Quick Answer: Engineering fits describe the intentional size relationship between two mating parts — typically a shaft and hole — that determines whether the assembled parts can move relative to each other, are accurately positioned, or are permanently locked. The three types are: clearance fit (shaft always smaller than hole — provides a gap for rotation or sliding, typical clearance 0.010–0.050 mm), transition fit (shaft and hole dimensions overlap in their tolerance zones — assembled parts may have slight clearance or slight interference, used for precise positioning with occasional disassembly), and interference fit (shaft always larger than hole — requires press or thermal assembly, creates friction-based load transmission, typical interference 0.010–0.030 mm). Fit type is defined by the overlap of tolerance zones between the shaft and hole, not by nominal size. In ISO 286, the combination H7/h6 produces a clearance fit; H7/k6 produces a transition fit; H7/p6 produces an interference fit — all at the same nominal diameter.
Why Engineering Fits Matter Beyond Dimensional Accuracy
Two parts can be machined to their nominal dimensions with acceptable individual tolerances and still assemble incorrectly — or fail in service — if the fit relationship between them was incorrectly specified. Fit selection is a functional design decision with direct consequences for how the assembly moves, transmits load, and survives its service environment.
The functional distinction: A shaft that rotates in a bearing requires a clearance fit — the shaft must be smaller than the bore so that the rotating surfaces are separated by a film of lubricant and never contact. If the tolerance specification accidentally produces an interference condition, the bearing seizes. Conversely, a gear hub mounted on a shaft and transmitting torque requires an interference fit — if clearance exists, the torque produces relative micromotion between hub and shaft surfaces (fretting), progressively wearing both surfaces until the connection fails.
The assembly consequence: An interference fit that is 50% tighter than calculated may require 10× the calculated press force (press force scales approximately as the square of the interference for elastic behavior). If a thin-walled component is pressed with this excessive force, the hoop stress may exceed yield, cracking the component during assembly. An interference fit that is 20% looser than calculated may transmit only 60–70% of the design torque before slipping.
The cost consequence: Specifying a tolerance that is one standard class tighter than functionally required increases machining cost by 30–80% for that feature. Applied across a complex component with many features, systematic over-tightening of tolerances without functional justification adds substantial cost without improving product performance.
The Three Types of Engineering Fits
Clearance Fit
In a clearance fit, the tolerance zones of the shaft and hole are arranged so that the shaft is always smaller than the hole at all combinations of their respective tolerances — even the largest allowable shaft diameter is smaller than the smallest allowable hole diameter. The assembly always has a gap (clearance) between the shaft and hole surfaces.
The clearance range: For a specific clearance fit, there is a minimum clearance (when the largest shaft meets the smallest hole) and a maximum clearance (when the smallest shaft meets the largest hole). For an H7/h6 fit at Ø20 mm:
- Hole range: 20.000–20.021 mm (H7)
- Shaft range: 19.987–20.000 mm (h6)
- Minimum clearance: 20.000 − 20.000 = 0 mm (limiting case)
- Maximum clearance: 20.021 − 19.987 = 0.034 mm
When to use: Any application requiring relative motion between the two parts: rotating shafts in bearings, sliding pistons in cylinders, linear guide carriages on rails, and any assembly that must be disassembled and reassembled repeatedly without controlled force. The amount of clearance determines the running accuracy — tighter clearances allow more precise rotation but require tighter manufacturing tolerances and more careful alignment.
Common clearance fit classes (ISO 286, hole-basis H7):
| Shaft Symbol | Clearance Type | Typical Application |
|---|---|---|
| H7/g6 | Close running | Precision rotating shafts, machine tool spindles |
| H7/h6 | Sliding/free running | General rotating shafts, pistons |
| H7/f7 | Free running | Rotating shafts with moderate clearance |
| H7/e8 | Loose running | Applications with poor alignment or temperature variation |
Transition Fit
In a transition fit, the tolerance zones of the shaft and hole partially overlap — the assembled pair may have slight clearance or slight interference depending on where within their tolerance ranges the individual parts were produced. Unlike clearance fits (always clearance) or interference fits (always interference), a transition fit can produce either condition.
