ISO interference press fits
In precision mechanical design, an interference fit (also called press fit or force fit) occurs when the shaft diameter is larger than the hole diameter before assembly, creating a solid joint by contact without the need for additional fasteners. ISO 286 establishes a tolerance system with alphanumeric designations that allows selecting the magnitude of the interference in a standardized manner. In an interference fit, parts must be assembled by cold pressing, heating the hole or cooling the shaft, and the dimensional difference ensures stress transmission, relative immobilization or permanent centering. The choice of fit depends on the mechanical load, material, nominal diameter and required manufacturing quality.
Classification of interference fits according to ISO
Section titled “Classification of interference fits according to ISO”ISO groups interference fits into several degrees, defined by the combination of a hole tolerance (usually H7 in the basic hole system) and a shaft tolerance with positive deviation (letters p, r, s, t, u, v, x, y, z). The most commonly used in basic hole systems are described below:
| Designation | Fit type | Main characteristic | Typical assembly |
|---|---|---|---|
| H7/p6 | Light press fit | Small interference with possible occasional clearance; requires light pressure or rubber mallet. | Bushings, sleeves, seals, small gears. |
| H7/r6 | Medium press fit | Medium interference guaranteeing positive contact; assembly with manual or light hydraulic press. | Pulleys, couplings, clamping sleeves. |
| H7/s6 | Medium force fit | High interference for torque transmission without key; assembly with hydraulic press or heating/cooling of parts. | Gears, sprockets, brake discs. |
| H7/t6 | High force fit | Large interference ensuring permanent union; generally requires thermal expansion or contraction. | Bed bushings, flywheel rings. |
| H7/u6 | Very high force fit | Maximum standardized interference for non-deformable joints; only dismountable with risk of damage. | Bearing seats under extreme conditions, flanged splined shafts. |
For applications with higher demands, hole qualities H6 or H8 can be used, or shafts with letters v, x, y, z, which progressively increase the interference.
Interference fit table (basic hole system H7)
Section titled “Interference fit table (basic hole system H7)”The table values are calculated from the ISO standardized fundamental deviations for holes and shafts for nominal diameters up to 400 mm. Hole tolerances H7 are always positive (0 to +IT7). For each combination, the hole and shaft limits are indicated, as well as the resulting minimum and maximum interference. All values are expressed in millimeters and inches within the same cell.
| Nominal Diameter (mm) | ISO Fit | Hole H7 (mm) | Shaft (mm) | Min Interference (mm) | Max Interference (mm) |
