Machine Design #15: Bearing Fits – Interference, Clearance and Surface Roughness
Machine Design #15: Bearing Fits – Choosing Interference, Clearance and Surface Roughness
In machine design, the bearing is a very familiar part.
When people talk about bearings, many immediately think of selecting the model: what bore diameter, what outer diameter, what width, how much load, what maximum speed.
But in practice, when it goes onto the drawing, the part that is easy to get wrong is usually somewhere else.
It is where the bearing mounts onto the shaft and into the housing.
Should the shaft be h6 or k6? Should the housing be H7 or K7? Does the inner ring need an interference fit or a clearance fit? Can the outer ring be left loose? What Ra should the surface roughness be? Do the shaft shoulder, fillet and chamfer need checking?
On the drawing it is just a few small symbols.
But if you choose wrong, the bearing can slip, heat up, get noisy, vibrate, wear quickly or fail early. In some cases the machine still runs fine at first, but problems only start after a few weeks or months. By then, going back to trace the cause is very time-consuming.
This article records the basic points when designing the bearing-mounting area, for machine-design engineers new to the field.
The article uses a few terms in parallel:
| Term | Meaning |
|---|
| Bearing | ball / rolling bearing |
| Shaft | the shaft |
| Housing | the seat / bearing bore |
| Inner ring | the inner ring |
| Outer ring | the outer ring |
| Fit / はめあい | the fit |
| Clearance fit / すきまばめ | loose fit (always a gap) |
| Interference fit / しまりばめ | tight fit (with interference) |
| Transition fit / 中間ばめ | transition fit |
| Surface roughness / 面粗さ | surface roughness |
| Internal clearance / 内部すきま | the bearing's internal clearance |
1. A bearing fit is not just "it goes in"
When designing the shaft or housing to mount a bearing, the goal is not just to make the bearing go in.
If you only think "machine it so it presses in" or "assemble it so there's no play," that is fairly dangerous.
A good bearing fit needs to ensure several things at once:
- The bearing does not rotate/slip on the shaft or in the housing
- It does not lose too much of the bearing's internal clearance
- It does not distort the bearing by pressing too tight
- It can still be removed when maintenance is needed
- The concentricity, runout and roundness are good enough for the bearing to run stably
- The mounting surface is good enough that the actual tightness does not change too much after running
In short:
A bearing fit is a balancing act between holding tight, running smoothly, being easy to assemble and lasting long enough.
Choosing as tight as possible is not automatically good.
Too tight and the bearing heats up, loses internal clearance, turns heavily, and may even fail early.
Too loose and the inner or outer ring can slip relative to the mounting surface. This is a very common fault in machines that run for a long time.
2. The three basic fit types: clearance, interference, transition
In a mechanical drawing, fits are usually divided into 3 main groups.
| English | Japanese | Meaning |
|---|
| Clearance fit | すきまばめ | always a gap, easy to assemble and remove |
| Interference fit | しまりばめ | has interference, usually needs pressing or heating to fit |
| Transition fit | 中間ばめ | may be slightly loose or slightly tight depending on the actual deviation |
For bearings, not every location should be an interference fit.
This is where beginners easily go wrong.
Some see that the bearing is a rotating part, worry about looseness, and want both the shaft and the housing tight. But that is not necessarily good. If both sides are tight, the bearing's internal clearance can drop a lot, the bearing heats easily and its life shortens.
Conversely, some want easy assembly and removal, so they leave it quite loose. Assembly is very easy, but when the machine runs a long time, the bearing ring can slip on the shaft or in the housing.
So when choosing a fit for a bearing, do not choose by feel.
You have to look at the real mechanism.
3. The key rule: see which ring carries the rotating load
A common way to remember it is:
The ring that carries the rotating load usually needs the tighter fit.
But this sentence must be understood correctly.
It is not that "whichever ring rotates" is automatically an interference fit. You have to see whether the load acting on that ring is a rotating load or a stationary load.
A simple example with a motor or an ordinary rotating shaft:
- The shaft rotates
- The inner ring rotates with the shaft
- The housing is stationary
- The load acts in a relatively fixed direction with respect to the housing
Seen from the inner ring, the load direction changes continuously as it rotates. The inner ring is carrying a rotating load.
In this case, you would usually choose:
- Inner ring to shaft: interference fit or tight transition
- Outer ring to housing: clearance fit or transition, depending on the structure and load
Conversely, in some conveyor roller mechanisms:
- The shaft may be stationary
- The roller or housing rotates
- The outer ring rotates with the roller
- The load acts on the outer ring in a rotating manner
Then the outer ring is the side that needs the tighter fit in the housing.
The important point is not to choose the tolerance just because "the shaft rotates" or "the housing is stationary."
