Windmill Gripper Dropping Sheets

Hi Everyone,

This has been talked about in a number of posts, so I’ll write about what I’ve already learned from those posts, and where I’m at now.

On my windmill, when running commercial, it prints so-so. It’s never totally on, but it is very close. However, I mostly run thicker stock, and when I run thinner stocks I run into issues. Every other print will be crooked.

http://www.briarpress.org/30705#comment-38093
http://www.briarpress.org/37287

From reading these posts, I stopped the press after it grabbed a sheet and drew a line where the gripper edge meets the paper, once for each gripper. One line was totally parallel with the sheet edge, and the other line was a few degrees crooked.

http://www.briarpress.org/15660

After reading this post, specifically “Is the sheet slipping while in the gripper?” I inched the press along to see what I could see. One gripper grabs the sheet fine, but the other pulls away a little before taking the sheet from the suckers. It’s almost as if it doesn’t have the strength to pull the sheet away, or that the suckers are holding it too tightly. I felt inside the gripper, and cleaned away some dust that was in it, but it didn’t make a noticeable difference.

https://www.youtube.com/watch?v=LF-4xby6syc
http://www.briarpress.org/46524

My issue is very similar to the problem that is in this video, from the post listed above. The only difference is that my press will bring the paper to the gripper, it will close on it, and then it will pull away and the paper will fall so that it is on top of the feed area and gets mashed by the sucker bar on the way down.

This is where I’m at now. I’m going to give it a second try in the morning. This press has more than once gotten better magically overnight. I may have a press repair gnome living in my shop. Let me know what you all think.

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Just a couple of thoughts ! Check your gripper for broken springs, grippers have a thin rubber strip which may have worn away so the gripper is not holding the sheet so well. Also don’t have the back stops on the pile too tight? Hope you get it sorted, windmills are capable of good tight registration.

First what Frank said. You have tried a lot so far, good post by the way a lot of homework. You may want to check the gripper gap to make sure they are open to the same gap as the sheet is being fed in with the suckers. Since one bar works use that gap amount. I was always afraid to try to bend or straighten my gripper bar support. I did loosen my not working bar and shimmed the bar to change it’s point of level at pickup. If the one bar is not level it might explain the grip shift. As the suckers are letting go you have an uneven contact lay point. I had broken springs and when I finally installed new gripper bars found a crack in one of the bars. Replacing my bars solved the problems you are having for me. You might try swapping the bar positions and see if the problem follows the gripper bar itself or where it is mounted. This may help you figure out what to fix.

If, on one of the gripper bars, the long narrow strip that goes across the gap left for the side guide is bent or broken out, that can create a difference. The part across the gap is intended to push slightly upcurved or very thin stock flat into the side guide; this, on one only, may interfere with positioning into the guide on thicker stocks. If both grippers are matched, then the problem is elsewhere. Usually, the actual problem is some fool little thing that all the sage heads have overlooked!
Frank.

Well, as fate would have it, I came in this morning and immediately found the culprit. I started where I left off yesterday, inching the press along and looking to see how the gripper was grabbing the sheet. The first thing I saw was the gripper grabbing the sheet AND the pink suckers I had left on from the previous job together. Took the suckers off, problem immediately stopped.

Now, I’m assuming that that shouldn’t be happening, and that I need to make and adjustment so that doesn’t happen in the future, but I’m just happy to be back to printing. Hard to do two color work when things are amiss.

I will say though, that the gripper that is having troubles must be a little bent. This isn’t my personal press, it’s one at work that I’m setting up to just run photopolymer, so I’m not sure what its history is. I think I’ll set this one up to print well for a particular thickness, photopolymer, boxcar base, and just leave it be and not worry about running commercial on it.

I use the pink suckers also for some card stocks. I found the gripper gap was the main issue. The suckers move the pickup point down and also the true infeed height of the sheet as it feeds into the gripper. Without the sucker it hits the bottom of the sucker shaft as it picks up. With the sucker the sheet is a little lower and hits the under pad area a little differently. I little wider gap to the gripper causing the issue might fix the problem.