Tolerance and Surface Roughness Design of Bearing Mounting Area｜Complete Explanation of Key Points

 

Here.How to use bearingsis most important."Fit Tolerance and Surface Roughness Design for Bearing Mounts"It is a note about.

 

Many websites explain the basics of fit tolerance, but the information is often fragmented and insufficient to systematically understand the overall design. For example, while they may touch upon how to choose a tolerance grade, there isn't much information that delves into how that affects the internal clearance of a bearing, or the balance between conditions requiring press-fitting and ease of assembly.

 

This article aims to comprehensively understand tolerance design for bearing mounting parts by connecting scattered pieces of information into a coherent whole.

 

__OPENROUTER_FAILED__Design considerations for related parts that are easily overlooked but can lead to critical defects　Note the

 

By reading this article to the end, you should be able to confidently perform optimal tolerance design.

"Fit" as the Basis for Tolerance Design of Bearing Mountings

To maximize bearing performance, the tolerance design of the mounting part is extremely important.This is. In particular, the concept of "fit," which determines the relationship between the shaft and the housing, forms the basis of all designs. Here, we will explain the fundamental knowledge for avoiding troubles caused by inappropriate tolerance settings and achieving stable mechanical performance.

 

What are the three types of fit?

There are mainly three types of fits for bearing installation.These are defined by the dimensional relationship between the shaft and the inner ring of the bearing, or between the housing and the outer ring of the bearing.

 

The first is "clearance fit." This refers to a fit where the hole's dimension is always set to be larger than the shaft's dimension, ensuring there is always a gap between the parts. The advantage of this method is that it makes assembly and disassembly of parts easier.

 

The second is called a "press fit." This is the opposite, where the hole is dimensioned smaller than the shaft, and it's a method of fixing parts securely by applying pressure to fit them together. It's used when you want to prevent parts from slipping and to transmit force reliably.

 

And the third is "intermediate fit," which has a nature between the two. This refers to a fit that can result in either a slight clearance or interference, depending on the dimensions of the parts being combined. It is utilized in a wide range of applications according to the design intent.

 

 

When to use tight fit and when to use clearance fit

The basic design principle is to use interference fits and clearance fits according to the load conditions applied to the bearing.Failure in this selection can not only prevent the bearing from performing at its best but also lead to the malfunction of the entire machine.

 

The specific criteria for distinguishing between them lie in whether the load "rotates" or is "stationary."For example, consider a case where the inner ring of a bearing rotates with the shaft, and the outer ring is fixed to the housing. In this case, it is common to apply a "press fit" to the rotating inner ring and a "clearance fit" to the stationary outer ring.

 

If you say it like this, you might wonder why a press fit is necessary on the rotating side. The reason is explained next.To prevent the harmful phenomenon called "creep"　It is.

 

 

Mechanism of creep

Creep is the phenomenon where the inner ring of a bearing slips slightly against the mating part and shifts in the circumferential direction when rotational load is applied to the bearing with insufficient retaining force on the fitting surface.This phenomenon significantly shortens the lifespan of bearings, so it must be addressed during the design stage.

 

 

What is an orbital wheel?

Here,"Bearing rings" refers to the inner and outer rings that are key components of a bearing.These ring-shaped parts have grooves (raceways) for rolling elements such as balls and rollers to roll in, and they serve as the tracks that support the smooth rotation of the bearing.

 

If a rotating race is installed with a slight interference fit, the race will flex slightly in the loaded area, creating a minute gap with the mating part on the opposite side. As the direction of the load shifts with rotation, this gap also shifts, resulting in the race slowly rotating as if it were slipping.

 

Once creep occurs, wear on the fitting surfaces progresses. Then, the metal powder generated by wear enters the bearing, contaminating the lubricant. This is extremely important for preventing creep, as it ultimately leads to catastrophic failures such as "seizure," which will be explained next.

 

Also, on NTN's website,Examples of creep damage with photos　The company has been

 

 

Seizure

Seizure is one of the most serious types of bearing failure.This refers to a phenomenon where rapid localized heating occurs during rotation due to poor lubrication, excessive load, or insufficient clearance, causing the surfaces of the raceway rings and rolling elements to melt and weld together. Once seizure occurs, the bearing becomes unable to rotate, leading to a serious malfunction that stops the entire machine. Contamination of the lubricant by creep is one of the main causes that trigger this seizure.

