A Machine Problem I Never Solved

Almost 30 years ago I worked for a large coffee company repairing the coffee packaging machinery. I took some pride in making sure the machines were always capable of running efficiently at top speed with few breakdowns. I had one somewhat intermittent problem that I never could solve and if a doper can solve this I will go to my grave a little bit happier.
These particular machines were the old style pouch pack machines packaging the 1.5 oz mylar coffee bags. A very simple low tec machine that used a cam operated carriage with heated jaws that would close at the top pulling the paper down while sealing the bag and cutting off the bag right below it at the same time. The length of the bag was controlled by small cams that controlled the durration of time a micro switch would remain closed allowing hydraulic fluid to close the jaws on the top and open them on the bottom by use of an electric solenoid.
The problem was upon start up the bags would be the normal length they were adjusted to but with each stroke they might loose about 1/200 of an inch in length, after about 10 min the bags would get so short that the machine would have to be shut off and restarted. No waiting neccessary, just shut off and restart and bags returned to normal length.
No sign of air in the hydraulic system, first suspect was faulty micro switches which were changed, sometimes this seemed to help leading me to believe it was micro switch related, other times it seemd it made no difference.
Any questions or clues?

My first guess would be that the feed rollers that fed the roll of stock into the sealing jaws was slipping.
Or, an idler driving the timing cam was slipping.
But without seeing it in action, it’s just a guess.

A progressive 1/200th is a big discrepancy for a mass production machinery item. If you let the machine run for a while longer would it settle on a size eventually or would it slowly but surely make a zero size bag (which would seem impossible).

If there is a point that it would settle at maybe a slightly larger starting bag would allow you to make longer uninterrupted runs.
For example desired bag size 4"

example: after 200 bags I now have a 3" bag but it will keep making 3" bags indefinitely. If so there is a design flaw in the tooling. Does the problem still occur if you are only feeding bags, but not product? Perhaps adjusting the tooling to a larger size spooling up the packaging, letting it run for a few min to settle to the desired size, then start feeding product?

This may be an unacceptable waste, but if you are losing product to this already, its probably better.
Could there be some kind of thermal expansion in the tooling?

We would actually start the bag a bit long and run it a bit short for a slightly longer run but it seemd to not settle in at any point. You could visually see the jaws closing later as if I adjusted for a shorter bag so I know it was jaw or jaw control related. What always through me was I could shut down and immediately start back up. We had about 15 of these machines that were all close to identical but purchased in lots of 2 or 3 a few years apart. Their seemed to be one group of 3 machies that experienced this problem the most often.
One off the wall theory I had years after the fact was that the 1/4" rod holding the cams for the micro switches might be flexing outward from centrifical force and then shutting it down allowed it to return to its starting position, but that is really an off the wall guess.

After 30 years – Need answer fast?

No, really don’t need answer at all, I have just been impressed with some of the answers I have seen on here and this particular problem had kind of bugged me over the years. I have been a mechanic of one kind or another most of my life and usually given enough time I was able to solve most all problems. This problem I had plenty of time to solve and just got to a point I couldn’t figure what else to eliminate.

Was the mechanism driven by a single motor that was then split out by gears and shafts to perform the different function or were the cams driven by a separate motor that could have drifted out of spec speedwise that would have affected the cutoff timing?

A single motor controlled the up and down of the main carriage and it was not adjustable or variable except speed.
The hydraulic pump had it’s own motor and only fed the jaws that opened and closed and were carried by the carriage assembly.
A small motor controlled the 1/4" shaft that held the on/off/duration cams for the oil solenoids.
Your question does bring up an interesting point however. Their were also a pair of foam jaws below the gripping and sealing jaws that pushed out the excess air before the bag was sealed. The timing of the foam jaws seemd to remain in sinc if I remember correctly or it would have pushed coffee into the seam. The foam jaws were air controlled and worked off the same cam shaft as the jaws, using a different cam. All the cams actuated micro switches.

Underpants gnomes moonlighting for spare cash.

You should have been able to hear or measure the difference in frequency of the jaws closing if the rate of operation changed. If it didn’t then it would seem to be something in the feed mechanism for the bag material. Was that material tensioned in some way before entering the mechanism or on the roll? Did it use a wheel for measuring the feed rate?

ETA: Oh yeah, was the amount of coffee in each bag remaining the same?

The rolls of mylar were threaded through a series of rollers to establish tension. The feed was simply however much the jaws grabbed and pulled down. For instance, the carriage had about a 12" stroke non- adjustable, the jaws within the carriage could be adjust to make a bag from about 10" long down to about 3" long depending on when the jaws closed and opened. The durration of this period was determined by the micro switch cams and the jaws were hydraulic.

This is kinda what I was wondering, whats the minimum. What stops it from being 2.5" Whatever that is, make it so the new minimum size is 4" so even if you were hoping for 4.25 but 4 was within spec, you could crank out product indefinitely.

Okay, so is the pull down by the jaws hydraulic? In that case it sounds like insufficient flow to the inlet or outlet on a hydraulic piston, shortening the stroke a little with each cycle. Turning it off allowed time for the fluids to get back to an equilibrium. Though I’m not sure what was controlling the microswitches. Was the switch function to start the seal and cut once the pull down was complete? Something had to go through a full range of motion to do that.

The micro switches were all controlled by one camshaft. The micro switches energised a solenoid for the oil to flow through to the jaws. The jaws were heated and would seal on the down stroke, the cutter engaged from the same oil flowing into the jaws. Each jaw had a sperate single piston to push them together. The switches all had a n/o and n/c function so one cam and switch would control opening and closing of the jaws. Keep in mind the jaws were located inside the carriage assembly.
Looking back, I believe the tensioner assembly on the guilty machines may have been slightly different than some of the older models and the paper threaded through a slightly different roller configuration.

May or may not be relevent, and probably not. But was the system ran by a programmable logic controller or just relays and pistons controlled by the cam triggered switches?

Also, do you remember the make/model so we can check for pics/videos on the web.

Let me see if I can find the machine on line, I don’t remember the model or manufactuer. They were called pouch pack machines. The newer ones had some basic logic used in the filler mechanism, but not the bag itself.