An h7 shaft tolerance can be suitable for some mounted ball bearing units, but it is not a universal installation requirement. Use the shaft tolerance specified for the exact insert, locking system, load, and speed. For an h7 shaft, the upper size deviation is zero and the lower deviation depends on nominal diameter. A 40 mm h7 shaft, for example, has size limits of 39.975–40.000 mm.
This guide explains how to read h7 limits, measure a shaft seat, and decide whether it meets the specified fit requirements before locking a mounted ball bearing in place. It applies mainly to insert ball bearing units with set screws, eccentric collars, adapter sleeves, or concentric locking systems. Other bearing types can require different shaft fits.
For the complete housing-positioning, fastening, and lubrication sequence, use our mounted bearing installation guide. This article focuses on the shaft-fit checks that support that installation process.
What Does h7 Mean for a Mounted Bearing Shaft?
The designation has two parts. Lowercase h identifies the shaft tolerance zone: its upper deviation is zero, so the permitted size does not extend above nominal. The number 7 identifies the IT7 tolerance grade. That grade sets the width of the tolerance zone for the relevant nominal-size range.
For a 40 mm h7 shaft, the deviations are 0 / −25 µm. Since 25 µm equals 0.025 mm, the limits are 40.000 mm maximum and 39.975 mm minimum. The designation does not mean ±0.025 mm, and it does not describe the bearing’s internal clearance.
Dimensional tolerance classes and limit deviations are defined in ISO 286-2:2010. The standard supplies the size limits; the bearing documentation determines which tolerance class is appropriate for a particular unit.
h7 vs. H7: Shaft and Hole Tolerances
Letter case matters. Lowercase h7 applies to a shaft; uppercase H7 applies to a hole. For H7, the lower deviation is zero and the tolerance zone extends above nominal size. The two classes have the same IT7 width for the same nominal diameter, but lie on opposite sides of nominal.
| Feature at 40 mm nominal | Upper / lower deviation | Permitted size limits |
|---|---|---|
| h7 shaft | 0 / −25 µm | 39.975–40.000 mm |
| H7 hole | +25 / 0 µm | 40.000–40.025 mm |
These are examples of two independently specified features. Do not assume that a mounted bearing insert has an H7 bore, or that its spherical housing seat is a cylindrical H7 hole. The bearing’s actual bore tolerance and housing-seat geometry must come from its own specification.
h7 Shaft Tolerance Chart for Common Bearing Sizes
The table covers selected nominal diameters from over 10 to 120 mm. Limits are in millimeters; deviations are in micrometers. Each range excludes its lower boundary and includes its upper boundary.
| Nominal shaft diameter | h7 upper / lower deviation | Example size limits |
|---|---|---|
| Over 10 to 18 mm inclusive | 0 / −18 µm | 15 mm: 14.982–15.000 mm |
| Over 18 to 30 mm inclusive | 0 / −21 µm | 25 mm: 24.979–25.000 mm |
| Over 30 to 50 mm inclusive | 0 / −25 µm | 40 mm: 39.975–40.000 mm |
| Over 50 to 80 mm inclusive | 0 / −30 µm | 60 mm: 59.970–60.000 mm |
| Over 80 to 120 mm inclusive | 0 / −35 µm | 100 mm: 99.965–100.000 mm |
Boundary check: exactly 30 mm uses the over-18-to-30 mm row, so 30 mm h7 is 29.979–30.000 mm. Exactly 50 mm uses the over-30-to-50 mm row, so 50 mm h7 is 49.975–50.000 mm. Use the nominal diameter to select the row, not a measured undersize diameter.
When Is h7 Appropriate?
