Machine for folding sheets of paper

In a machine for folding sheets of paper, particularly for folding large sheets leaving a printer or copier, including first, second and third flexible entrainment means for entraining a sheet to be folded, said first and third entrainment means having respective passes which are substantially parallel to each other and define a first folding channel and said second and third entrainment means having respective passes which are substantially parallel to each other and define a second folding channel, in which said first, second and third entrainment means are actuable alternately in opposite senses so as to supply a said sheet to be folded to said first and to said second folding channels alternately so as to form a fold each time said sheet passes from one of said folding channels to the other of said folding channels, and in which said first and second entrainment means cooperate with a first drive roller and with a second drive roller respectively which together define a feed passage for said sheet to be folded, the improvement comprising a third drive roller which cooperates with said third entrainment means and which is movable between a position in which it is pressed against said first drive roller and which it occupies during the phase of entrainment of said sheet in said first folding channel and a position in which it is pressed against said second drive roller and which it occupies during the phase of entrainment of said sheet in said second folding channel.