Driven by the rapid advance of automation, automatic screw-feeding systems have become ubiquitous in automated bolt-assembly lines. Compared with traditional manual work, they reduce repetitive labour, lower operator fatigue, guarantee highly consistent screw presentation and keep screws continuously available, effectively shortening the replenishment cycle.

Nevertheless, on real production lines the screws still jam. Inconsistencies in incoming screws, limited positioning repeatability, tolerances in the tightening mechanism and faulty motion-control logic all cause stoppages that require manual intervention and erode overall equipment effectiveness.

Jams inside the feeder structure

The mechanical design of the feeder itself is the first factor.
Step one is to convey screws from the hopper onto the linear-vibration rail. If the blow-off air pressure is unstable or the nozzle is set too high, non-conforming screws are not rejected in time and pile up, jamming the entrance.

Once on the rail the screws advance by vibration. After long use external disturbances detune the vibratory frequency; screws then advance irregularly. Excessive amplitude makes screws bounce vertically and stall. Oil and dirt carried by the screws also increase friction on the rail and cause stacking jams.

At the rail outlet the screw must transfer into the separator. If the rail mouth and separator inlet are even slightly misaligned, screws wedge or stack up and the separator cannot cut them off. A screw that has not fully entered the separator will block the next cycle.

Jams in the feeding tubes

After separation the screw is blown to the nose through a polyurethane or PA tube.
Variations in inner diameter, thin walls that collapse when the tube is bent through tight radii, or simply the wrong tube size for the screw length/diameter ratio all create high-risk choke points.

Jams in the blow- or pick-up nozzle

Screws come in many sizes and the tightening situation varies. If the length/diameter ratio is too small the screw can tumble at the three-way junction inside the nose module and jam. (See “Key factors for automating a single screw”.)

Incorrect motion-control logic

Even when hardware and screws are perfect, faulty PLC logic can ask for a screw when one is already present. A second screw is then blown into the nose, causing an immediate jam.
Electromagnetic interference from nearby equipment can also trigger spurious “screw request” signals and produce the same double-feed condition.