An agricultural harvesting header comprising a frame, a plurality of arms pivotally coupled to the frame to pivot up and down at their forward ends with regard to said frame and extending forward from said frame, and a conveyor belt (108, 112) disposed on said plurality of arms to carry cut crop material, the header comprising: a roller comprising a front end and a rear end and a central axis, wherein the conveyor belt (108, 112) is wrapped around the roller for approximately 180 degrees to form an end loop in said conveyor belt (108, 112); an elongate roller carriage comprising a front end and a rear end, said elongate roller carriage being configured to support the roller for rotation about the central axis, wherein the elongate roller carriage comprises a front carriage guide at the front end and a rear carriage guide at the rear end; front and rear guide members (224, 226) fixed with respect to an endmost pivoting arm of said plurality of arms, wherein the front guide member supports the front carriage guide and the rear guide member supports the rear carriage guide, wherein the front and rear guide members (224, 226) constrain the elongate roller carriage to translate with respect to the frame in a direction generally parallel to a direction of travel of the belt (108, 112); an elongate tensioning beam, the tensioning beam comprising a front end and a rear end, characterized in that a central portion of the tensioning beam is coupled to a central portion of the elongate roller carriage to apply pressure to the elongate roller carriage; that a spring assembly is fixed to the endmost pivoting arm, said spring assembly comprising a spring disposed to apply pressure to the rear end of the tensioning beam; that the elongate roller carriage defines an aperture which is disposed closer to the rear end of the tensioning beam than to the front end of the tensioning beam; and that a pin coupled to the elongate tensioning beam is disposed in the aperture to apply pressure to a surface of the aperture; and that the roller pivots on the endmost pivoting arm about an axis generally perpendicular to the longitudinal extent of the roller and perpendicular to a planar upper surface of the conveyor belt (108, 112). a roller comprising a front end and a rear end and a central axis, wherein the conveyor belt (108, 112) is wrapped around the roller for approximately 180 degrees to form an end loop in said conveyor belt (108, 112); an elongate roller carriage comprising a front end and a rear end, said elongate roller carriage being configured to support the roller for rotation about the central axis, wherein the elongate roller carriage comprises a front carriage guide at the front end and a rear carriage guide at the rear end; front and rear guide members (224, 226) fixed with respect to an endmost pivoting arm of said plurality of arms, wherein the front guide member supports the front carriage guide and the rear guide member supports the rear carriage guide, wherein the front and rear guide members (224, 226) constrain the elongate roller carriage to translate with respect to the frame in a direction generally parallel to a direction of travel of the belt (108, 112); an elongate tensioning beam, the tensioning beam comprising a front end and a rear end, characterized in that a central portion of the tensioning beam is coupled to a central portion of the elongate roller carriage to apply pressure to the elongate roller carriage; that a spring assembly is fixed to the endmost pivoting arm, said spring assembly comprising a spring disposed to apply pressure to the rear end of the tensioning beam; that the elongate roller carriage defines an aperture which is disposed closer to the rear end of the tensioning beam than to the front end of the tensioning beam; and that a pin coupled to the elongate tensioning beam is disposed in the aperture to apply pressure to a surface of the aperture; and that the roller pivots on the endmost pivoting arm about an axis generally perpendicular to the longitudinal extent of the roller and perpendicular to a planar upper surface of the conveyor belt (108, 112).