Fasteners
This section covers Fasteners. The exam tests temporary and permanent joints, thread terminology, bolt and screw selection, washers, rivets, keys, cotters and locking methods.
Core concepts
- Temporary fasteners can be removed without damage: bolts, nuts, machine screws, studs, washers, circlips and pins. Permanent fasteners include rivets and welded joints.
- A bolt passes through clearance holes and is tightened by a nut; a screw threads into the workpiece itself. A stud is screwed into one part and a nut is fitted on the other.
- Thread terms: pitch is axial distance between corresponding points on adjacent threads; lead is axial advance in one turn; major diameter is the largest diameter; minor diameter is the smallest; crest and root are the outer and inner edges.
- A plain washer spreads load and protects the surface; a spring washer or nylon-insert nut resists vibration loosening.
- A key transmits torque between a shaft and hub; a cotter locks parts and is tapered; a circlip retains parts in a groove.
Worked example
An M12 × 1.75 bolt is single-start. Its pitch is 1.75 mm, so five complete turns advance the nut by 5 × 1.75 = 8.75 mm. The reasoning is that for a single-start thread, lead equals pitch. If it were double-start, one turn would advance 3.50 mm and five turns 17.5 mm.
Traps that cost marks
Do not use a rivet where frequent dismantling is needed. Do not confuse pitch with lead on multi-start threads. Do not omit washers under soft surfaces or over-torque small screws. Left-hand threads are not interchangeable with right-hand threads.