An interference fit—also called a press fit or oversize fit—is a friction-locked shaft–hub connection in which components are joined with a deliberate interference. In the context of demolition works, rock excavation and tunnel construction, as well as natural stone extraction, it plays a central role: axle pins, bushings and tool carriers of concrete pulverizers, attachment shears or hydraulic wedge splitters are often accurately positioned and secured against relative motion via interference fits. The principle enables precise centering, high torque transmission and a robust, vibration-resistant joint—an advantage in dusty, impact-loaded environments typical of concrete demolition and deconstruction, building gutting and concrete cutting or other special-purpose operations.
Definition: What is meant by an interference fit
An interference fit is the joining of two components where the outside diameter of one part (e.g., shaft, pin) is larger than the inside diameter of the mating part (e.g., bore, bushing). During assembly—by cold pressing or by temperature methods such as shrink fitting/cryogenic fitting—a radial pre-stress with contact pressure is generated. This produces frictional locking that transmits forces and moments and secures the joint against relative motion. Interference fits are specified via tolerance classes according to ISO 286 (e.g., combinations such as H7/p6, H7/n6, P7/s6). They differ from clearance and transition fits by the deliberate positive interference. Typical applications are seating connections of gears, pulleys, coupling hubs, outer bearing rings, or wear-resistant bushings and pins at joint points of concrete pulverizers and steel shears.
Design, function and calculation fundamentals
The interference fit creates a circumferential contact pressure between shaft and bore. The interference leads to elastic deformation of both partners, thereby creating a contact pressure. Using the coefficient of friction μ (typically 0.08–0.20, depending on material pairing, surfaces and assembly aids), the transferable circumferential force and torque are determined. The larger the interference and the effective joining area (diameter × length), the higher the load capacity—up to a sensible maximum, since excessive interference can unduly increase assembly forces, notch stresses and plastic deformation. For design, tolerance position, material properties (modulus of elasticity, yield strength), temperature range, vibration/impact loading and surface roughness are considered. In practice, for joints of equipment with impact-type loads (e.g., concrete pulverizers when cutting through reinforced components), length/diameter guidelines are common: sufficient joining length (often L ≈ 1.0–1.5 × d) and finely finished surfaces (e.g., Ra ~ 0.4–1.6 μm) improve load capacity and repeatable centering. Temperature differences directly affect the fit: if the hub heats up or the shaft cools down, the contact pressure drops—this effect must be considered in assemblies near the hydraulic power pack. The transmissibility of the interference fit remains stable only if roundness, straightness and chamfers are properly machined, heat-affected edges are avoided, and assembly is axially guided.
Relevance in concrete demolition, deconstruction and tunnel construction
Interference fits are common in tool holders, pivots and bushings of concrete pulverizers, attachment shears, multi cutters and steel shears. They secure pins against rotational and axial play, center cutting arms and transmit torque without additional form elements. In hydraulic wedge splitters for rock and concrete, press fits are found, for example, on guide elements, housing bushings or load-introducing pins subjected to cyclic pressure shocks. In hydraulic power units, interference fits are used on coupling hubs, pump shafts or fan carriers to maintain drivetrain alignment over the long term. In rock demolition and tunnel construction as well as in natural stone extraction, precisely fitted bushings ensure that joints remain free of play even under dust, moisture and abrasive particles, with loads being evenly distributed.
Assembly methods: cold pressing, shrink fitting and cryogenic fitting
There are three principal assembly routes. In cold pressing, the shaft is pressed into the bore using a press; clean lead-in (chamfer), axial guidance and a controlled feed rate prevent scoring. In shrink fitting, the hub is heated (typically 80–200 °C, depending on material and heat treatment) so that the inner diameter temporarily expands; the cold shaft is then inserted quickly. Cryogenic fitting uses cooling of the shaft (e.g., dry ice) to reduce its dimensions. Assembly aids such as low-viscosity assembly oils or solid lubricants (used sparingly) lower joining forces but change the effective friction coefficient—this must be considered in the design. For on-site service—e.g., on concrete pulverizers or hydraulic wedge splitters—portable hydraulic presses are often used, powered by a hydraulic power pack. Clean mating surfaces, defined temperature control, reproducible joining forces and recording the press-in force as a quality characteristic are essential.
