The Conveyor That Jostled the Parts Off the Belt
We specified a belt conveyor to carry pallets between stations. The conveyor was rated for the load, the belt was the right width, and the speed matched the cycle time. On the floor, every time the pallet hit a transition between sections, it jumped 5 mm and a sensor was triggered falsely. The root cause: we used a standard fabric belt with a 2 mm nose bar at the transfer, but the pallet had a 10 mm bottom gap that caught on the transition. It looked fine in CAD. It didn’t work on the floor.
Conveyor system design for automation is one of those subsystems that everyone treats as a catalog item — pick the belt width, pick the length, pick the speed, done. But the details that determine whether it actually moves parts reliably are in the transitions, the drives, the accumulation logic, and how it interfaces with the rest of the machine. This article is what I’ve learned about conveyors that don’t drop, jam, or falsely trigger sensors.
Conveyor Type: Match It to the Part and the Application
Not all conveyors are belts. The conveyor type determines what parts it carries, how it accumulates, and how much it costs.
Belt Conveyor
The default. A flat fabric or plastic belt on a slider bed. Good for boxes, totes, pallets, and loose parts. Cheap, quiet, and forgiving. The downside: belts don’t accumulate well — if one part stops, the belt slides under it and parts pile up. For general transport between stations, a belt conveyor is fine. For accumulation (parts waiting in a queue), it’s not.
Modular Plastic Belt (MPB)
A belt made of interlocking plastic segments. It’s rigid, washdown-resistant, and can accumulate (parts can stop on the belt without the belt sliding). It’s more expensive than a fabric belt but handles heavier loads and harsh environments. For food, medical, or washdown applications, MPB is the default.
Chain Conveyor
Two or three chains carry a pallet or fixture. Good for heavy pallets, high-temperature environments, or where the part needs to be located precisely on the conveyor. The chain is the transport; the pallet sits on the chain and is stopped by a station stop. This is standard for pallet-based assembly lines.
Timing Belt / Precision Conveyor
A toothed belt that carries pallets with registered positioning. The pallets have a pin or notch that engages the belt timing profile, so the pallet position is known precisely. Used when the conveyor itself needs to position the part at a station (not just transport it). More expensive but eliminates the need for separate station locators on simple applications.
| Conveyor Type | Best For | Accumulation? | Cost |
|---|---|---|---|
| Fabric belt | Light parts, boxes, general transport | No (slides) | Low |
| Modular plastic belt | Washdown, heavy parts, accumulation | Yes (zoned) | Medium |
| Chain | Heavy pallets, high temp, precision locating | Yes (stops) | Medium |
| Timing belt | Precision positioning, lightweight pallets | No | High |
| Roller conveyor | Cartons, totes, free-flow accumulation | Yes (gravity or powered) | Low–medium |
Drive and Motor: Sizing for the Load, Not Just the Length
The conveyor motor moves the belt (or chain) plus the load on it. Sizing it wrong means the belt stalls under load or the motor overheats.
What the Motor Has to Move
- Belt weight: The belt itself has mass. A 10 m fabric belt weighs 20–30 kg. A modular plastic belt weighs more.
- Load weight: The parts (and pallets) on the belt. If the conveyor is full of 10 kg pallets at any time, that’s 10 kg × number of pallets on the belt.
- Friction: The belt sliding on the slider bed. This is the dominant force for long conveyors. A rough slider bed or a loaded belt has more friction.
- Acceleration: If the conveyor starts and stops frequently, the motor needs to accelerate the belt and load. Continuous-running conveyors don’t need acceleration margin.
Most conveyor suppliers have a sizing formula or calculator. Give them the belt width, length, load weight, speed, and duty cycle. They spec the motor. Don’t spec the motor yourself unless you know the friction coefficient of the slider bed material.
Speed: What the Cycle Time Requires
The conveyor speed is determined by the station pitch and the cycle time. If stations are 500 mm apart and the cycle is 6 seconds, the conveyor needs to move 500 mm in 6 seconds — 83 mm/s, or about 5 m/min. That’s slow. Most conveyor motors run 10–30 m/min. You need a gearmotor with the right output speed, or a variable speed drive (VSD) to adjust it.
For indexed conveyors (the conveyor moves, stops, moves, stops), use a motor with a brake or a servo/stepper drive that positions accurately. A standard gearmotor will coast when it stops — the pallet overshoots the stop. A brake motor holds position when power is off.
Transitions: Where Parts Actually Fall Off
The conveyor itself is a straight piece. The problems happen at the transitions — where one conveyor meets another, where a part transfers from a feeder to the conveyor, where the conveyor meets a station.
