calcium soap or lithium soap. In some cases, fine clay is used to form a structure that preserves and disperses the base oil. The thickener structure does not provide actual lubrication, but it is a storage tank that releases lubricant into the contact area.

Although thickeners have little effect on lubrication, they endow lubricating grease with unique characteristics that affect its applicability in certain applications or environments. Lithium and lithium composite thickened lubricating grease are the most common among them.

·Lithium - the most common, easy to manufacture, easy to store, good pumpability, and flowability allowing dirt to flow out;

·Calcium - Good water resistance, calcium soap helps to lubricate;

·Aluminum - has the highest tolerance to water, chemicals, and acids;

·Barium has high water resistance, but has certain toxicity;

·Sodium fibrous, water-soluble.

Another type of thickener is a non soap type thickener, typically used in applications where high temperatures cause other types of thickeners to undergo thermal degradation. Organic polyurea thickeners provide temperature range limitations similar to metal soaps, but also have antioxidant and wear resistance properties derived from the thickener itself.

·Clay and silica (insoluble powder, silica or clay platelets) - chemically modified structures and surfaces can be used as gel for greases. The maximum available temperature of these greases has been further increased.

·Polyurea polyurea grease is known as a high-performance grease due to its wide range of performance characteristics.

Figure 6 Schematic diagram of the "mixed" Stribeck curve

Figure 7 Schematic diagram of the "EHL" Stribeck curve

3.1.4 Additives

Additives can strengthen lubricating grease. The reinforced lubricating grease contains boundary and extreme pressure additives, as well as solid lubricants such as graphite and molybdenum disulfide.

·Corrosion and rust inhibitors - These are very common additives that can prevent corrosion and rusting of metal components in contact with lubricants. The working principle of these additives is to neutralize the acid and form a chemical protective barrier to repel water on the metal surface.

·Anti Wear (EP) - Anti wear additives and/or extreme pressure additives are chemical additives that protect metal surfaces during boundary lubrication. They form a protective film on the worn surface and chemically react with the metal surface to form a sacrificial surface facial mask. They are activated under high loads and high contact temperatures.

·Antioxidants: Most lubricating greases and oils contain antioxidants. This can extend the lifespan of the base oil. Oxidation can damage the base oil. Oxidation occurs at any temperature, but it accelerates with increasing temperature and in the presence of water, worn metals, and other pollutants.

·Viscosity Index (VI): These additives reduce the rate of change of viscosity with temperature.

·Pour point: Pour point additives improve the low-temperature working range.

·Thickener: These additives help the lubricant adhere to the metal surface during rotation.

3.2 Other characteristics

·Quantity: The amount of lubricant selected also affects the maximum operating speed and torque during startup and operation. Excessive amount of lubricating grease usually causes bearings to overheat. Generally speaking, as the rotational speed increases, the amount of grease injected decreases. In addition, as the load increases, the amount of grease injected usually also increases.

·Cleanliness: In small or micro bearings, lubricating oil can affect the noise level. It is recommended to use filtered lubricating grease and oil for micro or instrument bearings. The particle size greater than the thickness of the lubricating oil film can also cause the EHD film to rupture and produce wear debris. This may trigger a gradual process, leading to premature failure.

3.3 Shelf life

Shelf life refers to the period of time after the manufacturing of lubricating oil. During this period, we believe that lubricants are suitable for use without the need to retest their physical properties. Synthetic oil is essentially a stable material. Generally speaking, synthetic oils do not oxidize, polymerize, or evaporate at room temperature for 10 years or more. Ester oil (where ester bonds may undergo slight hydrolysis in the presence of water) becomes more acidic if it contains water. Fluorinated oil and silicone are unlikely to be affected by simple aging.

Lubricating grease will "age" in a more complex way. The quality of grease will be affected by the structural change of gel. If the gel shrinks, there will be obvious oil leakage, and the remaining oil lubricating grease will become hard. The gel structure may also become softer over time.

High quality grease is crucial for ensuring optimal bearing performance, and many greases meet military or other specification requirements. When the designer does not specify the type and quantity of lubricants, the lubrication of bearings should comply with industry standards.

The manufacturer declares that the specified shelf life applies only when oil and grease are correctly stored in their original, unopened containers.

4. Lubrication mechanism

The thickness of the fluid film determines the lubrication state or type of lubrication. The basic mechanism of fluid film lubrication is:

Hydrodynamic lubrication - two? The surface is separated by a fluid film.

·Elastohydrodynamic lubrication (EHL) - Two surfaces are separated by a very thin fluid film.

·Mixed lubrication - two surfaces partially separated and partially in contact.

·Boundary lubrication - Even with the presence of fluid, most of the two surfaces are in contact with each other.

In addition to fluid film lubrication, there is also solid film lubrication, where a solid film separates the two surfaces.

4.1 Lubricating Film: A Long Life Essential Product for Ball Bearings

Predicting Long Life Bearings Based on Correct Lubrication - Presence of Lubrication Film and Separation of Metal Surfaces. In the operation of radial ball bearings in electric motors or other equipment operating at similar speeds, proper lubrication means the presence of EHD (Elastohydrodynamic) films. In the calculation of bearing life, it is assumed that such a thin film exists.

