Capstan questions

[Chill315 at aol.com]

I always wondered what the tolerances were for the sizes of a  capstan.  
Reason for the question.
 
I was remembering that on a VR-1200 the same tape played on two different  
machines would show a positive or negative capstan error.  Then we time  
coded the same tape on the two machines and there was a difference in the final 
 time code number by several seconds.  Thus there had to be a small  
difference over the 32 minutes of tape.  
 
Now capstan sizes.  The VR series used basically the same size  capstan.  
As well as the AVR-2.  the AVR-1, ACR-25 used the vacuum  capstan that was 
substantially larger.  As I never inspected an AVR-3, I  can only look at the 
pictures and surmise that it was similar to the AVR-1  capstan.  What were 
the tolerances?  Were the vacuum capstans a  variable that enters into the 
equation?  I only worked with an ACR so do  not have the experience with the 
AVR-1.
 
Does anyone have insight for this set of questions?
 
Then the last question is we all know that the tape does have an  abrasive 
quality to it.  Being that the back side touches the capstan and  front side 
touches the roller, the tape would not wear down the  capstan like an audio 
head or guide.  Yet I have seen a number of capstans  that show a polished 
portion that could be taken for wear.  Has anyone ever  had to replace a 
capstan due to excessive wear?
 
Chris Hill
WA8IGN
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