Sectioning Conventions in a Protected Flanged Coupling
A candidate draws the half-sectional elevation and end view of a protected-type flanged coupling for a 40 mm shaft at 1:1. The key is 12 mm × 8 mm × 50 mm. In the section, the shaft, the key and both coupling halves are hatched; the key is hatched in the same direction and at the same 3 mm spacing as the shaft, and the two coupling halves are also hatched in the same direction as each other. The key is shown protruding 4 mm above the shaft in the end view. A note gives the keyway depth as 4 mm in the shaft and 3.5 mm in the hub. The candidate also draws a bolt through the flange with its shank sectioned along its axis. The teacher asks for a corrected drawing before it is sent for checking.
1. Identify the convention errors and state the correct practice with reasons.
Read the worked answer
Shaft and key are hatched. Longitudinally cut shafts, keys, pins and bolts are not sectioned; they are drawn in outside view even inside a section, because sectioning them would hide the very features the section is meant to show.
Key hatched like the shaft. Adjacent parts must be distinguished by opposite hatching directions (for example 45° and 135°) or by different spacing. Same-direction hatching makes the key and shaft read as one part.
Both coupling halves hatched alike. Adjacent halves must be hatched oppositely, so the joint between them is clear.
Key protruding 4 mm. With an 8 mm key, 4 mm in the shaft and 3.5 mm hub keyway depth, the key sits 4 mm in the shaft and 4 mm above it, leaving 0.5 mm clearance inside the hub. It must not protrude outside the hub; show it inside the hub bore, not above the shaft.
Bolt shank sectioned. Standard practice is not to section bolts, nuts, keys, pins or shafts when cut along their axis.
2. Check the key and keyway fit, and state how the key should be drawn in the section and end view.
Read the worked answer
For a 40 mm shaft the standard key is 12 mm wide × 8 mm high, so the given 12 × 8 × 50 key is correct; the 50 mm length fits within the hub. Check the fit: 4.0 mm shaft keyway + 3.5 mm hub keyway = 7.5 mm, so the 8 mm key has 0.5 mm clearance at the top, which is correct for a feather key that is clearance-fit on top and sides.
In the section, draw the key in the shaft keyway as an unsectioned rectangle 12 mm wide and 8 mm high, with 4 mm below the shaft surface and 4 mm above it. The hub keyway is 3.5 mm deep, so the top of the key lies 0.5 mm below the top of the hub keyway. In the end view, the hub hides the key, so show it only with hidden lines or omit it; do not show it protruding above the shaft. Keep the shaft and key outlines continuous through the section.
3. Justify the choice of a half section rather than a full section, and state what the end view should show.
Read the worked answer
The coupling is symmetric about its axis, so a half section is sufficient: the front half is cut away to show the shaft, key, hub and bearing surfaces, while the rear half remains in outside view to show the flange, bolt heads and external shape. A full section would repeat information and remove the outside outline that the shop also needs. In the half section, the bolt and nut can be shown in outside view in the unsectioned half and omitted or shown in the sectioned half according to standard practice, but never sectioned along the shank.
The end view should show the flange circle, the bolt circle with the correct number of bolts, the hub and the shaft; the key is inside the hub and is not a visible outline. Together, the half-sectional elevation and the end view give the dimensions and the fit without extra views.