You must ask yourself:
- Is the inner ring carrying a rotating or stationary load?
- Is the outer ring carrying a rotating or stationary load?
- Does the load change direction continuously?
- Is there vibration, impact or a large load?
- Does the mechanism need to be dismantled for maintenance?
Choosing a bearing fit means choosing by working conditions, not by habit.
4. What is creep?
Creep is the phenomenon of the bearing's inner or outer ring slipping very slowly on the mounting surface.
For example, the inner ring should grip firmly to the shaft. But because the fit is too loose, when the machine runs, the inner ring slips slightly on the shaft surface.
It does not spin loudly like a wheel losing traction. It slips very little, very slowly, but repeats continuously.
At first you may not notice anything clearly.
But after a while problems start:
- The shaft surface wears
- Metal powder is generated
- The lubricating grease gets dirty
- The bearing runs hotter than normal
- There is noise or vibration
- The mounting surface gets scratched, burned or seized
- The next bearing replacement also struggles to reach the original accuracy
In practice, creep is a rather annoying fault.
Because you only see the worn shaft or housing when you take it apart. By then, replacing the bearing alone is not enough. You may have to repair the shaft, make a sleeve, restore by plating or re-machine the housing.
So preventing creep is one of the main reasons to choose an interference fit for the ring carrying the rotating load.
5. A reference table of common fits
This section should only be seen as a table to understand the initial tendency.
Do not memorize it and copy it straight onto the drawing.
When designing for real, you must re-check the catalog of the exact bearing brand you use, such as NSK, NTN, SKF, JTEKT/Koyo, Nachi, FAG, IKO... Depending on the bearing type, size, load, speed, accuracy class, internal clearance and operating temperature, the recommended fit can differ.
Shaft tolerance / 軸の公差
| Symbol | Japanese search keyword | Fit tendency | Easy-to-understand note |
|---|
| g6 | g6 軸 公差 | Clearance | Shaft slightly smaller, easy to fit and remove |
| h6 | h6 軸 公差 | Light clearance or line-to-line | Common for ordinary shafts, but depends on load |
| js6 | js6 軸 公差 | Transition | May be slightly loose or tight depending on actual deviation |
| k6 | k6 軸 公差 | Transition to light interference | Often used when the inner ring must be held firmer |
| m6 | m6 軸 公差 | Interference | Used when better creep resistance is needed |
| p6 | p6 軸 公差 | Strong interference | Check the press force and internal clearance |
An easy example to picture:
For an ordinary rotating shaft, where the inner ring rotates with the shaft and carries a rotating load, h6 may not be enough in many cases. When the load is larger or there is a creep risk, you usually have to consider k6, m6 or a more suitable tolerance per the catalog.
But if that location has to be removed frequently, with light load and no big creep risk, choosing it too tight makes assembly and maintenance harder.
Housing bore tolerance / ハウジング穴の公差
| Symbol | Japanese search keyword | Fit tendency | Easy-to-understand note |
|---|
| H7 | H7 ハウジング 公差 | Clearance or base hole | Very common for ordinary housings |
| J7 / JS7 | J7 JS7 ハウジング 公差 | Transition | Used when you want the outer ring not too loose |
| K7 | K7 ハウジング 公差 | Transition to light interference | Can be used when the outer ring must be held firmer |
| M7 | M7 ハウジング 公差 | Interference | Watch for reduced internal clearance |
| P7 | P7 ハウジング 公差 | Strong interference | Common when the outer ring carries a rotating or large load |
For housings, H7 is a very common symbol.
But it does not mean everywhere should be H7.
If the outer ring carries a rotating load, a large load, or is prone to creep in the housing, H7 may not be enough. Then you should consider J7, K7, M7 or a fit suited to the catalog.
Conversely, if the outer ring needs to be able to shift slightly along the axis to absorb thermal expansion, fitting it too tight causes a different fault.
So the real question is not "can the housing be H7?"
The right question is:
- Does the outer ring need to be held tight?
- Does the outer ring carry a rotating load?
- Is this bearing the fixed side or the free side?
- Does it need to allow movement due to thermal expansion?
- Is the housing steel, cast iron or aluminum?
- Does the machine run hot?
- Does it need to be dismantled for maintenance?
6. Don't forget the bearing's internal clearance
A bearing has internal clearance. In Japanese it is 内部すきま.
This is a very important point.
When you press-fit the inner ring onto the shaft, the inner ring expands a little.
When you press-fit the outer ring into the housing, the outer ring is squeezed a little smaller.
Both cases reduce the bearing's internal clearance.