 

 

How to distinguish between rotating loads and stationary loads

As mentioned above,To prevent creep, it is essential to make the rolling bearing subjected to rotating load a press fit. Therefore, the designer must accurately determine whether the load is a "rotating load" or a "static load."

 

A rotating load is a load whose direction is relatively rotating with respect to the raceway.refers to. Meanwhile,Static load refers to a state where the direction of the load on a rolling element remains constant.means. Let's look at this difference with a specific case study.

 

 

Case Study 1: Electric Motor Rotor Shaft

• Situation: The rotor shaft of an electric motor is supported by bearings, and the shaft (inner ring) rotates at high speed. The housing (outer ring) is fixed. The load is applied in a spatially fixed direction, such as the rotor's own weight or belt tension.
• Analysis:
  • Inner ring: When viewed from the rotating inner ring, a stationary load (such as dead weight) acts as if it is rotating around you. This is called "rotating load."
  • Outer ring: When viewed from the fixed outer ring, the direction of the load is always constant. This is called a "static load."
• Conclusion: The inner ring, which receives a rotating load, is fixed to the shaft with an "interference fit," and the outer ring, which receives a static load, is mounted to the housing with a "clearance fit."

 

Case Study 2: Automotive Wheel (Hub Unit)

• __OPENROUTER_FAILED__
• Analysis:
  • Inner Ring: When viewed from the fixed inner ring, the direction of the load is always constant and vertically downward. This is referred to as "static load."
  • Outer ring: From the perspective of the rotating outer ring, the stationary load (vehicle weight) acts as if it is rotating around the inside of the outer ring. This is called "rotational load."
• Conclusion: The outer ring, which receives rotational load, is fixed to the hub with an "interference fit," and the inner ring, which receives static load, is mounted to the axle with a "clearance fit."

 

Case Study 3: Conveyor Idler Rollers

• Situation: The shafts supporting both ends of the roller are fixed to the frame. The roller body (housing and outer race) rotates with the movement of the belt. The load is the weight of the conveyed object and is always applied vertically downwards.
• Analysis: This structure is exactly the same as the wheel in Case Study 2. A "static load" acts on the inner ring attached to the fixed shaft, and a "rotational load" acts on the rotating roller body (outer ring).
• Conclusion: The outer ring is a "transitional fit," and the inner ring is a "clearance fit."

 

 

Basic Patterns for Fit Selection

Summarizing these relationships, the selection of fits can be organized as follows:

Operating conditions	Target race	Load type	Recommended fit	Main purpose/reason
Inner ring rotation, outer ring stationary	Inner circle	rotational load	tightening tear	Creep prevention
(motors, pumps, etc.)	Outer ring	static load	cracks in the ice	Ease of assembly and disassembly, absorption of thermal expansion
Inner ring stationary, outer ring rotating	Inner circle	static load	cracks in the ice	Ease of assembly and disassembly, absorption of thermal expansion
(Wheels, conveyor rollers, etc.)	Outer ring	rotational load	tightening tear	Creep prevention

This table shows the most basic concept in interference fit design.This indicates. In actual design, you will adjust the degree of interference and clearance by considering factors such as the magnitude of the load, temperature, and accuracy requirements.

 

 

Practical Bearing Mounting Hole Tolerance Selection Process

After understanding the basic principles of fit, the next step is to select specific tolerances. Here, we will explain a practical process for choosing dimensional tolerances and grades to indicate on drawings, as well as points that must be considered during selection.

 

The relationship between dimensional tolerance and clearance/interference fit

Dimensional tolerances and interference fits, which are often seen in design drawings, are closely related.It is located at. Dimensional tolerance is the range of variation in dimensions that is permitted when parts are manufactured.

 

Meanwhile, fit tolerances determine the relationship between two parts that are assembled together, such as a shaft and a hole, and are determined by the combination of the dimensional tolerances of each part. For example,The fit conditions, such as the aforementioned "clearance fit" and "interference fit," are determined by how the dimensional tolerances for the shaft and the hole of the bearing's inner ring are set.　It is a

 

Therefore, the designer must properly select the dimensional tolerances of the shaft and housing to achieve the desired fit (clearance, interference, etc.).

 

 

How to choose a tolerance grade (IT grade)

The International Organization for Standardization (ISO) has established "Tolerance Classes (IT Classes)" as an indicator of dimensional tolerance stringency.IT stands for International Tolerance. This grade refers not to the bearing itself, but to the mating parts into which the bearing is fitted.This applies to the machining accuracy of shafts and housings.The grade is represented by a number, with smaller numbers indicating a narrower tolerance range and higher processing accuracy.

