Types of Bearings and Correct Usage｜Knowledge Designers Need to Know

 

Here.Types of Bearings and Their Proper Usage｜Knowledge Every Designer Should Know.I am making a note of this.

 

When I first started my career as a mechanical designer, I had (and still have) many times of headaches in selecting bearings. I had to choose from a myriad of model numbers in catalogs, technical terms, and most of all, I had to ask myself, "Is this really the right choice? I think many designers have probably gone down a similar path.

 

There are many websites in the world that explain the different types of bearings,Most of them only introduce the individual characteristics of rolling bearings. While knowledge (dimensions and load capacity) can be obtained from catalogs for the information needed for actual design, a more in-depth basis for proper selection of bearings is required.

 

In this article, I will begin with a comparison of the two major systems of "rolling bearings" and "plain bearings," and present options that are fundamental to our design philosophy, in order to encourage you to rethink the stereotype of "putting in rolling bearings at any cost," which I had fallen into for a while.

 

First, the basic structure and characteristics of each type of bearing are clarified, and then the logical selection process according to load and rotational speed is concretely presented. The course then goes deeper into the correct design method for the "fit," which determines machine performance and is a stumbling block for many designers. Finally, it covers lubrication and sealing to maximize bearing life, leading you to the conclusion that you can confidently select the best bearings for your application.

 

This is not simply a guide to selecting parts, but a practical guide to learning design concepts to maximize overall machine performance. We hope you will find it useful.

Bearing Basics! Compare types and learn how to use them

The first step in selecting bearings is to understand their basic structure and classification. Here, we will explain the basics of bearings, including the difference between the two major bearing classifications "rolling bearings" and "plain bearings," the types of loads, and the concept of "rigidity," which is indispensable in design.

 

Structural Differences between Rolling and Plain Bearings

First of all,Many designers have an image of "bearings = rolling bearings However, this is a partial view of the whole.　　 Essentially, the term "bearing" refers to all "bearings" that support a rotating shaft, and they are classified into two main categories according to their operating principles.　　 A proper understanding and comparison of the characteristics of both types of bearings is the first step in selecting the optimum bearing.

 

The two major categories are rolling bearings and plain bearings. The choice between them is an important decision that fundamentally determines the characteristics of the machine.

 

Rolling bearings have a structure in which rolling elements such as balls and rollers are placed between the inner and outer rings to reduce friction as they roll.The main advantage is that the dimensions are standardized according to international standards (ISO/JIS).　　Since dimensions are standardized by international standards (ISO/JIS), they are highly interchangeable between manufacturers and are easy to obtain and replace, which is a major advantage.　　Another feature is that friction during startup is extremely low. On the other hand, since the rolling elements support the load with points and lines, they are relatively weak against shocks and prone to minute vibration and noise caused by the rolling motion.

 

Contrast,In a sliding bearing, the shaft and bearing surface make contact with each other at the "surface," and the motion is supported by "sliding" through a film of lubricating oil or the like between them.　　Its simple structure and particularly small radial dimensions contribute to machine downsizing.　　Another advantage is the large contact area and the oil film that acts as a cushion, resulting in excellent shock resistance and inherently quiet operation.　　However, it is important to design the machine to maintain ideal lubrication conditions, and friction tends to be high at startup due to the lack of oil film formation.

 

Thus, there is a clear difference in characteristics between the two. Rolling bearings are the leading choice when versatility and maintainability are important, while sliding bearings are the leading choice when shock resistance, quietness, and compactness are required.

 

Fundamentals of Radial and Axial Loads

Bearings are subjected to two main types of loads, depending on their direction　The two types of load are "radial load" and "axial load".　　These are "radial load" and "axial load.　　A proper understanding of these two loads is the most fundamental factor in selecting a bearing type.

Radial load is force applied perpendicular (at right angles) to the shaft.　　For example, the shaft of a pulley driven by a belt or the force generated by the meshing of gears fall under this category.In many machines, this radial load is the primary load.will be.

 

On the other hand, axial load is a force applied in the same direction as the shaft (axial direction).Also called thrust load.　　Examples include axial forces that occur when helical gears are used, or when a rotating body is supported vertically.

 

In actual machines, these loads are rarely applied alone. In most cases, they are "combined loads" in which radial and axial loads act simultaneously.　It is　　It is important to know exactly which direction and how large the load will be applied and to select a bearing that can handle it.

 

 

What is the bearing stiffness that is important in design?

Rigidity" in bearings is an index of resistance to deformation under load.　. is .　　A rigid bearing has low elastic deformation under load and can hold the position of the shaft precisely. This characteristic has a direct impact on overall machine performance.

