This is from 40 years ago and no longer important, but it still bugs me. I was working as a mechanic on packaging machinery for a large coffee co. These particular machines were what we called pouch pack machines where they packaged 1 1/2-ounce coffee in these little mylar bags. The problem I had was that the bags would get about .006 of an inch shorter with each stroke of the machine. This meant that about every 5 minutes the machine would have to be cut off and then restarted. No waiting period just restart machine and it would go back to normal. I will describe the relevant aspects of the machine. The cradle housed the jaws that would pull down, cut and seal the bags. The cradle had only one stroke and the length was controlled by when the jaws opened and closed. The jaws were controlled by a small cam that operated a micro switch that opened and closed a solenoid that would send and release fluid to a cylinder that actuated the jaws. The problem was somewhat intermittent in that it would come and go usually lasting for about two weeks and then for a few days start acting normal again. I changed micro switches and cylinders but don’t remember replacing the solenoids. Nothing I did seemed to make a difference. Now I am starting to think the solenoid was the culprit all along. Opinions??
I don’t know if the solenoid was a normally open or closed solenoid but if it was a normally open solenoid could the coils in the solenoid start acting like a capacitor and retaining some charge speeding them up a little.
As an auto mechanic in a former life (over 40 years ago) I can relate. I recall one particular customer’s car. It was a 1969 Chrysler New Yorker. Silver grey with a black vinyl top, 4dr hard top. 440 engine with a Holley 4bbl carb. This car was low mileage and in MINT condition. But what a lemon. It always suffered from some kind of running issues. Choke wouldn’t open, causing flooding, or choke wouldn’t close causing hard starting, needle and seat sticking, intermittent rough idling, intermittent flat spots on acceleration. Carb off numerous times, overhauled, ignition system completely gone through, etc, etc, etc. In spite of the obvious issues, it was have stalling & idling problems where there was no apparent cause. Likely a replacement new carb was perhaps going to have to be tried.
The part that was perhaps the most annoying about having trouble fixing this car were the owners were so patient, the type of customers you just wanted to personally go the distance for.
Finally, one day, the car comes in on the back of a tow truck. When there was a vacant stall in the shop I had it pushed into my spot. Upon quick inspection it was evident the timing chain had jumped. I thought to myself FINALLY I wonder if we’ll get to the bottom of what’s been a big issue with this car all along.
Quite the job on this large engine monster with AC and lots of other accessories. Anyway, got the chain and gears replaced and this thing ran LIKE A CHARM! The lady picks up her car, dives it not quite onto the street and the fu*king thing stalls! Upon inspection, the issue was nothing related to the timing chain but the float had decided to sink in the carb and the engine was flooded with gas. Talk about embarrassing.
After I left the mechanic trade the issues with that car bothered me for a while but you just have to let it go. Some machinery just doesn’t do what it’s supposed to do and you have to just accept it.
You probably looked at these things already but I’ll just ask anyway.
Have you eliminated slippage as problem? Another issue is the fluid in in the cylinder, is it getting foamy through use? Perhaps the reset just gives the fluid enough time to settle and operator with the right timing. That might be fixed just by changing the fluid. Is there some kind of ballast for the fluid?
Do you have a way to time the solenoids, or track current draw on them as they operate?
That sounds like it was not PLC controlled (and 40 years ago would be early days for PLC’s). I don’t remember how many mechanical problems I “fixed” in software.
Was the cam slipping? If each time it hit the microswitch it moved back just a tiny, tiny, tiny bit, over time that could add up. Or, since you mentioned the solenoids might act like capacitors, maybe the cam moving back that little tiny bit meant the button was held down a little too long.
If it was acting like a cap, I wonder if a bleeder resistor across the terminals (or some other way to ground the coil between cycles) would have fixed it.
It sounds like the vertical motion of the jaws is continuous, and getting the right length depends on the timing of closing the jaws. Is that correct? If so, was there any dead time at the bottom of the stroke? I.e., if it was a sinusoid motion, there is effectively some dead time, but not if it’s more like a sawtooth motion.
