Explaining the types of shafts and bearings! What are the guidelines for using them based on their relationship and roles?

Shafts and bearings are essential for rotating mechanisms. There are many different types of shafts and bearings, and they are used depending on the application. The performance of a rotating mechanism is not determined by the bearings or shafts alone, so the combination and design of each determines the lifespan and precision of the rotating structure.

This article will explain the relationship between shafts and bearings, their types, and guidelines for how to use them when designing.

Shaft Type

The shafts are used according to the power transmission method and connecting parts.The main types are as follows, and are selected according to the power transmission method and connecting parts.

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hollow shaft

A hollow shaft is a shaft that is lightweight and has a reduced moment of inertia due to its hollow interior.The mass can be reduced while maintaining the outer diameter, which contributes to improved responsiveness of the rotating body and energy savings.

Another major feature is that the internal space can be utilized to route wiring, piping, and cooling paths.They are often used in applications that require lightweight and high responsiveness, such as robot joints, servo mechanisms, and precision transport equipment.

On the other hand, because the wall thickness is thin, care must be taken when designing bending rigidity and fatigue strength, and it is important to set an appropriate wall thickness according to the conditions of use.

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Spline shaft

A spline shaft is a shaft that can reliably transmit high torque by providing multiple grooves (tooth profiles) on the outer circumference of the shaft.Compared to key connections, the contact area is larger and the load is distributed, making it superior in concentricity and less prone to rattle.

Furthermore, since torque can be transmitted while sliding in the axial direction, it can also be used in mechanisms that involve position changes.

They are widely used in industrial machinery and automotive drive systems, and their performance depends on high-precision tooth processing and wear resistance achieved through hardening treatment.

Serration shaft

A serrated shaft is a shaft with triangular or trapezoidal teeth that transmit torque by firmly meshing the parts together.They have fewer teeth than splines and are often designed with an emphasis on fastening strength, making them suitable for applications that aim to prevent slippage and misalignment.

They are used in places where secure fixation is required, such as steering mechanisms and precision drive parts. As assembly accuracy is directly linked to performance, it is important to control tooth profile accuracy and press-fit conditions.

Motor shaft

The motor shaft is a core component that transmits the rotation of an electric motor to the outside, and requires high runout accuracy and balance performance during high-speed rotation.Even slight deviations in eccentricity or surface roughness can affect vibration, noise, and bearing life, so controlling roundness, concentricity, and surface quality is extremely important.

Processing specifications must be optimized according to the usage environment. For example, wear resistance and fatigue strength are important for continuous rotation applications such as fans and pumps, while dimensional accuracy and surface roughness are important for servo motor applications.

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Transmission shaft

Transmission shafts are used inside the transmissions of automobiles and industrial machinery, and must be designed to withstand high torque and repeated impact loads.In many cases, multiple gears and bearings are assembled, and stepped structures and high-strength specifications with hardening treatment are common.

To ensure durability, not only is material selection important, but finish grinding after heat treatment and management of the surface hardened layer are also important design elements. Since long-term reliability is required, it is essential to achieve both dimensional accuracy and fatigue strength.

Bearing types

Bearings are components that support shafts and reduce friction.They are broadly divided into "slide bearings" and "rolling bearings."

1. Plain bearings
2. Rolling bearings

Plain bearings

A plain bearing forms an oil film between the shaft and bearing surface, reducing friction through fluid lubrication.The simple structure makes it suitable for large diameter and heavy load applications, and it has excellent shock absorption and quietness.

On the other hand, since oil film formation is affected by rotational speed and viscosity conditions, lubrication design is important.Wear is likely to progress in the boundary lubrication region, and the material selection and surface treatment will determine the lifespan.

Rolling bearings

Rolling bearings reduce resistance by converting it into rolling friction via balls or rollers.It has a low coefficient of friction and low starting torque, making it suitable for a wide range of equipment.

In addition to these, there are tapered roller bearings, spherical roller bearings, thrust bearings, etc., and the selection is based on the application and load conditions.

