Precision is the backbone of mechanical design — and nowhere is it more critical than in the fit between holes and shafts. When you’re working with steel bars or drawn wires, the choice of tolerance can make or break the success of your application.
In this post, we explain what hole and shaft tolerances are, why they matter, and how steel bar suppliers like Steelmet Industries help ensure your assemblies go together perfectly, every time.
🔧 What Are Hole and Shaft Tolerances?
In mechanical design, “fit” refers to how tightly or loosely a shaft goes into a hole. Because manufacturing cannot produce exact sizes every time, tolerances are defined to ensure interchangeability.
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The hole is usually stationary (e.g., in a housing)
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The shaft is the rotating or sliding part (e.g., steel bar, pin, axle)
Tolerances control the maximum and minimum allowable dimensions of the hole or shaft, ensuring that they can mate as intended.
🧩 Types of Fits Based on Tolerance Classes
| Fit Type | Result | Typical Use |
|---|---|---|
| Clearance Fit | Shaft is always smaller than the hole | Bearings, gears, moving parts |
| Interference Fit | Shaft is always larger than the hole | Press fits, permanent assemblies |
| Transition Fit | Depending on limits, may be tight or loose | Balanced fits, alignment parts |
📐 ISO Tolerance System for Holes & Shafts
Tolerances are classified using the ISO system with letters and numbers:
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Holes: Capital letters (e.g., H7)
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Shafts: Lowercase letters (e.g., h8)
The number indicates the grade of tolerance, and smaller numbers mean tighter tolerance.
🔍 Example: H7/h6 Fit
| Element | Tolerance Code | Type | Typical Range (mm) | Application |
|---|---|---|---|---|
| Hole | H7 | Clearance | +0.000 / +0.025 | Standard housing bore |
| Shaft | h6 | Clearance | -0.010 / 0.000 | Bright round steel shaft |
➡️ Result: Consistent sliding fit with minimal play — common in motor shafts, bearing seats, and precision guides.
📌 Why It Matters in Steel Bars & Wires
If you’re using steel bars or wires as shafts, dimensional control is everything. The wrong tolerance can lead to:
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Loose fits → vibration, wear, and noise
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Tight fits → assembly failure, press damage
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Inconsistent fits → rework, sorting, and delay
🔧 For example:
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A bright drawn bar at h9 tolerance might still be too loose for a H7 hole
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A ground bar at h6 or h5 ensures better repeatability and lower failure rate
🏭 How Steelmet Industries Ensures Proper Fit
At Steelmet Industries, we supply steel bars and wires in engineered tolerance classes:
✅ Cold drawn and ground bars in h5, h6, h7, h8, h9
✅ Bright flats, hexes, rounds produced with minimal variation
✅ Profile bars (e.g., round corner squares, D-shapes) tailored to fit your assembly
We maintain tight control of dimensional variation, ensuring consistent fits within and across batches.
📌 All measuring instruments are calibrated, and materials are traceable with test certificates.
💡 Tips for Selecting the Right Fit
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For sliding parts (shafts in bushings): use H7/h6 or H8/h7
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For press fits: use H7/p6 or H7/m6
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For location fits: use H7/k6 or H8/k7
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For steel wires used as axles or pins, choose drawn wires with h8 or h9 tolerance and consistent straightness
🔚 Conclusion
Understanding hole and shaft tolerances is key to successful assembly, smooth operation, and long service life. With precision steel bars and wires supplied in the right tolerance classes, you can avoid rework, improve speed, and reduce machine wear.
🎯 At Steelmet Industries, we deliver precision bright bars and wires tailored to your tolerance and application — backed by documentation and repeatability you can trust.
📩 Need help selecting the right shaft tolerance?
📞 Call us (India): 0712-2728071
📞 Call us (Intl): +91-712-2728071
🌐 Contact Page: https://www.steelmet.in/wp/contact-us/