 

For example, bearing rigidity is extremely important in applications such as machine tool spindles, which require machining accuracy in the micron range.　　If the rigidity is low, the spindle will tilt slightly due to cutting resistance, resulting in deterioration of machining accuracy.　　For this reason,For these applications, highly rigid bearings such as cylindrical roller bearings and angular contact ball bearings are the bearings of choice.

 

On the other hand, to increase rigidity, the clearance inside the bearing can be reduced or preloading (applying a load in advance) can be applied.　　However, these measures may lead to increased friction and heat generation, and may also reduce the allowable rotational speed.　　Highly rigid bearings also require high machining accuracy for peripheral parts, since even the slightest mounting error of the shaft or housing is difficult to tolerate.

 

Therefore, bearing rigidity is not simply a matter of high rigidity.　　It is important to take into account the rotational accuracy and load conditions required for the machine, and to select a bearing with optimum rigidity from the viewpoint of balancing it with other performance requirements.

 

 

Characteristics of deep groove ball bearings with high versatility

Deep groove ball bearings are the most representative of all rolling bearings and the most widely used type in the world.The main reason for this is its simple structure and extremely well-balanced performance.　The main reason for this is its simple structure and extremely well-balanced performance.

 

Structurally, there are deep arc grooves on the inner and outer rings, between which the ball rolls.The deep grooves allow for some axial loading in both directions as well as the main radial load.It is.　This versatility is one of the reasons why it is used in a wide variety of machines.

 

The ball contact at a point also has the advantage of low frictional torque, making it suitable for high-speed rotation.　　Because of these characteristics, the ball bearings are used in motors, generators, fans, conveyors, and many other devices that require high-speed rotation.　　Furthermore, its low noise and vibration characteristics make it ideal for applications that require quietness, such as home appliances and office automation equipment.

 

However, they may be inferior to other specially designed bearings for large axial and impact loads.　　Nevertheless, their excellent versatility, availability, and cost performance make them a good choice when selecting bearings for machine design,It is common to start with deep groove ball bearings as a starting point for consideration　It is.

 

 

How to use in selecting the type of rolling bearing

If deep groove ball bearings cannot meet the required specifications, other rolling bearings with more specialized performance should be considered.　　This section describes the features and effective use of high-precision "angular contact ball bearings," "cylindrical roller bearings" and "tapered roller bearings" that can withstand high loads, and "spherical roller bearings" that absorb mounting errors.

 

 

High-speed, high-precision angular contact ball bearings

Angular contact ball bearings are indispensable for applications requiring high speed rotation and high precision, especially in machine tool spindles.It is .　Its characteristic feature is a structure called the "contact angle". The orbits of the inner and outer rings are slightly offset from the axial direction, and the balls are designed to contact each other at an angle.

 

This contact angle allows angular contact ball bearings to support radial loads and large axial loads in one direction simultaneously.　However,Since they can only support axial loads in one direction, two of them are usually paired opposite each other (back-to-back or front-to-back) to accommodate axial loads in both directions.

 

Furthermore, by applying "preload" during this combination, the rigidity of the bearing can be greatly increased.　Preload is a technology that eliminates clearance and suppresses shaft shake to the utmost limit by applying a load to the inside of the bearing in advance.　　This technology achieves the high rigidity and rotational accuracy required for machines such as machine tools, which require precise positioning.

 

On the other hand, high precision is required for installation, and preload control is severe, so handling requires specialized knowledge.

 

 

Cylindrical roller bearings for high radial loads

As the name suggests, cylindrical roller bearings use cylindrical rollers as rolling elements.　　While balls make contact at a "point," rollers make contact with the raceway ring at a "line," resulting in a larger contact area.

 

Due to this structural difference,Compared to ball bearings, cylindrical roller bearings have the ability to support very large radial loads (load carrying capacity).　　It also features extremely high rigidity due to its low deformation under load.　　Because of this high load carrying capacity and high rigidity, it is widely used in harsh environments where large forces are applied, such as large electric motors and generators, gear units of machine tools, and axles of rolling stock.

 

Further,Cylindrical roller bearings also have excellent high-speed rotational performance.　　Depending on the shape of the "collar" provided on the inner and outer rings, various variations exist, such as a type that can carry a certain amount of axial load or a type that can absorb elongation due to thermal expansion of the shaft inside the bearing.　　This allows the designer to select the most suitable type according to the machine specifications.

