Ø30 H8/f7 Sleeve Batch: Fit Verification and Disposition of Out-of-Limit Parts
At a small pump repair works an ITI trainee is given a bearing-sleeve drawing whose seat is marked Ø30 H8/f7. The drawing quotes hole limits 30.000/30.033 mm and shaft limits 29.980/29.959 mm. The customer wants 120 sleeves and insists that assembly must be possible by hand with a thin oil film and must never need a press. The workshop owns a GO/NO-GO plug gauge for the bore, an outside micrometer reading to 0.01 mm for the shaft, and no snap gauge. On the first article the trainee measures a bore of 30.040 mm and a shaft of 29.968 mm, and asks for the pair to be accepted because they slide together nicely. The supervisor also wants a decision on the 14 sleeves already bored, which read between 30.036 mm and 30.041 mm, and on whether the micrometer is good enough to certify the shafts against the drawing tolerance.
1. From the quoted limits, determine the type of fit and compute the maximum and minimum clearance. Explain what the answer proves about the need for a press.
Read the worked answer
Limits on the drawing
- Hole: 30.000 to 30.033 mm (tolerance 0.033 mm)
- Shaft: 29.959 to 29.980 mm (tolerance 0.021 mm)
Clearances
- Maximum clearance = largest hole − smallest shaft = 30.033 − 29.959 = 0.074 mm
- Minimum clearance = smallest hole − largest shaft = 30.000 − 29.980 = 0.020 mm
Reasoning The hole tolerance zone (0 to +0.033) lies entirely above the shaft tolerance zone (−0.020 to −0.041). Whatever parts are picked from the batch, the difference is positive, so the assembly is always a clearance fit - specifically a running/sliding fit of the H8/f7 family. No press is required or permitted, because no combination can produce interference; the worst case still leaves 0.020 mm of diametral clearance, which is what carries the oil film.
The design intent is interchangeability: any conforming hole assembles with any conforming shaft. That is exactly why the individual limits, and not the feel of one trial pair, decide acceptance.
2. Decide the fate of the first article (bore 30.040, shaft 29.968) and of the 14 bores reading 30.036 to 30.041. Support the decision with numbers, and evaluate the trainee's argument.
Read the worked answer
Shaft 29.968 mm: 29.959 ≤ 29.968 ≤ 29.980, so it is conforming - accept.
Bore 30.040 mm: the upper limit is 30.033, so the bore is oversize by 0.007 mm and is non-conforming. A hole cannot be made smaller; reaming or honing only enlarges it. Practical options are: (a) scrap, or (b) bore out to a defined repair size and fit a matching oversize shaft - only with a written concession/deviation from the customer, and the repair size must be recorded on the drawing. It cannot be accepted as it stands.
The 14 bored sleeves (30.036–30.041): every one exceeds 30.033, so the whole lot is non-conforming and must be segregated, not mixed with good stock.
Why the trainee's argument fails: the pair clearance is 30.040 − 29.968 = 0.072 mm, which happens to sit inside 0.020–0.074. But acceptance is per part, not per pair. The same oversize bore paired with the smallest permitted shaft gives 30.040 − 29.959 = 0.081 mm, which exceeds the maximum clearance of 0.074 mm - a slack, noisy, oil-starved joint. Judging a fit by how one pair feels destroys interchangeability.
3. Assess whether the 0.01 mm micrometer is adequate to certify the shaft, and recommend measurement practice for 120 parts.
Read the worked answer
Tolerance being checked: 29.980 − 29.959 = 0.021 mm.
Rule of ten (gauge resolution): the measuring instrument should resolve at most one-tenth of the tolerance, i.e. about 0.002 mm. A micrometer reading to 0.01 mm resolves roughly half the tolerance band, so a part sitting near a limit can be accepted or rejected purely on rounding. It is not adequate for certification, although it is fine for rough checking.
Recommendation
- Use a 0.001 mm resolution digital micrometer (or a calibrated comparator) for the shafts, zeroed on a setting master and used at 20 °C.
- For 120 parts, an attribute GO/NO-GO snap gauge is faster and needs no reading: GO must pass, NO-GO must not enter. This matches the existing plug gauge philosophy for the bore.
- Confirm the plug gauge and snap gauge are within calibration, and record gauge identity on the inspection sheet so the concession discussion on the 14 sleeves is traceable.