Martin Quadra-Flex® is a proven design which offers long life, torsional flexibility, ease of installation, and withstands misalignment, shock, and vibration. Available in sizes 3 to 16 in bored to size, QD bushed, spacer type and plain bore for custom requirements. Martin Quadra-Flex® Spacer Coupling Hub, Bored to Size Type SC, 06 Coupling, 3/4 in Bore Dia, 2-1/2 in Hub Dia, 3/16 x 3/32 in Keyway, 6000 RPM Maximum, 1.21875 in Length Through Bore, 1/4-20 x 1/4 in (Set Screw)/1/4 x 1-3/4 in (Cap Screw) Screw, Steel