Press Fit Interference Calculator

Calculator Inputs

Shaft outside diameter mm

Hub bore diameter mm

Contact length mm

Shaft inner diameter mm

Hub outside diameter mm

Friction coefficient

Shaft elastic modulus GPa

Hub elastic modulus GPa

Shaft Poisson ratio

Hub Poisson ratio

Shaft yield strength MPa

Hub yield strength MPa

Service torque N·m

Service axial load N

Hub expansion coefficient µm/m°C

Shaft expansion coefficient µm/m°C

Assembly clearance mm

Formula Used

Diametral interference:

Δd = shaft diameter − bore diameter

Radial interference:

Δr = Δd / 2

Estimated contact pressure:

p = Δd / { d × [ ((Cs − νs) / Es) + ((Ch + νh) / Eh) ] }

For the shaft:

Cs = (d² + di²) / (d² − di²)

For the hub:

Ch = (Do² + d²) / (Do² − d²)

Normal force:

N = p × π × d × L

Axial slip capacity:

F = μ × N

Torque capacity:

T = μ × p × π × d² × L / 2

How To Use This Calculator

Enter the shaft and bore diameters in millimeters. Use actual measured values when available.

Enter contact length, hub outside diameter, and shaft inner diameter. Use zero for a solid shaft.

Add material properties for both parts. Steel often uses 210 GPa and 0.30 Poisson ratio.

Enter friction, yield strengths, service torque, and service axial load.

Press the calculate button. Review pressure, force, torque, stress, and safety factors.

Use CSV or PDF buttons to save the result for records.

Example Data Table

Case	Shaft mm	Bore mm	Length mm	Hub OD mm	Friction	Use
Light fit	25.012	25.000	20	50	0.10	Small gear
Medium fit	50.040	50.000	45	90	0.12	Pulley hub
Heavy fit	80.090	80.000	70	150	0.15	Rotor sleeve

Press Fit Interference Guide

A press fit joins two round parts by elastic strain. The shaft is made slightly larger than the hub bore. During assembly, the parts deform and create contact pressure. That pressure creates friction. Friction resists slip, rotation, and movement under load.

Why Interference Matters

Small interference can allow fretting. Fretting damages surfaces and loosens the joint. Excessive interference can crack the hub, buckle a thin shaft, or make assembly difficult. A good design balances torque capacity, material stress, surface finish, and production tolerance.

Important Inputs

The calculator uses shaft diameter, bore diameter, contact length, hub outside diameter, and shaft inside diameter. It also uses elastic modulus and Poisson ratio for both parts. These values describe how much each part deflects under pressure. Friction coefficient controls press force and torque capacity.

Reading The Results

Diametral interference is the size difference before assembly. Radial interference is half that value. Contact pressure is the estimated pressure at the mating surface. Normal force is pressure times cylindrical contact area. Axial press force is friction times normal force. Torque capacity is friction force times radius.

Stress And Safety

Hub hoop stress is usually critical. Thin hubs see higher stress than thick hubs. Compare calculated hoop stress with allowable material strength. Use a factor of safety for impact, heat, corrosion, and uncertain friction. For important machinery, verify the result with a detailed standard or finite element model.

Thermal Assembly

Heating the hub expands the bore. Cooling the shaft shrinks the shaft. Thermal assembly lowers press force and reduces surface damage. The tool estimates temperature change for clearance. Real shops should also consider handling time, part mass, coatings, and safe temperature limits.

Practical Design Tips

Use clean parts and controlled surface roughness. Chamfer the shaft end and bore entry. Apply a suitable lubricant when the design allows it. Measure parts at the same temperature. Record actual dimensions, because tolerance stack can change pressure greatly. Test one sample before approving a full production batch.

Limitations

Remember the result is an engineering estimate. Real assemblies depend on roundness, taper, plated layers, retained oil, and loading direction. Very short hubs can behave differently. When safety is critical, compare with measured pull tests and qualified design rules.

FAQs

What is press fit interference?

It is the size difference between a larger shaft and a smaller bore before assembly. This difference creates elastic pressure after the parts are joined.

What is diametral interference?

Diametral interference is shaft diameter minus bore diameter. It is measured across the full diameter, not only from the centerline.

What is radial interference?

Radial interference is half of diametral interference. It represents the radial compression needed at the contact surface.

Why does hub outside diameter matter?

A thicker hub is stiffer and usually has lower hoop stress. A thin hub expands more and can reach stress limits sooner.

How does friction affect the result?

Friction changes axial holding force and torque capacity. Higher friction increases holding capacity, but actual friction can vary with finish, lubricant, and assembly method.

Can this calculator handle hollow shafts?

Yes. Enter the shaft inner diameter. Use zero when the shaft is solid. Hollow shafts are usually more flexible.

Why estimate thermal assembly temperature?

Heating the hub or cooling the shaft creates temporary clearance. This can lower force and reduce scoring during assembly.

Is this suitable for final machine design?

Use it for estimation and comparison. Critical parts should be checked with design standards, material data, testing, and qualified engineering review.