 

For general industrial machinery, a tolerance class of IT6 is often recommended for the shaft where the bearing is mounted, and IT7 for the housing bore. However, for applications requiring extremely high rotational accuracy, such as the spindle of a machine tool, even tighter tolerance classes like IT5 or IT4 are selected.

 

WHEREAS,Relationship between IT grades and tolerance symbols defined by JISI will supplement the information regarding this. The notation for JIS fit tolerance (e.g., h6) is composed of two elements.

 

The alphabet (the 'h' part) is called the "tolerance class" and indicates where the tolerance is located relative to the basic dimension.For example, h represents the fundamental deviation, g represents a finer deviation, and the number "6" represents the "IT basic tolerance grade" itself.In other words,

• h5 means that the tolerance zone class is h and the basic tolerance grade is IT5.
• h6 means that the tolerance zone class is h and the fundamental tolerance grade is IT6.

 

JIS Fit Tolerance Class Examples (for Housing Materials of Cast Iron and Steel)

 

Of course, the tighter the tolerances, the higher the manufacturing costs. Therefore, it is important to select the optimal tolerance grade by considering the balance between the performance required of the machine and the cost.

 

 

Effect of temperature change on fit

During machine operation, bearings generate heat due to internal friction and external heat conduction.This temperature change significantly affects the fit condition, so it must be considered during the design phase.

 

Especially important is the tight fit between the inner ring and the shaft. Normally, during operation, the bearing's inner ring becomes hotter than the shaft. Even between steel materials, the inner ring, being at a higher temperature,Thermal expansion amountAs it expands, the interference fit with the shaft loosens as a result. The amount of this reduction in interference due to the temperature difference is very large.This can also be a dominant factor, especially in high-speed rotating machinery.

 

Therefore, designers must anticipate the temperature rise during operation and pre-set a larger interference fit at room temperature. Without this consideration, the effective interference fit will be lost when the operating temperature is reached, increasing the risk of creep.

 

 

Considerations for low-temperature environments

Conversely, in extremely low-temperature environments, such as with machinery used in cold regions, the opposite phenomenon of temperature rise occurs. When the entire machine is cooled, each part contracts. A point to note here is when a material different from steel, such as an aluminum alloy, is used for the housing.

 

Aluminum alloy has a higher coefficient of thermal contraction than steel, so in low-temperature environments it contracts more than the steel bearing outer ring.As a result, even if it is a clearance fit or an interference fit at room temperature, it may become a tight interference fit at low temperatures, excessively reducing the internal clearance of the bearing.

 

This is, of course, just a guess, but for example, transfer equipment operating in refrigerated warehouses or construction machinery designed for cold regions could fall under this category. In such cases, countermeasures are necessary, such as selecting a looser fit (with a larger clearance) at normal temperatures, anticipating shrinkage at low temperatures, or using bearings with larger internal clearances, such as C3 or C4.

 

 

Selecting tolerances based on housing material

The recommended values for standard fits often assume that the housing is made of cast iron or steelIf a light alloy such as aluminum alloy is used for the housing for purposes such as weight reduction, caution must be exercised in selecting tolerances.

 

Light alloys differ from cast iron and steel in two main respects.

1. Large coefficient of thermal expansion：The thermal expansion coefficient of aluminum alloy is about twice that of steel.Therefore, when the temperature of the entire machine rises, the aluminum housing expands more than the steel bearing outer ring, changing the fit in the direction of loosening.
2. Low rigidity (Low Young's modulus）：The rigidity of aluminum alloys is about 1/3 that of steel.Therefore, even if the outer ring is press-fitted with the same interference fit, the housing is likely to deform, and the expected holding force may not be obtained.

For these reasons,When using a light alloy housing, it is common to select a press fit (interference fit tolerance) that is one step tighter than for cast iron or steel housings.

Housing material	Load condition	Axial movement	Recommended tolerance class	Types of fit
Cast iron, steel	Outer ring rotation	unnecessary	P7	tightening tear
	Internal rotation (fixed side)	unnecessary	K7	Intermediate fit
	Internal rotation (free side)	necessary	H7	cracks in the ice
Light alloy	Outer ring rotation	unnecessary	R7	tightening tear
(Aluminum, etc.)	Internal rotation (fixed side)	unnecessary	M7	Intermediate fit
	Internal rotation (free side)	necessary	G7	cracks in the ice

 

 

What are the conditions that require press-fitting?

