liminfoShrink Fit Calculator
Free web tool: Shrink Fit Calculator
Calculate temperature change, interface pressure, and holding force for shrink/press fits. Dimensions in mm.
Interference
0.050 mm
ΔT Required
166.8 °C
Heat Hub To
187 °C
Cool Shaft To
-147 °C
Interface Pressure
101.9 MPa
Holding Force (est.)
119.8 kN
About Shrink Fit Calculator
The Shrink Fit Calculator computes the thermal and mechanical parameters needed to assemble a shaft into a hub using an interference fit. An interference fit (also called a press fit or shrink fit) is a fastening method where the shaft outer diameter is slightly larger than the hub bore diameter, creating a tight joint by elastic deformation. Given the shaft diameter, hub bore diameter, hub outer diameter, material selection, and ambient temperature, the tool calculates: the interference (shaft OD minus bore ID), the temperature change required to open the bore enough for slip fit assembly, the actual heating temperature for the hub or cooling temperature for the shaft, interface contact pressure after assembly, and estimated axial holding force.
Shrink fits are widely used in precision engineering applications where bolt-free, key-free connections are needed: gears and sprockets on shafts, bearing races in housings, coupling hubs on motor shafts, flywheel assemblies, and turbine disk-shaft interfaces. The thermal assembly method (heating the hub or cooling the shaft) is preferred over hydraulic pressing when the interference is large or when marring the bore surface would be unacceptable.
The contact pressure formula is based on thick-wall cylinder theory: P = E × δ / (d × ((1+k²)/(1-k²) + ν)), where δ is the diametral interference, d is the bore diameter, k is the ratio of bore to outer diameter (d/D), E is Young's modulus, and ν is Poisson's ratio (taken as 0.3). The holding force is estimated as F = P × π × d² × 0.15 (using a friction coefficient of 0.15). Seven standard engineering materials are included with their thermal expansion coefficients and elastic moduli.
Key Features
- Calculates diametral interference (shaft OD minus hub bore ID) in mm
- Computes required temperature change (ΔT) to achieve sufficient bore expansion for assembly
- Shows hub heating target temperature and shaft cooling target temperature
- Computes interface contact pressure in MPa using thick-wall cylinder theory
- Estimates axial holding force in kN (using friction coefficient 0.15)
- Seven material options: low carbon steel, stainless 304, aluminum 6061, cast iron, brass, copper, titanium
- Accounts for material thermal expansion coefficient (CTE) and elastic modulus (E)
- Adds 0.05 mm clearance margin to required expansion for reliable slip-fit assembly
Frequently Asked Questions
What is a shrink fit and how does it work?
A shrink fit is an interference fit where the shaft diameter is larger than the hub bore. When assembled thermally, the hub is heated (or the shaft is cooled) until the bore expands enough to slip over the shaft. As parts return to room temperature, the hub contracts onto the shaft, creating a compressive gripping force. No fasteners or keys are needed.
How much interference is typical for a shrink fit?
Typical interference is 0.001–0.002 times the nominal diameter (H7/p6 or H7/r6 ISO tolerances). For a 50 mm shaft, this is roughly 0.05–0.1 mm of diametral interference. Larger interferences create more holding force but require higher assembly temperatures and may risk yielding the hub material.
How hot should I heat the hub for assembly?
The required heating temperature is displayed in the blue "Heat Hub To" box. Add 20–30°C safety margin to the displayed value to account for heat loss during handling. Typical assembly temperatures range from 100°C to 250°C for steel components. Never exceed the material's tempering temperature or heat treatment threshold.
Can I use liquid nitrogen to cool the shaft instead of heating the hub?
Yes. The "Cool Shaft To" temperature shows the shaft cooling target using liquid nitrogen (-196°C) or dry ice (-78°C). The calculation adds 0.05 mm clearance to the minimum required expansion, making the cooling approach feasible for most interferences. Shaft cooling is preferred when the hub material cannot withstand high temperatures.
What is interface contact pressure and how does it affect holding force?
Interface contact pressure is the radial pressure between the mating surfaces after assembly, measured in MPa. It is proportional to the interference and elastic modulus, and inversely proportional to the bore diameter and geometry factor. The holding force (axial and torque capacity) is proportional to contact pressure × contact area × friction coefficient.
Why does the calculator add 0.05 mm to the required expansion?
The 0.05 mm clearance margin ensures that the bore is sufficiently larger than the shaft for a slip-fit assembly without risk of the parts binding before reaching the correct position. Without this margin, the bore would only just match the shaft size at the target temperature, leaving no practical assembly window during heat transfer.
Which material should I select if both parts are steel?
Select the material of the hub (outer ring) since that is the part being thermally expanded. If both parts are low carbon steel, select "Steel (low carbon)" with CTE = 12×10⁻⁶ /°C and E = 200 GPa. If the hub is stainless steel 304, select "Steel (stainless 304)" with CTE = 17.3×10⁻⁶ /°C.
What is the hub outer diameter used for in the calculation?
The hub outer diameter (D) is used to compute the thick-wall geometry factor k = bore_diameter / outer_diameter. This k ratio affects contact pressure because a hub with thin walls (high k) is more flexible and allows more bore expansion per degree of temperature change, but also generates lower contact pressure for the same interference.