It seems that all kinds of things could contribute to differences in the timing of the jaws actually clamping onto the bag, such as hydraulic pressure or even the voltage supply of the solenoids. I’m not quite sure how to square that with it correcting itself after a restart, but overheating of some components is a possibility.
Something else to consider is that maybe the motor that pulled the film forward was getting faster over time. I assume it wasn’t a stepper motor with a bad controller.
Was it’s (mains, I assume) power having small dips in voltage that would ever so slightly slow the motor and pull slightly less film? Maybe when something else kicks on?
From way out in left field:
Are we to assume the bags were printed correctly? GIGO, after all.
The paper was pulled through by the jaws just gripping it and pulling it. The roll was assisted by a motor hooked up to a tension bar. If there was too much tension on the paper it would activate the motor momentarily.
Yes opening and closing the jaws controlled the length, Depending on the length of the bag there was quite a bit of dead time at the bottom of the stroke, I think a typical bag was about 7" and we could make them as long as 12" if we chose.
There was no slipping on the cams, it never got out of time, it just shortened the duration of time it was closed.
I noticed the fluid in a cylinder point. The .006 measure is very minute. Could a temperature difference of the fluid cause such a variance? As the machine operated the fluid could change temperature and expand/contract. Ambient temperature may have changed enough to cause it too.
Just a guess.
Air is the system is what I had always attributed it to. I always had a hard time keeping the air out of the oil. I had about 15 machines and 2 machines were the biggest problem, they were right next to each other so they were probably purchased at the same time. I always made sure the pick up screens were clean. These machines were built in the early 1960’s through the mid 1970’s. I had up graded all of them to double production and I am sure that there was some aspect I missed during the upgrades. I increased the size of the pump, used higher wattage heaters and one pulley size change on the motor. I did not modify the solenoids or cylinders which may have been under rated after the upgrades. All the other machines worked fine. I still kind of feel it was air in the hydraulics related. The shutdown time was basically instant to correct it, thats what confused me so much.
I don’t think it was temperature as an instant off and on of the switch would correct it and start the cycle over.
Did off/on initiate a re calibration? Setting sensors and such back to zero at the current conditions?
This can correct the machine to current conditions, but then drift out again as conditions change later. This can actually make trouble shooting more difficult.
No sensors to reset, controlled by mechanical cams and micro switches.
How long is ‘instant’? But air in the hydraulics shouldn’t reset that easily. Just a second or two might allow a line to go glunk once and pop out an air bubble though, just depends on what’s going on in there. I hate problems like this. Especially when the equipment doesn’t belong to me and I can’t take a sledge hammer to it.
Did the fluid reservoir have an outlet to ambient pressure with a filter? A vented tank. Maybe negative pressure could build up. A start stop might have set the system in a state that could more quickly equalize the pressure, regaining proper flow rate. A clogged filter might impede flow rate? Or some other portion of the system built up a pressure differential that did not properly equalize until the start stop cycle.
I am enjoying this. But of course want to get my hands on that machine.
I highly suspect the hydraulic area. Pressure is a variable. Valve actuation interval time. Valve operation speed with pressure variations. It gets a lot fuzzier than direct mechanical linkages.
The hydraulic cylinders were designed for a machine with a maximum of about 50 strokes per minute but usually was operating on about 30 when I upgraded the machine it was now operating at about 70 strokes per minute so possibly it wasn’t big enough to handle that I have always felt it was hydraulic problems
Sounds likely to be the problem. Can you run it slower to see if the problem goes away? I would have bet against that working in the first place. I am a pessimist though.
At 70 per minute, there may have been flow rate problems. Did you alter any of the hydraulics to accommodate that? Higher pressure. Larger valve openings. Maybe even larger diameter tubing may have been required. Valves have a flow rate to pressure ratio. But they top out.