 

However,Note that there is little alignment (ability to absorb mounting errors), so the shaft and housing must be centered with a high degree of accuracy.It is.

 

 

Tapered roller bearings with high resistance to combined loads

Tapered roller bearings have a characteristic structure in which the rolling elements, the rollers, and the inner and outer ring raceways are tapered.This conical shape provides excellent load capacity for "combined loads" in which a large radial load and a large axial load in one direction are applied at the same time.I will do so.

 

As with angular contact ball bearings, they can only take axial loads in one direction.　Therefore, two of them are usually used facing each other.　This allows for axial loading from both directions and high rigidity.

 

Because of their high load carrying capacity and rigidity, they are widely used in places where large combined loads are constantly applied, such as automobile wheel hubs, differential gears, and transmissions.　　They are also indispensable in situations where reliability under severe conditions is required, such as reduction gears in construction machinery, agricultural machinery, and various industrial machines.

 

As a precaution, friction tends to be higher than in ball bearings because of the force that pushes the rolling elements against the raceway during rotation due to the structure.　　Therefore,Although not suitable for very high speed rotation, it is a very reliable bearing for heavy load applications up to medium speed range.　I am sure you will.

 

 

Spherical roller bearings absorbing shaft deflection

Spherical roller bearings are as the name implies,The most important feature is its excellent "auto-aligning ability".The structure consists of two rows of barrel-shaped rollers and a spherically machined outer ring raceway.　Structurally, it consists of two rows of barrel-shaped rollers and a spherically machined outer ring raceway.

 

This spherical outer ring raceway allows the inner ring and the entire rolling element to freely tilt to some angle relative to the outer ring.　　This action automatically absorbs any deflection of the shaft or mounting error between the shaft and housing during operation, preventing undue force from being applied to the bearing interior.

 

Also, because the "roller" is used as a rolling element,Much higher load carrying capacity than ball bearing type spherical bearings, capable of carrying very large radial loads and axial loads in both directions at the same timeThe first is

 

Because of their excellent alignment and high load capacity, they are widely used as the main bearings in large industrial machinery such as iron and steel making machinery, paper manufacturing machinery, mine crushers, and large blowers, which are subject to heavy loads, shaft deflection, and mounting errors.However, due to the structure that allows internal tilting, rigidity tends to be lower than other high-rigidity bearings.　There are

 

 

Learn how to use sliding bearing types and lubrication

Sliding bearings, which function on a different principle than rolling bearings, offer unparalleled performance in certain environments.　The following three types of bearings are used.　　Here, we explain how to use "sintered oil-impregnated bearings," which require no oil supply and are quiet, "resin-based bearings," which are resistant to special environments, and "PV values," which are important indicators for determining the performance limits of sliding bearings.

 

Sintered oil-impregnated bearings that can be used without lubrication

Sintered oil-impregnated bearings are a type of sliding bearing with "self-lubricating" properties that do not require external lubrication.　It is .　They are also called oilless bearings.　　The manufacturing method is very unique: powder metallurgy, in which metal powders (mainly iron or copper) are placed in a mold, compressed, and hardened at high temperatures.

 

This manufacturing process creates a myriad of invisible microscopic spaces (pores) inside the bearing.　　These pores are filled with lubricating oil to create a self-lubricating function.

 

When the shaft begins to rotate, heat from friction causes the oil inside to expand and centrifugal force causes the oil to slowly seep out to the sliding surfaces, forming a lubricating film. When rotation stops, capillary action causes the oil to be absorbed back into the internal pores.　This cycle enables long-term use without lubrication maintenance.

 

Since there is no rolling element, it is extremely quiet and is widely used in small motors such as fan motors of office automation equipment and audio equipment, where quietness is required.　　However, the amount of oil it contains is limited,It is basically suitable for light-load and medium-speed applications.

 

 

Resin-based bearings with excellent chemical resistance

As the name suggests, plastic-based bearings are sliding bearings made primarily of plastic.　They are.　　Polyacetal (POM) and polytetrafluoroethylene resin (PTFE),Polyetheretherketone (PEEK)He said,Engineering plastics with excellent mechanical strength and heat resistance are used.

 

It has many advantages over metal bearings, one of which is excellent chemical and corrosion resistance.　　They can be used without problems in water or seawater, where metal bearings would rust, or in environments where they are exposed to chemicals. Because of these characteristics, they are widely used in food processing machinery and chemical plants.

