fly

On a cylinder or platen printing press, the fly is a hinged frame fitted with spring-loaded fingers or grippers that catches each printed sheet as it leaves the impression point and carries it away from the rollers. The frame vibrates or rotates in synchrony with the press cycle, engaging the sheet at release and depositing it onto a delivery pile or conveyor. Without the fly, printed stock would jam between the cylinders or fall randomly into the press bed.

The fingers themselves are typically made from steel or hardened polymer and must be timed precisely to grip without marking the printed surface. The fly's motion is driven by a cam or linkage connected to the main crankshaft, so its rhythm locks to the press speed. On a typical 4-color offset or letterpress machine running 5,000 impressions per hour, the fly must engage and release each sheet in less than 0.1 seconds. Tension in the spring fingers is adjustable to handle different stock weights, from 60 gsm bond to 350 gsm card.

Variants and failure modes

Platen presses use a rotating fly wheel with multiple fingers arrayed around it; cylinder presses often employ a reciprocating beam fly that moves back and forth. Older machines use mechanical cams; newer systems may have pneumatic or servo-driven grippers. A fly that engages too early will crease the sheet; too late and it jams. Worn hinges, broken fingers, or sluggish return springs cause missed sheets and production stops. Ink transfer from the fly to the back of sheets is a chronic complaint; some operators add felt or silicone pads to the grip points.

The term arose from the fluttering, fly-like motion of the hinged frame in early hand-fed presses. As presses automated, the fly remained the core delivery mechanism; today it sits at the boundary between print formation and post-press handling. On digital inkjet or toner systems, pneumatic or electrostatic separation has largely replaced mechanical flies, but offset and letterpress shops still rely on them as the workhorse of sheet ejection.