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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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  5. ps355528

    ps355528 Regular member

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    It might be a kind of kebab?
     
  6. ddp

    ddp Moderator Staff Member

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  7. ps355528

    ps355528 Regular member

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    well I have heard of donor, doner and donner.. so why not?.. Not sure I fancy the donor ones... whereas a nice bit of horse testicle........

    I must say tho.. this has been a most educational and informative thread.. seems I forgot all that sort of stuff almost as soon as I walked out of school..
     
  8. rtrg

    rtrg Regular member

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    I misspelled the word DONOR!

    that is a deck I can take parts from. thanks for the info but we are only halfway there. I need to know the capstan motors rpm. if it is a single or 2 speed, 900/1800 is common, the drive pulley diameter in inches or mm/cm single pulley or 2 step, whether the belt is physically moved from step to step, or if the motor rpm is switched between the two rpms AND the flywheel's diameter in ches or mm/cm. in the first case the pulley would be a single unless it is sized for 50/60 hz, in which case only one pulley would be used and the motor would need to be 2 speed. in the second case where the pulley is only one size again the motor would need to be 2 speed. if the pulley is two step for either 50 OR 60 hz than the motor would be a single speed and a replaceable 2 step pulley for the other hz would be swapped out. any of these situations are possible. even a change in drive capstan is possible with a corresponding change in pinch roller to compensate for for the rollers linkage "throw". this is just for your own benefit. just so you get an idea of the various designs your deck uses. if you give me your decks model # such as the 7500, 7100, 1120, 1122, 1140, among others I can find the manual and get a better idea of its design. again the flywheel could be a single or 2 step size though a two step is not as likely as the drive pulley might be, unless it is used for hz conversion.

    with this info I can plug it into your formula or another i have that lets me find the drive,driven diameters/rpms if I know 3 out of the four. I can then use the same formula to find the other values as needed. ig you go to MGIF.com you will find it. known as pulley rpm calculator. or do a search for it or something similar. I have this on my desktop.please provide me with this additional info. you have been very helpful so far. the motor rpm will likely not have the rpm marked. you will need to use a tach, or at least give me the motors label info all of it. I might be able to find the rpm if I knew any of that. of course DOKORDER is long gone so finding this info will not be easy. please keep plugging away. thanks
     
  9. wither 1

    wither 1 Member

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    I have the 1120. Note that it comes in two flavors for the capstan drive. One is 117v/60Hz intended for the USA and the other is a Universal type which you can change the frequency and voltage. On that one, you move the belt to a bigger or smaller diameter part of the of the pulley to change from 60 Hz to 50 Hz.

    The thing weighs 64 pounds and it's sitting high on a shelf. I would need to disconnect all the cables and rotate it and then take the back off. Even then, I think the circuit board sits on top of it so I'm not sure how to get any information about the motor. I'm not going through that gyration. Note that I don't have a tachometer.

    If you go here-

    https://www.bing.com/images/search?...&form=IQFRBA&first=1&scenario=ImageHoverTitle

    the first photo is supposed to be a capstan motor for sale at E-bay. You can enlarge the photo and read some of the identifying information. You can also go to E-bay and ask the seller about it.
     

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