The Cylinder That Slowed at the End of the Line: Pressure Drop Over Distance
A machine with 10 workstations. Each station has a clamp cylinder. The first 8 stations clamped fast. The last 2 stations clamped slowly. The customer thought those cylinders were failing. We measured the air pressure at each station. The pressure dropped along the distribution line. The last stations were running at 4.5 bar instead of 6 bar.
How pressure drops in a loop
Air flows through a header (main line) and drops down drops (branch lines) to each station. As air moves through the header, it loses pressure due to friction. The first station sees full pressure. The last station sees the accumulated pressure drop of all the branches feeding before it.
The pressure drop scales with flow and with the 5th power of the tube diameter. A long, narrow header kills pressure at the end of the line.
What we measured
| Station | Pressure at station (bar) |
|---|---|
| 1 (nearest compressor) | 6.2 |
| 3 | 6.0 |
| 5 | 5.7 |
| 7 | 5.3 |
| 9 | 4.8 |
| 10 (farthest) | 4.5 |
The header was 10 mm ID, 15 meters long. At station 10, the pressure was 1.7 bar lower than at station 1. The cylinders at station 9 and 10 had less force and slower speed because they were running at 4.5 bar instead of 6 bar.
The fix: larger header, shorter drops
1. Upsize the main header. We replaced the 10 mm header with 20 mm. The pressure drop dropped by a factor of 32 (16x from the larger diameter). Station 10 now sees 5.9 bar. The cylinders run at full speed.
2. Use a loop (closed loop). Instead of a dead-end header (line goes out and ends at station 10), we looped the line back to the compressor. Air can flow from both directions to station 10. The pressure equalizes. This is standard for multi-station pneumatic systems.
3. Add a local regulator at the last stations. Even with the larger header, we added a regulator at station 9 and 10. If the pressure drops during peak demand, the regulator maintains 6 bar at the station. The cylinders see consistent pressure regardless of what’s happening elsewhere.
The pressure-to-force relationship
A cylinder’s force is proportional to the air pressure. For a Ø50 cylinder at 6 bar: F = P · A = 6 × 1963 = 11,778 N (cap end). At 4.5 bar: F = 4.5 × 1963 = 8,834 N. That’s 25% less force. If the clamp needs 10,000 N to hold the part, at 4.5 bar it only delivers 8,834 N — the clamp slips.
Speed also depends on pressure. Lower pressure means less air flow through the valve, which means slower cylinder movement. The last stations clamped in 1.2 seconds vs 0.8 seconds at the front. The cycle time was being set by the slowest station.
The rule for multi-station machines
- Size the main header for 6 m/s velocity (not the cylinder’s consumption).
- Use a loop (return line) so air flows from both ends.
- Add a local regulator at stations more than 10 m from the compressor.
- Check pressure at the farthest station, not just at the compressor outlet.
The pressure rule: If the end-of-line cylinders are slow, measure the pressure at the cylinder. Don’t trust the compressor gauge. The header is undersized if the pressure drops more than 0.5 bar between stations. Upsize the header, loop it, and add local regulators. The cylinders aren’t failing — they’re starving for air.