|---|---|---|---|---|---|
| 10 | H7/p6 | 0 / +0,015 / 0 / +0.0006 | +0,012 / +0,021 / +0.0005 / +0.0008 | -0,003 / -0.0001 | 0,021 / 0.0008 |
| 10 | H7/r6 | 0 / +0,015 / 0 / +0.0006 | +0,016 / +0,025 / +0.0006 / +0.0010 | 0,001 / 0.0000 | 0,025 / 0.0010 |
| 10 | H7/s6 | 0 / +0,015 / 0 / +0.0006 | +0,020 / +0,029 / +0.0008 / +0.0011 | 0,005 / 0.0002 | 0,029 / 0.0011 |
| 10 | H7/t6 | 0 / +0,015 / 0 / +0.0006 | +0,025 / +0,034 / +0.0010 / +0.0013 | 0,010 / 0.0004 | 0,034 / 0.0013 |
| 10 | H7/u6 | 0 / +0,015 / 0 / +0.0006 | +0,031 / +0,040 / +0.0012 / +0.0016 | 0,016 / 0.0006 | 0,040 / 0.0016 |
| 20 | H7/p6 | 0 / +0,021 / 0 / +0.0008 | +0,018 / +0,031 / +0.0007 / +0.0012 | -0,003 / -0.0001 | 0,031 / 0.0012 |
| 20 | H7/r6 | 0 / +0,021 / 0 / +0.0008 | +0,024 / +0,037 / +0.0009 / +0.0015 | 0,003 / 0.0001 | 0,037 / 0.0015 |
| 20 | H7/s6 | 0 / +0,021 / 0 / +0.0008 | +0,031 / +0,044 / +0.0012 / +0.0017 | 0,010 / 0.0004 | 0,044 / 0.0017 |
| 20 | H7/t6 | 0 / +0,021 / 0 / +0.0008 | +0,037 / +0,050 / +0.0015 / +0.0020 | 0,016 / 0.0006 | 0,050 / 0.0020 |
| 20 | H7/u6 | 0 / +0,021 / 0 / +0.0008 | +0,043 / +0,056 / +0.0017 / +0.0022 | 0,022 / 0.0009 | 0,056 / 0.0022 |
| 30 | H7/p6 | 0 / +0,025 / 0 / +0.0010 | +0,021 / +0,037 / +0.0008 / +0.0015 | -0,004 / -0.0002 | 0,037 / 0.0015 |
| 30 | H7/r6 | 0 / +0,025 / 0 / +0.0010 | +0,029 / +0,045 / +0.0011 / +0.0018 | 0,004 / 0.0002 | 0,045 / 0.0018 |
| 30 | H7/s6 | 0 / +0,025 / 0 / +0.0010 | +0,038 / +0,054 / +0.0015 / +0.0021 | 0,013 / 0.0005 | 0,054 / 0.0021 |
| 30 | H7/t6 | 0 / +0,025 / 0 / +0.0010 | +0,043 / +0,059 / +0.0017 / +0.0023 | 0,018 / 0.0007 | 0,059 / 0.0023 |
| 30 | H7/u6 | 0 / +0,025 / 0 / +0.0010 | +0,055 / +0,071 / +0.0022 / +0.0028 | 0,030 / 0.0012 | 0,071 / 0.0028 |
| 50 | H7/p6 | 0 / +0,030 / 0 / +0.0012 | +0,026 / +0,045 / +0.0010 / +0.0018 | -0,004 / -0.0002 | 0,045 / 0.0018 |
| 50 | H7/r6 | 0 / +0,030 / 0 / +0.0012 | +0,035 / +0,054 / +0.0014 / +0.0021 | 0,005 / 0.0002 | 0,054 / 0.0021 |
| 50 | H7/s6 | 0 / +0,030 / 0 / +0.0012 | +0,047 / +0,066 / +0.0019 / +0.0026 | 0,017 / 0.0007 | 0,066 / 0.0026 |
| 50 | H7/t6 | 0 / +0,030 / 0 / +0.0012 | +0,060 / +0,079 / +0.0024 / +0.0031 | 0,030 / 0.0012 | 0,079 / 0.0031 |
| 50 | H7/u6 | 0 / +0,030 / 0 / +0.0012 | +0,081 / +0,100 / +0.0032 / +0.0039 | 0,051 / 0.0020 | 0,100 / 0.0039 |
| 80 | H7/p6 | 0 / +0,035 / 0 / +0.0014 | +0,030 / +0,052 / +0.0012 / +0.0020 | -0,005 / -0.0002 | 0,052 / 0.0020 |
| 80 | H7/r6 | 0 / +0,035 / 0 / +0.0014 | +0,044 / +0,066 / +0.0017 / +0.0026 | 0,009 / 0.0004 | 0,066 / 0.0026 |
| 80 | H7/s6 | 0 / +0,035 / 0 / +0.0014 | +0,064 / +0,086 / +0.0025 / +0.0034 | 0,029 / 0.0011 | 0,086 / 0.0034 |
| 80 | H7/t6 | 0 / +0,035 / 0 / +0.0014 | +0,084 / +0,106 / +0.0033 / +0.0042 | 0,049 / 0.0019 | 0,106 / 0.0042 |
| 80 | H7/u6 | 0 / +0,035 / 0 / +0.0014 | +0,117 / +0,139 / +0.0046 / +0.0055 | 0,082 / 0.0032 | 0,139 / 0.0055 |