If the internal clearance drops too much, the bearing can:
- Turn heavily
- Heat up quickly
- Have increased friction torque
- Have a reduced life
- Seize when the temperature rises
- Fail early even with the right bearing model
This is why in some cases you must choose a bearing with larger clearance, for example C3.
But do not read it the other way as "with an interference fit, just choose C3."
It is not that simple.
C3 is only an option when the fit conditions, temperature, load and bearing type require larger-than-normal internal clearance. If C3 is chosen wrongly, the machine can be noisy or have poor rotational accuracy.
The correct approach is:
- Determine the actual interference of the fit
- Look at the shaft and housing material
- Look at the operating temperature
- Look at the load and speed
- Re-check the bearing maker's recommendation
A bearing is a standard part, but how it works after being fitted into the machine depends heavily on the surrounding design.
7. Temperature can change the fit
When the machine runs, the temperature changes.
And when the temperature changes, part dimensions change too.
For example, if the inner ring is hotter than the shaft, the tightness between the inner ring and the shaft can change.
Or an aluminum housing needs more attention. Aluminum has a larger thermal-expansion coefficient than steel. When the temperature rises, an aluminum housing bore can expand more than a steel outer ring. As a result, the fit between the outer ring and the housing can be looser than when cold.
So for an aluminum housing, do not just take the tolerance experience from a steel housing and apply it.
Be especially careful in these cases:
- The machine runs at high speed
- The machine runs continuously for many hours
- The mechanism is near a heat source
- The housing is aluminum
- The working environment is unusually hot or cold
- The bearing carries both load and heat
- The mechanism needs high rotational accuracy
Many bearing faults do not appear during a cold assembly.
You assemble it, turn it by hand and it feels light, run it for a few minutes and it is fine. But when the machine runs long enough, the heat rises, the fit changes, the internal clearance changes, and only then does it start to heat, get noisy or seize.
Machine design must look at the real running state, not just the assembly state.
8. Dimensional tolerance is not enough — you also need geometric tolerance
Many new designers only pay attention to the diameter.
For example, the shaft is noted φ20 k6, the housing φ47 H7.
That is only the dimensional tolerance.
But a bearing does not just need the right diameter. It also needs the mounting surface to be round enough, straight enough, concentric enough.
If the shaft is oval, tapered, or the shoulder is not square, the bearing still runs badly even though the diameter measured with a micrometer looks right.
Points to watch:
| Item | English | Japanese | Meaning |
|---|
| Roundness | Roundness | 真円度 | Is the surface truly round |
| Cylindricity | Cylindricity | 円筒度 | Is the cylindrical surface tapered, barreled or distorted |
| Runout | Runout | 振れ | Does it run out when rotating |
| Shoulder perpendicularity | Perpendicularity | 直角度 | Is the shoulder square to the shaft axis |
If the bearing mounting surface is oval, the bearing gets squeezed oval too.
If the surface is tapered, the bearing may only contact in one zone.
If the shaft shoulder runs out or is not square, the bearing can be mounted tilted.
A tilted bearing very easily produces an uneven load. The result is usually heat, noise, vibration and reduced life.
So for important locations, noting only h6, k6, H7 is not enough. You need to consider whether to add a runout, roundness, cylindricity or perpendicularity requirement.
9. How should surface roughness be noted?
Surface roughness (面粗さ) also affects the quality of the bearing fit.
If the surface is too rough, when you press the bearing in, the peaks of the surface get crushed down.
As a result, the actual tightness can drop compared to the original calculation.
In addition, a too-rough surface also tends to cause wear, scratching and an unstable fit quality.
Some common reference values:
| Location | Japanese search keyword | Reference Ra |
|---|
| Ordinary bearing-mount shaft | 軸 はめあい面 面粗さ | Ra 0.8 to 1.6 μm |
| Ordinary bearing-mount housing | ハウジング はめあい面 面粗さ | Ra 1.6 to 3.2 μm |
| Shaft with higher accuracy | 精密軸 面粗さ | Ra 0.8 μm or better |
| Housing with higher accuracy | 精密ハウジング 面粗さ | Ra 1.6 μm or better |
These are only reference values for initial design.
For a spindle, a high-speed machine, a measuring machine, or a mechanism needing low noise, small runout or high life, check the catalog and your own design standards.
One point to remember:
A surface is not "the smoother the better" in every case.
What matters is that it suits the fit requirement, the machining method, the accuracy needed and the cost. For an ordinary machine, noting too fine an Ra can raise machining cost without a matching benefit.
Good design is not choosing the highest grade for everything.
Good design is choosing just enough for the real working conditions.
10. Shaft shoulder and fillet: a small mistake with a big consequence
A bearing is usually located by the shaft shoulder or the housing shoulder.
This is a part that is very easily underrated.
