Press Fit Interference Calculator

Press Fit Interference Calculator

function calculatePressFit() { var dS = parseFloat(document.getElementById('shaftOD').value); var dH = parseFloat(document.getElementById('holeID').value); var di = parseFloat(document.getElementById('shaftID').value); var doHub = parseFloat(document.getElementById('hubOD').value); var L = parseFloat(document.getElementById('length').value); var mu = parseFloat(document.getElementById('friction').value); var E_gpa = parseFloat(document.getElementById('modulus').value); var v = parseFloat(document.getElementById('poisson').value); if (isNaN(dS) || isNaN(dH) || dS <= dH) { alert("Shaft diameter must be larger than the hole diameter for an interference fit."); return; } var E = E_gpa * 1000; // Convert GPa to MPa var delta = dS – dH; // Total diametrical interference var d = (dS + dH) / 2; // Nominal contact diameter // Lame's Equation for contact pressure (assuming same material for both) // P = (delta / d) / [ (1/E) * ( ((do^2 + d^2)/(do^2 – d^2)) + v ) + (1/E) * ( ((d^2 + di^2)/(d^2 – di^2)) – v ) ] var termHub = ((Math.pow(doHub, 2) + Math.pow(d, 2)) / (Math.pow(doHub, 2) – Math.pow(d, 2))) + v; var termShaft; if (di === 0) { termShaft = 1 – v; } else { termShaft = ((Math.pow(d, 2) + Math.pow(di, 2)) / (Math.pow(d, 2) – Math.pow(di, 2))) – v; } var pressure = (delta / d) / ((termHub / E) + (termShaft / E)); // Force F = P * Area * Friction = P * (pi * d * L) * mu var forceN = pressure * Math.PI * d * L * mu; var forceKN = forceN / 1000; // Torque T = F * radius = (P * pi * d * L * mu) * (d/2) var torqueNm = (forceN * (d / 2)) / 1000; document.getElementById('resInterference').innerText = delta.toFixed(4); document.getElementById('resPressure').innerText = pressure.toFixed(2); document.getElementById('resForce').innerText = forceKN.toFixed(2); document.getElementById('resTorque').innerText = torqueNm.toFixed(2); document.getElementById('results').style.display = 'block'; }

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Understanding Press Fit Interference

A press fit, also known as an interference fit or friction fit, is a fastening between two parts which is achieved by friction after the parts are pushed together. This is a common engineering practice used to secure bearings, bushings, and gears onto shafts without the need for additional fasteners like keys or bolts.

The Physics of Interference

The core principle relies on the elastic deformation of materials. When a shaft with a slightly larger diameter is forced into a hole with a slightly smaller diameter, both components experience stress. The shaft is compressed, and the hub (the part with the hole) is expanded. This creates contact pressure at the interface.

Key Calculation Factors

• Interference (δ): The physical difference between the shaft OD and the hole ID. In precision engineering, this is often measured in microns (μm).
• Contact Pressure (P): Calculated using Lame's equations for thick-walled cylinders. It depends on the material's Elastic Modulus (Young's Modulus) and Poisson's ratio.
• Assembly Force: The amount of axial force required to press the parts together. This is a function of the contact pressure, surface area, and the coefficient of friction.
• Transmissible Torque: The maximum rotational force the joint can withstand before slipping occurs.

Example Calculation

Suppose you have a 50mm steel shaft being pressed into a hub. You specify a 0.05mm interference (Shaft = 50.05mm, Hole = 50.00mm). If the engagement length is 40mm and the material is standard steel (210 GPa):

1. Interference: 50.05 – 50.00 = 0.05mm.
2. Pressure: Depending on the hub thickness, this might generate roughly 60-80 MPa of pressure.
3. Force: With a friction coefficient of 0.15, the assembly force would typically range between 40kN and 60kN.

Practical Tips for Engineers

When designing a press fit, consider the Yield Strength of your materials. If the contact pressure exceeds the yield strength, permanent (plastic) deformation will occur, which may weaken the joint or cause the hub to crack. Additionally, thermal expansion (shrink fitting) can be used to facilitate assembly by heating the hub or cooling the shaft.