Linear Shaft Tolerance for Linear Bearings: h6, h7 and Fit Selection

A linear bearing marked for a 20 mm shaft does not mean that every ordinary 20 mm steel rod will provide the correct fit. The actual shaft diameter must remain within a controlled tolerance, and the selected tolerance must match the bearing type, required radial clearance and housing design.

For many metric linear ball bearings, g6 or f6 shafts are used when normal operating clearance is required, while h6 may be selected when a smaller clearance is needed. Some bearing series use h7 under normal conditions or specify different tolerances altogether.

Therefore, h6 is not automatically the correct choice for every linear bearing. The bearing manufacturer's fit table should be checked before the shaft tolerance is specified.

What Does a Shaft Tolerance Such as h6 Mean?

A tolerance designation such as h6 contains two parts:

• Lowercase letter h: identifies the position of the shaft tolerance zone relative to the nominal diameter.
• Number 6: identifies the tolerance grade and therefore the width of the permitted size range.

For an h tolerance zone, the upper deviation is zero. This means the shaft may reach the nominal diameter but should not exceed it. The lower deviation depends on the nominal diameter and tolerance grade.

The letter must be written in lowercase when it refers to a shaft. An uppercase letter, such as H7, normally refers to a hole tolerance. Therefore, h6 and H6 do not describe the same component.

h6 and h7 Are Not Fixed Diameter Reductions

It is incorrect to say that h6 always means "0.01 mm smaller" or that h7 always permits the same deviation at every diameter. The permissible deviation changes with the nominal diameter range.

For example, under commonly used ISO limits and fits:

The h7 range is wider than h6. Both have the same maximum diameter in this example, but h7 permits a smaller minimum diameter.

These values are provided to explain how tolerance notation works. Before production, the applicable tolerance table, drawing standard and nominal diameter range should be confirmed rather than copying this 20 mm example to another shaft size.

Why Linear Bearings Need a Controlled Shaft Diameter

In a round-shaft linear ball bearing, the balls run directly on the shaft surface. The shaft therefore acts as part of the bearing raceway rather than merely passing through an inner ring.

The relationship between shaft diameter, bearing bore and housing condition affects:

• radial clearance;
• starting and running resistance;
• movement smoothness;
• vibration and noise;
• load distribution among the ball rows;
• positioning stability;
• wear of the balls and shaft surface.

If the shaft is too large for the selected bearing condition, the bearing may develop excessive preload or negative clearance. If the shaft is too small, the system may have more radial movement than the machine can accept.

Normal Clearance vs Small Clearance

For many standard metric linear bushing series, published fit tables distinguish between a normal-clearance arrangement and a smaller-clearance arrangement.

This table describes common selection directions, not a universal replacement for the bearing catalogue. Bearing series with different internal-clearance classes, outer-cylinder structures or accuracy grades may require different shaft tolerances.

Why h6 Is Not Always Better Than g6

An h6 shaft can be closer to nominal diameter than a g6 shaft of the same nominal size. This may reduce radial clearance, but it does not automatically improve the complete motion system.

Using a tighter shaft fit can also produce:

• higher starting force;
• higher running resistance;
• greater sensitivity to shaft straightness;
• greater sensitivity to parallelism errors in a dual-shaft system;
• more heat under continuous operation;
• shorter life if excessive negative clearance is created.

If normal operating clearance already meets the required positioning stability, selecting h6 only because it appears "more precise" may add assembly risk without providing a useful machine-level improvement.

The Housing Bore Can Change the Final Clearance

Shaft tolerance alone does not determine how the linear bearing will run. The bearing outer cylinder is installed in a housing, and the housing bore can affect its internal geometry.

A housing bore that is too small, out of round or tightened unevenly may compress the bearing outer cylinder. The bearing can then become tight even when the shaft diameter is within specification.

Housing-bore tolerances such as H7 are commonly recommended for loose-fit mounting of many standard linear bushing series. Some higher-accuracy arrangements use H6, J6, J7 or other model-specific fits.

Therefore, a shaft marked h6 and a housing marked H7 do not by themselves guarantee a particular clearance value. The tolerances of all components accumulate in the final assembly.

Do Not Treat a Linear Bushing Like an Ordinary Press-Fit Bearing

A common mistake is to apply ordinary rotating-bearing fit logic directly to a linear bushing. A standard linear ball bearing usually does not have a conventional inner ring fitted tightly onto the shaft. Its balls run directly on the hardened shaft.

The bearing must move along the shaft, so forcing an interference condition between the shaft and ball circuit can create binding and damage.

The bearing outer cylinder must also be secured correctly in the housing. It should not rely only on an uncontrolled press fit. Depending on the bearing style, axial retention may use:

• snap rings;
• retaining plates;
• flanged bearing construction;
• a manufacturer-designed bearing block or housing.

Do not tighten a pointed set screw directly against a thin bearing outer cylinder unless that retention method is explicitly permitted. Local deformation can reduce internal clearance and create a tight spot.

Open, Closed and Adjustable Bearings Need Different Checks

Closed Linear Bearings

A closed bearing surrounds the shaft and has limited ability to compensate for an oversized shaft or distorted housing. Shaft tolerance, housing accuracy and alignment must be controlled carefully.

