21020 | Characteristics and design of machines,apparatus, equipment |
21040 | Screw threads |
21060 | Fasteners |
21080 | Hinges,eyelets and other articulated joints |
21100 | Bearings |
21120 | Shafts and couplings |
21140 | Seals, glands |
21160 | Springs |
21180 | Housings,enclousers,other machine parts |
21200 | Gears |
21220 | Flexible drives and transmissions |
21240 | Rotary-reciprocating mechanisms and their parts |
21260 | Lubrication systems |