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Press fitting (also called interference fitting) is perhaps the most ancient form of metal joining, relying on elastic and plastic part deformation to couple an inserted part to a receiving part.
It is, in many ways, the metalworking equivalent of NAILING parts together! A Bronze Age swordsmith would very clearly understand the up-to-date approach to press fitting and the precision issues that it implies.
It’s a common and highly current mechanical assembly technique where two parts are joined by forcing one into the other, typically with a cylindrical interface, with precise dimensional tolerances – in modern processes.
The use of press fits predates all modern technologies, but the understanding of the fundamental behavior and the ability to manufacture parts with very high precision and tight tolerances has enabled the refined use of the approach to become a cornerstone in most mechanical assemblies and moving (and non-moving) part interactions.
The ability to selectively create tightly controlled interferences from the tightly fixed to the smoothly running creates options in friction and radial stress, ensuring a secure, permanent connection without fasteners or adhesives.
Two analysis methods are used in defining the fit of a shaft and a hole. These are termed ‘shaft basis’ and ‘hole basis’. As holes are simpler to manufacture with precision, the shaft-basis approach is more typical. Shaft-basis is typical when the shaft is a pre-existing and fully defined component – for example, when parts are to be made to fit a purchased precision bar/rod. Adjustment of a shaft to fit a pre-existing hole is how hole-basis operates.
The process and interference implications are identical. The difference lies entirely in which part is adjusted to define the fit.
This guide details the fundamentals, tolerances, engineering calculations, material options, surface finishes and common applications/troubleshooting of press fits. It is presented from the more common, hole-basis perspective.
What is a Press Fit?
A press fit relies on controlled interference between a shaft (male part) and a hole (female part). The shaft is slightly larger than the hole, requiring force for assembly. Once joined, friction holds the parts together.
It is appropriate to offer definitions of the terminology used, so that understanding can be clear:
- The interference in a fit is the difference between shaft and hole diameters (e.g., +0.05mm). This defines the degree of ‘overlap’ of the shaft/hole diameters that provides the resistance to fit and the binding forces between the two parts.
- The tolerance applicable in the feature diameters is the allowable deviation from nominal dimensions (e.g., H7/p6).
- The retention force covers two factors – the axial and rotational forces required to overcome the interference and either disassemble the fitted parts or cause relative motion in axial or rotational directions.
Types of Press Fits
Press fits are categorized by the interference level that binds the two parts together, resulting from the overlap of radial dimensions:
Press Fit Tolerance classes
Standardized under ISO 286, press fits use hole-basis systems (e.g., H7/p6):
- Hole Tolerance (H7): Always has a positive allowance (e.g., ⌀10.000–10.018mm). This implies that the hole diameter cannot be less than 10mm
- Shaft Tolerance (p6): Ensures interference (e.g., ⌀10.022–10.035mm).
Common ISO Tolerance Grades
Where the press fit is defined on a hole basis, the variances in diameter (and therefore clearance/tightness of fit) are defined in shaft diametral variations.
Engineering Calculations
Interference Calculation
Interference = D(shaft) – D(hole)
This example illustrates one possible fit tolerance:
- Hole: ⌀10.000 – 10.018mm (H7)
- Shaft: ⌀10.022 – 10.035mm (p6)
- Min. Interference: 10.022 – 10.018 = 0.004mm
- Max. Interference: 10.035 – 10.000 = 0.035mm
Assembly or Press Fit Force (F)
F = μ ✕ P ✕ π ✕ D ✕ L
Where:
- μ = Coefficient of friction (0.1–0.2 for steel)
- P = Contact pressure (from Lamé’s equations)
- D = Nominal diameter
- L = Engagement length
Contact Pressure (P)
Contact pressure is highly dependent on the wall thickness surrounding the hole. Elasticity in a thin-walled receptacle or local weak areas in this will alter the results considerably and render calculations moot. It is typically best to run a trial and approximation, where the hoop stress can be accommodated in these elastic ways. For a solid shaft in a thick-walled hub:
P = E ✕ δD ✕ (1−ν)(1+ν)
Where:
- E = Young’s modulus of the material(s) (e.g., 210 GPa for steel)
- δ = Radial interference
- ν = Poisson’s ratio of the material(s) (~0.3 for metals)
Material & Surface Finish Considerations
Material Selection
- Steel-on-steel: Most common (high strength, predictable expansion).
- Aluminum: Lower interference forces (softer material).
- Steel-on-phosphor-bronze: Typical of self lubricating bearing bushes, inserted into steel receptacles.
- Plastic-on-plastic or plastic-on-metal: Risk of creep—use light press fits to avoid relaxation and fracture.
Surface Finish
The optimal roughness (Ra measure): 0.8–3.2 µm for metals.
- Metal surfaces that are too smooth (<0.4 µm) experience a higher degree of micro-welding, creating a risk of galling and contact pressure loss as material is micro-damaged.
- Surfaces that are too rough (>6.3 µm) typically experience weakened retention force as peak contacts are smeared, reducing the planned interference.
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Applications
Troubleshooting Press Fits
Advanced Techniques
- Thermal or Cryogenic Assembly: Heat hub/cool shaft for easier insertion (e.g., bearings). This can induce high retention forces while allowing lower assembly forces, reducing the risk of part damage.
- Hydraulic Pressing: Controlled force application (avoids over-exhertion/impact damage and assists in fully asserted linearisation without initial skew-fit and bruising).
- Finite Element Analysis (FEA): Simulates stress distribution.
Conclusion
Press fits offer a simple yet powerful solution for joining components without the need for adhesives, fasteners, or welding. By relying on controlled interference between mating parts, press fits create durable, high-strength connections that are widely used in mechanical assemblies, from automotive drivetrains to consumer electronics.
The effectiveness of a press fit hinges on careful design considerations—such as material compatibility, tolerance control, and surface finish—which directly impact performance and reliability.
As manufacturing methods become more precise and materials science continues to evolve, press-fit technology remains a staple in both traditional and modern applications. The ability to provide clean, compact, cost-effective, and above all permanent but disassembly-capable joints makes it especially appealing for automated assembly and miniaturized devices.
With growing demand for sustainable manufacturing and disassembly for recycling, the reversible nature of many press-fit based designs also aligns with circular economy goals. Press fit as a technique its worth as both a timeless and forward-thinking engineering solution.
