On a used sheetfed press, the printing pressure screen is the one inspectors most often gloss over — yet on workhorses like the Heidelberg SM102 and CD102 it is a precision servo system: a 24 VDC, 5 W servo motor moves the blanket cylinder through a gear train, and behind the numbers sit three concepts that must never be conflated — cylinder gap (the minimum blanket-to-impression distance once the clamp closes), printing pressure (the gap times -1), and impression pressure (what the substrate actually feels). Mix them up and every setting reads wrong.
- Auto-positioning: enter stock thickness T and the servo sets the gap to T - 0.1 mm, so standard impression pressure is always 0.100 mm; 0.07 mm coated paper gives a -0.03 mm gap, 0.50 mm board opens it to 0.40 mm.
- Gap limits differ by model: -0.75 to +0.30 mm on SM102, -1.20 to +0.30 mm on CD102, -0.55 to +0.20 mm on SM74, -0.40 to +0.20 mm on SM52.
- Impression-pressure ceilings: 0.55 mm on SM/CD102, 0.45 mm on SM74 and SM52; each keypress steps 0.01 mm on printing units, 0.005 mm on coating units.
- Hard acceptance check: in the service menu, servo incremental values must read 616 ± 8 at +0.30 mm and 3550 ± 8 at -1.20 mm on a CD102 printing unit; 478 ± 4 and 3617 ± 4 on SM102.
The manual headroom
After stock thickness is entered, CD102/SM102 starts at 0.1 mm impression pressure, and manual adjustment can add at most 0.45 mm before hitting the 0.55 mm software ceiling; on SM74/SM52 the headroom is only 0.35 mm. If a used unit sits at its increment limit yet still cannot reach target pressure, suspect gear-train backlash or feedback potentiometer drift — not the stock.
Reading the over-limit fault
When the servo travels past the impression ceiling, a pressure symbol appears in the fault menu and the feeder is locked out. Check the servo motor and its 10 VDC, 10 kΩ feedback potentiometer before resetting anything. On site, zero every unit, enter the same thickness unit by unit, and compare the readings — a visible spread means the adjustment mechanisms are out of sync.
Bottom line: printing pressure is a closed-loop servo system with a standard value, hard limits and increment tolerances — not a feel-based tweak. Walk through concepts, auto-positioning, limits and the increment check, and half an hour at the console tells you whether the pressure system has been mistreated.
