Abstract: A speed reducer comprised of a casing (1) obtained in a single block, which comprises a broad opening (2) through which it is possible to mount the different internal elements, among which there is a flange (4) which permits the assembly of a bottom shaft (7) with its pinion (8) and meshing (9), resting at one end of the bearings (10) and at the other on a bridge (11). It also includes some coaxial shafts (12, 13) resting on the flange (4) and on bearings (14, 15). The shaft (12) is the one that is coupled to the motor outlet (16), while the shaft (13) protrudes through the opposite part and constitutes the structure for effecting the transmission. The special distribution of the described internal elements permits that none of the gearing is overhanging; in other words, all the shafts have two supports with bearing, determining a greater balance and greater resistance of the unit.