A timing belt assembly trained about drive and idler pulley assemblies for supporting optical scanner units and moving the same in a continuous closed path upon rotation of the drive pulley assembly. The effective circumference of the drive pulley assembly is slightly greater than one-third the effective circumference of the timing belt assembly; the effective circumference of the idler pulley assembly is slightly less than one-third the effective circumference of the timing belt assembly; biasing means impose a lateral force on the timing belt assembly for maintaining the latter under substantially constant tension; and the cylindrical face means of the idler pulley assembly, about which the timing belt assembly, comprised of endless flexible tape means, is trained, has an axial extent at least equal to the width of the tape means.