Interference Fit Tolerance Calculator

Calculator Inputs

Nominal diameter mm

Hole minimum mm

Hole maximum mm

Shaft minimum mm

Shaft maximum mm

Fit length mm

Friction coefficient

Shaft elastic modulus GPa

Hub elastic modulus GPa

Shaft Poisson ratio

Hub Poisson ratio

Thermal expansion ppm per C

Material yield strength MPa

Service torque N m

Example Data Table

Nominal mm	Hole range mm	Shaft range mm	Length mm	Interference range mm	Typical note
25	24.990 to 25.000	25.012 to 25.025	20	0.012 to 0.035	Light press fit
50	49.980 to 50.000	50.030 to 50.055	40	0.030 to 0.075	Medium press fit
80	79.960 to 80.000	80.070 to 80.110	65	0.070 to 0.150	Heavy press fit

Formula Used

Minimum interference = shaft minimum - hole maximum.

Maximum interference = shaft maximum - hole minimum.

Mean interference = average shaft diameter - average hole diameter.

Compliance = ((1 - shaft Poisson ratio squared) / shaft modulus) + ((1 - hub Poisson ratio squared) / hub modulus).

Contact pressure = diametric interference / (nominal diameter × compliance).

Press force = friction coefficient × contact pressure × pi × diameter × fit length.

Torque capacity = friction coefficient × pressure × pi × diameter squared × fit length / 2.

Thermal change = maximum interference / (thermal expansion coefficient × nominal diameter).

How To Use This Calculator

Enter the nominal shaft and hole diameter.
Add the minimum and maximum hole limits from the drawing.
Add the minimum and maximum shaft limits from the drawing.
Enter the press fit contact length.
Set friction, material stiffness, Poisson ratio, and expansion rate.
Add yield strength and service torque for quick checks.
Press the calculate button.
Review the result above the form.
Download CSV or PDF files for records.

Interference Fit Planning Guide

Why Interference Fits Matter

An interference fit joins a shaft and a hole by controlled overlap. The shaft is made slightly larger than the hole. Assembly creates pressure at the shared surface. That pressure produces friction. The friction helps resist sliding, spinning, and vibration.

What The Calculator Reviews

This calculator estimates the main values needed during early fit planning. It compares the largest shaft with the smallest hole. It also compares the smallest shaft with the largest hole. These two checks show the maximum and minimum interference. A positive value means a press fit exists. A negative value means clearance may occur.

Formula Meaning

The pressure estimate uses a simplified elastic model. It treats the shaft and hub as flexible parts. The model uses diameter, material stiffness, and Poisson ratio. This gives a practical planning value.

Assembly Force

Press force depends on pressure, contact area, and friction. Longer contact length increases the force. Higher friction also increases the force. Surface finish, lubrication, coatings, and temperature can change real results. Use conservative values when the process is uncertain.

Thermal Assembly

Thermal assembly can reduce press effort. Heating the hub expands the hole. Cooling the shaft shrinks the shaft. The required temperature change depends on interference, diameter, and expansion rate. Always confirm safe temperature limits for materials, seals, and treatments.

Design Checks

Good tolerance planning prevents many failures. Too little interference can cause slip. Too much interference can crack the hub. It can also yield the shaft surface. Review pressure against material strength. Check torque capacity against real service torque. Add a safety factor for shock loads.

Workshop Records

Record each input before production. Keep units consistent. Measure real parts from the same batch. Compare measured results with drawing limits. Update the fit plan when machining changes. This habit keeps workshop decisions traceable and repeatable.

Example Use

The example table gives typical situations only. It helps users compare small and large fits. Actual design limits can differ by standard. Material grade, bore thickness, shaft form, and speed matter. Use the output as a careful guide. Then confirm dimensions with approved drawings. Recheck after plating, heat treatment, or grinding. Document assumptions before changing machines or suppliers during production runs.

FAQs

What is an interference fit?

It is a fit where the shaft is slightly larger than the hole. Assembly creates contact pressure. That pressure creates friction and helps lock the parts together.

What does negative interference mean?

Negative interference means the hole may be larger than the shaft. That condition creates clearance. It may not hold torque without another locking method.

Can this replace a design standard?

No. This calculator is a planning tool. Use relevant standards, material data, drawings, testing, and engineering review for final designs.

Why does fit length matter?

Fit length changes contact area. A longer contact length usually increases press force and torque capacity. It can also change assembly difficulty.

Why is friction coefficient important?

Friction affects press force and torque capacity. Lubricated parts need less press force. They may also hold less torque after assembly.

What temperature change is shown?

The calculator estimates the temperature change needed to overcome maximum interference. It can represent heating the hub, cooling the shaft, or both.

What safety factor is used?

The pressure safety factor compares material yield strength with estimated maximum pressure. It is a simple screen, not a full stress analysis.

Why do real press forces differ?

Real forces change with finish, roundness, lubrication, speed, coatings, alignment, and measurement error. Always compare estimates with shop data.