Keys, Couplings and Bearings
This domain covers the machine elements that connect shafts and support them: keys, couplings and bearings.
Keys
A sunk key fits into keyways in both shaft and hub and transmits torque by shear and crushing. Forms include square, rectangular, parallel, taper, gib-head, Woodruff, feather and saddle. A saddle key relies on friction and is used for light duty. Approximate proportions for a shaft diameter d: key width d/4 and thickness d/6; standard sizes are then selected. A gib head helps withdrawal of a taper key.
Couplings
Rigid couplings include muff and flange types. A muff coupling is a sleeve keyed over the two shaft ends; a flange coupling uses two flanges bolted together. Flexible couplings (Oldham, universal, bush-pin) accommodate misalignment. Bearings are sliding (journal, footstep, collar, thrust) or rolling (ball, roller); a rolling bearing has inner race, outer race, rolling elements and cage.
Worked example
A 40 mm shaft needs a key. Width = 40/4 = 10 mm; thickness = 40/6 = 6.7 mm, so adopt the standard 10 × 8 mm key. Key length is commonly about 1.5d = 60 mm. Reasoning: the key must be strong in shear and crushing but not so deep that it weakens the shaft.
Traps
Using a saddle key for heavy torque; cutting the keyway too deep; forgetting the key in a coupling assembly; mixing up a footstep bearing with a journal bearing; drawing a ball bearing without a cage.