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The ISO 286 Guide to Limits and Fits: Clearance, Interference & Metrology

The Engineer's Bible: ISO 286 Limits and Fits

In the world of Precision Metrology and CNC machining, adhering to the ISO 286 standard for limits and fits is non-negotiable. Whether you are designing a bearing press fit or a sliding shaft, understanding these metric standards is the difference between a smooth assembly and expensive scrap.

1. The Big Picture: Hole Basis vs. Shaft Basis

Before calculating numbers, you must choose a system.

  • Hole Basis System (Most Common): We keep the hole size constant (e.g., exactly 20.00 mm with a tolerance of H7) and machine the shaft to fit. This is preferred because drills and reamers come in standard sizes.
  • Shaft Basis System: We keep the shaft size constant (e.g., standard cold-rolled steel bar) and bore the hole to fit. Used for long shafts like textile rollers.

2. Common Fits for Design Engineers

While the standard lists thousands of combinations, 95% of engineering uses just these three:

A. Clearance Fit (Slide) - H7/g6

The shaft is always smaller than the hole. Used for:
Sliding gears, clutch discs, and pivot pins.

B. Transition Fit (Tap) - H7/k6

The zones overlap. It might be tight or loose. Used for:
Locating dowels, pulleys on shafts, and coupling hubs. Requires a light tap with a mallet.

C. Interference Fit (Press) - H7/p6

The shaft is always larger than the hole. Used for:
Permanent bearing mounts, bushings, and seal rings. Requires a hydraulic press or thermal shrink fitting.

3. Decoding the Numbers: IT Grades

The "Number" in the code (e.g., the '7' in H7) tells you the Cost of Manufacturing.

IT Grade Typical Process Application
IT 01-4 Lapping / Superfinishing Gauge blocks, Fuel injectors
IT 5-7 Precision Grinding / Reaming Bearings, Engine pistons
IT 8-11 Turning / Milling General brackets, keyways

4. Mathematical Calculation (IT Formula)

For general engineering (IT6 - IT16), the standard tolerance unit (i) in microns is calculated as:

i = 0.45 × ∛D + 0.001D

Where D is the geometric mean of the size range.

Tolerance Grade Multipliers

GradeTolerance Value
IT610i
IT716i
IT825i
IT940i

5. Thermal Considerations (Shrink Fits)

For heavy interference fits (like a railway wheel on an axle), simple pressing is dangerous. We use Shrink Fitting.
By heating the hole (expanding it) and freezing the shaft (shrinking it) using liquid nitrogen, we create a temporary clearance gap.
Formula: ΔL = L × α × ΔT

Conclusion

A design engineer must balance precision with cost. Specifying an H7 fit on a garden gate hinge is a waste of money; specifying H11 on a turbine bearing is a catastrophe.

Next Post: We will calculate specific examples using Excel in Standards of limits and fits (Part 2).

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