21 : Mechanical systems and components for general use

21.020

Characteristics and design of machines, apparatus, equipment

21.040

Screw threads

21.060

Fasteners

21.080

Hinges, eyelets and other articulated joints

21.100

Bearings

21.120

Shafts and couplings

21.140

Seals, glands

21.160

Springs

21.180

Housings, enclosures, other machine parts

21.200

Gears

21.220

Flexible drives and transmissions

21.240

Rotary-reciprocating mechanisms and their parts

21.260

Lubrication systems