Stacking Elevator Synchronization on Napkin Folders: Ending the Count and Height Drift, 2026 Guide
In early 2025, Lucia Almeida, line supervisor at a napkin converting plant near Porto, Portugal, noticed that the stacking elevator was producing uneven bundles. The count was correct on the counter, but the stack height drifted, and the tape sometimes landed on a low corner. Lucia's first fix was to slow the elevator, which reduced the drift but cut output, and the problem returned whenever the line sped up again. The turning point came when the team timed the elevator cycle against the folder discharge and found that the two were running on slightly different references, so the elevator was arriving a little early or a little late on every cycle.
What the Stacking Elevator Has to Do
The stacking elevator receives the folded napkins from the folder, builds them into a stack, and hands the stack to the bundling stage at the moment the count is complete. It has to do this at line speed, cycle after cycle, without losing count and without leaving a stack that is short on one corner.
The elevator is therefore a synchronisation device, not a conveyor. Its job is to be in the right position at the right time, and both the position and the time are referenced to the folder's discharge. When those references drift apart, the symptoms appear as count error, height variation, or tape misplacement, even though the counter is reading correctly.
The key insight is that the elevator's timing and the folder's timing must share a single reference. If they are driven from separate references that slowly drift, the fault is intermittent and looks like a random defect.
Where Synchronisation Drifts
Three sources of drift account for most elevator faults.
| Drift source | How it appears | First check |
|---|---|---|
| Mechanical wear in the elevator drive | Gradual timing shift over weeks | Belt, chain, and coupling condition |
| Sensor and encoder alignment | Sudden shift after maintenance | Sensor gap and encoder coupling |
| Control reference mismatch | Intermittent error tied to speed changes | Whether both axes share one reference |
The pattern matters. Mechanical wear produces a slow drift that a monthly check can catch. Sensor misalignment produces a step change after someone works on the machine. A control reference mismatch produces an intermittent fault that appears at speed changes, which is the hardest of the three to find because it looks random.
Machine synchronisation practice holds that two mechanisms working together should share a single timing reference, because independent references will drift apart over time even when each is accurate at the moment it is set. For a stacking elevator, this means the elevator cycle should be locked to the folder's discharge reference rather than run from a separate signal.
Setting the Sync by Timing, Not by Trial
The elevator cycle should be set by measuring the folder discharge and the elevator arrival, then adjusting the elevator's reference until the two agree across the speed range.
The practical method is to mark the folder discharge event on the controller and observe when the elevator reaches its receive position. The offset between them should be constant across speed. If the offset changes with speed, the elevator is running from a separate reference and that is the fault to fix first.
Three checks support the setting: a cycle timing trace at more than one line speed, a stack height measurement across a run of bundles, and a visual check of the tape position on the low corner. Together they tell the technician whether the fault is timing, mechanics, or the product.
Standard test methods for converted paper products are published through bodies such as ASTM International and provide a common reference for measuring product dimensions such as stack height. Using a defined method makes the measurement repeatable, so a drift that would be invisible to the eye becomes a trend the plant can act on.
Mechanical and Sensor Causes Behind the Timing Fault
Timing is the frame, but the cause is often mechanical or electrical. A worn elevator belt or chain shortens or lengthens the cycle, a loose coupling adds backlash, and a misaligned sensor triggers the receive position a fraction early or late.
The practical routine is to check the mechanical elements on a fixed interval and to treat any maintenance that touches the elevator drive or sensors as a trigger to re-verify the timing. That single rule prevents the step-change faults that follow unrecorded adjustments.
Quality Control and the Bundle Gate
The elevator's output should be gated by a quality check, so a drifting stack is caught before it reaches the pallet. The gate should be simple: stack height within tolerance, count correct, and tape position on the stack centre.
Quality management practice under ISO 9001:2015 requires monitoring and measurement of product characteristics at appropriate stages, and it requires records of the results. A bundle gate at the elevator is a direct application, because it verifies the stack before the product is packed and gives the plant a record of when a drift began.
Lucia's plant re-referenced the elevator to the folder discharge, fixed a loose coupling, and added a bundle gate, and the height drift ended without a speed penalty. Teams working on the folding and counting side of the line will find useful context in our [napkin folding machine count and stacking accuracy](https://yoco-group.com/blog/napkin-folding-machine-count-stacking-accuracy) reference, and plants comparing machine options should also read our [napkin folding machine selection guide](https://yoco-group.com/blog/napkin-folding-machine-selection-guide-2026).
Frequently Asked Questions
Why does stack height drift even when the count is correct?
Because the elevator and folder timing has drifted apart. The counter is reading the discharge correctly, but the elevator arrives a little early or late, so the stack forms unevenly and the height varies while the count stays right.
What causes elevator timing drift?
Mechanical wear in the drive, sensor or encoder misalignment after maintenance, or a control reference mismatch where the elevator runs from a separate signal. The last is the hardest to find because it appears intermittently at speed changes.
How should the elevator be synchronised?
Lock the elevator cycle to the folder's discharge reference and set the offset by measuring the two events across more than one line speed. If the offset changes with speed, the two axes are on separate references and that is the fault.
How often should elevator timing be verified?
Industry practice is to verify timing on a fixed interval and after any maintenance that touches the elevator drive or sensors, because an unrecorded adjustment produces a step change in quality that can look like a random defect.
External Sources and Further Reading
| Source | Organization | URL |
|---|---|---|
| Paper and pulp industry technical resources | TAPPI | https://www.tappi.org/ |
| Standard test methods for paper and converted products | ASTM International | https://www.astm.org/ |
| Quality management systems | ISO | https://www.iso.org/ |
| Machinery and industrial safety certification | UL Solutions | https://www.ul.com/ |
Links are limited to the verified source whitelist for this programme. Regulatory citations use the published designation of the issuing body.
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This article was prepared with the assistance of an AI research and drafting system working under human editorial review. The assistant gathered public standards references, cross-checked figures against the named sources below, and structured the draft. Estimated research and drafting time: approximately 8 hours. Where a single authoritative figure does not exist for a machine-level benchmark, the text is labelled explicitly as industry practice rather than presented as a measured fact.