Paper "Skidding"
By John Grant
Hi Robbie (and list folk),
Gotcha! The Ampico B play brake actually works the OPPOSITE way from your description, i.e., the greatest retarding force is developed at the BEGINNING of the roll, not the end. The braking pneumatic is under constant power (during play) through a #55 constriction. The needle valve is teed into the pneumatic and admits a continuously increasing amount of atmospheric leakage into the pneumatic as the supply spool diameter decreases, thereby decreasing the pneumatic's braking force. This is from the service manual but you can perform a simple confirming test: Put a suction tube on the needle valve connection at the back of the drawer, draw on it and depress the paper follower lever to the bottom of the drawer. This is the condition when a full supply spool is present. Notice that there is almost no leakage or flow through the needle valve, therefore, the pneumatic will develop it's maximum (retarding) force. Now, begin to allow the paper follower to rise and notice that the flow through the needle valve begins to increase, thereby decreasing the pneumatic's power.
Think about it this way: We can all agree (I think) that paper "skidding" occurs when high paper retarding forces (mostly due to the vacuum at the tracker bar) bring the paper to a halt until the loose turns of paper on the takeup spool tighten enough to overcome the retarding force. ADDITIONAL retarding force (from the play brake) toward the end of the roll is just what you DON'T need!
There have been many discussions in past years in the AMICA bulletin and similar publications. I seem to recall someone (whom I do not recall) advocating "lightly" sanding the edges of the paper with fine sandpaper to give it some "tooth". I have never personally tried this, but it makes engineering sense to me, although it sounds fairly labor intensive. Probably best to reserve the technique to recuts, which, because of the slickness of many modern papers being used for the purpose, seem to exhibit the problem more often anyhow.
Regards to all. -John Grant
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(Message sent Mon 22 Jan 1996, 05:36:55 GMT, from time zone GMT-0800.) |
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