Nose Bar or Tail Pulley
At the end of a belt conveyor, the belt wraps around a tail pulley. The diameter of that pulley determines how small a gap the part can cross. A 50 mm tail pulley has a 25 mm radius — small parts with a flat bottom might tip into the gap. A nose bar (a series of small rollers at the end) reduces the effective diameter to 10–15 mm, letting small parts cross smoothly.
For small parts (under 100 mm), specify nose bars at every transition. For large pallets, a standard tail pulley is fine.
Transfer Between Conveyors
When two conveyors meet end-to-end, the gap between them must be smaller than the smallest footprint of the part. A 5 mm gap is fine for a 200 mm pallet. It’s a problem for a 20 mm electronic component. For small parts, use a flat transition plate between the conveyors (a thin stainless strip bridging the gap).
Side Rails and Guides
Parts need to stay on the conveyor. Side rails (aluminum extrusion or bent sheet metal) contain the part. The gap between the rails should be 2–3 mm wider than the part (or pallet) on each side. Too tight and the part jams. Too loose and the part wanders into the station misaligned.
For pallets, the side rails are the guides. They should be adjustable (slotted holes) so you can tune the gap when the part changes. Fixed rails are wrong on a quick-change machine.
Accumulation: When Parts Need to Wait
If the downstream station is busy, parts need to queue up on the conveyor. A standard belt conveyor doesn’t accumulate — parts push each other, and the belt slides under a stopped part. This causes wear and can tip light parts.
How to Accumulate
- Zone accumulation: Divide the conveyor into zones, each with its own drive or clutch. When a part stops in a zone, the zone’s belt stops. Upstream parts keep moving until they hit the stopped zone. This is standard for pallet transport systems.
- Roller conveyor with zero-pressure accumulation: Each roller zone has a sensor and a clutch. When a part stops, the rollers in that zone disengage. Parts don’t touch each other.
- Queue with a stop: For simple systems, a pneumatic stop (a pop-up stopper at the station) stops the pallet at the station. The belt keeps running under the pallet. This works for heavy pallets on a chain conveyor — the chain is the drive, the pallet is stopped by the stopper.
The stopper rule: Every station that needs a part waiting has a pneumatic stopper that pops up, catches the pallet, and retracts when the station is ready. The stopper is actuated by the PLC. Without it, parts don’t know where to stop.
Sensors and Control: Knowing Where Every Part Is
A conveyor without sensors is a black box. You need to know when a part arrives, when it’s stopped, and when it leaves.
What to Sense
- Part at infeed: A photo-eye at the conveyor entry that tells the system a new part has arrived. Triggers the conveyor to start moving it to the next station.
- Part at station: A proximity switch or photo-eye at each station that confirms a pallet is in position before the station cycles.
- Pallet on stopper: A sensor that confirms the stopper has caught the pallet and it’s seated. Prevents the station from cycling on a pallet that’s only partially stopped.
- Queue full: A sensor at the end of the queue that tells the upstream conveyor to stop feeding. Prevents parts from piling off the end.
Frame and Supports: The Conveyor Doesn’t Float
A conveyor is a long, thin structure. Without supports, it sags in the middle. For conveyors over 2 m, add supports every 1.5–2 m. The supports should be adjustable (levelling feet) so you can set the belt height level with the stations it connects.
The conveyor frame should be rigid enough that it doesn’t flex when a pallet slams into a stopper. A flexing frame causes the pallet to bounce, which causes false sensor triggers. Use aluminum extrusion or welded tube, not sheet metal.
A Conveyor Design Checklist
- What type of conveyor matches the part? (Belt, MPB, chain, roller, timing)
- Is the motor sized for the belt weight + load + friction? (Don’t guess — use supplier calculator)
- What speed does the cycle time require? (Does the gearmotor or VSD deliver it?)
- For indexed motion: does the motor have a brake or servo positioning?
- Are transitions small enough for the smallest part? (Nose bars, transition plates)
- Are side rails adjustable for part variation?
- Does the conveyor accumulate? (Zoned, zero-pressure, or stopper-based?)
- Are stoppers at each station pneumatic and PLC-controlled?
- Are sensors at infeed, each station, and end-of-queue?
- Is the frame supported every 1.5–2 m and levelled to match the stations?
The Bottom Line
Custom conveyor integration isn’t buying a belt on a frame. It’s selecting the right conveyor type, sizing the drive for the actual load, designing transitions that small parts cross, adding stoppers and sensors so the PLC knows where every pallet is, and supporting the frame so it doesn’t sag. The conveyor that moves parts reliably without jamming, dropping, or false-triggering isn’t the cheapest catalog option — it’s the one where the transitions, accumulation, and sensors were designed as carefully as the belt itself.
