H7/f7 fit: maximum and minimum clearance
On the drawing of a bronze plain guide bearing, the fit with the steel journal is specified as ⌀50 H7/f7. The inspector has to work out the maximum clearance, to avoid runout under load, and the minimum clearance, to guarantee the lubricating oil film at rated speed.
How to calculate the clearances of an ISO H7/f7 fit
| Nominal size ⌀50 mm | H7 hole (mm) | f7 shaft (mm) | Resulting clearance (mm) |
|---|---|---|---|
| Upper limit | 50.025 | 49.975 | Max clearance = 50.025 - 49.950 = 0.075 |
| Lower limit | 50.000 | 49.950 | Min clearance = 50.000 - 49.975 = 0.025 |
All values are given for the reference environmental conditions of 20 °C.
Maximum and minimum clearance
The f7 deviation makes the shaft strictly smaller than nominal (es = -25 µm, ei = -50 µm for the 30–50 mm range). Max clearance = H7 max - f7 min = 50.025 - 49.950 = 0.075 mm (75 µm). Min clearance = H7 min - f7 max = 50.000 - 49.975 = 0.025 mm (25 µm). To compare with other grades, read the js9 deviation and its limits.
Why minimum clearance matters for hydrodynamic lubrication
A minimum clearance of 25 microns lets the oil wedge form under the bearing surface. If the clearance drops below 10 microns because the parts heat up in service, metal-to-metal contact makes the bearing seize.
Example: inspecting a journal and bushing pair
Measured hole: 50.018 mm. Measured shaft: 49.960 mm. 1. Actual clearance: 50.018 - 49.960 = 0.058 mm. 2. Allowed range: [0.025 mm; 0.075 mm]. Verdict: the actual clearance (58 µm) conforms.
Shop-floor pitfalls when checking running fits
The first pitfall is confusing the f7 position with g6. The second is forgetting the roundness error of the bearing surfaces. The third comes from measuring the shaft and the bushing at different temperatures. To automate the analysis of your drawings, see CoteCote and its inspection module.