Press-fitting is a typical assembly method for achieving a tight fit.This is used when parts need to be securely fixed, especially in machinery subjected to large loads or in areas where torque needs to be reliably transmitted.

 

For example,For bearings that receive extremely large loads, such as those in railway vehicles or crushers, a tight fit is essential to prevent creep, and installation is performed by press-fitting.To perform press-fitting, specialized equipment such as a press machine is required, and significant force is necessary for assembly and disassembly.

 

As a point of caution, it may be difficult to disassemble parts without causing damage when they are press-fitted. Therefore, for areas that require disassembly for regular maintenance, it is necessary to avoid press-fitting or devise a structure that facilitates easy disassembly.

 

 

Always take into account changes in internal clearance.

When selecting interference fit, one of the most important design considerations is the change in bearing "internal clearance."These are internal clearances. Internal clearance is the small amount of play built into a bearing; without it, a bearing cannot rotate smoothly.

 

When the inner ring is press-fitted onto the inner ring with a tight fit, the inner ring is pushed outward. This deformation reduces the internal clearance of the bearing. Similarly, when the outer ring is mounted to the housing with a tight fit, the outer ring is compressed inward, also reducing the internal clearance.

 

If this reduction is too large and the internal clearance becomes zero or negative (preloaded state), excessive force will be generated inside the bearing. As a result, this will cause abnormal heat generation and increased rotational torque, leading to premature bearing failure. Therefore, when selecting an interference fit, it is essential to calculate the amount of clearance reduction and ensure that appropriate clearance remains under operating conditions.

 

 

Geometric tolerances for bearing mounting surface shape

To properly bring out bearing performance, dimensional tolerances alone that control the diameter of the mating part are insufficient.It is necessary to simultaneously consider "geometric tolerances" that regulate the precision of the "shape" of the parts themselves. Here, we will explain geometric tolerances and related part design, which are often overlooked but extremely important.

 

The importance and types of geometric tolerances

What is Geometric Tolerance?It regulates how much the shape of a part deviates from geometrically correct circles, straight lines, and planes.Even if the diameter is within the dimensional tolerance, the bearing will not function correctly if the shape is elliptical or distorted.

 

Improperly fitting bearings onto shafts or housings with incorrect shapes will deform the raceways. As a result, the load within the bearing becomes uneven, causing vibration and noise, and ultimately leading to premature failure.

 

When installing bearings, particularly important geometric tolerances include "roundness" and "cylindricity," which regulate the shape itself, and "runout," which regulates the positional relationship with the mounting reference surface.and others.

 

 

How to Specify Roundness and Cylindricity

Roundness is an indicator that shows how much a part's cross-sectional shape deviates from a perfect circle.Also, cylindricality regulates how much the overall shape of the part deviates from a perfect cylinder, and it can be considered a stricter tolerance that encompasses roundness, straightness, and other factors.

 

If these geometric errors exist on the fitting surfaces, the contact pressure will be uneven when the bearing is fitted. In areas of high pressure, the bearing's outer or inner ring can deform locally, and conversely, in areas of low pressure, it can become a starting point for creep.

 

Generally, the cylindricality of mating surfaces is required to be one to two tolerance grades tighter than the dimensional tolerance IT grade.For example, if the dimensional tolerance for an axis is specified as IT6, it is recommended that cylindricity be controlled with IT5 or IT4.

 

 

The runout accuracy of the shoulders determines the lifespan.

Bearings are axially positioned by resting their end faces against steps called "shoulders" on the shaft or housing.The accuracy of this shoulder surface, especially the "runout" accuracy, is a critical factor that directly affects the lifespan of the bearing.It is.

 

Runout is the value that indicates how much a part's surface deviates from its position when rotated. If the shoulder surface of a shaft is tilted with respect to the center of rotation (large axial runout), the bearing will be installed in a tilted state.

 

This inclination generates excessive force within the bearing, causing a phenomenon called "edge loading" where excessive stress concentrates at the ends of the raceway. Since edge loading is one of the most detrimental factors that significantly reduces bearing life, shoulder runout must be strictly controlled.

 

 

Surface roughness standards for mating surfaces

The finished state of the mating surfaces, in other words, "surface roughness," also affects the quality of the fit.If the surface is too rough, the peaks of the surface irregularities may collapse during press-fitting, potentially preventing the designed interference fit effect from being achieved. In addition to this reduction in effective interference, a rough surface can cause localized stress concentrations, contributing to a decrease in fatigue life.