Start with the complete unit designation and its shaft-fit recommendation. An h7 requirement can be appropriate for certain set-screw or eccentric-collar units under specified operating conditions. A different combination of load, speed, and locking design may permit a wider tolerance or require a narrower one. There is no single shaft class for every product in the mounted bearings range.
| Situation | What to verify | Why h7 alone is insufficient |
|---|---|---|
| Set-screw or eccentric-collar insert | Recommended shaft class for the actual load and speed. | The gap at the shaft seat can affect running behavior and locking performance. |
| Adapter sleeve or concentric locking system | Shaft size, form, finish, and the locking system’s assembly requirements. | A clamping system may have different permitted shaft tolerances and tightening controls. |
| Insert intended for an interference fit | Specified shaft fit and bearing bore tolerance. | A shaft within h7 limits does not automatically create an interference fit. |
| Existing shaft being reused | Measured size, wear, screw damage, runout, and the approved repair criteria. | A nominal diameter or one acceptable reading does not establish seat condition. |
h6, h7, h8, and h9: What Changes?
For the same nominal diameter, these h classes share an upper deviation of zero. The grade changes the width below nominal. At 40 mm, their limits are:
h6: 39.984–40.000 mm; tolerance width 16 µm.
h7: 39.975–40.000 mm; tolerance width 25 µm.
h8: 39.961–40.000 mm; tolerance width 39 µm.
h9: 39.938–40.000 mm; tolerance width 62 µm.
A shaft measuring 39.970 mm is below the h7 lower limit at 40 mm nominal, although it falls within the h8 size limits. That does not make it acceptable for a unit requiring h7. Likewise, choosing h6 does not automatically increase bearing life: size tolerance does not establish surface finish, shaft straightness, or the suitability of the locking mechanism.
Calculate Shaft-to-Bore Clearance Separately
Shaft tolerance describes only one side of the interface. To calculate possible diametral clearance, obtain the bearing bore limits as well:
Minimum diametral clearance = minimum bore diameter − maximum shaft diameter.
Maximum diametral clearance = maximum bore diameter − minimum shaft diameter.
Illustrative calculation, not a product specification: suppose a cylindrical bore has limits of 40.005–40.015 mm and its shaft is specified as 40 mm h7, or 39.975–40.000 mm. The possible diametral clearance is 0.005–0.040 mm: 40.005 − 40.000 = 0.005 mm, and 40.015 − 39.975 = 0.040 mm.
These assumed bore limits are not H7 and are not attributed to a LILY bearing model. Replace them with the actual insert limits before evaluating a real assembly. This shaft-to-bore clearance is also different from radial internal clearance between the bearing rings and rolling elements.
How to Measure a Shaft Seat Before Installation
Measure the seat where the bearing will actually run, rather than an unworn area beside it. Isolate the machine, support the shaft, and clean the measuring surfaces. Allow the shaft and instrument to reach stable measurement conditions; a recently machined, washed, or heated shaft can give misleading results.
Confirm the specification. Record nominal diameter, required tolerance class, and any separate form, finish, hardness, or runout requirements.
Check the instrument. Use a calibrated outside micrometer with a suitable range and measurement capability. Resolution alone does not establish accuracy or measurement uncertainty.
Take multiple readings. As a practical initial check, measure at three axial positions across the seat, in two perpendicular directions at each position: six readings in total. Add positions or use more suitable equipment where form control is important.
Record the individual readings. Compare each with the specified size limits; do not accept the seat on the average diameter alone.
Check features the micrometer does not establish. Inspect burrs and wear, and verify specified surface finish, runout, and form using appropriate methods. Two directions at three positions cannot prove complete roundness or cylindricity.
Example: Checking a 40 mm h7 Shaft
| Axial position | Direction A | Direction B |
|---|---|---|
| Near one end of the seat | 39.991 mm | 39.989 mm |
| Middle of the seat | 39.987 mm | 39.985 mm |
| Near the other end | 39.982 mm | 39.980 mm |
All six indicated values fall within 39.975–40.000 mm. The largest-to-smallest reading spread is 0.011 mm. This warrants checking the seat’s form against its drawing, but does not by itself prove a specific taper, roundness error, or cylindricity value.
The smallest reading is 0.005 mm above the lower size limit. Whether the seat can be formally accepted depends on measurement uncertainty, the agreed acceptance rule, and all other specified requirements. If a reading is close to a limit, repeat it under controlled conditions rather than rounding it into compliance.
