Paper grippers losing paper mid print

We are printing thinnish adhesive winelabels on a Heidelberg Platen and the paper grippers just lose every second sheet during printing. Needless to say the paper gets destroyed every time. It only happens when we put on the impression, so i am assuming that the impression is pulling the paper out of the gripper.
Surely there must be a way to tighten the gripper or to avoid this from happening. I have only been using the windmill for about 6 months and this is the first time this is happening, but it is also the first time that we are using such thin stock. Please help!!

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sometimes a piece of masking tape inside the gripper will solve the problem.

Your tacky forme is holding the sheet probably , you need then to use your stripper fingers/ Frisket fingers/ grippers or whatever you know them as . The idea for best use is to set the job down with either room in the grip and tail of the sheet or a space in the centre of the form to allow room for frisket finger to fit see page96 of the operators manual . The reason its every alternate sheet may indicate one gripper is holding tighter than the other for some reason test by feeding a sheet into the gripper arm one then the other and stop sheet with gripper pointing cornre to corner of platen pull the sheet from the tail with a gentle wiggle and see if its is too easily released partly or whatever to determine the possible problem twist in arm , bent , setting of release too tight on one arm ? you have to answer the questions as they are asked its not just one possibility !

Thank you for the prompt reply, I will try your advise today and report back.

I’m not sure about a Heidelberg platen, but on a C & P you can stretch rubber bands across the grippers which do a great job of holding the paper to the platen. Just make sure they fall between the printed area and the gauge pins on two sides of the form. The thicker the rubber band, the better.

Hi there, tried the masking tape, no luck. I do not have access to Frisket fingers, so that is not a option.

I slowed everything down and noticed that the paper does not go all the way in when initially gripped. Is the a quick solution to fix this?

your gripper could be bent, there are a few things that can cause this besides the bent gripper. Peter is better at explaining this then me, he must be taking a nap (old guy thing) check to see if the stock hits the gripper before it closes on it, you could also try lowering your pile and see if it grips better, it seems if you are running thick stock ok that it might be a simple adjustment to get thinner stock to run.

Lets try a few checks the east way first then .
clear feeder board and raise to the maximum height with the hand wheel (the point just before it drops half inch)
Now turn the press over by hand until the sucker bar is just off the feed board ,is the sucker bar parallel to the surface of the feed board and is it also parallel with the head board (the bit the blowers seperate the sheets through ) IF IT ISNT THEN YOU NEED TO BEND THE BAR STRAIGHT TILL ITS PARALLEL IN THE TWO PLANES .
If more needed check the grippers them self are both the same distance off the surface of the platen and the distance observed is pretty even down the length of the gripper (This is best done with the grippers pointing corner to corner at an angle across the platen) .
If this still needs more looking loosen the two bolts holding the headboard in place (the one you use to loosen headboard in order to twist it )completely lift the headboard off and make sure no one has put a shim beneath it ,if clear replace make sure no crud gets under it.
level the headboard till the indicator is central and lock the bolts .
Check the headbord is parallel with the gripper when the gripper just settles in the open position at the feeder ,check the sucker bar again ,all must be parallel .Set a pile of decenly cut stock in the feeder and make sure seperator springs etc are not interfering with the raising of a sheet , use a sheet of reasonably heavy stock but not requiring rubber suckers and try to run a couple of sheets ,if your machine has the toggle you use on thin stocks to brake the sheet make sure it is not activated as this imparts a vacuum which drags the sucker bar across the sheet as its taken away (the control says pull lever when running thin stocks or something like that ) .
next .
feed a sheet or two through the machine and when you can time it right stop a sheet in the gripper whilst the vacuum is still holding it ,you will hear it hissing get a pencil and draw a line down the sheet following the gripper as a guide ,do this and mark the gripper and sheet ,do the same with the other gripper ,the drawn lines need to be relatively paralell with sheet edge and relatively similar distances from the edge of the sheet .
watch as you did yourself and look see if the gripper contacts the sheet when the sheet approaches . The sheet must pick up properly at all stages and not be distorted when its fed into the gripper arm .if the sheet approaches cleanly and the gripper closes neatly you should see the sheet is held by a stip the width of the sheet and is held by about 1/8 to 3/16 if there is a greater discrepancy it will need setting to within that range by shimming beneath the headboard .
try to run the sheets through starting with the middle of the sucker bar tilt and alter till it feeds sheets .Does the problem occur at minimum or max tilt ? all through the range ? Need input for further suggestions!
Did the grippers both have a good grasp on the sheet when checked ?

the problem was that the arms where not straight. after some bending it all works fine now. thanks for all the advise

It’s really good that Peter’s advice worked, thank you for posting -sadly this very likeable talented knowledgeable printer died more than a couple of years ago aged only 53, and he is still much missed.