Why Your Heidelberg SM 74 Sheets Curl at the Delivery

Sheet curl at the delivery is one of the most frustrating faults on a Heidelberg SM 74, because the press appears to be running perfectly. Print quality in the units is good, registration is holding, the sheet feeder is behaving — and yet every sheet comes off the pile with a curl, a fluttered edge or a dog-eared corner. Operators often chase the problem in the wrong place: they adjust the pile, add anti-setoff spray, or slow the press down. In most cases the cause is inside the delivery itself, and it comes down to four things: worn slowdown bands, a tired sheet brake, gripper release timing, and pile or air settings.

This guide is written for press operators and maintenance technicians working on Heidelberg SM 74 presses. Work through it in order — cheapest and most common cause first.

How the SM 74 delivery is supposed to handle a sheet

By the time a sheet reaches the delivery, it has already been printed, transferred and accelerated. The delivery chain carries it at press speed on gripper bars, and then three things happen in rapid sequence:

• The grippers release the sheet at the opening cam.
• The slowdown bands (slow-down bands) decelerate the sheet so it drops onto the pile at a controlled speed instead of being thrown.
• The sheet brake unit and the front stops catch the sheet and align it against the pile front edge.

Curl appears when any one of those three stages stops doing its job: the sheet is released too early, decelerated unevenly, or braked harder on one side than the other. Because the symptoms overlap, diagnose before you order parts.

Four causes of sheet curl at the SM 74 delivery

1. Worn or glazed slowdown bands

Slowdown bands are a consumable. They run in constant contact with the sheet at high speed, and they glaze, flatten or harden with age and pressroom chemistry. A glazed band loses friction, so the sheet is no longer decelerated evenly across its width — one edge travels faster than the other and the sheet curls or skews as it lands. This is the single most common cause of curl that appears gradually over weeks rather than overnight.

Signs to look for: bands that look polished or shiny, flat spots, a hard “plastic” feel instead of a soft, grippy surface, visible wear ridges, or a visible difference in height or colour across the band set. If the bands are more than a year old in a busy shop, treat them as suspect.

2. Sheet brake out of adjustment or worn

The sheet brake catches the sheet as it reaches the pile and stops it. If brake pressure is too high, or one side is set harder than the other, the sheet is stopped unevenly and curls at the leading edge. Worn brake components — glazed brake surfaces, tired springs, damaged drive wheels — make the brake grab in steps instead of cushioning the sheet. A brake that has been running on worn parts for months often cannot be adjusted back into spec, only rebuilt.

3. Gripper release timing and pad condition

If the delivery grippers hold the sheet a fraction too long, the sheet is still being pulled when the bands take over: it gets tensioned and curls. If they release too early, the sheet flies into the pile and folds. Hardened, cracked or glazed gripper pads change the release characteristics, and so do tired gripper bar springs. Release timing is set at the opening cam and should be re-checked against the press manual whenever the delivery has been disturbed for any reason.

4. Pile height, delivery air and static

Curl that is uniform across the whole pile and appears only at high speed is often not a wear problem at all. Pile height too low increases the drop distance; too much delivery air flutters the sheet as it falls; static in dry conditions makes the sheet cling to the bands and curl when it finally releases. Check these three before ordering anything.

Diagnosing: match the curl to the cause

• Curl on one edge only, worse on one side of the machine → uneven slowdown bands or an unbalanced sheet brake.
• Curl that worsens gradually over weeks → band glazing or brake wear.
• Sheets marked or dog-eared as well as curled → gripper pads and release timing.
• Curl only at production speed, clean when running slow → pile height, delivery air or static.
• Random curl plus delivery jams → gripper bar springs and opening cam timing.

Change one variable at a time. Make an adjustment, run a couple of hundred sheets, and inspect the pile — not just the sheet in your hand. Curl that only shows up in the bottom third of the pile is a different fault from curl on every sheet.

Fix 1 — Replace glazed slowdown bands

Bands are the cheapest fix and the one that solves the problem most often. Work one gripper bar at a time with the press locked out, and replace the whole set rather than just the two worst bands — mixing new and glazed bands on the same bar simply moves the uneven deceleration to a different part of the sheet.

Before ordering, confirm the band height and profile against the part you take off the machine. SM 74 and CD 102 presses share several band types, and soft foam bands are not interchangeable with the harder green or pink bands used in other configurations. RelyOffset stocks the SM74/102 foam slow down bands and the SM74/CD102 green slowdown bands — match the profile physically rather than ordering from memory.

Fix 2 — Service the sheet brake unit

Clean the brake surfaces, then check that both sides contact the sheet evenly at the same pressure. Replace glazed, scored or flattened components. Where a brake has been running on worn parts for a long time, the springs and drive wheels are usually finished as well; a brake that grabs and releases in steps leaves a curl that no amount of band changing will remove. Replacement SM102/72 sheet brake units are also used across the SM 74 delivery, so confirm fitment against your machine’s delivery configuration before fitting.

Fix 3 — Restore gripper release timing

Set release timing at the opening cam to the value in your press manual. Fit new pads if the old ones are hardened or glazed, and check the gripper bar springs: a soft or broken spring lets the gripper float and release late, which produces exactly the tensioned, curved sheet you are trying to eliminate. After any timing change, jog the press slowly and watch the release point at the pile before running up to production speed.

Preventive checks that keep curl away

• Inspect slowdown bands at every scheduled service and replace the set at the first sign of glazing.
• Clean sheet brake surfaces at every delivery service and confirm even contact on both sides.
• Check gripper pad condition and release timing at each scheduled maintenance interval.
• Re-verify pile height and delivery air whenever the paper stock, thickness or format changes.
• Log the date bands and brake parts were fitted — tracking consumable life is the fastest way to spot a part that is failing early.

Getting the right SM 74 delivery parts

Order by physical match: profile, thickness and mounting pattern from the component you removed. Sheet curl is almost always a wear or adjustment fault, and in most shops bands first, brake second, timing third will clear it inside a single maintenance window.

RelyOffset parts are OEM-compatible aftermarket components. Brand names are trademarks of their respective owners.