How Shaft Fit Affects Locking
A locking mechanism does not remove the need for a suitable shaft seat. Extra tightening cannot restore the diameter, geometry, or contact conditions of a worn shaft.
Set screws: check the recommended shaft size, surface condition, and hardness. Tighten to the exact unit’s specified torque; do not increase torque to compensate for an undersize seat.
Eccentric collars: confirm the approved shaft fit and rotation direction. Engage the collar according to its instructions. A collar is not a repair for a worn shaft.
Adapter sleeves: verify sleeve compatibility, shaft size and form, and the specified drive-up or tightening method. Excess drive-up can reduce bearing internal clearance.
Concentric locking systems: check the permitted shaft tolerance and surface conditions. Tighten using that design’s sequence; different collar and clamping constructions are not interchangeable procedures.
Do not assign a universal speed limit, axial holding capacity, or tightening torque to a locking category. Do not reduce screw torque by a fixed percentage simply because the shaft is stainless steel. Those values require the specification for the actual unit and assembly conditions.
What to Do If the Shaft Does Not Meet the Requirement
| Finding | Next action | Avoid |
|---|---|---|
| Below the specified minimum diameter | Confirm the reading; assess approved shaft repair or replacement, or a different engineered bearing arrangement. | Extra screw torque, improvised shims, or assuming a collar will make up the missing diameter. |
| Above the specified maximum diameter | Check for burrs, deposits, temperature effects, or an incorrectly made seat. Correct and remeasure to the specification. | Hammering a slip-fit insert onto the seat or heating the unit as a workaround. |
| Large variation between positions | Inspect wear and verify shaft form and runout against the drawing. | Using the average diameter to disregard local undersize or oversize readings. |
| Size passes, but the assembly binds | Check base support, alignment, shaft straightness, seals, and locking adjustment. | Treating an h7 size check as proof that the whole installation is correct. |
Retaining compounds, coatings, and other repairs change the interface and future removal procedure. Use them only under an approved repair specification for the service conditions. An apparently tight assembly is not evidence that the shaft fit has been restored.
Final Fit Checks Before Locking and Startup
Once the shaft seat is accepted, follow the mounting procedure for the exact unit. The fit-related handover should confirm:
The complete bearing and locking-system designation matches the shaft specification.
The inspection record includes actual readings and any separately required form, finish, and runout results.
The seat is clean and free of raised damage; any assembly lubricant is permitted for that interface.
The mounting sequence and locking torque come from the unit instructions.
Housing support and alignment are checked separately from shaft diameter.
Thermal expansion is accommodated where needed. An h7 shaft fit does not create a non-locating bearing position, and self-alignment does not provide axial float.
After installation, investigate binding, unexpected heating, or repeated locking loosening before returning the machine to normal service. These symptoms can have causes other than shaft size; the tolerance designation alone cannot diagnose them.
FAQ
Can a general “0 to −0.002 inch” shaft allowance be treated as h7?
No. A 0.002 inch allowance equals 0.0508 mm, or 50.8 µm. The h7 width depends on nominal diameter; at 40 mm it is 25 µm. A general inch allowance is not interchangeable with a size-dependent ISO tolerance class.
Does an inch-size shaft automatically use the nearest metric h7 limits?
No. One inch is exactly 25.4 mm, not 25 mm. Use the actual nominal size and the tolerance specified for the inch-bore unit. Do not assume that a metric shaft-fit recommendation also applies to the inch version.
Does a coating count toward the finished shaft diameter?
Yes. Check the finished bearing seat after coating unless the drawing specifies a different inspection stage. For a uniform coating of radial thickness t on a cylindrical surface, the diameter increases by 2t. Coating thickness variation, finish, and compatibility with the locking system also need review.
What information is needed if the shaft tolerance is missing from the product page?
Provide the full unit designation, nominal shaft diameter, locking design, speed, radial and axial loads, rotation direction, and shaft material and condition. Request the approved shaft-fit and installation information before machining or accepting the seat.
For help checking a shaft specification or a compatible bearing insert, contact us with the unit number, drawing, and measured seat dimensions.
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