What does this mean?
According to the dictionary, concentric means “having the same center”. A very simple definition.
What this should mean to a printer is also very simple: If the rollers you are using are not concentric, they are “defective”.
For a roller to be concentric, all parts of that roller must be made and kept true to the center hole. The original center hole of the journal is machined into the core when it is manufactured. The rubber covering is applied and is built to that same center hole, ensuring the concentricity of the journal “to the core” to the rubber.
The importance of concentricity
If you have a roller recovered just once, there is a 20% chance that it is no longer concentric. The odds multiply each time you have that roller recovered. During the recover process, your core is handled and turned on a lathe several times. Each time this is done, there is one more opportunity for the center hole to be “wallowed out”. Each time a “bearing puller” is used there is a chance of nicking or damaging the uniformity of the center hole.
“Defective” (non-concentric) rollers are the underlying cause of many of the problems that create so much frustration for a printer. If you have ever spent the afternoon chasing “dry-ups” you will know exactly what we are talking about. The lack of concentricity in a roller causes uneven pressure, which in turn causes vibration, which creates heat. These “heated spots” then dry up the fluids near them; thus disrupting your ink and water balance. Premature wear and end-pitting on a roller may also be attributed to the fact that the roller lacked concentricity. Also, you cannot hope to set this “defective” roller correctly if with each quarter turn of the roller, the stripe size varies anywhere from 1/16th of an inch to 1/8th of an inch.
When you consider that it only requires approximately .006 inch movement to create a correct stripe, if you set a roller on the low side and it was out of concentricity by as little as .003 inch you would be squeezing the high side .009 inch. If you set the roller on the high side and it is out enough, this will result in winking. Being out by .003 inch is common on used roller cores; we have seen them out as much as .008 inch.
Of course, if your running your press at only 5,000 impressions per hour, you really don't have anything to worry about. However, if you are a high-speed printer, concentricity is essential to you.
The only way
to be sure
is to start out
with a new core