On a sheet-fed offset press the plate cylinder, the blanket cylinder and the impression cylinder carry the image, transfer the ink and press the sheet against the blanket. How those cylinders are supported at each end decides whether printing pressure stays even from one side of the sheet to the other, and it also sets the ceiling on running speed. For most of the twentieth century the standard answer was a plain sliding bearing. Nearly every new press sold now uses a rolling bearing instead. The change came from the surrounding engineering, not from one clever part.

What sliding bearings did well

At low and medium running speeds, cylinder journals turned in sliding bearings. The design has a large length-to-diameter ratio and works through low pair contact, so the oil film absorbs shock and the drive runs smoothly. The long contact line stiffens the cylinder, deflection under impression pressure falls, and pressure stays consistent along the axis. A sliding bearing is also easy to machine, easy to fit and easy to maintain. At the speeds presses ran then, it met every process requirement, which is why it was used so widely.

The speed ceiling

From the 1980s press builders began to raise output, and speed was the first lever they pulled. The sliding bearing resisted. Its friction coefficient is high, the journal contact area is large, the clearance in the friction pair is small and heat is carried away poorly, so a modest speed increase brings the risk of the shaft seizing in the bush. Open up the clearance to avoid seizure and the frame loses rigidity, transmission accuracy drops and ink streaks appear on the sheet. At that point the sliding bearing was the bottleneck, and attention moved to rolling elements. Better machine design and better manufacturing had by then removed the obstacles. A rolling bearing has a small length-to-diameter ratio and converts low pair contact into high pair contact, which restores the stiffness the cylinder body loses under impression load.

Why builders hesitated, and what changed

The old objection was deflection. Under heavy impression pressure a cylinder bends, its deflection curve sags, the middle of the cylinder keeps less pressure than the ends, dots and type print weak in the centre and the sheet fails the standard. Finite element analysis changed the calculation. Designers now optimise the cylinder structure and its internal cavity, distributing ribs and web plates by number, shape and section size so overall stiffness rises and deflection under load stays inside the permitted range.

Cylinder manufacturing improved on the same timeline. Material and hardness are now controlled through melting, casting and heat treatment. A press cylinder also has to hold dynamic accuracy, and accuracy after fatigue and creep, not only static accuracy and stiffness. Dynamic accuracy depends on how mass is distributed and on the metallurgical structure and chemical composition of the metal. Runout is measured with the cylinder almost at rest, yet at running speed the error differs because mass distribution and grain structure are never perfectly uniform.

Bearing supply matured as well. Selection, fitting, assembly and inspection are now routine, and high precision bearings are available from domestic and imported sources without a technical barrier. Neighbouring parts improved to match. Cylinder gear accuracy reaches grade four or better in both the kinematic group and the smoothness group, the same class used in aviation gears. Frame bores and end faces are machined in a single clamping on a CNC boring and milling machine, so dimensions, form tolerance and surface roughness satisfy the over-constrained frame structure. With those parts improved, presses run more steadily, vibration sources fall and the cylinder system needs less shock absorption. Heidelberg, Mitsubishi, Komori and KBA all fit rolling bearing supports as standard, and the experience gathered abroad gives domestic builders a clear reference.

Bearing types in current use

The first decision is which bearing structure to fit, and builders answer it differently.

  • Heidelberg favours needle roller bearings, with working clearance set at assembly by measuring the needles and fitting them in matched groups.
  • Roland builds around multi-row short cylindrical solid roller bearings combined with thrust bearings, radial and axial loads adjusted separately. The bearing maker guarantees working clearance, and the precision requirement is high.
  • Mitsubishi uses tapered roller bearings, which carry radial and axial load together in a simpler layout, with clearance set during assembly and a higher demand on fitting skill.
  • Some domestic builders fit hollow cylindrical roller bearings made to a US patent, run with high preload and slight interference.

All of these arrangements serve an offset press. Each has its own character, and in theory any of them can support a cylinder. The choice follows the builder's design capability and process level, how well the technical team understands each bearing type and the condition of the plant, so that objective limits and engineering judgement meet in the same place. Across every option, clearance control is the common concern.

Preload and working clearance

The second technical problem is preload. Clearance between the bearing pair has to be adjusted to raise system stiffness, and the clearance left after preload at installation decides both final print quality and machine life. To raise stiffness and cut noise, a rolling bearing should run with negative working clearance.

Theory and shop experience agree on the target. Fatigue life is longest when working clearance sits at about -10 μm. Push the negative value further and fatigue life falls sharply. When bearing temperature rises hard, thermal growth makes the effective clearance more negative than the value set on the bench, so assembly should leave a margin for that shift.