Limits and fits for a gearbox output shaft and bush
A gearbox output shaft is nominal Ø40 mm where it runs in a bronze bush. The drawing is to use ISO limits and fits. The candidate has dimensioned the shaft as Ø40 ±0.010 and the bush bore as Ø40 +0.025/0, with a note 'close running fit'. The shaft also has a 120 mm overall length, a 40 mm shoulder distance and a 30 mm distance from the other end; all three are dimensioned from different faces, so the dimensions form a closed loop. The surface finish symbol on the bearing seat is a general triangle with no value, and the general tolerance note says 'ISO 2768-mK' but the title block says 'mH'. The workshop has a comparator and a micrometer, but no air gauging. The client has asked for a guaranteed clearance of at least 0.005 mm and no more than 0.060 mm.
1. Evaluate whether the candidate's shaft and bush tolerances guarantee the requested clearance. Calculate the actual minimum and maximum clearance and state the decision.
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Shaft limits: 40 ± 0.010 gives 39.990 mm to 40.010 mm. Bush limits: 40 +0.025/0 gives 40.000 mm to 40.025 mm.
Minimum clearance = hole minimum − shaft maximum = 40.000 − 40.010 = −0.010 mm. That is an interference, not a clearance. Maximum clearance = hole maximum − shaft minimum = 40.025 − 39.990 = 0.035 mm.
The requested range is 0.005 mm to 0.060 mm. The maximum is acceptable, but the minimum fails because the assembly can interfere by 0.010 mm. The note 'close running fit' is therefore false. The tolerances do not guarantee clearance; the scheme must be changed.
2. Recommend a standard ISO fit that meets the requirement and show the limits. Use the fact that for Ø40 mm, IT7 = 25 µm and IT6 = 16 µm.
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A suitable fit is Ø40 H7/g6. For a 40 mm nominal size, H7 gives the hole 40.000 mm to 40.025 mm. The g6 shaft for the 30–50 mm range has an upper deviation of −9 µm and a lower deviation of −25 µm, so the shaft is 39.975 mm to 39.991 mm.
Minimum clearance = 40.000 − 39.991 = 0.009 mm. Maximum clearance = 40.025 − 39.975 = 0.050 mm. Both lie inside the required 0.005 mm to 0.060 mm band.
H7/f6 would give a minimum clearance of about 0.025 mm and a maximum of about 0.066 mm, exceeding the upper limit. H7/h6 would give a minimum clearance of zero, failing the lower limit. H7/g6 is therefore the correct choice for a running fit with guaranteed clearance.
3. Advise on the dimensioning scheme and the general tolerance conflict. Explain how to avoid over-dimensioning and what must be corrected in the notes.
Read the worked answer
The shaft has a closed dimension loop: 120 mm overall plus 40 mm and 30 mm from different faces can contradict one another. Use one datum face, preferably the shoulder that locates the bearing, and dimension the 40 mm and 30 mm from that datum. The 120 mm overall can then be shown as a reference dimension in parentheses, or omitted if it is the sum of the controlled dimensions. Do not give both the overall and every chain dimension as mandatory.
The general tolerance note must match the title block. Choose either ISO 2768-mK or ISO 2768-mH and apply it consistently; mK and mH are different tolerance classes. The surface finish symbol is incomplete: a general triangle with no value does not specify roughness. Add an Ra value, for example Ra 1.6 µm on the bearing seat, and a machining process note if required. The workshop can then inspect the part with its micrometer and comparator.