A spiral coil dispenser is a horizontal helical spring — visually similar to a stretched-out slinky — mounted on a motor-driven shaft inside a vending machine shelf. Each product row sits nested between the coils, one item per turn of the spring. When a customer makes a selection, the shelf's drive motor rotates that row's coil through a fixed number of turns, and each full rotation advances the product forward by exactly one coil pitch — the spacing between adjacent loops.
The coil keeps advancing the front-most product until it clears the last loop and reaches the shelf edge, where gravity takes over and the item drops into the delivery chute below. The rotation count is fixed at installation and depends only on how far back the product rests and how wide the coil pitch is.
This is why operators talk about "coil calibration" almost as much as restocking — the same coil geometry has to work across a whole range of snack widths just by changing how many rotations the motor is told to run.
Adjust the coil pitch, product width, and motor speed below, then press Dispense to watch the coil rotate the product forward until it clears the shelf edge and drops into the chute.
The number of rotations a coil needs is purely geometric: how far the product's resting point is from the shelf edge, divided by how much ground one rotation covers. Dispense time follows directly once you know the motor's rotation speed.
| Symbol | Meaning | Unit |
|---|---|---|
| N | Rotations required to reach the shelf edge | turns |
| D | Resting distance from product start to shelf edge | mm |
| P | Coil pitch (spacing between loops) | mm |
| RPM | Motor rotation speed | rev/min |
| t | Dispense time from selection to drop | seconds |
The coil pitch P must always be built slightly larger than the product width W — usually by 3–6 mm — or the product jams between loops. N should also land on a fractional turn the controller can actually command; stepper-driven coils typically resolve to 1/8 or 1/16 of a rotation.
A vending operator is loading a wider bag of pretzels into a slot that previously held a narrower candy bar. The coil pitch on that row is fixed at 24 mm, the resting position sits 380 mm from the shelf edge, and the drive motor runs at 20 RPM.
Result: the slot is programmed for 15.875 rotations at 20 RPM, a 47.6-second dispense cycle. If the drop sensor doesn't trigger within roughly 50 seconds, the controller flags a possible jam and reverses the coil half a turn before trying once more.
Elevator/lift systems, drop-shelf gates, and robotic pick arms trade dispensing speed, product range, and mechanical complexity differently than a coil.
| Property | Spiral Coil | Elevator / Lift | Robotic Pick Arm |
|---|---|---|---|
| Best for | Snacks, chips, candy | Bottles, cans, fragile items | Mixed/irregular, fresh food |
| Dispense Time | 15–50 s | 5–12 s | 8–25 s |
| Mechanical Complexity | Low | Medium | High |
| Jam Rate (typical) | 2–5% | <1% | <1% |
| Cost per Slot | Low | Medium–High | Very High |
This is almost always a coil-rotation mismatch, not an empty slot. If the coil was calibrated for a narrower product and someone reloaded it with a wider one, the item never reaches the drop sensor within the programmed rotation count, so the controller assumes the row is empty. Recalibrate using the D/P formula above.
3–6 mm is standard. Too little and the product gets pinched between loops; too much and slim products can rattle sideways off the coil track before reaching the edge.
Usually a worn or stretched coil spring — if the pitch has crept wider than spec, the product ahead of the intended one can clear the last loop on the same rotation count. A spring stretched unevenly needs replacing, not recalibrating.
Within its clearance tolerance, yes — a 24 mm pitch coil comfortably handles roughly 18–21 mm wide products. Outside that range, jam rate rises or the SKU should move to a better-matched row.
A separate infrared drop sensor across the chute confirms it, independent of the coil's rotation count. If the beam doesn't trigger, most controllers attempt one recovery rotation before refunding the purchase.