 

It is also self-lubricating, requiring no lubricant or grease, making it ideal for clean environments where oil contamination is not desired.　In addition, it is lightweight and,Another important feature is its insulating property that does not conduct electricity.The first is

 

On the other hand, one of the disadvantages is that it is more vulnerable to heat than metals and is prone to thermal expansion.　　There are also limits to the load and speed that can be tolerated, so it is important to fully check the conditions of use before making a selection.　　Oiles Industry is known as a representative manufacturer in this field.

 

 

Importance of PV value to indicate performance limits

In the selection of plain bearings, especially those used without lubrication, the "PV value" is an indispensable index for determining the performance limits.The PV value is　　　The PV value is expressed as the product of the "surface pressure (P)," which is the pressure applied to the bearing, and the "sliding velocity (V)," which is the speed at which the shaft rotates.

 

Surface pressure (P) is obtained by dividing the load applied to the bearing by the projected area of the bearing (inside diameter x length). The unit is usually in MPa (megapascals). Sliding velocity (V) is the distance a shaft surface travels in one second and can be calculated from the shaft diameter and number of revolutions. The unit is m/s.

 

The PV value (unit: MPa-m/s) obtained by multiplying these two values gives an approximate indication of the amount of frictional heat generated on the sliding surfaces of the bearing; if the PV value is too high, the risk of wear or, in the worst case, melting and burning of the sliding surfaces due to frictional heat will increase.

 

For this reason, each manufacturer defines a limit value called "allowable PV value" for each bearing material in their catalogs and other sources.　The designer must ensure that the PV value calculated from the conditions of use is within theAlways check that the PV value is below the allowable PV value　It must be done.　　This is a very important process because failure to perform this check can lead to premature bearing failure.

 

 

How to use different types of bearings to extend their life

Knowledge of lubrication and sealing is essential for long and stable bearing lifeIt is.　　This section explains the difference between "grease lubrication" and "oil lubrication," which are typical lubrication methods, "life calculation" to ensure reliability, and the roles and performance differences of "seals" and "shields" to protect bearings from external contaminants.

 

Lubrication Basics: Difference between Grease and Oil Lubrication

Lubrication is extremely important to maintain bearing performance and extend bearing life.　　There are two main types of lubrication methods: grease lubrication and oil lubrication.

 

Grease lubrication is a method of sealing semi-solid grease inside the bearing　Grease is a low-fluidity material that is resistant to leakage.　　The greatest advantage of grease is that it has low flowability, so it does not leak easily, and a relatively simple sealing device can be used. For this reason, this method is used in many rolling bearings.　　It is easy to handle and requires little maintenance, but little cooling effect can be expected.　　Therefore, it is recommended to use the following types of rolling bearings,Suitable for general applications at medium and low speeds and where operating temperatures are not so high.

 

On the other hand,Oil lubrication is a method of using liquid lubricant　It is.　　Because of its high fluidity, the oil is able to penetrate to every corner of the bearing, providing excellent lubrication performance.　　In addition, circulating the oil provides a high cooling effect, allowing heat generated by friction to escape to the outside.

 

From this characteristic,Essential for use in high speed and high temperature conditions, such as machine tool spindles　It is.　　However, complex and reliable sealing devices are required to prevent oil leaks, and the overall system tends to be large.

Thus, the choice of which lubrication method to use must be based on a comprehensive consideration of the machine's rotational speed, heat generation, cost, and ease of maintenance.

 

 

Lifetime calculation methods to ensure reliability

The life of a rolling bearing is not infinite.　With continued operation, the raceways and rolling element surfaces are subjected to repeated stresses that fatigue the material and cause scaly delamination called "flaking.The total number of revolutions before this flaking occurs is defined as the "fatigue life" of the bearing.　The following is a summary of the results of the project.

 

The designer performs a life calculation to verify that this fatigue life meets the required life of the machine.The most basic calculation is the "basic rated life (L10). This is the total number of revolutions that a group of identical bearings, 90% of them, can reach without causing flaking when operated under the same conditions.

The formula is as follows L10​=(C/P)p

 

where C is the "basic dynamic load rating" (catalog value), P is the "dynamic equivalent load" (actual load applied to the bearing), and p is an exponent (3 for ball bearings and 10/3 for roller bearings).　This formula indicates that the smaller the applied load and the higher the basic dynamic load rating, the longer the life of the bearing.

 

In recent years, the concept of "modified rated life" has been introduced to compensate for more realistic conditions such as lubricant cleanliness and viscosity.　　This allows for even more accurate life prediction.　　These calculations are complex, but can be easily performed by using the technical calculation tools provided by each bearing manufacturer.