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The relationship between shafts and bearings

The performance of shafts and bearings is not determined independently. As a rotating body, the shaft acts as an axis, the bearings that support the shaft, and the housing that secures and holds the bearings are combined, and the accuracy is determined by the sum of each element.

Here we will summarize each relationship.

1. The basic structure of the rotating structure is the shaft, bearing, and housing
2. Shaft accuracy determines rotation accuracy and lifespan
3. Shaft and bearing roles

The basic structure of the rotating structure is the shaft, bearing, and housing

Bearings are components that not only support loads but also define the center of rotation.A rotational support mechanism using rolling bearings consists of three main elements.

Additionally, the radial and axial loads applied to the shaft are transmitted to the housing via the bearings.

Shaft accuracy determines rotation accuracy and lifespan

The basic rating life (L10 life) of a rolling bearing is calculated on the assumption that the contact stress distribution between the rolling elements and bearing rings is uniform.Theoretical life is the total number of revolutions that 90 out of 100 identical bearings can perform without experiencing rolling fatigue (flaking).

However, if the shaft's roundness, cylindricity, runout accuracy, and concentricity are insufficient, the inner ring will rotate eccentrically, and uneven loads will act on the rolling elements. Uneven loads will locally increase the Hertzian contact stress at the contact point, causing early flaking.

Furthermore, if the shaft diameter tolerance is not appropriate, the fit may change and the clearance inside the bearing may deviate from the design value.

If the internal gap is small, it can cause heat generation and seizure, and if it is large, it will increase vibration and noise.

Shaft and bearing roles

The shaft and bearings have the following roles:

For example, in high-speed rotation applications, low friction characteristics are required for bearings, but runout accuracy and concentricity are more important on the shaft side.Separating and understanding the roles will lead to rationalization of the design.

Guidelines for using shafts and bearings in design and product selection

In addition to selecting the type of bearing, it is necessary to consider the conditions for combining it with the shaft.

Here we will summarize the main points that become important during the drawing design stage.

1. Mating tolerance: shaft diameter/housing diameter
2. Fitting: Tight fit/Clearance fit
3. Surface roughness: Surface quality of the contact surface
4. Hardness and heat treatment
5. Lubrication conditions: oil, grease, no lubrication

Mating tolerance: shaft diameter/housing diameter

Dimensional tolerances for rolling bearings are specified by standards, and the fit between the inner and outer rings is determined by the combination of the tolerance range class of the shaft diameter and housing diameter.

Since the internal clearance changes during actual operation, if the tolerances with the mating parts are inappropriate, there is a possibility that the risk of vibration, heat generation, abnormal noise, etc. increases.Therefore, when increasing the precision grade to improve rotational accuracy, it is important to also design the mating tolerances based on the same design concept.

Fitting: Tight fit/Clearance fit

Generally, the ring that receives the rotating load is called an interference fit, and the ring on the non-rotating side is called a clearance fit.It is important to consider the load direction and rotation conditions, taking into account which wheel will be subjected to the rotational load.

If the fit is too tight, the internal gap will be reduced, increasing the risk of heat generation and seizure. On the other hand, if the gap is too large, micro-slip will occur, causing slippage and vibration.Therefore, it is important to select a product that takes into account the load, temperature difference, maintainability, and other usage conditions.

Surface roughness: Surface quality of the contact surface

The surface quality of the bearing contact surface is related to the stability of load support and the suppression of micro-slip.If the surface is too rough, contact will be concentrated at certain points, leading to poor fit of the seating surface, accelerated wear, and increased vibration.

On the other hand, smoothing the surface more than necessary will increase finishing costs.Therefore, it is important to manage the Ra value according to the application.

Hardness and heat treatment

At the point where the shaft and bearing meet, repeated loads and minute slippage combine, leading to wear and galling of the bearing surface, which directly impacts the bearing's lifespan.High-frequency induction hardening and carburizing are sometimes used to ensure wear resistance, but simply increasing hardness is not the only solution.