Disassembly and maintenance in service
For disassembly of press-fitted pins or bushings, hydraulic pullers, press-out fixtures or heat/cold methods are used. Local heating of the hub or targeted cooling of the pin reduces the contact pressure. Blows without proper support can expand bores or chamfer edges—this degrades future fits. After disassembly, roundness, surface condition and dimensional accuracy should be checked; worn bushings in joints of concrete pulverizers and steel shears are preferably replaced and then finish-reamed. Run-in marks, fretting corrosion or chatter marks indicate insufficient sealing, an incorrect friction level or an inadequate joining area. After reassembly, the joint should be checked under operating conditions, e.g., by measuring bearing clearance, breakaway torque or through acoustic/thermal anomalies during a test run.
Standards, tolerances and fit selection
Fit selection follows ISO 286 (tolerance classes and positions) as well as common calculation rules for shaft–hub connections. For light to medium loads, ranges such as H7/m6 to H7/n6 are common; for higher loads, H7/p6 or P7/s6. The appropriate choice depends strongly on diameter, length, material pairing, load case (torque, transverse forces, impact), temperature and required disassemblability. In vibration- and impact-loaded applications—such as in concrete crushing—more robust tolerance positions and sufficiently long joining areas are preferred. Key influencing factors are:
- Diameter range and joining length
- Material properties (modulus of elasticity, yield strength, thermal expansion)
- Surface quality and form tolerances
- Operating loads: static, alternating, impact
- Temperature and environment: moisture, dust, slurry, corrosion
- Maintenance strategy: disassemblable or permanent joint
Materials, surfaces and corrosion protection
Common pairings are hardened pins (induction- or through-hardened) with wear-resistant steel or bronze bushings. A fine surface texture promotes uniform area contact and reduces notch effects. For harsh use—such as in rock excavation and tunnel construction—seals and grease chambers protect mating surfaces from particles; for longer downtimes, a light, suitable corrosion protection prevents fretting. Galvanically unfavorable combinations should be avoided or electrically insulated. Where high temperatures occur (e.g., near pump shafts in hydraulic power packs), thermal expansion must be considered in fit selection to prevent a loss of contact pressure.
Calculation example: interference and load capacity
Example: A joint pin d = 60 mm, joining length L = 80 mm, shall transmit a frictional torque of 1.2 kN·m. With a conservative friction coefficient μ = 0.12, the required circumferential force is T = M / (0.5·d) ≈ 1.2 kN·m / 0.03 m = 40 kN. The required mean contact pressure p follows from T = μ · p · A, with A = circumference · length = π·d·L ≈ 3.1416·0.06 m·0.08 m ≈ 0.0151 m². Thus p ≈ 40,000 N / (0.12 · 0.0151 m²) ≈ 22 MPa. The associated interference results from the elasticities of the parts (simplified cylinder assumption). Depending on modulus of elasticity and wall thickness, an interference on the order of a few tens of micrometers typically results. This example serves only for orientation; the actual design requires a detailed consideration of materials, geometry, temperature and safety factors.
Typical failure patterns and remedies
In practice, recurring patterns occur that can be avoided through targeted measures:
- Fretting corrosion: Moisture/slurry ingress; remedy: sealing, suitable corrosion protection, clean fit edges.
- Chatter marks/Brinelling: Micro relative motion due to insufficient contact pressure; remedy: adjust fit, increase joining length, stabilize friction level.
- Bore expansion: Disassembly by hammering without support; remedy: hydraulic press-out, heating/cryogenic methods, proper press-out tools.
- Assembly damage: Burr formation, scoring due to missing chamfers; remedy: deburr, chamfer, ensure axial guidance, appropriate assembly speed.
- Thermally induced loosening: Contact pressure drops in warm operation; remedy: match fit selection to temperature profile, review material pairing.
Quality assurance and inspection
To ensure a durable interference fit, dimensional and form tolerances are measured (internal/external micrometers, probes), surfaces are evaluated, and the press-in force is documented. A blue contact pattern check reveals localized high spots. After assembly, runout, alignment and—at joints of concrete pulverizers and hydraulic wedge splitters—the breakaway and operating torque should be checked. In heavily loaded applications, regular visual inspections for rust nests, chips and play help. For safety-relevant components, non-destructive testing may be advisable depending on the application area in concrete demolition and special demolition or other special operations.
Safety and general notes
Work on interference fits requires appropriate personal protective equipment and safe handling of heat, cold and hydraulic forces. Shrink and cryogenic methods must be carried out with care and proper ventilation. Welding in the area of press fits changes material properties and should only be performed after expert evaluation. Dimensional and fit specifications are application-dependent; the applicable standards and design specifications are authoritative. In case of doubt, expert verification is advisable—especially on assemblies from Darda GmbH that operate in impact- and vibration-intensive environments.