Open Linear Bearings

An open bearing has a longitudinal opening so it can operate on a fully supported shaft. The opening does not mean that shaft diameter can be ignored. The shaft remains the rolling surface and must meet the required diameter, hardness and finish.

Clearance-Adjustable Bearings

Some split housings allow limited radial-clearance adjustment around a compatible bearing. The adjustment should be made gradually. Excessive clamping can create high preload, uneven ball contact and premature wear.

Clearance adjustment should correct an intended assembly condition; it should not be used to compensate for a bent shaft, oversized diameter or poor alignment.

How to Measure Linear Shaft Diameter Correctly

A vernier caliper is useful for quick identification, but it is usually not sufficient for verifying an h6 shaft. The tolerance may be only several micrometres wide, so a suitable outside micrometer should be used.

1. Clean the shaft and micrometer measuring faces.
2. Allow the shaft and measuring equipment to reach a stable temperature.
3. Measure near the two ends and at several positions along the working length.
4. At each position, measure in at least two directions approximately 90° apart.
5. Record the maximum and minimum values rather than only one reading.
6. Compare all readings with the specified upper and lower limits.

Measurements in different angular directions help reveal out-of-roundness. Measurements at different longitudinal positions help identify taper, local grinding variation or plating-thickness variation.

When micrometre-level tolerances are involved, temperature matters. A long steel shaft measured immediately after grinding, transport or exposure to direct sunlight may not have the same diameter after reaching the inspection-room temperature.

Diameter Tolerance Is Only One Shaft Requirement

A shaft can meet h6 diameter tolerance and still produce poor linear motion. The following properties should also be checked:

For general shaft selection, see How to Choose a Linear Shaft for Linear Bearings.

Chrome-Plated Shaft Diameter Must Be Checked After Plating

For a chrome-plated linear shaft, the required final tolerance applies to the finished running surface, not only to the steel base before coating.

If the plating thickness is uneven, the shaft may show:

• a larger diameter at one end;
• different diameters in different angular directions;
• local high spots;
• a correct average diameter but excessive variation along the stroke.

The shaft should therefore be measured after all grinding, polishing and plating operations are complete. Learn more in Chrome Plated Linear Shaft Quality: Plating Thickness, Surface Finish and Inspection.

Why a Bearing Can Be Tight Even When the Shaft Is Within h6

If a linear bearing feels tight, do not assume that the shaft diameter is the only possible cause. Check the complete assembly.

A useful diagnostic method is to test the bearing on the shaft before and after housing installation. If the bearing runs smoothly before installation but becomes tight afterward, the housing fit, clamping method or assembly alignment is more likely to be responsible.

Why a Bearing Can Feel Loose

Excessive radial play may result from:

• a shaft below its minimum diameter;
• a tolerance zone that provides more clearance than the application requires;
• bearing internal clearance at the upper end of its tolerance;
• worn balls or bearing raceways;
• shaft wear along the most frequently used travel area;
• a clearance-adjustable housing that has not been set correctly.

Do not measure only an unused section near the shaft end. A shaft that has operated for a long time may meet the original diameter at the ends but be worn smaller along the working stroke.

Practical Fit-Selection Process

1. Identify the exact bearing model. Do not select the shaft fit only from the nominal bore diameter.
2. Check the bearing accuracy and clearance class. Standard and precision grades may use different recommendations.
3. Define the required operating clearance. Decide whether smooth normal clearance or reduced radial play is more important.
4. Select the shaft tolerance from the model-specific table. Confirm whether the bearing requires f6, g6, h6, h7 or another zone.
5. Confirm the housing-bore tolerance. Include the effect of the outer-cylinder fit and retention method.
6. Check shaft hardness, finish and straightness. Diameter tolerance alone is not sufficient.
7. Inspect the completed assembly. Check running resistance and radial movement over the complete stroke.

Information to Provide When Ordering a Linear Shaft

For a shaft intended to run with linear bearings, specify:

• nominal shaft diameter;
• required shaft tolerance, such as g6 or h6;
• linear bearing model and accuracy grade;
• shaft material and surface treatment;
• surface hardness requirement;
• surface-finish requirement;
• straightness requirement;
• total length and working stroke;
• supported or unsupported installation;
• end machining, threads, keyways or retaining-ring grooves;
• working environment and corrosion requirements.

For DLY linear shaft production and inspection, the bearing model and required fit should be confirmed before the final diameter tolerance is assigned. This is particularly important for custom-machined or chrome-plated shafts, because the finished running diameter must match the intended bearing condition.

Conclusion

The correct linear shaft tolerance cannot be selected from nominal diameter alone. h6 and h7 describe different tolerance widths, while f6, g6 and other zones position the shaft diameter differently below the nominal size.

For many compatible linear bearing series:

• f6 or g6 may be used for normal operating clearance;
• h6 may be used when a smaller clearance is required;
• h7 or another tolerance may be specified for a different bearing series;
• the housing-bore fit must be checked together with the shaft.

The final goal is not simply to use the tightest tolerance. It is to achieve the required clearance, smooth movement, stable load distribution and acceptable service life after the bearing is installed in the complete machine.

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