Criteria for Fitting Bearings｜Case Studies and Design Points

Here.Criteria for determining whether to press-fit bearings｜Examples and design pointsI am making a note about the

For many mechanical designers, information on bearing press fitting tends to be a matter of following tolerance tables in catalogs, but this article is not just about choosing numbers, but about the engineering rationale behind them and theI would like to clarify the reason why bearings should be pressed-in based on my practical experience and detailed research, and to clarify "clear criteria for situations where bearings should be pressed-in and situations where they should not be pressed-in.to the

While many other sources of information only list general recommendations, this book goes into the effects of "dynamic environments" such as vibration and thermal expansion on the fit, covers criteria that can be used immediately in the field, and explains the entire design process, including installation methods using press-fit jigs, burn-in procedures, and future maintainability. The book also explains the entire design process, including installation methods using press-in jigs, burn-in procedures, and future maintenance.

Judgment of press fitting of bearings and basis of design based on JIS

Basic use of clamping and crevice joints

When incorporating a bearing in a machine design, the condition of the coupling with the shaft and housing (bore), i.e., theThe selection of the "fit" is the first gateway to determining the life and performance of the machine.　　Fits can be broadly classified into two types: "clamping fits" and "clearance fits," and the physical characteristics and roles of each must be clearly understood and used separately.

Clamping is a design technique in which the shaft diameter is slightly larger than the bearing bore diameter or the bearing outer diameter is slightly larger than the housing bore diameter.　　In this state, the parts are geometrically interfering with each other before being assembled, and are only put together when a strong force is applied to "press-fit" them together.

The press-fitted bearing and shaft are firmly adhered and integrated by restoring force (surface pressure) due to elastic deformation.　　It is mainly used in drive shafts that need to transmit rotational torque, in locations subject to high vibration and shock, and in critical security locations where high positioning accuracy and rigidity are required.

On the other hand,Clearance is a design in which a small gap (clearance) is intentionally left between the bearing and the mating material.　　In this case, the bearing can be threaded onto the shaft or inserted into the housing by hand without resistance.　　It is mainly applied on the side that receives the static load, on the "free side bearings" that need to absorb the expansion and contraction of the shaft due to thermal expansion, and where frequent disassembly and replacement maintenance is performed.

There is also an "intermediate bame" that falls between these two, though,In practice, the basic approach is to use a tight fit on the side that must not move and a clearance fit on the side that ensures assembly.　This is the case.　　If this basic distinction in use is made incorrectly, it can lead to serious problems, such as bearings spinning on the shaft during operation, which can wear the shaft, or conversely, bearings that are too tight, which can damage parts during assembly.

Proper fit of inner and outer rings determined by rotational classification

The decision as to whether the inner or outer ring of a bearing should be press-fitted is based on a logical rule called "rotation classification," rather than on a rule of thumb or intuition.　　This rule is derived by analyzing which raceways rotate and in which direction the load is applied.

The most standard case is the "rotating inner ring, stationary outer ring" condition.　　This configuration is found in many parts of industrial machinery, such as motor shafts and fan drive shafts.　　In this case, the inner ring rotates with the shaft, and the position where the load is applied (point of action) moves one after another on the inner ring raceway.This is called "rotating load.　　If there is a gap between the inner ring, which receives the rotating load, and the shaft, the inner ring will be displaced in the circumferential direction from the shaft as it rotates.　　To prevent this, the inner ring must be secured to the shaft by a "clamping fit.　　In contrast, since the stationary outer ring is subjected to a load in a fixed direction (static load), a "clearance fit" is generally selected for the housing to give priority to ease of assembly.

Conversely, there are cases of "rotating outer ring and stationary inner ring.　　Idler rollers in conveyors and wheel hubs in automobiles fall into this category.　　In this situation, the outer ring must be "clamped" against the housing (roller or hub) because the outer ring is subjected to the rotational load.　　On the other hand, since the shaft is fixed and the inner ring is subject to static load, the inner ring should be designed to have a "clearance fit" so that it can be easily pulled off the shaft during maintenance.

The following table summarizes the basic fit selection criteria based on the nature of the rotational state and load.

