Heidelberg Headstop Suction

[Pressmanscooter]

On my CD 102, the little ports across the feed plate (directly in front of the head stops) are not sucking the sheet down. There is air blowing out of them. I know its basically a venturi, and is probably clogged with paper dirt and powder....

Is there an easy way to get them to suck again?

 

[Cornishpastythighs]

Pardon my ignorance If there is air blowing out of them how can they 'suck' the sheet?

 

[Al Ferrari]

Google is your friend:

http://www.google.com/url?sa=t&rct=...xDMuIC7Z_jlHArcXuaS_F-w&bvm=bv.45107431,d.b2I

Al

 

[Pressmanscooter]

I understand how a Venturi works, the question is how do I get it unclogged without completely disassembling my infeed.

 

[Al Ferrari]

The Google link I posted was intended for Cornishpastythighs.

Are all of the venturies equally clogged, or some more than others? Does it seem that at an earlier time there was more pressure than now?

I have never dealt with such a problem. Sorry I am not able to help more.

Al

 

[Mike Herndon]

What year is your CD? If it's a feeder plus you have 3 filters underneath the infeed that need to be cleaned regularly. The older presses do not have the filters and the venturi holes requires some disassembly to clean them. Have you been running a lot of two pass work? The infeed venturi get clogged easily with spray powder.

Also, usually the air for the infeed venturi comes off the main air supply tube underneath the drive side catwalk. Check the KLM modules in your air cabinet to make sure all of your blowers are working. If you have a blower out you can still run but your air volume will be low affecting different things ( like the infeed air venturi).

Mike

 

[Al Ferrari]

Mike,

How does powder from the first pass on the sheets clog the venturi while those sheets pass over them? After all, the venturi are blowing air out.

Al

 

[Deleted member 16349]

Mike,

How does powder from the first pass on the sheets clog the venturi while those sheets pass over them? After all, the venturi are blowing air out.

Al

The exhaust of a venturi is blowing out but the suction section could take in particles. In your google example, the venturi suction section creates a negative static pressure with no air flow since the suction section is connected to a closed system (u tube). But when the suction section is open, the negative pressure results in an air flow into the venturi. That is probably what is causing the clogging.

Possibly the particles are going through the venturi and clogging a filter on the exhaust side. Maybe?

 

[Mike Herndon]

Mike,

How does powder from the first pass on the sheets clog the venturi while those sheets pass over them? After all, the venturi are blowing air out.

Al

Thats a good question, we've seen it happen in a few different ways. We've printed certain jobs where we had to turn the venturi air down or even off to prevent the job from marking on the 2nd pass (we see this sometimes with specialty varnish jobs). At the end of the run going to the next job the venturi air will not be the same, it's as if the holes are clogged. We take a paper clip and turn the air up as high as possible and clean the debris form the holes.

On our feeder plus press the software signals the air to blast out of the venturi three times on start up. This greatly reduces the chance of clogging.

It's interesting because the venturi does blow out. When we rebuild the side guides on our older machines we'll go ahead and remove the feeder plate exposing the venturi channels; the passages are always clogged to some degree.

This is what we see in our shop, it could be different depending on the application.

 

[Mike Herndon]

Ok, I looked through my notes because I remembered there is a specific reason why the venturi at the infeed can get clogged. I asking this question a few years ago when I was touring the demo room in Kennesaw. One of the engineers in laymen's terms explained to me the venturi system on the infeed is not a true venturi per say. Instead it works closer to the Bernoulli's principle where you have flow (paper) and a pressure differential (air). Bernoulli's principle is usually applied to fluids, gases and aviation(lift).

According to my notes from our conversation, the channels in the infeed are designed to create a low pressure when you have a stream of paper essentially sucking the sheet down to the infeed. While the press is running particles/dust etc can be drawn in also but remain somewhat suspended. When the air is shut off the particle/dust can drop into the channels/holes.

Basically the Engineer concluded by telling mw Heidelbergs infeed is not really a venturi, it's called that for simplicity, there's no simple term to completely describe how the air flow works.

I'm sure someone here can explain this concept more, not trying to muddy the waters.

Mike

 

[Al Ferrari]

Perhaps that Wikipedia article on the venturi effect was not the best reference for us in discussing the use of air flow to create a down suction effect for a sheet at the head stops on a printing press. I see that it has introduced confusion because there are tubes mentioned in the Wikipedia and there are tubes directing air blasts at the head stops on the press. But these are very different tubes!

Forget the Wikipedia article and lets think about how "lift" is created on an airplane wing or an air foil. The greater curvature on the upper side of the wing compared to the lower, causes air to flow faster above the wing than below it. The lift is created by push of the higher air pressure of the slower moving air on the lower side of the wing.

In our printing press, the sheet at the head stops is the "wing." But it is flat, not curved. The differential air flow is created artificially by blowing air under the sheet. The press room air above the sheet is moving "slower," thus with greater pressure. This is what forces the sheet down!

Now for the clogging of the air blast tubes, this is just the consequence of dirty press room air. I'll let some one else tackle the 2nd pass problem.

Al

 

[Mike Herndon]

Yes, Al is correct high versus low air pressure is really how the system works. I mistakenly left that out of my post - the pressroom air above the sheet has greater pressure and that is what brings the sheet down.

Interesting the principles behind something that seems so simple.

 

[Alois Senefelder]

Aero - dynamics

Aero - dynamics

Hello fellow Lithographers,


M.A.N. Aero- dynamics in Sheetfed Offset Presses,

hopefully the PDFs will be of interest and value !!!!!


Regards, Alois

 

[Alois Senefelder]

Aero- dynamics part 2

Aero- dynamics part 2

M.A.N. Aero-dynamics part 2




Regards, Alois

 

[Pressmanscooter]

Thanks Mike. You called it, some of the hoses were off of the air bar. The CD is a '92. I looked in the parts book but could not find a good picture of the "venturi". I was thinking about taking off the hoses (one by one) and hooking them to a vacuum pump. Thought maybe I could suck the dirt out (reverse direction). It will be about another year before i'm ready for a side guide rebuild, and I was hoping to get them working before then. And Yes, I do run a lot of 2 sided work.

 

[Al Ferrari]

... In your google example, the venturi suction section creates a negative static pressure with no air flow since the suction section is connected to a closed system (u tube)....

Boy, do I ever regret posting that link to the Wickipedia article. I see that even our own trained engineer got "venturied" (sucked in) by it.

See my 10:05 AM post for a clarification.

Al

 

[junker1984]

Scooter - Given your press is model year 1992, it's possible that you still have the plastic "s" connectors at the blast air ports. These were changed to stainless steel, as the plastic connectors (or nipples) fractured over time and the "venturi" failed. Good Luck, Steve