Pivot bushing: a clearance note the drawing's own fits cannot deliver
You are checking a detail drawing for the pivot joint of a package conveyor. A bronze bushing is pressed into a steel housing: the housing bore is Ø30 H7 (+0.021/0), the bushing OD is Ø30 with a stated interference of 0.015–0.035 mm, and the bushing bore is dimensioned Ø20 H8 (+0.033/0). The mating steel pin is Ø20 h6 (0/−0.013). A general note states 'diametral clearance 0.02 to 0.06 mm to maintain the grease film'. The joint runs at 80 °C and is machined and assembled at 25 °C. Steel α = 11.5 × 10⁻⁶/K, bronze α = 18 × 10⁻⁶/K. A second note reads 'bushing bore finished before pressing'. Forty bushings have already been made to the drawing and the shop is asking whether to release the pin batch. Nothing on the sheet mentions a temperature, a measurement condition, or any allowance for bore contraction after pressing.
1. Compute the minimum and maximum diametral clearance at 25 °C and at 80 °C and decide whether the clearance note can be met. Show limits and arithmetic.
Read the worked answer
Limits from the drawing:
- Pin Ø20 h6: 19.987 / 20.000 mm
- Bushing bore Ø20 H8: 20.000 / 20.033 mm
At 25 °C (assembly):
- Minimum clearance = 20.000 − 20.000 = 0.000 mm (line-to-line contact is possible)
- Maximum clearance = 20.033 − 19.987 = 0.046 mm
Thermal shift, ΔT = 55 K, diametral:
- Pin: 20 × 11.5×10⁻⁶ × 55 = 0.0127 mm growth
- Bore: 20 × 18×10⁻⁶ × 55 = 0.0198 mm growth
- Net clearance gain = 0.0198 − 0.0127 ≈ 0.0071 mm
At 80 °C: min ≈ 0.007 mm, max ≈ 0.053 mm.
Verdict: the note needs 0.02 minimum and the fit gives 0.000 cold and 0.007 hot, so the lower limit fails at both temperatures; only the upper limit is satisfied. The 80 °C expansion does not rescue the design - bronze grows faster than steel, but 0.007 mm is far short of 0.02 mm.
The press fit makes it worse: pressing a Ø30 bushing with 0.015–0.035 mm interference contracts the Ø20 bore by roughly the same order (typically 0.010–0.030 mm for this wall thickness, to be confirmed by trial), which can remove the clearance entirely and seize the pin. The note 'bore finished before pressing' is therefore a second defect: the finished bore size is not the assembled bore size.
2. Specify a corrected fit and the drawing notes/process needed so the shop can hold the requirement. Justify with numbers.
Read the worked answer
Work backwards from the note. Required cold minimum = 0.02 − 0.007 thermal gain ≈ 0.013 mm minimum at 20 °C before any press-fit correction, plus a press-fit allowance.
Candidate fits (18–30 mm grade sizes: IT6 = 13 µm, IT7 = 21 µm, IT8 = 33 µm):
- Ø20 H8 bore + Ø20 f6 pin (−0.020/−0.033): 0.020–0.054 cold, 0.027–0.061 hot - upper limit marginally exceeded.
- Ø20 H6 bore (+0.013/0) + Ø20 f6 pin: 0.020–0.046 cold, 0.027–0.053 hot - inside 0.02–0.06 across the whole temperature range. This is the fit to adopt.
Process and notes needed:
- Change the bore callout to Ø20 H6 and pin to Ø20 f6.
- Replace 'bore finished before pressing' with 'bore to be reamed to Ø20 H6 after the bushing is pressed into the housing; verify after assembly' - or, if pre-press finishing is kept, state a pre-press bore size validated by a press trial and inspect the assembled bore.
- Restate the clearance note as 'diametral clearance 0.02–0.06 mm at 20 °C, measured after assembly'.
- State the interference as limits on the bushing OD matched to the Ø30 H7 housing, and add a note that press-fit bore contraction is included in the bore callout.
Without items 2 and 3 the shop can hold every dimension and still deliver a joint that will not turn.
3. Forty bushings already exist with H8 bores. What is the cheapest technically defensible disposition, and what risk must be stated to the customer?
Read the worked answer
The bushings are recoverable by reaming after pressing rather than scrapping.
Route: press the existing bushings into the H7 housings, then ream the assembled bore to Ø20 H7 (+0.021/0) and pair with Ø20 f6 pins (−0.020/−0.033). Resulting diametral clearance:
- Cold: 0.020 to 0.054 mm
- Hot (80 °C, +0.007): 0.027 to 0.061 mm
This meets 0.02–0.06 for all but the extreme worst-case stack (0.061 vs 0.06), which is a 1 µm overshoot - engineering noise compared with the 0.000 mm of the released drawing. The drawing note should be revised to '0.02–0.065 mm at 20 °C' if the customer accepts it, otherwise add a selective-assembly rule (match pins and bores into two clearance bands).
Risks to state in writing:
- Reaming after pressing removes the original bore finish and any pre-applied lubricant; the bore must be cleaned and re-greased.
- Reaming to H7 reduces the bushing wall by up to 0.033 mm relative to H8, slightly lowering load capacity and press-fit retention; check the remaining wall.
- The rework route only works if the bore has enough stock - confirm the as-pressed bore is smaller than Ø20 H7 before committing the batch.
What must not be done is to release standard h6 pins and rely on the grease to cope: at 0.000 mm minimum the joint is a press fit when cold.