 

 

Sealing role to prevent foreign objects from entering

To ensure that bearings maintain their performance over a long period of time, it is essential to keep the inside of the bearing clean at all times.　It is.　　However, many machines operate in environments where substances harmful to bearings exist, such as dust, cutting particles, moisture, and corrosive gases.

 

If these foreign substances enter the bearing, they degrade the lubricant grease or lubricating oil and significantly degrade its performance.　　In addition, hard foreign substances cause indentations and scratches on the raceways and rolling elements, which in turn become a source of noise and vibration.　　Furthermore, these scratches can be the starting point for the aforementioned flaking (fatigue delamination) to occur prematurely, significantly shortening the life of the bearing.

 

The "sealing device" plays the role of preventing foreign matter from entering and, at the same time, preventing the lubricant sealed inside from leaking to the outside.There are various types of sealing devices, but "shields" and "seals" are typical ones that are incorporated into the bearing itself.Selection of the appropriate sealing device according to the harshness of the operating environment is an extremely important design item to ensure bearing reliability and achieve the planned life.

 

 

Structure and performance differences between seals and shields

Typical sealing devices incorporated in bearings are "shields" and "seals.Although they are similar in name, they differ greatly in structure and performance.It is important to understand the characteristics of each and choose the one best suited for the environment in which it will be used.

 

The shield is a disk-shaped plate made of pressed steel sheet and fixed to the outer ring.　　A small gap (labyrinth gap) is provided between the shield and the inner ring to prevent contact.

 

Seals, on the other hand, are made of elastic materials such as synthetic rubber and are fixed to the outer ring.　There are two types of seals: a "contact type" in which the tip (lip) contacts the inner ring, and a "non-contact type" in which the lip does not.

 

The performance differences between the two are summarized in the table below.

The choice between the two is determined by balancing the required level of dustproofing and waterproofing with the allowable rotational speed.For example, the shielded type is considered suitable for high-speed motors used in clean rooms, while the contact seal type is suitable for agricultural machinery used outdoors.

 

 

Understanding fit and mastering bearing installation

It is no exaggeration to say that "fit" design is the most important factor in maximizing bearing performance.　It is necessary to understand the basics of fitting, the difference between a tight fit and a tight fit, and the phenomenon of "creep" caused by a poor fit.

 

Fit" refers to the dimensional relationship between the shaft and bearing inner ring, and the housing (bearing housing) and bearing outer ring.　　Specifically, it is determined by the difference between the diameter of the shaft and the inner ring, and the difference between the housing bore and the outer ring outer diameter.　　If this fit is improper, not only will the bearing not perform as it should, but it will also lead directly to serious problems such as abnormal noise, vibration, and premature failure.

 

The choice between a tight fit and a tight fit is at the heart of fit design.　For its selection,Absolute basic principles to prevent creep　In addition to the aforementioned fit tolerances (dimensional tolerances), it is equally important to control geometric tolerances in order to maximize bearing performance.

 

Put these together.Tolerance design of bearing mountings　Please also refer to the page titled "How to make the most of your time in the office" for a more detailed summary.

 

 

 

Summary of optimal bearing type and usage

This article has provided a comprehensive overview of the selection and use of bearings in machine design.　　For optimal design, it is essential not only to know the individual pieces of knowledge piecemeal, but also to understand them in an integrated manner and to compare them with the required specifications of the overall machine.　　Finally, we summarize the main points of this article.

 

• Bearings are broadly classified into rolling bearings and plain bearings.
• Rolling bearings are standardized and highly versatile, while plain bearings are superior in shock resistance and quietness.
• There are "radial load" and "axial load" in the load direction.
• Stiffness indicates the resistance of the bearing to deformation and affects the accuracy of the machine
• Deep groove ball bearings are the most versatile and the first choice
• Angular contact ball bearings are suitable for high-speed, high-precision applications, especially for machine tool spindles.
• Cylindrical roller bearings and tapered roller bearings are used in applications where large loads are applied.
• Spherical roller bearings can absorb shaft deflection and mounting errors.
• There are different types of plain bearings that can be used without lubrication, such as sintered oil-impregnated bearings and resin-based bearings.
• PV values, which indicate performance limits, must be checked when selecting a plain bearing.
• Lubrication includes grease lubrication and oil lubrication, which are used according to rotational speed and cooling properties
• Rolling bearing life can be predicted by the basic rating life (L10) formula
• Sealing devices include shields and seals, which are selected based on the environment in which they are used.
• The purpose of fit design is to prevent creep phenomena that cause wear
• The basic principle of fit design is "to make the raceways subjected to rotational load into a tight fit.

 

That's it.