The engineering purpose: Transition fits exist for applications that require more accurate positioning than a clearance fit provides, but where the need for occasional disassembly makes a full interference fit impractical. The slight interference that may result from a transition fit can be broken by hand force or light tapping; the slight clearance is so small (typically 0–0.010 mm) that positional accuracy is maintained.
Example — H7/k6 at Ø20 mm:
- Hole range: 20.000–20.021 mm (H7)
- Shaft range: 20.002–20.015 mm (k6)
- Possible clearance: 20.021 − 20.015 = +0.006 mm (if smallest shaft, largest hole)
- Possible interference: 20.000 − 20.002 = −0.002 mm (if largest shaft, smallest hole)
When to use: Bearing locations that require alignment but where the bearing must be removable for service (outer ring to housing); precision fixture locating features; coupling hubs that carry light torque and need occasional removal; alignment pins that must be hand-pressed in rather than slip-fitting. Transition fits are widely used in machine tool and precision instrument assemblies where alignment accuracy is critical but permanent fixation is not.
Common transition fit classes (ISO 286, hole-basis H7):
| Shaft Symbol | Character | Typical Application |
|---|---|---|
| H7/js6 | Slight clearance dominant | Bearing outer rings, light location |
| H7/k6 | Near neutral | Precision location, removable |
| H7/m6 | Slight interference dominant | Accurate coupling, hand press |
| H7/n6 | Light press | Fixed location, moderate force |
Interference Fit
In an interference fit, the tolerance zones are arranged so that the shaft is always larger than the hole — even the smallest allowable shaft diameter is larger than the largest allowable hole diameter. Assembly requires applying force (press fit) or thermal expansion (heating the outer part and/or cooling the inner part to create temporary clearance for assembly).
Load transmission mechanism: The interference creates compressive hoop stress in the outer part and tensile stress in the inner part, with the interfacial normal pressure producing a friction force that resists relative rotation or axial movement. The torque that an interference fit can transmit is:
T = μ × P × A × r
where μ is the coefficient of friction at the interface, P is the interfacial pressure (determined by the interference amount and material properties), A is the contact area, and r is the mean radius. Increasing the interference increases P and therefore the transmissible torque — but also increases the assembly force and the stress in both parts.
When to use: Permanent assemblies where relative motion would cause failure: gear hubs on shafts (prevents fretting from torque-induced micro-motion), wheel hubs on axles, bearing inner rings that rotate with the shaft (must have interference with shaft to prevent creep), and any connection that must transmit load without a mechanical key or fastener.
Common interference fit classes (ISO 286, hole-basis H7):
| Shaft Symbol | Interference Level | Typical Application |
|---|---|---|
| H7/p6 | Light press fit | Hub and shaft connections, light torque |
| H7/r6 | Medium press fit | Gear hubs, moderate torque |
| H7/s6 | Drive fit | High torque transmission |
| H7/u6 | Force/shrink fit | Heavy-duty connections, thermal assembly |
The ISO 286 and ANSI Fit Systems
ISO 286 (Metric System)
ISO 286 defines a system of standardized tolerance grades and deviation codes that define the tolerance zone position and width for any nominal diameter. Understanding the notation:
For holes: Capital letter (A through ZC) defines the fundamental deviation (position of the tolerance zone relative to the zero line); number (1 through 18) defines the tolerance grade (width of the tolerance zone). H is the standard code for the reference hole in a hole-basis fit system — H has its lower deviation at zero (the hole is never smaller than nominal).
For shafts: Lowercase letter defines the fundamental deviation; number defines the tolerance grade. h has its upper deviation at zero (the shaft is never larger than nominal). Letters g, f, e, d produce clearance fits with H holes; letters k, m, n produce transition fits; letters p, r, s, t, u produce interference fits.
