Heidelberg platen (13x18) - Inconsistent gripping

gjt666
on 21 Mar 14 (10:42)

I am seeing deviation in registration from one sheet to the next (i.e. every other sheet). I am running commercial register so it looks to be the position of the sheet as its is picked up by the gripper and taken into place. It’s every other sheet and so it seems to be one of my gripper bars (I have watched closely so I know which one it is).

I bought the press second hand a few weeks ago and have been teaching myself to get it setup. When it arrived it had loads of gunky electrical tape in the grippers, which I cleaned off. Could this be the reason for the difference I am seeing do you think? I am wondering whether the tape was there to improve the grip of the gripper and it is now slipping between being delivered by the suckers and getting into position for impression. It is the right side slipping further out of the gripper, which is what makes me think the slipping is occuring after pickup but before impression.

What do you reckon? Do people generally tape inside the gripper bars to improve grip rather than having (what I have, which is) bare metal.

Thanks in advance for your collective wisdom.

Gary

AnonyMouse
on 21 Mar 14 (10:51)

You might try stopping the press just after pickup and tracing a pencil line along the grippers (and marking “A” and “B” per gripper) to assess if the pickup is consistent before and after impression.

If it is consistent, the gripper may be bent.

Original grippers had a rubber pad, which some folks replace with tape. See the manual for details.

gjt666
on 21 Mar 14 (11:07)

Hi Anonymous

Hmmmmm. Just inching it through and it looks like the right front corner of the sheet is not clearing the underside of “dodgy” gripper bar prior to being gripped. As the sheet hits the underside of the gripper it is knocking it out of position (pushing the right side out of the gripper). Would this indicate it is bent slightly do you think? Any solution beyond buying a replacement?

Thanks
Gary

AnonyMouse
on 21 Mar 14 (11:35)

I would check the front lay standard for squareness first. It’s the vertical plate with the air-blast and fingers.

Another more involved gripper diagnostic is to swap the grippers and see if the issue follows the gripper. If so, it is often easiest to just replace the bad gripper.

gjt666
on 21 Mar 14 (11:36)

…….if I inching it slow enough the corner that catches on the gripper pushes past to arrive in the gripper in the right place (i.e. identical to the other gripper). So it is “just” the position of the bottom of the gripper bar which seems to be the problem. It can only be a fraction of a mm.. Argh. Any ideas?

phase4
on 21 Mar 14 (11:50)

you can mark a sheet when it is grabbed by the gripper an then use the same sheet to check for diference with the other one,also check paralelism using the tympan as guide line.
i have seen operators that has bent them by hand when it is a small differnce in paralelism that is in case youre already thinking of buying a new one and can work .i have also seen some machinist that has taped the grippers with paper tape the on that painters use (works good for me)that can be easiliy cleaned with gasoline between prints or when diecutting adhesive stock and also cheap and fast to replace.

phase4
on 21 Mar 14 (11:50)

gjt666
on 21 Mar 14 (12:08)

Thanks all, back to tinkering to get this sorted. I will check that the front lay is parallel to the gripper, thanks!