would it be due to advances in joining materials, like adhesives etc? if you can glue something to the same strength what's easiest- cost effective to manufacture at that point.
Is it Possible that you have never known how to make Seamless Mylar Belts ? Because I do !
During the 1970s, I was
employed as a Technical Assistant to all the Technicians of a Company
who manufactured Flight Data Recording Equipments here in the UK.
Royston Instruments (The Designers) and Royston Engineering (The Manufacturers) made two recorders, one a Maintenance Recorder which recorded 144 sets of data onto one inch tape, and an Aircraft Crash Recorder.
My remit was to keep Engineers and Draughtspersons up to date with developments in all areas which were of interest to them. As a result of that I sourced Catalogues, Periodicals and Documents etc. from Libraries, Manufacturers etc.
One most important to us at that time was a 'Technical Note' I discovered from NASA, referred to the Tape recorders designed for use in the early Meteorological Satellites called Tiros (sometimes called Tyros).
This document as I recall was either stamped Unclassified or Declassified, also my memory told me it was a D**** however ; the only related reference I can find now, is to the following.....
Licht,J.H.,and White,A.,"Polyester-FilmBelts,"Mach.Des.32(22):137-143, October27,1960; also NASA Technical Note D-668,May1961
(Mach.Des. Is for Machine Design).
The strange thing is that I cannot locate either document.
Nevertheless and for those of you who are interested, here are the basic details behind the 'endless belt'.....
Someone had realized that a large washer, proportioned like the letter 'O' cut or stamped out of a sheet of mylar (DuPont's polyester film) if stretched between rollers, does indeed become a seamless belt, albeit of wedge shape.
Further development of the belt by heat treatment was also carried out and barrelled rollers were designed which induced the belts to centralize on the rollers so that the belt did not 'run off'.
I understand mylar belts are available today but none of which is seamless and therefore I assume and logically, they must be inferior.