Fasteners, Threads, Keys, Couplings and Bearings
Threads
Thread forms: ISO metric 60°, BSW and BSF 55°, square, ACME 29°, buttress. Pitch is the axial distance between crests; lead = pitch × number of starts, so a double-start screw advances twice the pitch per turn. Right-hand threads are assumed and left-hand threads are marked LH. Conventionally a thread is drawn with two lines, crest thick and root thin: on a bolt the major diameter is thick, the minor thin; on a nut shown in section the minor (crest) diameter is thick and the major diameter thin.
Fasteners and keys
Bolts, nuts, washers, studs, cap screws, machine screws, set screws, rivets and foundation bolts have standard proportions: for a bolt of nominal diameter d the head is about 1.5d across flats, the nut about 0.8d high, thread length about 2d + 6 mm. Keys: sunk keys (rectangular, square, parallel, taper 1:100, gib-head), saddle keys, round keys and Woodruff keys; as a working rule key width ≈ d/4 and thickness ≈ d/6, the keyway depth split between shaft and hub.
Couplings and bearings
Sleeve (muff) coupling, protected and unprotected flange couplings, bush-pin flexible coupling, Oldham coupling, universal joint. Bearings: journal, footstep, collar, pivot, and ball and roller bearings, all drawn with standard proportions.
Worked example
For a 40 mm shaft, key width = 40/4 = 10 mm and thickness = 40/6 ≈ 7 mm, length equal to the hub length. A double-start thread of 3 mm pitch has a lead of 6 mm - state the lead, not only the pitch, when the helix must be shown.
Traps
Crest and root line weights interchanged; hatching a key or shaft in an assembly; omitting the LH mark; drawing a key sitting on top of the shaft instead of half sunk.