Unless you consider factors such as the depth of the hardened layer, the toughness of the base material, and the loss of roundness, it will be difficult to improve rotational accuracy.

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Lubrication conditions: oil, grease, no lubrication

The lubrication method is selected based on rotational speed, temperature rise, sealing ability, and maintainability. Oil lubrication is effective for high-speed rotation, and grease application is common when a sealed structure or easy maintenance is required.

In addition, self-lubricating plain bearings may be used to reduce the number of times they need to be lubricated. In this case, it is necessary to optimize the lubrication mechanism on the bearing side and the surface roughness and hardness conditions on the shaft side.

Sanwa Needle Bearings are strong in difficult and high-precision machining

To optimize the rotating structure, it is essential to maximize the performance of both the shaft and bearings.Sanwa Needle Bearing has established an in-house integrated production system and can assist in the design of optimal combinations of structures, including grinding.

We also have a track record of high-precision machining of difficult-to-cut materials and high-hardness materials.

Rollers are almost essential parts for bearings

The rollers are the key components that determine the performance of rolling bearings. The rollers, located between the inner and outer rings, roll while bearing the load, significantly reducing friction.

In particular, with linear contact types such as cylindrical rollers and needle rollers, the machining precision of the rollers is directly linked to bearing performance.

1. Roller Basics
2. The role of rollers in bearings
3. Examples of roller products

Roller Basics

Rollers are the general term for rolling elements and are the main element that bears radial loads. Load distribution changes depending on the shape.

The role of rollers in bearings

Variations in the roundness and dimensions of rollers are directly related to the distribution of load, and large variations can cause increased vibration and noise.In particular, small diameter, slender needle rollers can ensure high radial load capacity in a limited space.

On the other hand, since it is a line contact structure with a larger contact area than point contact, although it can reduce local stress, it is easily affected by variations in roundness, cylindricity, dimensional tolerance, surface roughness, etc.

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Examples of roller products

Precision grinding of rollers is directly linked to performance stability.Sanwa Needle Bearing manufactures high-precision rolling elements such as needle rollers and cylindrical rollers.

For small and thin diameter rollers, we have a track record of supplying products with outer diameter tolerance control in μm units, roundness control at the submicron level, and high-hardness materials such as SUJ2, as well as products for the medical and precision equipment fields.

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Shafts and Bearings FAQ

Here we will introduce some frequently asked questions about shafts and bearings.

Q1. What are the different types of shafts and what are their roles?
Q2. How do rollers relate to the performance of rolling bearings?
Q3. Which precision should be prioritized: shaft or bearing?

Q1. What are the different types of shafts and what are their roles?

The shapes vary depending on the application, such as power transmission, positioning, linear guide, etc., and the connecting parts and load conditions also vary. Below are examples of shaft types and their applications.

Q2. How do rollers relate to the performance of rolling bearings?

Rollers are elements that share the load, and if there is variation in outer diameter or roundness error, the load will be concentrated in one area, leading to problems such as early flaking, increased vibration and noise, and deviation from the theoretical life (L10).

Especially for cylindrical rollers and needle rollers that make line contact, geometric tolerance control is directly linked to their lifespan. High machining precision is also important.

Q3. Which precision should be prioritized: shaft or bearing?

The combination of precision on both the shaft and bearing is crucial.If only the shaft or bearing is highly accurate, the expected performance cannot be achieved.

Summary | Performance of shafts and bearings is determined by their combined design

The performance of a shaft and bearing is determined only by their combined design. Life and reliability depend on a comprehensive design that takes into account tolerances, fitting, surface roughness, hardness, and lubrication conditions.The precision of the rollers used in the bearings is also important.

Therefore, if you want to achieve high-performance rotation, it is best to consult a specialist manufacturer.

At Sanwa Needle Bearings, we have built an integrated in-house production system, leveraging our strength in sub-micron grinding processing technology.We have a wealth of experience in processing difficult-to-cut materials, so please feel free to contact us if you would like to improve the precision of your equipment.