Table 1: Basic matrix of rotation classification and fit selection

Reference Source：NTN Rolling Bearing General Catalog

Reference Source：How to Select JTEKT Bearings

Like this,The principle of "press-fitting the ring on the side that receives the rotational load."　If you understand the following, you will be able to make the right decisions without hesitation under complex design conditions.

Procedures for selecting JIS tolerances and recommended tightening allowance

If it is determined that press fitting is necessary, the next step is to determine the specific level of dimensional tolerance (tolerance range class) to be indicated on the drawing.　　This selection is based on the Japanese Industrial Standard (JIS B 0401) and takes into account the magnitude of the load and the presence or absence of impact.

For the tolerance range classes of shafts, the following are used depending on the load level in general inner ring rotation applications.

For precision equipment that requires light loads and high rotational accuracy, "k5" and "js5" are chosen.　　They have a very small or near-zero tightening allowance, minimizing stress on the bearing inner ring.

For general industrial machinery with normal loads, "m5" or "m6" is standard.　These settings create a secure press-fit condition and are less likely to loosen over long periods of operation.

Furthermore, in environments where heavy or severe impact loads are applied (construction machinery, rolling mills, etc.), strong clamping clamps such as "n6" and "p6" are selected.　However, since these are difficult to press-fit at room temperature, assembly using thermal expansion such as baking fit is required.

The same considerations apply to the tolerance range class of the housing.In the general case where the outer ring is stationary, "H7" is the most standard.　It is easy to process and provides adequate clearance.　　For more clearance or to release thermal expansion, "G7" or other types may be selected.

On the other hand, if the outer ring rotates, or if the housing is thin-walled with low rigidity, a tight fit tolerance such as "M7" or "N7" is selected to prevent the outer ring from spinning, and the housing hole is made smaller than the bearing outer diameter.

Table 2: Guideline for selecting shaft and housing tolerances for radial bearings (Class 0)

Reference Source：NSK (NSK) Selection of Rolling Bearings

In design, the process of determining the optimum tolerance class is important, based on the catalog recommendations and after calculating the "equivalent radial load" of the target machine and checking the ratio of this to the basic dynamic load rating (light load, normal load, or heavy load).

Internal gap reduction after press-fitting and points to note when selecting

One of the physical phenomena that designers must pay the most attention to when press-fitting bearings together is the "reduction of internal clearance.The surface pressure created by the press fit elastically deforms the raceways.

Specifically, when the inner ring is press-fitted (clamped) onto the shaft, it is pushed apart from the inside and expands in the direction of the outer diameter.　　This increases the diameter of the raceway groove in which the ball or roller rolls.　　Conversely, when the outer ring is press-fitted into the housing, it is tightened from the outside and contracts in the inner diameter direction.The total amount of expansion of the inner ring and contraction of the outer ring will reduce the "radial internal clearance" that the bearing originally had.

If a tight tolerance such as m6 or n6 is set for a bearing with a standard gap (CN gap), the residual gap after assembly could be less than zero (negative gap).When operated in a negative gap condition, the rolling elements are always strongly pinched, resulting in increased rotational torque, abnormal heat generation, and premature seizure damage.

To avoid this problem, when a clamping joint is employed,Especially in designs with a large tightening allowance, it is standard practice to select bearings with a C3 or C4 clearance, in which the internal clearance is set larger than the standard clearance.　　During design, the following factors are considered to identify the final effective gap

1. Gap reduction due to fitting(inner ring expansion + outer ring contraction)
2. Gap reduction due to temperature difference between inner and outer rings(During operation, the inner ring tends to be hotter and thus expands)

Even if these are deducted,Professional design approach is to select the initial gap so that a slightly positive gap (or appropriate preload for the purpose) remains during operation(Bearing selection is difficult when you get down to it.　(Bearing selection is difficult when you get down to it.)

Managing the risk of inner ring cracking due to hoop stress

Another risk often overlooked in press-fit design is cracking of the inner ring due to hoop stress (circumferential tensile stress).　　Bearing steel used in bearings is extremely hard and wear-resistant, but it does not have a high degree of toughness (tenacity) and is brittle under tensile forces.

