Mechanical Components
Mechanical Components
This domain covers the parts that hold a machine together and let it move: threaded fasteners, keys and splines, couplings, bearings, seals, springs and retaining devices. The exam asks you to select a component for a duty, decode its designation, or explain a failure mode.
Fasteners and joints
- ISO metric thread designation M10 × 1.5 means 10 mm nominal diameter and 1.5 mm pitch; a fine series has a smaller pitch.
- Property class 8.8 means tensile strength 800 MPa and yield strength 0.8 × 800 = 640 MPa. Class 10.9 gives 1000 MPa and 900 MPa.
- Torque controls preload. Too little preload lets a joint fatigue and loosen; too much strips the thread. Use the correct tightening sequence for a flanged joint.
- Keys transmit torque; a parallel key is for location, a taper key for drive. Woodruff keys suit small shafts. Splines carry higher torque.
Bearings, couplings and seals
Bearing designation 6205: 6 = deep-groove ball, 2 = light series, 05 = 25 mm bore. Bore codes 00, 01, 02 and 03 mean 10, 12, 15 and 17 mm; from 04 multiply by 5. Rigid couplings need accurate alignment; flexible couplings absorb misalignment; an Oldham coupling takes parallel offset. Lip seals fit with the lip toward the lubricant. Preload removes internal clearance but excessive preload overheats the bearing.
Worked example
A drawing specifies bearing 6308. Decode: 6 = deep-groove ball bearing, 3 = medium series, 08 = 8 × 5 = 40 mm bore. The same shaft carries a class 8.8 M12 bolt. Its proof strength is 640 MPa; with a stress area of 84.3 mm² the clamp load is roughly 640 × 84.3 = 54 kN before yielding, so the torque wrench setting must produce noticeably less than that.
Traps
Reading a bearing bore as the last two digits themselves; reversing a lip seal; assuming a flexible coupling cures angular misalignment of a rigid bearing arrangement.