The 60 mm Sliding Joint That Nineteen Trainees Got Wrong
You are the trade theory instructor for a batch of 28 Turner trainees in week 6 of a 12-week block. The topic is limits, fits and tolerances. In the last class test, the item 'Select the fit for a 60 mm steel shaft that must slide axially through a bronze bush under light load with occasional lubrication' was answered correctly by only 9 trainees. Nineteen chose H7/h6, and four of those wrote the note 'clearance fit, so oil can enter'. The same nineteen trainees correctly defined IT7 and IT6, and read the IS 919 tables without error, so the weakness is not table reading. In two weeks the batch sits the sectional test in which fits carry 12 marks. The section owns one set of fit charts, no plug or ring gauge library, and the lathe shop can release one hour of machine time for a demonstration. The training officer wants the topic revised before the test but has not released any extra classroom hours.
1. Explain what the nineteen trainees actually reasoned, and show with numbers why H7/h6 is the wrong choice for this joint.
Read the worked answer
What they reasoned. They matched a label to a label. H7/h6 is the textbook example of a 'clearance fit', and the question said 'slide', so the label fitted. They never converted the fit into a minimum clearance and compared that with what the joint has to do.
The arithmetic for 60 mm.
- H7 for 50–80 mm: ES = +0.030, EI = 0.000
- h6 for 50–65 mm: es = 0.000, ei = −0.019
- Clearance range: 0.000 to 0.049 mm
Why it fails. A minimum clearance of zero means that at worst case the shaft and bush are line-to-line. There is no room for an oil film, so axial sliding under load scuffs and galls the bronze. Worse, the steel shaft reaches running temperature before the bush, so the zero-clearance condition turns into interference. The letter 'h' means the shaft's upper deviation is zero by definition - h-shafts are designed to touch the nominal size. That is a locational clearance fit, not a sliding or running fit.
The defensible answers.
- H7/g6: clearance 0.010 to 0.059 mm - a close sliding/guide fit where alignment matters.
- H7/f7: clearance 0.030 to 0.090 mm - a lubricated running fit.
The rule to install. Choose the letter from the minimum oil-film clearance the function needs, then choose the IT grade from the accuracy the function can tolerate. A fit is a function, not a name.
2. The training officer asks what would change if the bush were aluminium and the joint ran at 70 °C. Answer using differential expansion, and state whether that rescues H7/h6.
Read the worked answer
Expansion at ΔT = 50 K (20 °C to 70 °C), nominal 60 mm.
- Steel shaft, α = 11.5 × 10⁻⁶/K: 60 × 11.5 × 10⁻⁶ × 50 = 0.0345 mm growth of diameter.
- Aluminium bush, α = 23 × 10⁻⁶/K: 60 × 23 × 10⁻⁶ × 50 = 0.0690 mm growth of bore.
- Change in diametral clearance = 0.0690 − 0.0345 = +0.0345 mm.
Effect on the candidate fits.
- H7/g6 at 20 °C: 0.010–0.059 mm → at 70 °C roughly 0.045–0.094 mm.
- H7/f7 at 20 °C: 0.030–0.090 mm → at 70 °C roughly 0.065–0.125 mm.
- H7/h6 at 20 °C: 0.000–0.049 mm → at 70 °C roughly 0.035–0.084 mm.
Judgement. The hot condition is comfortable even for H7/h6, but that is luck, not design. A fit is specified at the reference temperature of 20 °C, and the assembly, the start-up and the first few strokes all happen cold. H7/h6 at assembly still has a zero minimum clearance.
The aluminium bush also changes the life story: the clearance grows 0.0345 mm hot, so a g6 shaft may rattle and lose alignment at temperature even though it is correct cold. That is an argument for H7/f7 with a wider band, or for a different bush material - not for H7/h6.
Teaching point for the batch. Fit selection has two checks: the cold worst case and the hot worst case. Do both, then choose.
3. Design the 40-minute remedial session, using the one hour of lathe time, and state how you will know the misconception is gone.
Read the worked answer
Structure (40 min classroom + lathe slot used in parallel by rotation).
- 10 min - confrontation with the numbers. Give each trainee the 60 mm H7/h6, H7/g6 and H7/f7 values and one instruction: compute the minimum and maximum clearance. Put the three minimum clearances on the board: 0.000, 0.010, 0.030. Ask only one question: 'Which of these can hold an oil film?' No lecture until they answer.
- 10 min - function first. Three cards, three joints: sliding plunger (needs alignment), continuously rotating lubricated shaft (needs oil film), dowel pin that must never move (needs interference). Trainees match letter to function and write one line of justification each. This attacks the label-matching habit directly.
- 10 min - lathe demonstration. Turn two 60 mm test plugs to the actual g6 and h6 limits (0.010/0.029 below nominal and 0/0.019 below), then try each in the bronze bush by hand. The h6 plug drags and marks; the g6 plug slides with a film of oil. Physical evidence replaces the argument.
- 10 min - written exit check. One new item, different size (say 40 mm H7/g6 vs H7/h6), requiring the clearance range and a one-sentence justification.
Success criterion. At least 24 of 28 get both the range and the justification right on the exit check. If fewer, the misconception is still label-deep and the topic goes back on the revision list before the sectional test - do not rely on the marks alone, because the exit check is the only evidence you will get before the exam.