Materials and Hardware
Core content
Materials and hardware covers ferrous and non-ferrous metals, alloys, heat treatment, corrosion, composites, fasteners, threads, rivets, bearings, cables, pipes, seals and springs. You must know the difference between strength, hardness, ductility and toughness, and why aluminium alloys are used with protective treatments. Understand galvanic corrosion, stress corrosion, fretting and exfoliation, and how to prevent them. Fastener knowledge includes thread types, grip length, torque, locking methods and identification markings. Bearings must be selected for radial or axial loads, and control cables need correct tension and routing.
Traps
Common traps include confusing coarse and fine threads, assuming all bolts are interchangeable, and ignoring grip length. A bolt must be long enough for the unthreaded shank to carry shear loads, and washers change the effective grip. In corrosion questions, remember that dissimilar metals in the presence of an electrolyte form a galvanic cell. Another trap is lockwire direction: it must oppose the loosening rotation. Heat treatment terms such as annealing, normalising, hardening and tempering are easily mixed up.
Worked example
A bolt must clamp two plates with a total thickness of 18 mm, and a washer adds 2 mm. The required grip length is therefore 20 mm. Select a bolt whose grip is close to 20 mm, so the thread does not sit in the shear plane. Rule of thumb: grip length should equal the total thickness of the parts plus washers, and the bolt should be long enough to show full thread beyond the nut.
Key points
- Grip length equals material thickness plus washers.
- Avoid placing threads in a shear plane.
- Dissimilar metals plus electrolyte cause galvanic corrosion.
- Lockwire must be installed in the tightening direction.
- Bearings are chosen for radial or axial load capability.