21 MECHANICAL SYSTEMS AND COMPONENTS FOR GENERAL USE

#	Name
21.020	Characteristics and design of machines, apparatus and 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	Housing, enclosures, other machine parts
21.200	Gears
21.220	Flexible drives and transmissions
21.240	Rotary-reciprocating mechanisms and their parts *Including pistons, piston-rings, crankshafts, etc. for general engineering
21.260	Lubrication systems