Why Fit Selection Matters More Than Tolerance Tightening
A common response to assembly problems is to tighten tolerances. Tighter tolerances cost more to machine, take longer to produce, and increase rejection rates — and they often do not solve the actual problem if the root cause is incorrect fit selection rather than inadequate precision. Understanding the ISO system of limits and fits allows the designer to specify exactly the functional relationship needed between two mating parts, using the coarsest (cheapest) tolerance that achieves it.
This guide explains the ISO fit system, walks through the most commonly used hole-basis fits, and provides selection criteria with practical examples drawn from real mechanical assemblies.
The ISO System: Fundamental Concepts
ISO 286 defines a system of tolerances and deviations for cylindrical features (and by extension, parallel features like keys and slots). The system uses two parameters for each tolerance:
- Fundamental deviation: a letter code that positions the tolerance zone relative to the nominal size. For holes, uppercase letters (H, G, F, …) are used. For shafts, lowercase letters (h, g, f, …) are used. The letter H means the lower deviation is zero — the minimum hole size equals the nominal size. The letter h means the upper deviation is zero — the maximum shaft size equals the nominal size.
- Tolerance grade (IT grade): a number (6, 7, 8, …) that sets the width of the tolerance zone. Lower numbers are tighter. IT6 is tighter than IT7, which is tighter than IT8.
A fit is specified by combining a hole tolerance and a shaft tolerance: H7/g6 means a hole with fundamental deviation H and grade 7, mating with a shaft with fundamental deviation g and grade 6.
Hole-Basis vs. Shaft-Basis System
In the hole-basis system, the hole tolerance is fixed (always H) and the shaft tolerance varies to produce the required fit. In the shaft-basis system, the shaft tolerance is fixed (always h) and the hole varies. The hole-basis system is standard for most applications because holes are harder to adjust after machining — drills, reamers, and boring bars produce fixed sizes that are expensive to change. Shafts can be turned to any diameter.
Use the shaft-basis system only when a standard shaft diameter (such as a rolling bearing outer ring or a hydraulic cylinder rod) must be used unchanged and the mating bore must be adjusted to suit.
The Three Classes of Fit
All fits fall into one of three functional classes:
- Clearance fit: the shaft is always smaller than the hole; assembly requires no force; parts can slide or rotate relative to each other. Used for rotating bearings, sliding guides, and parts that must be regularly disassembled.
- Transition fit: the shaft may be slightly smaller or slightly larger than the hole depending on where actual sizes fall within their tolerance bands. Assembly may or may not require light force. Used for location fits where parts must align accurately but also be removable.
- Interference fit (press fit): the shaft is always larger than the hole; assembly requires force (pressing or thermal expansion). Used for permanent or semi-permanent joints that must transmit torque or force without fasteners.
Commonly Used Fits and Their Applications
| Fit Designation | Fit Class | Typical Application | Assembly Method |
|---|---|---|---|
| H7/g6 | Clearance (close running) | Precision sliding fits, spindle bearings, precision guides | Hand assembly; slight resistance |
| H7/h6 | Clearance (close sliding) | Locating fits; accurate positioning without rotation | Hand push; no perceptible play |
| H8/f7 | Clearance (free running) | General bearing journals, shafts in plain bearings | Easy hand assembly; noticeable clearance |
| H9/d9 | Clearance (loose running) | Loose pulleys, agricultural equipment, non-precision fits | Slides freely; significant clearance |
| H7/k6 | Transition | Gear hubs, pulley hubs, accurate location on shaft | Light press or mallet; hand-removable |
| H7/n6 | Transition (close) | Tight location; retained by key or press | Arbor press; requires key for torque transmission |
| H7/p6 | Interference (light press) | Permanent bushings, bearing races in housings | Arbor press or thermal; semi-permanent |
| H7/s6 | Interference (medium press) | Gear blanks pressed on shafts; high torque applications | Hydraulic press or thermal expansion |
Practical Selection Logic
For Rotating Shafts in Plain Bearings
H8/f7 is the standard starting point for general plain bearing applications. For higher speeds or precision spindles, use H7/g6. Calculate the minimum oil film thickness at the design operating conditions — the clearance must be sufficient to maintain hydrodynamic lubrication at the minimum load and maximum speed condition.
For Gear and Pulley Hubs on Shafts
H7/k6 or H7/n6 with a parallel key is the most common arrangement. The fit provides accurate radial location; the key transmits torque. Avoid relying on interference alone for torque transmission unless the press force and resulting friction have been calculated and verified sufficient.
For Bearing Inner Rings
Rolling bearing manufacturers specify shaft tolerance classes directly. A typical recommendation for a rotating inner ring under normal load is k5 or m5 on the shaft. Using a loose fit (h6 or g6) on a rotating inner ring allows the ring to creep on the shaft, which rapidly destroys both the ring bore and the shaft surface.
Tolerance Stacking in Assemblies
Individual fit selection must be evaluated in the context of the complete assembly. If three clearance fits are stacked in series, the total positional variation is the sum (or RSS combination) of the individual variations. A chain of three H7/g6 fits may produce positional error large enough to affect function even though each individual fit is correctly specified.
Before finalizing tolerance specifications, perform a tolerance chain analysis on any critical assembly dimension. Identify the assembly gap or clearance that must be maintained, trace all the contributing dimensions, and verify that the combination of specified tolerances produces an acceptable result at worst-case and statistical conditions.
FAQ
Q: When should I use an interference fit instead of a key to transmit torque?
Interference fits are preferred when the joint must be smooth (no keyway stress concentration), when the assembly sees dynamic or reversing loads (keyways fret under such loading), or when assembly alignment is critical and a keyway would add rotational indexing uncertainty. Calculate the transmitted torque based on the interference, contact pressure, coefficient of friction, and contact area before committing to this approach.
Q: The machinist says H7 is expensive for this bore. Is H8 acceptable?
H8 has a wider tolerance than H7 — it is cheaper to produce. Whether it is acceptable depends on the mating shaft tolerance and the required functional fit. If you change the hole from H7 to H8 without also changing the shaft specification, the resulting fit will be different (typically looser). Evaluate the new fit against the functional requirement. Often H8/f7 provides adequate running clearance at lower cost than H7/g6.
Q: How do I specify fits on non-cylindrical features like keyways and slots?
ISO 286 applies to cylindrical features. For parallel features, ISO 2768 and the relevant key and keyway standards (ISO 773, ISO 774) provide width tolerances with analogous letter-number codes. The logic is the same: select the deviation letter and tolerance grade combination that produces the required functional fit class (clearance, transition, or interference) for the specific feature size.
Reference Standard
The exact rules behind these callouts trace back to one standard that most drawings ultimately answer to.
ASME Y14.5-2018: Dimensioning and Tolerancing
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