We went through this last year on a retrofit line — 2x 150T Komatsu servos that were originally installed pre-2017. Here's what actually bit us during the audit:
The STO/SS1 configuration was the big one. Our drives technically had the safety functions but they'd never been properly parameterized or validated. The auditor wanted to see the safety function response time documented — specifically that SS1 brings the slide to zero within the time used in the safety distance calculation per ISO 13855. We had to hire the drive manufacturer's field engineer to come out and do the timing measurements. Two days of downtime plus $4,800 in service fees.
Second issue: our light curtains were sized for the old mechanical press stopping time. Servo presses stop faster, so technically the safety distance could be reduced, but you need to re-validate the whole calculation with the actual measured stopping time, not the catalog spec. Our Sick rep helped us with this — turns out we could move the curtains 80mm closer to the die, which actually improved ergonomics for the operators.
Third — and this one surprised me — the muting function for part ejection. On a mechanical press the muting window is tied to cam angle. On servo you need to define it by slide position, and the safety PLC needs to verify the slide is actually in the correct position window before allowing the mute. We had to add a second independent position feedback (linear encoder) because the safety standard doesn't accept the drive encoder alone for SIL 2.
Budget-wise: plan $30-50K for a full safety retrofit on an older servo press. New installations from Aida/Komatsu/Schuler come with most of this sorted, but verify the documentation package includes the safety validation report — some dealers skip it.