Galv. rectangular, v-groove, medium duty, no seals, 2000 FPM, 5″ w.c. max. The CD35 commercial control damper offers sturdy, steel construction with interlocking frame design. It locks together without bolts, screws, or rivets that could shake loose. It also features: Frame corners that are internally braced to reduce racking.Axles that positively lock to blades without screws or welds.Non-stick, noncorrosive bearings that assure long life and ease of operation. Low maintenance operation.