An iris mechanism valve apparatus for a fluid transmission line, the valve apparatus comprising: a housing; a drive mechanism; a leadscrew selectively rotatable by the drive mechanism in a first or second leadscrew direction; a drive gear mounted to the housing, the drive gear configured to be selectively rotatable in a first or second drive gear direction during rotation of said leadscrew in the first or second leadscrew direction, the drive gear defining a throughhole; a fastener threadingly engaged with the leadscrew and engaged with the drive gear for transferring torque from the leadscrew to the drive gear during rotation of the leadscrew by travelling along a length of the leadscrew during rotation thereof, and wherein the fastener has a rectangular construction; a lever arm engaging the fastener and the drive gear; a bushing engaging the fastener and the lever arm; a plurality of leaves disposed about the throughhole, each of said leaves including a curvilinear portion and a pair of ends and being mounted to the drive gear adjacent one of said ends, and each of said leaves being oriented to extend radially inward into said throughhole to define an orifice; and a plurality of engagement members biasing said leaves to reduce the orifice when the drive gear rotates in said first drive gear direction and to increase the orifice when the drive gear rotates in said second drive gear direction to adjust a volume of transmission of fluid therethrough; characterised in that the valve apparatus further comprises a singular stationary shaft adjacent and extending parallel to the leadscrew, the fastener having a semi-circular bore slidably engaged with the shaft for balancing the torque transferred from the leadscrew to the fastener, and in that each of said leaves terminates in a fin substantially perpendicular to the curvilinear portion. a housing; a drive mechanism; a leadscrew selectively rotatable by the drive mechanism in a first or second leadscrew direction; a drive gear mounted to the housing, the drive gear configured to be selectively rotatable in a first or second drive gear direction during rotation of said leadscrew in the first or second leadscrew direction, the drive gear defining a throughhole; a fastener threadingly engaged with the leadscrew and engaged with the drive gear for transferring torque from the leadscrew to the drive gear during rotation of the leadscrew by travelling along a length of the leadscrew during rotation thereof, and wherein the fastener has a rectangular construction; a lever arm engaging the fastener and the drive gear; a bushing engaging the fastener and the lever arm; a plurality of leaves disposed about the throughhole, each of said leaves including a curvilinear portion and a pair of ends and being mounted to the drive gear adjacent one of said ends, and each of said leaves being oriented to extend radially inward into said throughhole to define an orifice; and a plurality of engagement members biasing said leaves to reduce the orifice when the drive gear rotates in said first drive gear direction and to increase the orifice when the drive gear rotates in said second drive gear direction to adjust a volume of transmission of fluid therethrough; characterised in that the valve apparatus further comprises a singular stationary shaft adjacent and extending parallel to the leadscrew, the fastener having a semi-circular bore slidably engaged with the shaft for balancing the torque transferred from the leadscrew to the fastener, and in that each of said leaves terminates in a fin substantially perpendicular to the curvilinear portion.