Heidelberg SM 74 Delivery Gripper Cam Replacement Guide

Delivery gripper opening cams are among the cheapest parts on a Heidelberg SM 74 — and among the most expensive to ignore. When the cam ramp wears, the grippers no longer open at the moment the sheet is over the pile. They open late and pull the tail edge back, or they open early and drop the sheet short. Either way the operator compensates with gripper springs and then with speed, and the press never quite runs clean.

This guide covers what the cam does, how to tell cam wear from gripper wear, and how to replace and re-time it.

What the opening cam does on an SM 74 delivery

On an SM 74, the printed sheet is carried from the last unit to the delivery by the delivery gripper bars. Each bar holds a row of grippers that are closed by spring force and opened mechanically. As the bar travels around the delivery chain path, the gripper hook — the follower on the gripper shaft — rides over the opening cam. The cam profile lifts the shaft and opens the grippers at the exact point where the sheet is over the pile, holds them open long enough for the sheet to settle, then lets the springs close them again before the bar picks up the next sheet.

Two parts of the profile matter. The opening ramp controls how fast the grippers release; a worn ramp opens the grippers over a longer distance, so the release becomes soft and late. The nose of the cam controls how long the grippers stay open. Wear at the nose shows up as sheets that are released too briefly, or that get caught by the closing grippers and pulled forward into the delivery stop.

Typical wear patterns are a polished flat on the ramp, a groove worn by the follower running off-centre, and a burr or step at the transition between ramp and nose. The hook itself flat-spots at the same time, so it is worth replacing cam and hook together — a new cam running against a flat-spotted hook will wear to the same shape within a few weeks.

Symptoms that point at the cam rather than the grippers

• Tail-edge marks or scuffing. Grippers that release late drag the sheet back slightly, leaving short marks or a polished band on the tail edge.
• Dog-eared or torn corners. The sheet is still gripped while the front edge has already hit the pile or the delivery stop.
• Sheets landing short or cockled. Early release drops the sheet before it is over the pile; it lands on the delivery stop and buckles.
• A metallic tick or squeal once per sheet. The follower is riding over a flat or a burr. If the noise repeats with the chain pitch, it is the cam, not a bearing.
• Jams only at high speed. Cam wear is speed-sensitive: the follower skips across the worn ramp instead of following it, so the press runs at 6,000 sheets/hour and jams at 12,000.
• Pile needs constant jogging. Sheets arrive at slightly different angles because the release point varies from bar to bar.

If the delivery also shows curling or sheets that slide forward on the pile, check the slowdown bands and sheet brake before blaming the cam — those two systems do overlapping work, and a worn slowdown band produces a very similar pile. Our SM 74 delivery sheet curl troubleshooting guide covers that side of the delivery.

Diagnose before you order anything

Work through this order, because a worn cam is usually hidden behind two other problems:

1. Gripper pads and springs. Glazed or compressed pads hold the sheet unevenly; over-tightened springs mask a worn cam and then break gripper hooks.
2. Gripper hook (follower). Look for a flat spot or a step. Any flat means the hook has been running against a worn cam for a while.
3. Opening cam. Clean the ramp, then run a fingernail across it. You should feel a smooth curve. A detectable step, groove or polished flat is wear.
4. Chain tension. A slack delivery chain changes the relationship between the bar and the cam. Check it before you adjust anything.

Tools and parts

• Hex keys (4–6 mm) and 10/13 mm sockets, torque wrench
• Feeler gauges, dial indicator with magnetic base
• Degreaser, clean rags, lithium assembly grease
• Marker pen and a phone for reference photos
• New opening cam (and the matching SM 74 delivery gripper hook if the follower is flat-spotted)
• Spare SM 74 delivery gripper bar complete only if the shaft or bushings are worn — see the last section

Step-by-step replacement

1. Isolate the press properly. Main switch off, lockout/tagout applied, air supply closed and bled down. The delivery gripper bars are spring-loaded and the chain can move when drive or air is released — never put a hand into the delivery path with the press live.
2. Bring the bar to a working position. Turn the press by hand until the affected gripper bar sits at the front of the delivery, above the pile, where the cam mounting screws are accessible.
3. Photograph the cam before loosening it. The mounting is slotted, and the cam is timed against the bar. Reference photos plus the witness marks on the bracket save a full re-timing later.
4. Remove the retaining screws (typically two). Some brackets use a dowel — if the cam will not come off after the screws are out, check for a dowel instead of prying.
5. Clean and inspect the mounting face and stud. Burrs or old thread-locker on the face will tilt the new cam and change the opening point.
6. Fit the new cam and bring it to the witness marks, screws hand-tight only.
7. Check follower contact by hand. With the grippers closed, the hook should sit on the base circle with the gripper shaft at rest and no preload. Crank the press through one full bar revolution slowly and watch the grippers open and close — nothing should bind, and the hook should never leave the cam face.
8. Torque the screws to the value in your machine documentation, then re-check the witness marks. Do not guess torque values: the bracket takes the full spring load of the bar.
9. Repeat on the opposite side if your press carries a cam on each side of the gripper bar. Replacing one side only leaves the bar opening unevenly across the sheet width.

Setting the opening point

Static check first. Turn the press by hand and watch the grippers relative to the pile: the sheet should be fully released while the bar is over the front part of the pile, with enough open time for the sheet to settle before the grippers close. If the grippers close before the sheet has landed, the cam is timed late; if they open while the sheet is still short of the pile, it is timed early.

Then run the dynamic check. Start at low speed with waste sheets, step up in stages, and stop at the first speed where the pile starts to scatter. Re-inspect the cam ramp for fresh witness marks from the follower — a mark that appears on only part of the ramp means the cam is running out of alignment, not that the new part is defective.

Finally, run a sheet test at production speed and look at the tail edge of the delivered sheets under a light. A clean tail edge with no gripper marks and a square pile is the pass criterion. If marks persist with a new cam and a new hook, go back to gripper pad condition and spring tension.

Opening cam or complete gripper bar?

Replace the SM 74 delivery gripper opening cam alone when the wear is confined to the ramp and the gripper shaft still moves freely with no lateral play. Replace the complete bar when any of the following is true:

• Visible play in the gripper shaft bushings, or the shaft does not return smoothly
• Gripper pads will not hold the sheet even with correct spring tension
• The bar is bent, or gripper spring pockets are worn oval
• Individual grippers have been replaced several times on the same bar

A complete bar arrives assembled and set, turning a two-day job of chasing individual grippers into a few hours of chain work.

Preventive maintenance

Inspect the opening cam and hook at every scheduled lubrication service: clean the ramp, check for a step with a fingernail, and look at the hook for flat spots. On two-shift operation, expect to replace cams well before the rest of the delivery train shows wear, because the follower load is concentrated on a very small contact area. Keep the chain correctly tensioned and the cam face lightly greased — a dry ramp wears several times faster and takes the hook with it.

When you change a cam, change the hook in the same visit. It is the biggest factor in cam life.

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