The hole-basis system: Most precision machining uses the hole as the fixed reference because hole-making tools (drills, reamers, boring bars) are produced in standardized sizes and are more difficult to adjust than shaft-turning parameters. The H7 tolerance class is the most widely used hole specification in general engineering — H7 at Ø20 mm tolerates 20.000–20.021 mm.
The shaft-basis system: When the shaft diameter is determined by a purchased component (a motor output shaft, a standard shaft from catalog stock), the hole-basis system is reversed — the shaft tolerance is fixed and the hole tolerance varies. This is common in bearing selection (the shaft is the given; the housing is designed to produce the required fit with the bearing outer ring).
ANSI Fit System
ANSI (American National Standards Institute) defines engineering fits through named classes that describe the application rather than the tolerance zone geometry:
| ANSI Class | Name | Equivalent to ISO | Application |
|---|---|---|---|
| RC1–RC9 | Running/Sliding Clearance | Clearance fits | Rotating and sliding motion |
| LC1–LC11 | Locational Clearance | Clearance fits | Accurate positioning, easy assembly |
| LT1–LT6 | Locational Transition | Transition fits | Accurate positioning, can press assemble |
| LN1–LN3 | Locational Interference | Tight transition/light interference | Accurate alignment, moderate force |
| FN1–FN5 | Force/Shrink | Interference fits | Permanent, high load transmission |
Thermal Expansion and Fit Design
A fit specified at room temperature (20°C per ISO 1) may behave differently in service if the operating temperature is significantly different from the assembly temperature, or if the two mating parts are made from different materials with different coefficients of thermal expansion (CTE).
The critical calculation: The change in dimension due to temperature change is ΔL = α × L₀ × ΔT, where α is the CTE of the material, L₀ is the nominal dimension, and ΔT is the temperature change from assembly temperature.
A common failure case: Steel shaft (α ≈ 11.7 µm/m·°C) in an aluminum housing (α ≈ 23.1 µm/m·°C). At assembly temperature (20°C), the interference fit is 0.020 mm. If the assembly operates at 80°C (ΔT = 60°C), the aluminum housing expands more than the steel shaft:
- Aluminum expansion at Ø50 mm bore: 23.1 × 10⁻⁶ × 50 × 60 = 0.069 mm
- Steel shaft expansion at Ø50 mm: 11.7 × 10⁻⁶ × 50 × 60 = 0.035 mm
- Net change in fit: +0.034 mm (aluminum grows more than steel)
- Fit at operating temperature: 0.020 − 0.034 = −0.014 mm (clearance — the interference is gone)
A design where an interference fit at room temperature becomes a clearance fit at operating temperature will fail — the connection loosens. The correct approach is to calculate the required room-temperature interference that, after accounting for differential thermal expansion, produces the required minimum interference at the maximum operating temperature.
Fit Selection for Common Assembly Types
Bearings: The fit requirements for bearings are defined by bearing manufacturers (SKF, FAG, NSK) in application guides and depend on whether the inner or outer ring is the rotating element. The general rules: the rotating ring should have an interference fit with its seat (inner ring with shaft if shaft rotates; outer ring with housing if housing rotates) to prevent creep; the stationary ring should have a transition fit or light interference.
For a typical shaft-rotating application (shaft rotates, housing stationary): inner ring to shaft uses H7/k6 or H7/m6 (transition to light interference); outer ring to housing uses K7/h6 or M7/h6 (transition to light interference). The specific class depends on bearing load and speed — heavy loads require heavier interference; high speeds with light loads may use looser fits to avoid excessive temperature rise from bearing preload.
Gears and hubs: Gears transmitting torque through friction (interference fit only, without key) require sufficient interference to develop the required friction force at the interface. The required interference is calculated from the design torque, material properties, diameter, and contact length. Typical interference for steel gear on steel shaft at Ø40 mm: 0.015–0.030 mm for moderate torque (500–2,000 N·m); increase to 0.030–0.050 mm for high torque or shock loading.
