Loss of Pneumatic Pressure

My Classic is losing pneumatic pressure in unit 2 and then stops after printing 5-10 sheets. What causes this?

 

Short stroke piston is the most common one.

 

I agree with Mark. This post talks a little bit about replacing the short stroke pistons:

http://www.colorprintingforum.com/direct-imaging-offset-press/broken-plastic-must-replaced-6773.html

 

Thank you Paul and Marc. Replacing the short stroke piston solved the problem. Would either of you know of a reputable QMDI technician that can help me recondition and adjust general parts on my press?

 

If you call your service coordinator here, there are two very good technicians in your region who are located in the Chicago area.

 

measuring short stroke piston

Paul Cavanaugh et al. I replaced a leaking short stroke piston on a QMDI and did my best to measure the length from the inside of the mounting hole to the outside of the drive pin with the cylinder extended using a digital caliper to within about 0.1 mm. The problem is that although my new piston stroke is 10 mm the old one has only about a 9 mm stroke. Apparently the old piston swelled or something. Exactly how should the measurement be made. I did it a few ways to be safe, caliper on the piston from the inside mounting hole to the outside of the pin with the piston extended by hand; pen mark on side plate of press next to actuated arm; and dial indicator on actuated arm under load but none of the previous positions may be right with the old cylinder because of the 9 mm stroke problem. Is the only critical position when the cylinder is extended? Where is the best place to measure or check gear lash to determine if the cylinders are parallel? Thanks for your help.

 

This adjustment is not able to be made with out a custom guage set which was provided by the factory. It consisted of differents sets of feeler guages which were specific to each unit. A special bar was slid through the sideframes in front of the the plate and blanket cylinder and the feelers were used to measure the distance from the bar to the bearers. These measurements were carried out both on impression and off impression. Adjustments were made to the short stroke pistons, long stroke pistons as well as various eccentrics for the blanket and plate cylinder. Along with this adjustment there was also a special electronic tool used to measure the bearer pressure in which a piece of tissue like paper was ran between the bearers while on impression and measured with the optical device.