When an inner ring is press-fitted onto a shaft with a strong clamp, the inner ring is constantly subjected to a force that stretches it in the circumferential direction.　　If this tensile stress (hoop stress) exceeds the allowable limit of the material, the inner ring may crack in a straight line in the axial direction at the moment of impact or thermal stress during operation.　　The risk is particularly high when press-fitting into a hollow shaft or when used in a low-temperature environment, such as in cold regions, because the material tends to become brittle.

To avoid this risk, the designer should check the calculation of the stresses occurring at the maximum tightening allowance.　　General,Safe allowable stress in bearing steel is 120 MPa (megapascal) or lessIt is considered to be a

If the calculated result exceeds this value, design changes should be made, such as changing to a looser tolerance class, increasing the bearing size to ensure a thicker inner ring, or considering a combination of adhesive fixation instead of press-fitting.　　It is important to ensure that the safety factor is sufficient for the criticality of the machine.

Physical Phenomena Requiring Press-Fitting of Bearings Learned from Case Studies

Mechanism of Creep Occurrence due to Insufficient Press Fit

Creep is one of the most feared failure phenomena in bearing mating selection.Creep is a phenomenon in which the raceways of a bearing gradually shift in relation to the shaft or housing and rotate (move circumferentially) when subjected to a rotational load with a gap in the fit.refers to

This is not simply a phenomenon of slipping due to lack of friction.　　If there is a gap between the inner ring and shaft, the inner ring is slightly elastically deformed and deflects at the contact point under radial load.　　This deflection causes a difference in circumference (difference in travel distance) between the contacting shaft and inner ring.　　　When the shaft rotates, the inner ring moves forward by this slight difference, as ifRotates with strong driving force like planetary gearsThis is a very powerful force.　　This force is very powerful,Difficult to stop with set screws or general frictional forces aloneIt is.

When creep occurs, the contact surfaces are scraped and wear debris is generated, which penetrates into the bearing and damages the rolling surfaces.　　In the worst case scenario, the shaft becomes thin and thin, leading to a serious accident in which the machine rattles and stops.

As mentioned above,For raceways subjected to rotational loads, the most reliable and engineering-correct measure to prevent creep is to completely eliminate physical gaps by press-fitting (clamping) and to strongly integrate the rings by the repulsive force of elastic deformation.　The first two are the following.

Secure fastening to prevent fretting wear

Fretting (fine motion wear) occurs when a vibration load is applied with an insufficient press fit or with a slight gap.　　This is a wear phenomenon that occurs when parts that appear to be stationary to the naked eye repeat minute back-and-forth sliding at the microscopic level.

When a clearance-mounted bearing is subjected to vibration or variable loads, the metal surfaces rub against each other to a very small extent on the contact surface.　　As a result, the oxide film on the surface is repeatedly destroyed and regenerated, resulting in a large amount of fine reddish-brown wear particles (iron oxide) called "fretting corrosion.　　Although it is easily mistaken for corrosion because it resembles rust in appearance, it is essentially wear.

As fretting progresses, the gap between the mating surfaces widens and rattling increases, leading to a vicious cycle of even more severe vibration.　　In addition, the hard wear particles generated act like an abrasive and rapidly wear the shaft and housing.

The most effective way to prevent this is to provide an appropriate tightening allowance and press fit to physically contain the minute relative motion at the contact surface.　　Also,In areas where press-fitting is unavoidable, the technique reduces the risk of fretting by applying a high-viscosity paste lubricant or by using anaerobic adhesives in combination to completely fill micro gaps.is taken.

Special selection criteria for disproportionate load environments

Special loading conditions called "disproportionate loads" occur in vibrating motors, vibrating sieve machines, or machines that handle rotating bodies with an unbalanced center of gravity.　　In this environment, the designer must be especially careful because the normal fit selection rules (inner ring press-fit for inner ring rotation) may be reversed.

A disproportionate load is the centrifugal force generated by the rotation of an eccentric weight (weight). The direction of this centrifugal force is always outward, synchronized with the rotation of the shaft.

This can be analyzed from the bearing perspective as follows.

• Inner ring (rotates with the shaft): The eccentric weights also rotate with the shaft.　This means that, from the viewpoint of the inner ring, centrifugal force is always applied to the same single point (in the direction of the weights).　This is a relative "static load" condition.
• Outer ring (fixed to housing): The direction of the load (centrifugal force) comes around and around as the shaft rotates. In other words, the entire circumference of the outer ring raceway is sequentially loaded, which is a "rotating load" condition.

