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open reel capstan drive

Discussion in 'All other topics' started by rtrg, Mar 24, 2020.

  1. rtrg

    rtrg Regular member

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    can anyone provide me with a formula I can use to either machine a flywheel, capstan, pulley or use motors, flywheel, capstan from various donar decks to produce a linear tape speed of 3.75 ips? various makers use different dimensions/rpms to produce standard ips speeds. would it be better to get motor,flywheel, pulley, capstan, bushing from a specific model as a matched set? I would prefer to machine my own. if I could know the capstans rpm as it relates to ips speed for a given diameter that would be a good starting point. all advice welcome.
     
  2. wither 1

    wither 1 Member

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    To determine the the diameter of the capstan which produces 3.75 ips, you would need to know the rotation speed. For instance, assume the capstan rotates once per second. If the capstan is .25 inches in diameter, the circumference is pi x diameter or pi x .25 inches. The circumference is .7854 inches (rounded off). Thats' the distance the tape would travel in 1 second. If you want 3.75 inches per second, you would divide 3.75 by .7854. Thats, 4.775 rps. So, you need to know the rps of the capstan to get back to the required diameter. If you had that, call it X. Then the diameter would be 3.75\X times pi.
     
    Last edited: Mar 27, 2020
  3. rtrg

    rtrg Regular member

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    could you reduce that to a formula(s)? with a description of each element? I used a tach on a working deck which read 303 rpm for a 6mm diam capstan which is the typical size .2367 inches I believe. for .25 that would reduce the capstan rpm to something less than 303. in addition to this is the addition drive motor rpm, known or measured, motor pulley, known or measured, flywheel diam, measured carrying the centered capstan being supported by a bushing. the driving components would be machined to any dimension as needed. starting with the capstan and building out from there. if I could find the capstans rpm to yield 3.75 ips on its own, I could than build the flywheel to accomodate a given motor rpm and a pulley of the needed size. does any of this make sense? I can use measuring tools and a tach to measure rpm. any math help would be welcome. I have a speed/diameter calculator on my desktop. all I need is the specific sizes and rpms to plug into it. the flywheel/capstan relationship is unknown to me since the capstan is not driven by a belt but the flywheel is. a simple ratio of 4 to 1 comes to mind between capstan and flywheel. then I would need to find the needed flywheel rpm while the capstan rpm would be 1/4 that. that seems to be a good place to start. as an aside I used the same formula to make my own turntable, motor at 1800 rpm, platter at 12 inches, platter rpm at 33.3, required pulley .222. the center pin size/rpm was not involved. this is similar except that the center pin IS involved. that throws a wrinkle into the calculations I do not fully understand. I do not have access to a drive system I can measure and copy.
     
  4. wither 1

    wither 1 Member

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    D (diameter) = 3.75 inches per second/X (revolutions per second) x pi

    If you divide the RPM (303) by 60 seconds, the rps (revolutions per second), that's 5.05 rps. Using the above formula-

    D= 3.75 ips/5.05 rps x pi

    the result is .236 inches which is 6 mm, consistent with your value.

    If you used a larger capstan, the speed would increase to more than the 3.75 ips.

    I have a Dokorder reel to reel which plays at either 3.75 ips or 7.5 ips. The capstan is .314 inches (7.9756 mm) in diameter. So the capstan is turning at 3.801 rps or 228 rpm for 3.75 ips playback.

    What is Donar?
     

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