A tractor feed mechanism for feeding print forms comprises a continuous flexible pin belt wrapped around a pair of drive pulleys rotatably supported by a tractor body. Pins projecting from the flexible belt engage the forms via perforation in the forms. A door pivotally connected to the tractor body and is rotatable between an open condition in which paper can be placed on or removed from the pins and a closed position in which the forms can be restrained from becoming disengaged from the pins during feeding by the pin belt. A cam rotatable with the door and a spring is operable therewith produce a first restraining force on the door in its closed position which allows the door to be lifted a predetermined amount by the forms during feeding by the pin belt to accommodate variations in the thickness of the forms caused by buckling or folds. The cam and spring produce a second restraining force while the door is in the closed position of a magnitude which prevents lifting of the door beyond the predetermined amount. The cam is structured such that the first restraining force produced by the spring means remains substantially constant up to the point where the door is lifted said predetermined amount.