Based on this logic, in vibrating machines,The rotating inner ring is set in a "clearance fit" and the stationary outer ring is set in a "clamping fit" (press-fit).In some cases, the selection of the motor is the opposite of that of a normal motor.　If the outer ring is mistakenly set to a clearance fit, severe creep and fretting will occur between it and the housing, causing the housing bore to wear into an ellipse.

Table 3: Paradoxes of fit selection in disproportionate load environments

Reference Source：NTN Rolling Bearing General Catalog

Thus, it is not just a question of whether it is turning or not,Determining whether the load vector is moving with respect to the ring is an advanced design perspective that prevents problems.It is.

Measures and calculations for thermally expanding housings

In recent years, light alloys such as aluminum and resin are increasingly being used for housings to reduce the weight of machines,The big pitfall here is the "Difference in coefficient of thermal expansion".　　While the coefficient of linear expansion of iron (bearing steel) is about 12.5 × 10^-6 /K, aluminum has a coefficient of expansion of about 23 × 10^-6 /K, nearly twice as high.

For example, suppose an outer ring is press-fitted into an aluminum housing at room temperature (20°C) with an appropriate fit tolerance.　　However, when the machine is operated and the temperature rises to 60°C to 80°C, the aluminum housing expands more than the steel bearing outer ring, causing the hole diameter to widen.　　As a result, the tightening allowance is lost only during high-temperature operation, resulting in an unintended change to a "clearance fit.

this (something or someone close to the speaker (including the speaker), or ideas expressed by the speaker)When "missing tightening allowance" occurs, the outer ring creeps only during operation, leading to a nasty problem that wears down the aluminum housing and enlarges the bore.

Many conservationists in the field must see this phenomenon.

As a countermeasure, when aluminum housings are used, thermal expansion at the maximum operating temperature should be calculated and a very tight tolerance (P7, R7, etc.) should be set at room temperature.　　Alternatively, it is also effective to adopt a structure in which an iron sleeve (insert ring) is cast or press-fitted into the bearing mating area of the aluminum housing to make the contact surface with the bearing "iron-to-iron.　　Although difficult, the designer may need to be careful to accurately estimate the operating ambient temperature and determine tolerances after simulating dimensional changes due to heat.

Practical techniques for press fitting and maintenance of bearings

Correct insertion method of bearings using a press-fit jig

No matter how perfect the tolerances selected on the design drawings, if the bearings are improperly assembled on the manufacturing floor, they will be fatally damaged before they reach their full potential.The ironclad rule that must absolutely be followed in press-in operations is to "press directly on the raceway ring on the side to be press-fitted.

To give a specific example, when press fitting an inner ring to a shaft, the object to which force should be applied must always be the "inner ring.　　If the force is accidentally applied by pressing the outer ring, the press fit force will be transmitted to the inner ring via the balls and rollers (rolling elements).　　At this time, the rolling elements will be pressed hard against the raceway, resulting in minute indentations (Brinell indentations) or scratches.　　This causes poor rotation and abnormal noise from new.

For correct installation, a special "press-fit jig" (application tool) is used.　　This is a pipe-shaped tool precisely machined to apply force evenly across the entire end face of the inner ring.　　Striking the bearing directly with a hammer is strictly prohibited.　　Ideally, a hand press or hydraulic press should be used to press the bearing straight against the shaft at a slow, constant speed through a jig.

The designer must design the shape to ensure in advance that there is enough space (height of the shaft shoulder and clearance) for this jig to securely hit the inner ring end face.　For mass production,Appropriate surface roughness and geometric tolerances of mounting points　andControl of Press-in Force　The method of managing the tightening fee is also taken, for example, by

Temperature control and procedure for heating insertion with yakibame

In the case of large industrial bearings and "n6" and "p6" bearings with a large tightening allowance for heavy loads, it is physically difficult to press-fit at room temperature, and if forced, the contact surface will be damaged due to galling (metal-to-metal adhesion).　　Therefore, the commonly used method is the "quenching" method.

