Loss of printing pressure

Has anyone else suffered from sporadic loss of printing pressure on one of the printing units on a heidelberg CD74.Basically sometimes the fourth unit on our press doesnt seem to fire on impression.Anyone else evered had this problem?

 

There are a couple of things that can cause this. First I would check the ensure the impression ram is operating like normal. When going on and off impression this ram should operate in two seperate stages. If you watch the ram it should extend from each end seprately. Just watch another unit and make sure your unit 4 is the same. If this is not happening it could be a loose wire going to the solonoid on the ram, or a bad solonoid. If it is the same also make sure the heads that are screwed into each end of the ram are tight and not working their way out of the ram. Usually this will result in an excess of pressure. Next I would make sure your printing pressure servo motor is working correctly. Its very easy to check the make sure this motor is positioned correctly. Select a sheet thickness of .10 in the printing material screen. Manually put the unit on impression and inch the maching to a position where the blanket cylinder surface and impression cylinder surface are facing each other. Use a long feeler of .35mm (.014 in), this is the gap between the blanket cylinder bearer and impression cylinder "bearer" surface. It should be very close to that. Try these three things.

 

We have had this problem often, its always the ram / piston that doesnt drop to the second stage. The wires leading to the piston tend to go brittle after some time, we replace the wire(s) and sometimes the solonoid as well. As exheidmech has mentioned, compare with another unit, it should also make two sounds when been activated, if it only makes one sound, then its not dropping onto the second stage.

 

ive seen a failing servo motor "HUNT" to try and find its selected setting. if your on a cptronic vintage speedmaster you will know if its "HUNTING" by a flashing readout of the impression setting for that unit. if the servo continues to hunt for the correct setting sometimes a reboot of the press will get it to settle on the selected setting. one thing ive done on servo motors is to drill a little hole at the lowest point of the plastic casing at the bottom of the servo motor. this will allow any oil that has accumulated there to drain out. ive been told by a reliable source that the buildup of oil in this plastic cover will cause this problem on occasion. if it is a hunting problem thats causing your lack of impression on that unit its a pretty good indicator of a motor thats due to fail soon. good luck in finding a solution!!!