1. A belt conveyor, comprising a carrying belt unit which includes an endless carrying belt having an upper load-carrying run adapted to carry loads and movable in a direction of transportation, and a plurality of supporting rollers arranged to support and guide said load-carrying run of said carrying belt unit; a friction belt unit which includes an endless friction belt having an upper run which is movable in said direction and is in frictional contact with said load-carrying run of said carrying belt, a plurality of bearing rollers arranged to support and guide said one run of said friction belt, and a plurality of deviating rollers which are located between and offset downwardly relative to said bearing rollers so that said one run of said friction belt moves about said deviating rollers and is alternately guided by the same into and out of frictional contact with said load-carrying run, said upper run of said friction belt being guided by said bearing rollers and some of said deviating rollers; a frame having side walls which are formed as a parallel linkage mounting said bearing rollers and said deviating rollers on said frame and including a plurality of flat bars which cross and are articulately connected with each other, said parallel linkage having upper knee points in which axes of said bearing rollers are located, and cross points in which axes of said deviating rollers are located; and a conveyor frame including a plurality of uprights and two longitudinal walls formed as a further parallel linkage extending between two adjacent uprights and each having a plurality of bars which cross and are articulately connected with each other, said longitudinal walls having end portions...
