Why the Boxes Jammed at the Corner: Conveyor Flow, Explained
A Z-shaped conveyor line. Boxes came in on a straight section, then turned 90° onto a cross-conveyor. Every 20th box jammed at the corner. The operator cleared it in 10 seconds, but the downtime added up. We didn’t change the conveyor speed. We changed the corner geometry.
What happens at a 90° turn
A box moving on a belt conveyor reaches a 90° transfer. The incoming belt pushes the box forward. The outgoing belt moves perpendicular. At the corner, the box’s leading edge hits the side of the outgoing conveyor. If the box isn’t perfectly aligned, it skews. If it skews enough, it jams.
The jam isn’t random. It happens when the box’s length along the incoming direction is shorter than the conveyor’s width at the corner. A 300 mm long box on a 400 mm wide conveyor: the box reaches the corner, but the trailing edge is still on the incoming belt. The incoming belt pushes. The outgoing belt pulls sideways. The box rotates. The corner wall catches it. Jam.
The corner types, and when they jam
| Corner type | How it works | Jams when |
|---|---|---|
| Live corner (belt turn) | Curved belt, 90° around a pulley | Boxes too small or light to track |
| Transfer (90° pop-up) | Box stops, pop-up chain pushes sideways | Pop-up timing off, boxes skew |
| Sweep (angled belt) | Outgoing belt at 30–45°, box sweeps around | Boxes longer than the sweep radius |
| Turntable | Box stops, rotates 90°, exits | Cycle time adds up |
What we changed
The original corner was a simple 90° transfer — the incoming belt ended, and a cross-belt started. No transition. The boxes hit the edge of the cross-belt and skewed. We replaced it with a sweep corner: the outgoing belt enters at 30° to the incoming belt. The box rides the angle and naturally turns. No hard corner to catch on.
We also added a side guide rail on the incoming conveyor, 50 mm before the corner. The rail centers the box. A centered box enters the sweep straight. A skewed box enters at an angle and catches. The rail prevents the skew.
The geometry that matters
For a sweep corner, the incoming belt and outgoing belt overlap by at least the box length. The box never leaves the belt — it’s always supported. If the belts are adjacent (no overlap), the box bridges the gap and tips or skews.
Overlap ≥ L_box (box length in feed direction)
For a 300 mm box, overlap ≥ 300 mm. The two belts overlap by a foot. The box is always on a belt. No bridging, no tipping, no skew.
The sweep angle is 30°–45°. Too shallow (15°) and the box doesn’t turn. Too steep (60°) and the box side-swipes the guide. 30° works for most boxes.
Speed and spacing
Boxes should be spaced at least one box-length apart at the corner. Two boxes close together at a 90° turn: the first one turns, the second one shoves into it. Jam. We added a gap sensor on the incoming conveyor — if the boxes are closer than 500 mm, the conveyor slows. The boxes space out by the time they reach the corner.
What didn’t help
We tried slowing the conveyor. The jam rate dropped from 1 in 20 to 1 in 50, but the throughput dropped by 30%. The jam wasn’t a speed problem — it was a geometry problem. Speeding it back up with the sweep corner: jam rate 1 in 500. Throughput back to spec.
The corner checklist
- Overlap the belts by at least one box length. No gap.
- Use a 30° sweep, not a 90° hard corner.
- Center the box with side guides before the corner.
- Space boxes by one box length at the turn.
- Run the outgoing belt slightly faster than the incoming (10% faster). This pulls the box through the corner, not pushes it.
The corner rule: Boxes don’t jam because they’re moving too fast. They jam because the conveyor geometry forces a skewed transition. Overlap the belts, sweep the angle, center the box. A corner that jams is a badly designed corner, not an operator error.
