An apparatus for conveying a substrate or sheet through a printing machine comprises a drive roller and a baffle assembly forming a path for the substrate past the drive roller. The baffle assembly includes an idler roller having an axle rotatably supported within the baffle assembly to cooperate with the drive roller to exert a nip force on the substrate. A nip spring connected to the baffle assembly bears against a bushing carrying the idler roller axle to exert a nip force on the idler roller. A nip force adjustment apparatus is mounted within the baffle assembly and is operable to apply an adjustment force on the bushing of the idler roller to augment the nip force generated by the nip spring. The adjustment apparatus includes an actuator movable between a neutral position and an activated position, a force transmission element movably supported within the baffle assembly to engage, in an operable position, the bushing of the idler roller to exert the adjustment force, and a linkage connecting the actuator to the force transmission element to move the force transmission element into the operable position when the actuator is moved from the neutral position to the activated position.