Calculate the basic rated life of ball bearings using ABMA Standard 9. The method includes adjustment factors for reliability, special bearing characteristics, and operating conditions. The adjustment coefficient a3 is used for operating conditions. If the kinematic viscosity of the lubricant drops below 13 cSt or the speed is very slow (i.e. no EHD oil film is formed), then the adjustment coefficient a3 is less than 1. 50% of the basic life can be adjusted to 20% of the rated life.

4.2 Formation of Oil Film and Stribeck Curve

The thickness of the fluid film is determined by factors such as fluid viscosity, the load supported by two surfaces, and the relative velocity of motion between the two surfaces. This in turn determines the lubrication mechanism. How these factors affect friction loss and how they correspond to different mechanisms will be shown on the Stribeck curve. Engineers use this tool to evaluate lubricants, design bearings, and understand lubrication conditions (Figure 3).

The combination of low fluid viscosity, low speed, and high load will generate boundary lubrication. The characteristic of boundary lubrication is that there is less interface fluid and greater surface contact. We can see that on the Stribeck curve, this causes a very high frictional force (Figure 4).

As the viscosity and velocity of the fluid increase, and/or as the load decreases, the two surfaces begin to separate and the fluid film begins to form. This fluid film is very thin, but it can support more and more loads. Mixed lubrication is the result, and it is easy to see a sharp decrease in friction coefficient on the Stribeck curve. The decrease in friction is the result of reduced surface contact and increased fluid lubrication.

As the speed or viscosity increases, the two surfaces will continue to separate until a complete fluid film appears and there is no surface contact. The friction coefficient will reach its minimum value and transition to a hydrodynamic lubrication state. At this point, the load on the interface is completely supported by the fluid membrane. Due to the presence of a full fluid film and the absence of solid solid contact, friction is low and hydrodynamic lubrication is wear free (Figure 6).

The Stribeck curve shows an increase in friction in the hydrodynamic region. This is due to fluid resistance (frictional force generated by the fluid) - higher speeds may result in thicker fluid films, but also increase the fluid resistance on the moving surface. In addition, higher viscosity will increase the thickness of the fluid film, but it will also increase resistance.

Before transitioning to a hydrodynamic lubrication state under normal operating conditions (high speed and thick film), the machine is usually in a boundary lubrication state (low speed and thin film) during startup and shutdown. Throughout the Stribeck curve, it is shown that the motor or machine exhibits the highest friction and wear during startup and shutdown (Figure 7).

The reason why hydrodynamic lubrication is named is because the fluid film is generated by the relative motion of the solid surface, and the fluid pressure increases this result. There are small protrusions (peaks) on the surface, and direct contact should be avoided. In hydrodynamic lubrication, a fluid film separates two surfaces to prevent wear and reduce friction.

When the geometric shape, surface motion, and fluid viscosity are combined to increase the fluid pressure enough to support the load, a hydrodynamic film is formed. The increased pressure forces the two surfaces to separate and prevent contact. Therefore, in hydrodynamic lubrication, one surface floats on another surface. The increase in fluid pressure forces two surfaces to separate, resulting in an increase in fluid dynamics.

5. Application examples

Typical environments include factories and industrial sites, dust and pollutants, damp and flushing areas. Typical characteristics of lubricants suitable for most motor applications include:

·NLGI Level 2;

·Mineral or synthetic base oil;

·Thickener formula provides durability against mechanical shear forces;

·Low noise characteristics;

·Corrosion prevention;

·The operating temperature range is approximately -20oF to+350oF.

High speed operation - DN value (bearing inner diameter mm ×  Rpm can be used to determine whether the bearing is running at high speed. DN values exceeding 1.5 million ensure the performance of high-speed lubricants. Alternatively, a safe rule of thumb is: if the bearing is operating at more than 70% of the allowable speed values listed in the catalog, high-speed lubricants should be chosen. A typical high-speed lubricating grease contains a base oil with low kinematic viscosity. During high-speed operation, high viscosity generates excessive heat. In addition, the hardness of the lubricating grease should also be included. Usually, lubricating grease with flow characteristics is required. When the bearing rotates, the flowing lubricating grease is more likely to be pushed aside by the rolling element and kept to one side. This can reduce stirring and temperature gain. Lubricating grease that is not flowing or sliding back into the ball channel may cause excessive heat generation.

High temperature - Bearings that continuously operate at temperatures above 300 to 350oF should use high-temperature lubricating grease. At higher temperatures, lubricants undergo thermal degradation. This may be a challenge for lubrication engineers.

More about KYOCM Combined Bearing：

Composite bearing is a kind of roller bearing which can bear both radial load and axial load. The structure of the combined bearing is axial and radial bearing running at 900 °, the main load is borne by the radial bearing, and the axial bearing bears the lateral thrust.

Combination bearing is used together with profile guide rail or channel steel. There are two main types of profiles, I and C profiles. The combined bearing slides into the profile guide rail to generate the required linear movement. The combined bearing is welded with the flange plate for installation according to the application requirements. The assembly is mainly used for precise heavy vertical and horizontal movement.

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