Machine Drawing: Fasteners, Keys, Couplings and Bearings
This domain tests standard representation of threads and fasteners, keys, couplings and bearings, with proportional dimensions. You must know conventional thread lines and the proportions used for bolt heads, nuts and keys.
Threads and fasteners
External threads: major diameter continuous thick, minor diameter continuous thin; internal threads: major diameter thin, minor diameter thick, with hatching crossing the thin line. A bolt is specified by nominal diameter, length and thread form, e.g. M20 × 80. Proportional dimensions for a hex bolt: across flats 1.5d, head height 0.7d, nut height 0.8d, washer thickness 0.15d.
Keys and couplings
Sunk key: width d/4, height d/6 for shaft diameter d. Woodruff key uses a semicircular seat; feather key is fixed and allows sliding. A flange coupling has two flanges, a boss, a key and bolts; a muff coupling is a hollow sleeve keyed to both shafts. A universal coupling transmits motion through an angle.
Bearings
Journal bearing supports a rotating shaft; ball and roller bearings use rolling elements; thrust bearings take axial load. Show bearing symbols and hatching correctly.
Worked example
Draw an M20 hex bolt 80 mm long with nut. Nominal diameter d = 20 mm. Across flats = 1.5d = 30 mm; head height = 0.7d = 14 mm; nut height = 0.8d = 16 mm; thread length = 2d + 6 = 46 mm. Draw the bolt axis, the head as a hexagon across flats 30, the shank Ø20, and the nut at the end. The reasoning is that all dimensions are proportional to d, so the drawing stays consistent.
Traps
Drawing the thread as a series of V shapes instead of conventional lines loses marks. Do not use the same line thickness for crest and root. Forgetting the keyway in a coupling section is common. Check whether the question asks for a drawing or a projection; proportions must be calculated, not guessed.