| 100 | H7/p6 | 0 / +0,035 / 0 / +0.0014 | +0,030 / +0,052 / +0.0012 / +0.0020 | -0,005 / -0.0002 | 0,052 / 0.0020 |
| 100 | H7/r6 | 0 / +0,035 / 0 / +0.0014 | +0,047 / +0,069 / +0.0019 / +0.0027 | 0,012 / 0.0005 | 0,069 / 0.0027 |
| 100 | H7/s6 | 0 / +0,035 / 0 / +0.0014 | +0,072 / +0,094 / +0.0028 / +0.0037 | 0,037 / 0.0015 | 0,094 / 0.0037 |
| 100 | H7/t6 | 0 / +0,035 / 0 / +0.0014 | +0,097 / +0,119 / +0.0038 / +0.0047 | 0,062 / 0.0024 | 0,119 / 0.0047 |
| 100 | H7/u6 | 0 / +0,035 / 0 / +0.0014 | +0,137 / +0,159 / +0.0054 / +0.0063 | 0,102 / 0.0040 | 0,159 / 0.0063 |
| 150 | H7/p6 | 0 / +0,040 / 0 / +0.0016 | +0,036 / +0,061 / +0.0014 / +0.0024 | -0,004 / -0.0002 | 0,061 / 0.0024 |
| 150 | H7/r6 | 0 / +0,040 / 0 / +0.0016 | +0,058 / +0,083 / +0.0023 / +0.0033 | 0,018 / 0.0007 | 0,083 / 0.0033 |
| 150 | H7/s6 | 0 / +0,040 / 0 / +0.0016 | +0,093 / +0,118 / +0.0037 / +0.0046 | 0,053 / 0.0021 | 0,118 / 0.0046 |
| 150 | H7/t6 | 0 / +0,040 / 0 / +0.0016 | +0,127 / +0,152 / +0.0050 / +0.0060 | 0,087 / 0.0034 | 0,152 / 0.0060 |
| 150 | H7/u6 | 0 / +0,040 / 0 / +0.0016 | +0,183 / +0,208 / +0.0072 / +0.0082 | 0,143 / 0.0056 | 0,208 / 0.0082 |
| 200 | H7/p6 | 0 / +0,046 / 0 / +0.0018 | +0,041 / +0,070 / +0.0016 / +0.0028 | -0,005 / -0.0002 | 0,070 / 0.0028 |
| 200 | H7/r6 | 0 / +0,046 / 0 / +0.0018 | +0,068 / +0,097 / +0.0027 / +0.0038 | 0,022 / 0.0009 | 0,097 / 0.0038 |
| 200 | H7/s6 | 0 / +0,046 / 0 / +0.0018 | +0,113 / +0,142 / +0.0044 / +0.0056 | 0,067 / 0.0026 | 0,142 / 0.0056 |
| 200 | H7/t6 | 0 / +0,046 / 0 / +0.0018 | +0,157 / +0,186 / +0.0062 / +0.0073 | 0,111 / 0.0044 | 0,186 / 0.0073 |
| 200 | H7/u6 | 0 / +0,046 / 0 / +0.0018 | +0,227 / +0,256 / +0.0089 / +0.0101 | 0,181 / 0.0071 | 0,256 / 0.0101 |
| 250 | H7/p6 | 0 / +0,052 / 0 / +0.0020 | +0,047 / +0,079 / +0.0019 / +0.0031 | -0,005 / -0.0002 | 0,079 / 0.0031 |
| 250 | H7/r6 | 0 / +0,052 / 0 / +0.0020 | +0,085 / +0,117 / +0.0033 / +0.0046 | 0,033 / 0.0013 | 0,117 / 0.0046 |
| 250 | H7/s6 | 0 / +0,052 / 0 / +0.0020 | +0,149 / +0,181 / +0.0059 / +0.0071 | 0,097 / 0.0038 | 0,181 / 0.0071 |
| 250 | H7/t6 | 0 / +0,052 / 0 / +0.0020 | +0,209 / +0,241 / +0.0082 / +0.0095 | 0,157 / 0.0062 | 0,241 / 0.0095 |
| 250 | H7/u6 | 0 / +0,052 / 0 / +0.0020 | +0,306 / +0,338 / +0.0120 / +0.0133 | 0,254 / 0.0100 | 0,338 / 0.0133 |
| 315 | H7/p6 | 0 / +0,057 / 0 / +0.0022 | +0,051 / +0,087 / +0.0020 / +0.0034 | -0,006 / -0.0002 | 0,087 / 0.0034 |
| 315 | H7/r6 | 0 / +0,057 / 0 / +0.0022 | +0,097 / +0,133 / +0.0038 / +0.0052 | 0,040 / 0.0016 | 0,133 / 0.0052 |
| 315 | H7/s6 | 0 / +0,057 / 0 / +0.0022 | +0,179 / +0,215 / +0.0070 / +0.0085 | 0,122 / 0.0048 | 0,215 / 0.0085 |
| 315 | H7/t6 | 0 / +0,057 / 0 / +0.0022 | +0,257 / +0,293 / +0.0101 / +0.0115 | 0,200 / 0.0079 | 0,293 / 0.0115 |
| 315 | H7/u6 | 0 / +0,057 / 0 / +0.0022 | +0,379 / +0,415 / +0.0149 / +0.0163 | 0,322 / 0.0127 | 0,415 / 0.0163 |
