Press Fit Design: Interference & Best Practices

Press fits join parts through elastic deformation—an oversized shaft forced into an undersized hole. When designed correctly, the resulting friction locks parts together without fasteners, adhesives, or welding. When designed incorrectly, parts crack, slip, or won’t assemble.

This guide covers the engineering fundamentals and practical considerations for successful press fit design.

How Press Fits Work

When a shaft is pressed into a smaller hole, both parts deform elastically. The hole stretches and increases in diameter while the shaft compresses and decreases in diameter. This mutual deformation creates contact pressure at the interface, and friction at this interface resists relative motion between the parts.

The resulting connection transmits torque and axial loads proportional to the contact pressure and friction coefficient. A well-designed press fit can handle substantial loads without any mechanical fasteners.

ANSI Fit Classes

The American National Standards Institute defines standard interference fits across five classes, ranging from light assemblies that can be pressed together at room temperature to heavy fits requiring thermal assistance.

FN1 - Light Drive Fit

This lightest class uses interference of 0.0002-0.0006 inch for a 1-inch diameter. Parts can be assembled using press force at room temperature without heating or cooling. FN1 fits suit applications requiring easy disassembly or where loading is light.

FN2 - Medium Drive Fit

With interference of 0.0004-0.0010 inch for a 1-inch diameter, FN2 represents the workhorse of press fits. Assembly requires press force, and lubrication may help reduce the required force. This class suits permanent assemblies under moderate loads.

FN3 - Heavy Drive Fit

Interference increases to 0.0006-0.0016 inch for a 1-inch diameter. Assembly typically requires either heavy press force or shrink fitting. FN3 fits serve high-load permanent assemblies where significant holding force is required.

FN4 - Force Fit

At 0.0012-0.0025 inch interference for a 1-inch diameter, FN4 generally requires shrink fitting using heat or cold. This class handles applications like gears on shafts and heavy machinery components where maximum force transfer is needed.

FN5 - Shrink Fit

The tightest class uses 0.0016-0.0030 inch interference for a 1-inch diameter. Assembly requires shrink fitting only—the interference is too great for mechanical pressing. FN5 applies when maximum holding force is absolutely required.

Scaling with Diameter

Interference scales roughly with diameter. The following table shows approximate FN2 interference ranges for various nominal diameters.

Nominal Diameter	FN2 Interference Range
0.5″	0.0003-0.0006″
1.0″	0.0004-0.0010″
2.0″	0.0006-0.0014″
4.0″	0.0010-0.0020″

These values are approximate—use published tables for exact specifications on critical applications.

Calculating Interference Requirements

From Required Torque

If the press fit must transmit torque T, calculate the required interface pressure using:

p = 2T / (π × d² × L × μ)

Where p is the required interface pressure, T is torque, d is interface diameter, L is engagement length, and μ is the friction coefficient (typically 0.1-0.2 for steel-to-steel).

From Required Axial Force

If the press fit must resist axial load F:

p = F / (π × d × L × μ)

Interface Pressure to Interference

For a solid shaft in a thick-walled hub:

δ = p × d × [(do² + d²) / (E₂ × (do² - d²)) + (1 / E₁)]

Where δ is diametral interference, do is hub outer diameter, d is interface diameter, and E₁ and E₂ are the elastic moduli of shaft and hub respectively.

For most designs, published tables or online calculators prove more practical than manual calculation.

Design Considerations

Surface Finish

Surface finish affects both assembly force and holding strength. Both mating surfaces should achieve Ra 32 µin (0.8 µm) or better.

The finish matters because rough surfaces have higher initial friction, making them harder to assemble. During assembly, asperities deform and reduce effective interference. However, very smooth surfaces may reduce the friction coefficient and thus the holding force. See our surface finish guide for measurement details.

Lead-In Geometry

Parts must align and start before full interference engagement begins. The shaft needs a chamfer or radius on the leading edge—typically 0.015-0.030 inch at 45 degrees. The hole should have a similar chamfer on entry. For critical assemblies, a short pilot section at clearance helps alignment before interference engagement begins.

Wall Thickness

The hub must have sufficient wall thickness to withstand hoop stress without yielding or cracking. As a rule of thumb, hub wall thickness should be at least 0.25 times the bore diameter for steel, with more material required for weaker materials.

Verify that hoop stress remains below yield stress at maximum interference:

σ_hoop = p × (do² + d²) / (do² - d²)

Engagement Length

Longer engagement increases holding force proportionally but also increases assembly force by the same proportion. Typical engagement length runs from 0.5 to 1.5 times the diameter. Use at least 0.25 times the diameter as a minimum to avoid stress concentrations at the engagement edges.

Stress Concentrations

Avoid features that concentrate stress near the press fit interface. Keyways weaken the hub and can initiate cracks. Sharp corners at the end of engagement create stress risers. Cross-holes through the press fit zone interrupt the contact pressure distribution. Rapid section changes near the fit can lead to unexpected failures.

