The subject of acceptance criteria for machines is worthy enough that it deserves an entire chapter in a book. See the Web Machine Buying Guide (Destech, 2011) for details. Still, we can capture a few key ideas here. You might encounter this challenge if you help write an RFQ (Request For Quote) during the early part of the buying process. There are three general ways an RFQ and subsequent purchasing contract can go astray.
The first error is to not specify something essential. Examples in the web handling arena include product width range, tension range and wound roll sizes. I also recommend that you consider web handling quality items such as roller cylindricity and alignment precisions. Similarly, control precisions should also be considered such as holding tensions (as read by a responsive load cell) to less than 5% variation from a setpoint during steady state and 10% during speed changes. Don’t forget to insist that all readouts are calibrated and calibrateable (procedure given in maintenance manual) in engineering units. If your company strongly prefers a certain brand of drive or PLC controls, then write it down. (The corollary is if there is no strong preference, leave that up to the builder).
I also strongly recommend that you include a couple of carefully selected reliability metrics, such as MTBF (Mean Time Between (electrical) Fault) or minimum consecutive successful roll transfers. There are many things you can’t reasonably specify, even if they are important. An example here is ‘to wind good rolls.’ What the heck is a ‘good’ roll anyway? Will the builder agree to something they don’t have complete control over, such as raw material? However, electrical faults and missed splices is something they have complete or nearly complete control over.
The second error is to specify something that is not essential (such as the brand of PLC if it is not that important to you). Avoid getting into micromanagement of machine design. Examples here might be specifying load cells instead of dancer feedback or electric motor instead of a pneumatic unwind brake. While these choices certainly matter and perhaps even a lot to you, they are already covered in the tension quality spec given above. The goal is to define goals, not the path to that goal. Every item you add to the RFQ will risk limiting the number of builders willing to quote as well as risk increasing the price.
The third error is to not hold back the last check (usually in the 10-20% range) contingent on an acceptable startup. If you don’t do this AND something goes wrong, you have little significant leverage to motivate the builder to correct something. An exception here might be if you are a very large company that buys a lot of equipment from that builder.
Still, even with all diligence in negotiating the best RFQ and purchasing contract, things can go wrong that either could not or were not anticipated. Here defining whether the root cause of the issue was largely machine design or largely product design (such as raw material quality) or largely operation or some combination might be daunting. So, I am going to give you two examples at the ends of the grey scale just for illustration. An example of a largely machine design issue is if the web had a diagonal wrinkle in one section of the machine not related to a process (such as adding heat or water) and perhaps even then. An example of a largely web issue would be if a defect appeared on one side of the web but flipped to the other side if that supply roll were flipped. Situations like these require upping your troubleshooting game.
I have a summary series on this in my Web401 section of my AllWebHandling channel on YouTube as well as two books and a two-day short course.