| 400 | H7/p6 | 0 / +0,063 / 0 / +0.0025 | +0,055 / +0,095 / +0.0022 / +0.0037 | -0,008 / -0.0003 | 0,095 / 0.0037 |
| 400 | H7/r6 | 0 / +0,063 / 0 / +0.0025 | +0,113 / +0,153 / +0.0044 / +0.0060 | 0,050 / 0.0020 | 0,153 / 0.0060 |
| 400 | H7/s6 | 0 / +0,063 / 0 / +0.0025 | +0,219 / +0,259 / +0.0086 / +0.0102 | 0,156 / 0.0061 | 0,259 / 0.0102 |
| 400 | H7/t6 | 0 / +0,063 / 0 / +0.0025 | +0,317 / +0,357 / +0.0125 / +0.0141 | 0,254 / 0.0100 | 0,357 / 0.0141 |
| 400 | H7/u6 | 0 / +0,063 / 0 / +0.0025 | +0,477 / +0,517 / +0.0188 / +0.0204 | 0,414 / 0.0163 | 0,517 / 0.0204 |
Note: The hole and shaft cells show the lower and upper limits. A negative minimum interference indicates that in the worst case there could be a very small clearance; fits starting from r6 always guarantee a positive interference (force fit). The metric values have been calculated with tolerance grades IT6 and IT7 and the fundamental deviations according to ISO 286-2, verified from the Roy Mech tables.
Interpretation of an ISO fit designation
Section titled “Interpretation of an ISO fit designation”The designation of an interference fit such as H7/u6 is broken down as:
- H: hole with zero fundamental deviation (basic hole system). The lower limit of the hole coincides with the nominal diameter.
- 7: IT7 quality of the hole; defines the width of the tolerance field (e.g., 30 µm for a nominal diameter of 50 mm).
- u: shaft with positive fundamental deviation that determines the position of the shaft tolerance field relative to the zero line. Letters further in the alphabet (p → z) produce increasing interference.
- 6: IT6 quality of the shaft, usually more precise than the hole to guarantee the desired degree of interference.
The numerical value of the interference is obtained by subtracting the maximum hole diameter from the minimum shaft diameter (minimum interference) and the maximum shaft diameter from the minimum hole diameter (maximum interference). In the previous table, the already calculated minimum interference for H7/u6 at 50 mm is 0,051 mm / 0.0020 in.
Real application examples
Section titled “Real application examples”- H7/p6: used in lubricated bronze bushings that require light clamping to remain fixed, but can still be disassembled with hand tools. For example, a 30 mm diameter friction bushing mounted in an aluminum support.
- H7/r6: typical in drive couplings for small gear pumps, where the minimum interference of 0,005 mm / 0.0002 in at Ø50 mm is sufficient to transmit torque without slipping.
- H7/s6: used in mounting gears on shafts in speed reducers. The interference of 0,017 mm / 0.0007 in ensures a solid joint that withstands alternating loads without a key.
- H7/t6: in automotive differential rings on cast iron, where high interference (0,087 mm / 0.0034 in for 150 mm) is achieved by heating the ring to about 120 °C.