On the drawing, the shaft shoulder is just a small step. But if designed wrong, the bearing may not seat on the right face, may tilt, or may catch on the fillet.
There are two points to check.
Shoulder height
The shoulder must be high enough for the bearing to seat firmly against the end face.
If the shoulder is too low, the bearing is not located well.
But if the shoulder is too high, it can make the bearing hard to remove, or in some cases catch the bearing's seal/shield.
So check the shoulder diameter per the bearing catalog; do not guess.
Fillet radius
At the root of the shaft shoulder there is usually a fillet to reduce stress concentration.
This is good for the shaft.
But if the fillet radius is larger than the bearing's chamfer, the bearing will not seat correctly against the shoulder face. It can rest on the R corner.
Then, from outside, it looks pressed home, but the bearing is actually mounted tilted or not in proper face contact.
Check these parameters:
| English | Japanese | Meaning |
|---|
| Shoulder diameter | 肩径 | shoulder diameter |
| Chamfer dimension | 面取り寸法 | the bearing's chamfer dimension |
| Fillet radius | 隅R | the fillet radius |
| Abutment dimension | 取付関係寸法 | the related seating/mounting dimension |
Take these from the bearing catalog.
Do not draw the shaft shoulder by feel.
In practice, the fillet-R-catching-the-bearing fault is not rare. Especially with small shafts, small bearings, or parts turned out of habit to "round the R for looks."
Rounding the R is right, but the R must not make the bearing sit up.
11. Fixed side and free side also need attention
In many shaft assemblies, the bearing not only supports the radial load.
It also relates to locating the shaft axially and absorbing thermal expansion.
There is usually a fixed side and a free side.
- Fixed side: the side that locates the shaft, holding its axial position
- Free side: the side that lets the shaft expand or shift slightly along the axis
If both sides are locked rigidly along the axis, when the temperature rises the shaft expands but has no room to escape. The bearing can then take an unwanted axial load.
So when designing a long shaft assembly or a machine that runs hot, look at:
- Which bearing is the fixed side?
- Which bearing is the free side?
- Does the outer ring or inner ring need to shift slightly?
- Are you accidentally locking both ends rigidly?
- Does the chosen fit destroy the free-side function?
This is a part many beginners easily skip, because they only focus on the mounting diameter.
But for a rotating shaft assembly, the axial locating method is just as important.
12. A checklist before finalizing the drawing
Before finalizing the shaft or housing drawing for a bearing mount, quickly check the following.
About the bearing
- Which bearing model is used?
- Is it a deep groove ball bearing, angular contact, tapered roller or another type?
- Does the accuracy class have any special requirement?
- Is the internal clearance C0, C3 or another type?
- Does it have a seal/shield?
- Does the maker's catalog recommend a specific fit?
About load and motion
- Does the inner ring or outer ring rotate?
- Which ring carries the rotating load?
- Which ring carries the stationary load?
- Is the load large or small?
- Is there impact?
- Does the load reverse direction?
- Is there a creep risk?
About the fit
- Should the shaft be g6, h6, js6, k6, m6 or p6?
- Should the housing be H7, J7, K7, M7 or P7?
- Does the chosen fit reduce the internal clearance too much?
- Do you need to choose a C3-clearance bearing?
- Does it need to be easy to remove for maintenance?
About temperature and material
- Does the machine run hot?
- Is there a large temperature difference between the inner and outer rings?
- Is the housing steel, cast iron or aluminum?
- Does thermal expansion change the fit?
- Are the fixed side and free side clearly defined?
About machining and the drawing
- Is the surface roughness Ra noted?
- Do roundness and cylindricity need to be specified?
- Does the shaft-shoulder runout need checking?
- Is the shoulder face square to the shaft axis?
- Is the shoulder diameter correct per the catalog?
- Do the bearing chamfer and the shaft fillet interfere?
- Is there enough room to remove the bearing during maintenance?
Just going through this checklist greatly reduces the chance of a wrong bearing-fit drawing.
Conclusion
Designing a bearing mount should not just look at one tolerance symbol.
h6, k6, m6, H7, K7 or P7 are only the final result noted on the drawing.
Before noting that symbol, you need to understand how the mechanism works.
Which ring rotates? Which ring carries the rotating load? Do you need to prevent creep? Does the interference fit reduce the internal clearance too much? Does the machine run hot? Is the housing steel or aluminum? Is the machined surface good enough? Does the shaft shoulder tilt the bearing?
A bearing is a standard part.
But the bearing mount is the machine designer's responsibility.
Choosing the right tolerance is not choosing it as tight as possible to be safe.
Choosing right is choosing just enough so the bearing does not slip, does not distort, does not overheat, is easy to assemble and runs stably under the machine's real conditions.
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