BIZOMAT MAKEOS · Power Transmission
Bearing Fit and Operating Internal Clearance
How much radial internal clearance will this bearing have once it is pressed on and running warm, and could it go into preload?
Operating radial internal clearance of a rolling bearing: the initial clearance range, the reduction from the inner ring expanding on an interference shaft fit and the outer ring contracting in an interference housing fit (as shares of the effective interference), the reduction from the inner ring running hotter than the outer, and the resulting operating clearance range with a preload check.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Initial radial clearance, minimum | µm | Lower limit of the bearing’s radial internal clearance group (from the bearing maker for the size and group). |
| Initial radial clearance, maximum | µm | Upper limit of the clearance group. |
| Bore diameter | mm | Bore diameter. |
| Outside diameter | mm | Outside diameter. |
| Effective shaft interference | µm | Effective interference of the inner ring fit (measured interference less surface smoothing). |
| Share of shaft interference reaching the raceway | Share of the inner ring interference that expands the raceway (about 0.7–0.9 for solid steel shafts). | |
| Effective housing interference | µm | Effective interference of the outer ring in the housing (0 for a clearance fit). |
| Share of housing interference reaching the raceway | Share of the outer ring interference that contracts the raceway (about 0.6–0.8 for steel housings). | |
| Inner ring hotter than outer ring by | K | Temperature difference between inner and outer ring when running. |
| Bearing steel expansion coefficient | µm/m·K | Expansion coefficient of bearing steel. |
| Smallest operating clearance acceptable | µm | Least operating clearance you accept (0 = just no preload; negative values allow a small preload). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Initial radial clearance, minimum
- What it is
- The smallest radial play the bearing can have as supplied, before it is mounted: the lower limit of its internal clearance group (normal, C3, C4 and so on) for this bore size.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 1000 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The minimum operating clearance moves one for one with this value, so it decides the preload check.
- Get it right
- Take the limit from the bearing maker’s table for the exact size and group; radial and axial clearance are different values.
What this means: Initial radial clearance, maximum
- What it is
- The largest radial play the bearing can have as supplied: the upper limit of the same clearance group, used to show the loosest bearing you could receive.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 1000 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 40 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The maximum operating clearance moves one for one with this value.
- Get it right
- It must not be below the minimum. It only sets the upper end of the operating range, not the preload verdict.
What this means: Bore diameter
- What it is
- The diameter of the bearing’s inner ring bore, which fits on the shaft. With the outside diameter it gives the mean raceway diameter used for the thermal reduction.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger bore raises the mean diameter, so the same temperature difference removes more clearance.
- Get it right
- Use the bearing’s nominal bore from the designation or catalogue, not the measured shaft size.
What this means: Outside diameter
- What it is
- The diameter of the bearing’s outer ring, which sits in the housing. Averaged with the bore it gives the mean raceway diameter d_m.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 3000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 90 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger outside diameter raises the mean diameter and so the thermal reduction.
- Get it right
- It must be larger than the bore; check you have not typed the housing bore or a width.
What this means: Effective shaft interference
- What it is
- How much bigger the shaft seat is than the inner ring bore after mounting: the measured diametral interference less the amount lost as surface peaks are smoothed when pressed on.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 500 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The clearance falls by this interference times the share reaching the raceway.
- Get it right
- Use the diametral interference (shaft minus bore), not the value per side, and allow for surface smoothing rather than using the raw drawing limits.
What this means: Share of shaft interference reaching the raceway
- What it is
- The fraction of the inner ring interference that actually expands the inner raceway. The rest is taken up by the ring and shaft deforming; it depends on ring section and shaft type.
- Units and format
- Enter a plain number with no unit. The form accepts 0.3 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.8 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Multiplies the shaft interference, so a higher share removes more clearance.
- Get it right
- Type it as a fraction, not a percent. Hollow shafts and thin-section rings change the share; the bearing maker’s method is the better source.
What this means: Effective housing interference
- What it is
- How much smaller the housing bore is than the outer ring after mounting, as an effective diametral interference. A clearance or transition fit that does not grip the ring counts as zero.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts 0 µm to 500 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The clearance falls by this interference times the housing share.
- Get it right
- Use 0 for a loose housing fit; do not type the clearance as a negative value here.
What this means: Share of housing interference reaching the race…
- What it is
- The fraction of the housing interference that contracts the outer raceway. It depends on the outer ring section and how stiff the housing is.
- Units and format
- Enter a plain number with no unit. The form accepts 0.3 to 1. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.7 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Multiplies the housing interference; it has no effect when that interference is zero.
- Get it right
- Type it as a fraction. Thin-walled or light-alloy housings change the share; the bearing maker’s method is the better source.
What this means: Inner ring hotter than outer ring by
- What it is
- How much warmer the inner ring runs than the outer ring in service. The inner ring grows more than the outer and takes up clearance; a negative value means the inner ring is cooler.
- Units and format
- Enter a number in K: kelvin (a difference of 1 K equals 1 °C). The form accepts -50 K to 100 K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5 K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The thermal reduction is α × ΔT × d_m, so it grows in direct proportion.
- Get it right
- This is a difference between the rings, not the bearing’s running temperature or its rise above ambient.
What this means: Bearing steel expansion coefficient
- What it is
- How much the bearing steel grows per metre of length for each kelvin of temperature rise. It converts the ring temperature difference into a change of raceway diameter.
- Units and format
- Enter a number in µm/m·K: micrometres per metre per kelvin. The form accepts 0.000001 µm/m·K to 30 µm/m·K. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12 µm/m·K as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The thermal reduction is directly proportional to this coefficient.
- Get it right
- One µm per metre per kelvin is the same as 10⁻⁶ per kelvin. Take the value for the actual ring material from the bearing maker.
What this means: Smallest operating clearance acceptable
- What it is
- The least operating clearance you will accept at the tightest combination of fits and temperature. Zero means just no preload; a negative value means you accept a slight preload.
- Units and format
- Enter a number in µm: micrometres (µm, microns). The form accepts -50 µm to 200 µm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 µm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- It does not change the clearance figures, only whether the check passes.
- Get it right
- Set it from the bearing maker’s advice for the application; it is a pass mark, not a calculated quantity.
Method and formulas
- Fit reduction = f_i × I_i + f_o × I_o.
- Thermal reduction = α ΔT d_m, with d_m = (d + D) ÷ 2 the mean raceway diameter.
- Operating clearance = initial clearance − fit reduction − thermal reduction (for both limits).
- Example: 25–40 µm initial clearance runs at 4.8–19.8 µm after the fit and a 5 K difference.
Limitations
- Approximate shares; the bearing maker’s method uses ring and shaft geometry.
- Thin-walled housings and hollow shafts change the shares.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose a clearance group (normal, C3, C4) for a fit and temperature
- Check that a tight shaft fit will not preload the bearing
- Explain bearing overheating after a fit change
Who it is for
- Design engineers
- Maintenance engineers
- Power transmission engineers
What you get
- Clearance reduction from the fits
- Clearance reduction from temperature
- Operating clearance range and preload check
Interpreting the result
- If the minimum goes negative, choose the next larger clearance group (C3, C4) or a looser fit.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2365–2416. Topic reference; no source tables or recommended settings are reproduced.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Bearing Fit and Operating Internal Clearance calculator