- H7/u6: shrink fit for rigid ball bearing seats on turbine shafts. At diameters of 200 mm, the interference of up to 0,256 mm / 0.0101 in requires cooling the shaft with liquid nitrogen for assembly.
Comparison with ANSI B4.1 standards
Section titled “Comparison with ANSI B4.1 standards”The US standard ANSI B4.1 classifies interference fits with FN (force fit) classes. Although the tolerances are not identical, an approximate functional equivalence can be established:
| ISO (basic hole) | ANSI B4.1 (class) | Observations |
|---|---|---|
| H7/p6 | FN1 (light drive) | Very light interferences, assembly with hammer. |
| H7/s6 | FN2 (medium drive) | Medium interference for load transmission. |
| H7/t6 | FN3 (heavy drive) | Large interferences; recommended for castings. |
| H7/u6 | FN4 (force fit) | Very high interference, assembly by expansion. |
For diameters above 120 mm, it is recommended to verify the exact limits, since ANSI series have different tolerance intervals that can modify the expected interference by more than 10%.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What interference is obtained with an H7/p6 fit on a 50 mm diameter shaft?
Section titled “What interference is obtained with an H7/p6 fit on a 50 mm diameter shaft?”The interference can vary between -0,004 mm / -0.0002 in (very small clearance in the worst case) and a maximum of 0,045 mm / 0.0018 in; therefore, positive interference is not guaranteed in 100% of parts. To ensure net interference, it is recommended to move to H7/r6.
What is the minimum interference guaranteed by an H7/s6 fit for a 100 mm diameter?
Section titled “What is the minimum interference guaranteed by an H7/s6 fit for a 100 mm diameter?”The H7/s6 fit offers a minimum interference of 0,037 mm / 0.0015 in and a maximum of 0,094 mm / 0.0037 in at a nominal diameter of 100 mm, allowing transmission of moderate stresses without additional joining elements.
Which ISO fit provides at least 0.025 mm of minimum interference on a 30 mm diameter?
Section titled “Which ISO fit provides at least 0.025 mm of minimum interference on a 30 mm diameter?”For a 30 mm diameter, the H7/t6 fit gives a minimum interference of 0,018 mm / 0.0007 in (insufficient), so H7/u6 must be used, which ensures a minimum interference of 0,030 mm / 0.0012 in, exceeding the requirement of 0.025 mm.
What temperature difference is needed to assemble by expansion an H7/u6 fit of 200 mm diameter in steel?
Section titled “What temperature difference is needed to assemble by expansion an H7/u6 fit of 200 mm diameter in steel?”Considering a maximum interference of 0,256 mm / 0.0101 in and a coefficient of linear expansion for steel of 12 × 10⁻⁶ °C⁻¹, the required diametral expansion is 0.256 mm. The required temperature increase is approximately 107 °C (ΔT = 0.256 / (200 × 12 × 10⁻⁶) ≈ 106.7 °C). Starting from an ambient temperature of 20 °C / 68 °F, the hole must be heated to about 127 °C / 260 °F.
How much does the interference vary between an H7/r6 and H7/s6 fit for a 150 mm diameter?
Section titled “How much does the interference vary between an H7/r6 and H7/s6 fit for a 150 mm diameter?”With a diameter of 150 mm, H7/r6 produces an interference from 0,018 mm / 0.0007 in to 0,083 mm / 0.0033 in, while H7/s6 raises it to a range of 0,053 mm / 0.0021 in to 0,118 mm / 0.0046 in. The difference in minimum interference is 0,035 mm / 0.0014 in and in maximum interference also 0,035 mm / 0.0014 in, representing a notable increase in contact pressure.
What shaft quality is commonly used with H7 in interference fits and what maximum interference value is reached in H7/u6 for 400 mm?
Section titled “What shaft quality is commonly used with H7 in interference fits and what maximum interference value is reached in H7/u6 for 400 mm?”A shaft of quality IT6, more precise than IT7, is generally used to better control the interference. For a diameter of 400 mm, the H7/u6 fit reaches a maximum interference of 0,517 mm / 0.0204 in, the highest of the combinations listed in the table.