In the burn-in fit method, the entire bearing is heated to cause thermal expansion, temporarily widening the inside diameter before being inserted into the shaft.　　Upon cooling, the bearing returns to its original dimensions and a strong clamping fit is obtained.　　The most important control item in this process is the "heating temperature.

The upper temperature limit at which the microstructure of a typical bearing steel remains unchanged and hardness is120°CThe temperature at which the steel is heated above this level will temper and soften the steel.　　Heating above this temperature will temper and soften the steel, dramatically reducing the rated life of the bearing.　　In practice, it is recommended to work at temperatures below 100°C to ensure a safety margin.

The best heating method is to use a dedicated "induction heater.　　This device can heat bearings evenly and quickly, has accurate temperature control, and automatically demagnetizes them after operation. Direct heating with a gas burner should never be used because of the high risk of local overheating due to the high degree of temperature irregularity.

In addition, immediately after hard-facing, the bearing will try to shrink in the axial direction as it cools. Therefore, after insertion, it is essential to continue applying pressure in the axial direction with a shaft nut or the like until the bearing cools completely, and to tighten the bearing so that there is no gap between the shaft shoulder and the inner ring (retightening).

Design innovations that take future maintenance into consideration

Press-fitted bearings are extremely rigidly fixed in place, making them extremely difficult to remove when they need to be replaced over the life of the machine or for periodic inspections.　　If maintainability was not taken into consideration during the design phase, on-site workers would have no way to remove the bearings, which could result in burning them out with a burner or destroying the entire shaft.

Skilled designers draw their drawings with "disassembly" as well as assembly in mind.　　Specific innovations include the following designs.

1. Installation of cutouts for pullers: A groove or notch is provided in the shoulder of the shaft (where the bearing butts up against it) to catch the claw of the puller tool (puller).　　This allows the puller to securely grasp the inner ring and pull it out.
2. Placement of jack bolt holes: When a bearing outer ring is press-fitted into a housing, a threaded hole (service hole) through the bottom or back side of the housing should be provided.　　When disassembling, bolts can be screwed in here to press the bearing outer ring out evenly from the back side.
3. Oil groove for hydraulic removal: For extra-large bearings, a hydraulic pump connection hole and groove may be machined in the shaft center to allow "hydraulic draining," in which high-pressure oil is fed into the mating surface and drained out while expanding it.

The designer must be able to determine not only "how to put it in" but also "how to take it out" with imagination.

Summary: Understanding Bearing Press Fit for Optimum Design

The design of press fitting of bearings is an important engineering aspect that underlies the performance and reliability of the machine. The main points discussed in this article are summarized below.

• A tight fit is the only reliable means of physically preventing creep from occurring due to rotational loading.
• The clearance fit is applied to the free side, which requires ease of assembly and disassembly and shaft clearance due to thermal expansion.
• The basic principle in selecting a fit is to "press-fit the raceway ring on the side that receives the rotational load.
• In the case of a general rotating inner ring and stationary outer ring, the inner ring should be "clamped" and the outer ring should be "clearance fit".
• For outer ring rotation (conveyor, etc.), the outer ring must be "clamped".
• In some cases, the outer ring is press-fitted in the opposite direction to normal because the load vector rotates under disproportionate load (vibrating machine).
• For JIS tolerances (k5, m5, n6, etc.), select the appropriate rank depending on the magnitude of the load and the impact
• Select C3 or C4 gap to compensate for the decrease in internal clearance of the bearing when press-fitting is performed, if necessary.
• Excessive tightening allowance may risk inner ring cracking due to hoop stress, so it is necessary to check the allowable stress.
• Tolerance settings and insert measures are taken to account for temperature changes because gaps in aluminum housings widen due to thermal expansion.
• When press-fitting, always press the ring on the side to be press-fitted with a jig, and do not transmit impact loads to the rolling elements.
• When baking, strictly control the heating temperature so that it does not exceed 120°C.
• Incorporate cutouts for pullers and jack holes in the design to facilitate replacement during maintenance
• Since fretting wear occurs through minute gaps, secure fastening or the use of adhesives is effective
• When in doubt, determine the value based on each bearing manufacturer's catalog recommendation, while taking into account your own environmental factors.

That's it.