Assembly Methods

Press Fitting at Room Temperature

Room temperature pressing works for light to medium fits. Use a hydraulic or mechanical press with force monitoring. Clean both parts thoroughly, apply assembly lubricant if specified, align parts carefully, press at a steady rate while monitoring force, and verify final position.

Regarding lubrication: light oil or assembly paste reduces force by 30-50% but also reduces holding friction. Use lubrication when force is the limiting factor; avoid it when maximum holding strength is required.

Shrink Fitting with Heat Expansion

Heat the hub uniformly to the calculated temperature, insert the shaft while the hub is expanded, then allow the assembly to cool as the hub contracts onto the shaft. Verify final position before the parts lock together.

Calculate the required temperature rise using:

ΔT = δ / (α × d) + margin

Where α is the thermal expansion coefficient (about 6.5 × 10⁻⁶ per degree Fahrenheit for steel).

For example, achieving 0.002 inch interference on a 2-inch diameter in steel requires:

ΔT = 0.002 / (6.5 × 10⁻⁶ × 2) = 154°F rise needed

Add 50-100 degrees Fahrenheit margin for handling time, heating the hub to approximately 250 degrees above ambient.

Exercise caution with temperature: heating above 400 degrees Fahrenheit can affect heat treatment. Use an oven or induction heating for uniform temperature distribution.

Freeze Fitting with Cold Contraction

Cool the shaft in dry ice (-109 degrees Fahrenheit) or liquid nitrogen (-320 degrees Fahrenheit), insert the contracted shaft into the hole, and allow the shaft to warm and expand into the hole. Verify final position.

Freeze fitting works well when heating the hub risks damage or distortion, for large interferences where heating alone provides insufficient clearance, and for temperature-sensitive materials.

Combined Methods

For maximum interference assemblies in the FN5 class, combine heating the hub and freezing the shaft to achieve sufficient clearance for assembly.

Material Combinations

Steel into Steel

This most common combination works well across all fit classes. The friction coefficient ranges from 0.10 to 0.15 dry, decreasing when lubricated.

Steel into Aluminum

The aluminum hub expands more than the steel shaft with temperature changes, so consider operating temperature effects on interference. The friction coefficient ranges from 0.12 to 0.18. Because aluminum yields at lower stress than steel, verify hub stress at maximum interference.

Steel into Cast Iron

Cast iron’s brittleness limits allowable interference to prevent cracking. The friction coefficient ranges from 0.12 to 0.16. Stick to lighter fits in the FN1-FN2 range and verify tensile stress in the casting.

Plastics

Plastics creep under sustained stress, meaning press fits may loosen over time. Use metal inserts for critical fits, or accept some loosening for non-critical applications. See our engineering plastics guide for more details.

Tolerance Stack Analysis

Real assemblies have tolerance variation, requiring analysis of both extremes.

Minimum interference occurs when the shaft is at its minimum size and the hole is at its maximum size. This condition must still provide adequate holding force for the application.

Maximum interference occurs when the shaft is at its maximum size and the hole is at its minimum size. This condition must not crack or overstress parts and must be assemblable with available force or temperature differential.

Example Calculation

For an FN2 fit at 1.000-inch diameter:

From tables, the shaft ranges from 1.0010 to 1.0016 inches and the hole ranges from 1.0000 to 1.0006 inches.

The interference range works out to:

• Minimum: 1.0010 - 1.0006 = 0.0004 inch
• Maximum: 1.0016 - 1.0000 = 0.0016 inch

Verify both extremes meet requirements before releasing the design.

Common Problems and Solutions

Parts Won’t Assemble

When parts refuse to go together, the interference may be too large, alignment may be poor, or lead-in features may be inadequate. Verify actual dimensions on both parts to confirm they’re within tolerance. Add larger chamfers to improve alignment. Consider shrink fitting instead of pressing. Review the interference calculations for errors.

Parts Slip Under Load

Slippage indicates the interference is too small, the surfaces were lubricated when they shouldn’t have been, or temperature effects are reducing interference in operation. Verify the minimum interference condition provides adequate holding. Clean and degrease surfaces before assembly. Consider a tighter fit class. As a backup, add secondary retention through set screws or pins.

Hub Cracks

Cracking results from excessive interference, inadequate wall thickness, or material defects. Reduce interference by selecting a lighter fit class. Increase hub wall thickness if geometry permits. Inspect incoming material before assembly. Consider using a more ductile hub material.

Working With NextGen Components

We manufacture press fit components with the precision tolerances these assemblies require. Our capabilities include precision bore grinding for hole tolerances to 0.0002 inch, OD grinding for shaft tolerances to 0.0002 inch, surface finish to Ra 16 µin or better, and complete documentation of dimensions.

Questions about press fit design or component manufacturing? Contact our engineering team.