In a coating line, the conveyor is not transport. It is the fixture that presents the part to the spray gun, carries it through the oven and returns it to unload. Every hanger and every transition affects the finish.
Key takeaways
- Hanger design controls part orientation, overspray and oven clearance.
- The conveyor must be electrically grounded for electrostatic coating, and the ground path must be maintained through the hanger.
- Oven transitions are the most common source of chain wear and finish defects.
- Accumulation before load and after unload decouples the line from operator pace.
Hanger design before conveyor selection
The hanger holds the part at the angle the process needs. For a flat panel, a slight tilt lets the coating drain and avoids runs. For a box section, the hanger must rotate or offset the part so the spray reaches all faces. For a heavy casting, the hanger needs two support points and a centre of gravity below the hook.
Hanger load is not the part weight alone. Add the fixture, the paint that accumulates on the fixture, and the dynamic load during a stop. A hanger that carries 12 kg when new may carry 16 kg after a month of paint build-up. The conveyor chain and the hanger hook should be rated for the dirty weight, not the clean weight.
Grounding for electrostatic coating
Electrostatic coating relies on a grounded part. The ground path runs from the part through the hanger, the conveyor chain, the track and the building earth. Paint is an insulator, and a film of cured paint on a hanger can break the path. The result is a finish defect that looks like a gun problem but is actually a grounding problem.
Design the hanger with a dedicated contact point that is cleaned on a schedule, and measure resistance from the part to earth at the load station. A reading above the coating supplier's limit means the hanger needs cleaning or the track needs a ground strap. Do not rely on the chain alone.
Oven transitions
Where the conveyor enters and leaves the oven, the chain expands and the lubricant behaves differently. A transition that is too tight pulls the chain against the track and wears the chain and the track together. A transition that is too loose lets the hanger sway and the part touch the oven wall.
Design the entry and exit with a straight section of at least one hanger pitch before the oven opening, and use a heat-resistant chain lubricant rated for the oven temperature. If the oven runs above 200 °C, consider a chain that runs dry with a graphite or PTFE coating rather than oil, which will carbonise.
Accumulation and line balance
Overhead lines usually accumulate before load and after unload. At load, accumulation lets operators work at their own pace without stopping the line. At unload, it gives the packers a buffer. The accumulation length should cover the longest expected stoppage, typically 5–10 minutes of line output.
Accumulation stops must be designed for the hanger, not the chain. A stop that pushes on the chain will wear the chain and may derail it. A stop that holds the hanger trolley is safer and more precise. For heavy hangers, use a double stop to avoid swinging.
Chain speed and pitch
Chain speed is set by the process, not by the conveyor. In a spray booth, the speed must match the gun traverse and the coating thickness. In an oven, the speed must give the required residence time. A typical coating line runs at 2–6 m/min, with a hanger pitch of 600–1,200 mm.
The pitch determines the number of parts per hour: at 4 m/min and a 900 mm pitch, the line delivers about 266 hangers per hour. If the part has two pieces per hanger, the output doubles. This calculation should be done before the booth and oven are sized, because it sets the length of both.
| Process stage | Typical speed | Why |
|---|---|---|
| Load / unload | 2–3 m/min | Operator reach and hanger handling |
| Spray booth | 3–5 m/min | Gun traverse and film thickness |
| Flash-off | 3–5 m/min | Solvent release before baking |
| Oven | 3–6 m/min | Residence time for cure |
| Cooling | 4–6 m/min | Handling temperature at unload |
Maintenance that prevents finish defects
Overhead conveyors are often neglected because they are above eye level. A monthly inspection should cover chain tension, trolley wear, hanger straightness, ground resistance and lubricant condition. A chain that is too tight will wear the track; a chain that is too loose will jump the sprocket at the drive.
Keep a set of spare hangers and trolleys. A damaged hanger can stop the line, and a replacement is cheap compared with a shift of lost output. Clean hangers on a schedule, not when the finish fails.
Frequently asked questions
Can an overhead conveyor pass through a curing oven?
Yes, with a heat-resistant chain, high-temperature lubricant or dry-running chain, and a track designed for expansion. The drive and any sensors should be outside the hot zone.
How do I stop hangers from swinging?
Use a hanger with a low centre of gravity, add a stabiliser rail at the booth entry, and avoid abrupt speed changes. A pendulum hanger will swing at every stop if the stop is applied too quickly.
What ground resistance is acceptable for electrostatic coating?
Follow the coating supplier's specification, which is usually below 1 MΩ from the part to earth. Measure at the hanger, not at the track, and clean the contact points on a defined interval.
Send the part drawing, coating process and oven temperature to our engineers through the contact page, and we will specify the hanger, chain and transitions together. The overhead conveyor section covers the mechanical options.