 

Generally, the recommended arithmetic average roughness Ra for fitting surfaces varies depending on the size of the bearing. For example,For shafts where small bearings are fitted, Ra of 0.8 μm or less is recommended.is done. On the other hand,The holes in the housing do not require as strict a finish as the shaft, with a target of 1.6 μm or less.　The first two are the following.

 

These surface roughness values are generally achievable with turning, but grinding is recommended when particularly strict requirements for runout or acoustics exist.

 

 

Recommended values for overall accuracy and surface roughness

The following table summarizes the recommended dimensional tolerances, geometric tolerances, and surface roughness according to bearing grade.

Bearing grade	Parts	subject (of taxation, etc.)	Dimensional tolerance grade	Geometric Tolerance Grade (Cylindricity/Runout)	Surface roughness Ra (μm)
Normal class	軸	Fit part	IT6	IT5	Less than or equal to 1.6
		Shoulder	-	IT5 (Axial Runout)	-
	Housing	Fit part	IT7	IT6	<=3.2
		Shoulder	-	IT6 (Axial Runout)	-
Precision grade	軸	Fit part	IT5	IT3 - IT4	Less than or equal to 0.8
(P5, P4)		Shoulder	-	IT3 (Axial Runout)	-
	Housing	Fit part	IT6	IT4 - IT5	Less than or equal to 1.6
		Shoulder	-	IT4 (Axial Runout)	-

(Note) The above is a general guideline. More stringent tolerances are required for bearings with higher precision.

 

 

Design criteria for shoulder height and corner rounding

There are two important rules for designing the shape of the shoulder that supports the bearing: "shoulder height" and "corner radius."It is.

 

First, regarding the shoulder height, a height sufficient to adequately support the bearing end face is necessary. Specifically, the shoulder must be designed to be higher than the bearing's chamfer dimension. This ensures that the bearing end face makes firm contact with the shoulder, enabling stable support.

 

On the other hand, you also need to be careful not to raise the shoulders too high. If the diameter of the shoulders is too large, it may leave no room for tools like pullers when disassembling the bearing, or it may interfere with the seal if it is a sealed bearing. Therefore,It is safest and most reliable to design based on the shoulder diameter (da, Da) values recommended by the bearing manufacturer in their catalog dimension tables.It is.

 

Next is the corner rounding (fillet radius). To avoid stress concentration, a fillet is always provided at the base of the shoulder, but if this radius is too large, it will interfere with the chamfer of the bearing. Therefore,The radius of the corner rounding must always be designed to be smaller than the chamfer dimension of the bearing.If this rule is not followed, the bearing will be installed tilted, causing premature failure.

 

 

Key points for optimal bearing tolerance design

As explained so far,Optimal tolerance design for bearing mounting seats is not simply about deciding on a single number.First, based on the basic principles of fits, select either "interference fit" or "clearance fit" based on the load conditions. Next, determine the dimensional tolerances and tolerance grades, and it is essential to always check the resulting changes in internal clearance.

 

And, it is key to bring out 100%of bearing performance not only by specifying dimensions but also by properly specifying geometric tolerances such as roundness, cylindricity, and runout, and furthermore, by considering details such as surface roughness and shoulder shape.Let's comprehensively consider these elements and aim for an optimal design while balancing the performance and cost required of the machine.

 

• There are three types of fits: "clearance," "interference," and "transition."
• For bearing rings subjected to rotational load, use a press fit.
• An orbiting wheel subjected to a static load can be a "clearance fit."
• Tight fitting is essential to prevent creep.
• Adjust the strength of the clamp according to the magnitude of the load.
• The fit condition is determined by the combination of dimensional tolerances.
• The smaller the number for the tolerance grade (IT grade), the higher the precision.
• The shaft is IT6, and the housing is IT7, which are common standards.
• A press fit reduces the internal clearance of a bearing.
• Thermal expansion due to temperature rise also reduces internal clearance.
• Choose a clearance class (e.g., C3) anticipating a reduction in internal clearance.
• Specifying geometric tolerances, as well as dimensional tolerances, is important.
• The roundness and cylindricity of the fitting surface should be stricter than the dimensional tolerance.
• The freedom from runout prevents bearing tilt and protects its lifespan.
• The shoulder height should be higher than the chamfer of the bearing.
• The corner radius should be smaller than the bearing chamfer
• Properly manage the surface roughness of the fitting surfaces.

 

That's it.

 

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