Dowel pins: Locating dowel pins in fixtures and precision assemblies typically use H7/n6 or H7/m6 (light to medium interference) in the fixed component and H7/g6 (close clearance) in the mating component that must be removed. This maintains precise location while allowing the assembly to be separated.
Common Fit Selection Mistakes and Their Consequences
| Mistake | Consequence | Correct Approach |
|---|---|---|
| Specifying tightest fit “for safety” | Excessive press force; part cracking in thin-walled components; high machining cost | Match fit type and tightness to functional requirement — movement, alignment, or fixation |
| Ignoring thermal expansion (dissimilar materials) | Interference lost at operating temperature (connection loosens) or increases (binding) | Calculate fit at operating temperature; specify room-temperature interference that accounts for differential expansion |
| Interference fit for serviceable components | Part damage during disassembly; high downtime | Use transition or close clearance fit where regular removal is required |
| Excessive clearance in vibrating assemblies | Fretting wear; loosening; noise | Use transition or light interference in vibrating environments; avoid clearance fits in high-cycle vibration |
| Applying nominal tolerances without standard fit system | Unpredictable fit behavior; inconsistent production | Use ISO 286 or ANSI fit classes; specify full tolerance zone codes (H7/h6, not just ±0.010 mm) |
| Specifying tolerances beyond manufacturing capability | High scrap; delays; cost increase | Verify Cp/Cpk capability for the specified tolerance before releasing drawings |
Key Takeaways
- Engineering fits define functional behavior, not just dimensional accuracy: a clearance fit enables motion; a transition fit provides alignment with limited force for assembly/disassembly; an interference fit creates permanent, load-transmitting connection through interfacial friction.
- ISO 286 fit notation (H7/h6, H7/k6, H7/p6) completely defines the fit relationship: the capital letter and number specify the hole tolerance zone; the lowercase letter and number specify the shaft tolerance zone; the combination determines whether the assembly always has clearance, may have either, or always has interference.
- Thermal expansion must be considered for all fits in dissimilar material assemblies: steel shaft in aluminum housing (or any combination with different CTE values) changes its fit condition as temperature changes; an interference fit designed at 20°C may become a clearance fit at 80°C if differential expansion is not accounted for.
- Bearing fits require specific guidance from bearing manufacturers: the rotating ring must have interference with its seat to prevent creep; the stationary ring may have transition or controlled interference; specifying these fits generically without consulting bearing application data is a common source of premature bearing failure.
- Over-tightening tolerances is a major cost driver: one tolerance class tighter than necessary increases machining cost by 30–80% for that feature; applying this systematic over-tightening across a complex component adds substantial cost without improving function.
- The hole-basis system (H as the fixed hole tolerance) is the standard for most CNC machining applications: holes are cut with standardized tools; the shaft diameter is turned to produce the required fit type relative to the standard hole.
- For OEM procurement and design teams: when drawing callouts specify a fit (e.g., Ø20 H7/h6), verify that the machining supplier’s process capability can hold both tolerance zones independently and consistently. A tolerance zone of ±0.010 mm requires a process with standard deviation of approximately ±0.003 mm to achieve Cp ≥ 1.67. If the supplier’s capability is ±0.010 mm standard deviation, the tolerance should be relaxed to ±0.025 mm for the same confidence level, and the fit design should be re-evaluated.
Frequently Asked Questions
What are the three types of engineering fits?
The three types of engineering fits are clearance fit (the shaft is always smaller than the hole, providing a gap for rotation or sliding), transition fit (the shaft and hole tolerance zones overlap, so assembled parts may have slight clearance or slight interference depending on where they fall within their tolerance ranges), and interference fit (the shaft is always larger than the hole, requiring press or thermal assembly and creating load-transmitting interfacial friction). The type is determined not by nominal size but by the relationship between the tolerance zones of the shaft and hole. In ISO 286, changing the shaft tolerance code from h (clearance with H7 hole) to k (transition) to p (interference) changes the fit type while the nominal diameter and hole specification remain the same.
