Even More Paper Skidding
By John Grant
Hello John Rhodes (and list readers),
I need to clear up a slight misconception about my theory on the design goal(s) of the Ampico B variable play brake:
On Mon, 29 Jan 96 John Rhodes wrote in Automatic Music List Digest (96.01.28):
>
> From: "John D. Rhodes" <jrhodes@mail.teleport.com>
> To: automatic-music@foxtail.com
> Date: Sat, 27 Jan 1996 13:34:21 +0000
>
> Subject: Paper Skidding (continued)
>
> <Partial snip>
>
> With proper adjustment of the feed-spool brake (*reducing* hold-back
> torque as the roll is fed to maintain a constant web tension -- as
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> succinctly described by John Grant), the Ampico-B should be capable
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> of nearly flawless feeding of even large rolls.
>
>-- John Rhodes
>
Please note: I never said (and in fact do not believe) that the Ampico B variable drag play brake was designed or intended to maintain CONSTANT PAPER TENSION during the playing of a roll. The tension in the paper (what units of measurement would you use for this?) is clearly a very complex "value" that depends on many variables including the friction of the brake itself, the number of tracker bar holes open (or more importantly, closed), the instantaneous strength of the vacuum appearing at the tracker bar, the instantaneous ratio of the supply and takeup spool diameters, the speed-torque characteristics of the roll drive motor, and the friction characteristics of the paper's surface. There may be others, such as temperature, relative humidity, atmospheric pressure, and the amount of change in your pocket, but their affects on the value are probably negligible. The only one of these addressed by the variable drag brake is the spool diameter ratio. The rest of the "dynamic" factors cannot possibly be compensated for in "real time" by a simple, slowly decreasing drag force from the play brake. I can envision our theoretical "web tension" meter bouncing all over the place as the roll plays. A fully compensating drag brake system would need, at the very least, a feedback loop from the stack vacuum level in order to maintain constant paper tension.
The absolute value of tension in the paper is not a problem UNLESS and UNTIL the combined retarding forces on the paper exceed the slip-friction value between the turns of paper on the takeup spool. As soon as that happens, the paper will "slip" until the tightening event INCREASES the tension enough to overcome the combined retarding forces. Intuitively then, the problem would not occur if the retarding forces, a major component of which is from the drag brake, had not risen high enough to cause it.
How then to accomplish this? Make sure there is high paper tension at the beginning of the roll so that there is little or no "room" for tightening between the takeup spool layers. Then as the layers build up and the danger of tightening becomes more probable, slowly REDUCE the paper tension by decreasing the drag brake force. Guess what? This is EXACTLY the way the system operates! In an ordinary wind motor player system this might lead to an increase in tempo due to lower retarding forces, but the speed regulation is so damn good in the B motor I don't think you could measure the effect.
So my engineering "sense" is that the design goal was NOT to maintain constant paper tension, it was to gradually REDUCE the paper tension through out the playing of the roll in an effort to insure that at all times the retarding forces were less than the value that would cause the paper to slip. If anyone can cite anything in the (vintage) technical literature or in Dr. Hickman's writings that allude to "constant paper tension", I would appreciate knowing about it. Even then I would take the position that the problem was perhaps not well understood but serendippedly solved none-the-less.
-John Grant
"Every problem can be solved with the materials in the room."
-Edwin H. Land, inventor of the Polaroid Land Camera |
(Message sent Tue 30 Jan 1996, 09:45:33 GMT, from time zone GMT-0800.) |
|
|
Mechanical
Music Digest™ Archives
