Press fits
Specifying interference joints through size limits, material response, assembly control and demonstrated retention.
Tags: parts-and-design, features, fits
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A press fit assembles parts with dimensional interference by applying force. Evaluate the size stack separately from installation force and retained load.
In the ideal example, shaft limits of 30.020–30.030 mm and hole limits of 29.995–30.005 mm at the same reference condition give 0.015–0.035 mm diametral interference, or 0.0075–0.0175 mm radial overlap. These dimensions do not establish press force, torque capacity or freedom from cracking.
For press-fit pins, SPIROL distinguishes solid pins, which deform the host material, from spring pins, which compress to conform to the hole. It also identifies friction, mating materials/coatings and interference as contributors to retention. A solid shaft/hub joint, a knurled pin and a coiled spring pin should therefore not inherit the same hole recommendation simply because all are pressed into place. SPIROL, Hole Preparation for Press Fit Pins
Establish the dimensional fit
For a cylindrical shaft and hole, define diametral interference as I = shaft diameter − hole diameter. This sign convention gives positive interference. Do not mix diametral interference with radial overlap, which is half as large for ideal concentric cylinders.
Use the complete size limits:
I_min = shaft_min − hole_max.I_max = shaft_max − hole_min.
Positive values throughout the range establish dimensional interference. A range crossing zero permits both clearance and interference; it does not guarantee a press fit in every assembly. These are size calculations, not contact-pressure or strength calculations.
ISO 286-1:2010 establishes the code system and terminology for linear-size tolerances and fits, including cylinders and two opposite parallel surfaces. ISO's listing identifies edition 2, April 2010, confirmed in 2026, and lists Corrigendum 1:2013 separately. The scope explicitly treats fit terminology without orientation and location constraints. A fit designation therefore does not replace the drawing's geometric requirements. ISO, ISO 286-1:2010
State nominal size, applicable standard/edition and both tolerance classes, or provide explicit limits. Resolve any disagreement between numeric limits and a fit code before manufacture. Do not assume that a familiar hole class by itself defines interference: the mating component's limits also matter.
Original worst-case example
Assume an illustrative shaft limit of 30.020–30.030 mm and hole limit of 29.995–30.005 mm, at the same reference condition. Then:
| Calculation | Result |
|---|---|
| Minimum interference | 30.020 − 30.005 = 0.015 mm |
| Maximum interference | 30.030 − 29.995 = 0.035 mm |
| Radial overlap range for ideal cylinders | 0.0075–0.0175 mm |
The 0.020 mm spread in interference equals the 0.010 mm shaft tolerance plus the 0.010 mm hole tolerance. These are invented teaching dimensions, not a recommended 30 mm fit or values decoded from an ISO class. They do not establish insertion force, torque capacity or freedom from cracking.
If production uses selective assembly, record the pairing rule and measured sizes. Do not silently replace worst-case interchangeable assembly with selected pairs while retaining an interchangeable-part claim.
Material, geometry and surface response
Equal nominal interference can create different contact conditions in a thick steel hub, a thin sleeve, a hollow shaft or a polymer housing. Establish the actual wall geometry, elastic/plastic behavior, service loads and acceptable deformation. Include whether the joint transmits torque, resists axial movement or mainly locates components. If both torque and axial loads act, they compete for the available frictional resistance.
Hole quality also matters. SPIROL warns that reaming does not correct a bell-mouthed, crooked, oversized, out-of-round or badly chattered starting hole. Its pin guidance cannot be reduced to a nominal drill-size instruction. Choose a manufacturing and inspection method capable of controlling the required size and form along the engagement. SPIROL, Hole Preparation for Press Fit Pins
NSK explains that rough surfaces can lose effective interference through local plastic deformation during pressing. In rolling bearings, excessive interference can crack an inner ring or reduce internal clearance enough to cause abnormal heating. These bearing-specific consequences illustrate why increasing interference is not a universal remedy for poor retention. NSK, Fits and Internal Clearance
Keep coating thickness, texture, cleanliness and specified lubricant in the joint definition. A change that reduces assembly force can also change retained friction. A successful force calculation requires a justified contact-pressure model and friction assumptions; interference alone is insufficient to assign a press load or guaranteed pull-out strength.
Temperature and long-term condition
Evaluate assembly and service temperatures separately, including unequal part temperatures. For a first-order screening calculation, free thermal size change is ΔD = αDΔT. It does not by itself model the constrained assembled joint.
Original sensitivity example: use the previous 30 mm dimensions and assume hypothetical constant expansion coefficients of 20 × 10⁻⁶/K for the hole-containing part and 10 × 10⁻⁶/K for the shaft. For a common 60 K rise, the hole grows approximately 0.036 mm while the shaft grows 0.018 mm. Free dimensional interference decreases by approximately 0.018 mm, moving the illustrative 0.015–0.035 mm range to approximately −0.003 to +0.017 mm. The example shows sensitivity to differential expansion; it is not a material-specific operating prediction.
For actual polymers, account for the selected grade's time, temperature and conditioning response. Do not assign a metal dowel's interference rule to a plastic host or infer long-term retention solely from an initial press-out test.
Product-specific assembly boundaries
igus specifies that its standard press-fit iglide bushings reach their intended inner dimensions only after installation in the recommended H7 housing bore. Its FAQ recommends even pressing force and warns that centering/calibration pins can damage the bearing or enlarge clearance. This is a manufacturer-designed bushing system, not a general instruction that every press-fit hole should be H7. Inspect the installed bore condition as well as the free component. igus, iglide FAQ
For rolling bearings, apply assembly force to the ring being fitted, with appropriate support; do not use the rolling elements as the force path. NSK describes pressing through an inner-ring mounting tool and using a tool contacting both rings when both require a tight fit. It also describes thermal mounting for larger bearings. Follow the exact bearing's temperature, seal and lubricant restrictions rather than adopting a generic heating recipe. NSK, Mounting
Release and validation information
Make the assembly instruction reviewable:
- Identify both components, materials/conditions, engagement length and size limits.
- Specify form, surface, lead-in/chamfer, cleanliness and lubricant requirements.
- Define support, alignment, insertion direction, seating depth and allowed assembly conditions.
- Establish appropriate force/displacement acceptance and final dimensional checks.
- Demonstrate retention under representative loads, temperature and life conditions.
Do not interpret a high insertion-force peak as proof of good retention: misalignment, burrs or damage can also increase load. Inspect the completed interface or relevant functional dimensions, and define removal/reuse policy before service. Pressing a part out can change the contacting surfaces; original retention should not be assumed after reassembly.
Source scope
ISO's public listing supports the edition and scope above, not unviewed tolerance tables. The SPIROL PDF and cited NSK/igus technical pages do not state technical revision dates. Their application-specific guidance is preserved without converting it into universal interference, pressure, temperature or retention limits.