When should a clearance fit be used?
A clearance fit should be used whenever relative motion between the mating parts is required or intended: rotating shafts in plain bearings or bushings, pistons moving in cylinders, linear guides sliding on rails, and assemblies that must be easily disassembled and reassembled. The amount of clearance required depends on the motion requirements — a machine tool spindle bearing might use a close clearance (H7/g6, clearance 0.004–0.029 mm at Ø50 mm) while a low-precision sliding guide might use a looser fit (H8/f7, clearance 0.025–0.075 mm at Ø50 mm). The critical design rule: never specify a clearance fit for a part that must transmit torque through friction — the inevitable micro-motion from torque will cause fretting corrosion, progressively deteriorating the interface.
What is a transition fit and why is it used?
A transition fit is used when the application requires more accurate positioning than a clearance fit provides, but where the part must still be removable without significant damage. The tolerance zones of shaft and hole overlap, meaning that an individual pair may assemble with slight clearance or slight interference depending on where within their tolerances each part was produced. In practice, transition fits allow assembly by hand pressure or light tapping, with enough positional accuracy (near-zero clearance) for alignment-critical features. Typical applications: bearing outer rings in housings (precise alignment required, bearings must be removable for replacement), locating pins in tool fixtures, and coupling hubs that transmit light torque and require occasional disassembly. In ISO 286, shaft codes k, m, and n produce transition fits with a standard H7 hole.
How do you calculate the required interference for an interference fit?
The required interference is calculated from the torque or axial force that must be transmitted, the coefficient of friction at the interface, the contact geometry, and the material properties. The transmissible torque is T = μ × P × A × r, where P (interfacial pressure) depends on the interference amount, the diameter, and the material elastic moduli through the Lamé equation for thick-walled cylinders. For a solid steel shaft in a steel hub at Ø30 mm, typical torque capacity per 0.010 mm of interference and 30 mm contact length is approximately 100–200 N·m. The actual required interference is then calculated from the design torque (with an appropriate safety factor) using these relationships. Practically, published tables from standard references (Machinery’s Handbook, Shigley’s Mechanical Engineering Design) provide interference values for common materials and sizes, avoiding the need for the full Lamé calculation in routine design.
What is the difference between H7/h6 and H7/k6?
Both H7/h6 and H7/k6 use the same hole tolerance (H7), but specify different shaft tolerances. H7/h6 specifies a shaft with its upper deviation at zero (never larger than nominal) and a total tolerance of IT6 grade. With a H7 hole (lower deviation at zero, IT7 grade), the combination always produces a clearance — from near zero to a maximum of IT6 + IT7. H7/k6 specifies a shaft with a positive fundamental deviation (the shaft tolerance zone is shifted upward, into or above the hole’s tolerance zone). The combination can produce slight clearance (when the shaft is at minimum and hole at maximum) or slight interference (when shaft is at maximum and hole at minimum). The ISO designation of k indicates a transition fit — the fit may be either clearance or interference within the tolerance range.
Written by the CMS engineering team with 15+ years of precision CNC machining experience producing shaft-hole assemblies, bearing seats, gear hubs, and precision locating features to ISO 286 and ANSI fit specifications for automotive, industrial automation, medical device, and aerospace OEM manufacturing programs. Technical references: ISO 286-1 (Limits and Fits — Bases), ANSI B4.1 (Preferred Limits and Fits for Cylindrical Parts), Shigley’s Mechanical Engineering Design (Shaft and Hub Fits chapter), Machinery’s Handbook (Preferred Fits and Tolerances), SKF and FAG Bearing Fit Selection Application Guides.
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