1. Rest in Peace Paul (Kegger) If you would like to help the family in this time of great sorrow and need, you may donate on their GoFundme page: https://www.gofundme.com/mckechnie-medical-and-funeral-fund?
    Dismiss Notice

Idler wheel size = platter speed?

Discussion in 'Turntables' started by JohnMac, Dec 12, 2011.

  1. mfrench

    mfrench Super Member

    Messages:
    4,252
    Location:
    Friendly Village of Fallbrook
    I'd feel better about the compression theory, if,.. I hadn't spent so much time creating the least amount of contact pressure possible. I can watch the action of the mechanisms, and its not that.
     
  2. pustelniakr

    pustelniakr Silver Miner at Large Subscriber

    Messages:
    6,182
    Location:
    Tucson, AZ
    If anybody would know about idlers & rollers, it would be Mr. Witt here.

    Nice to see you my friend. Its been a long time.

    Enjoy,
    Rich P
     
  3. goldear

    goldear Certifiable Audio Junkie

    Messages:
    8,666
    Location:
    Seattle
    That is incorrect. Platter speed is independent of idler diameter. But the idler speed will vary as a function of diameter though (even though this is irrelevant here).

    Platter speed IS dependent upon the pulley diameter, and the platter diameter though, and I think that this is what you probably meant.
     
  4. tewitt1949

    tewitt1949 New Member

    Messages:
    11
  5. JohnMac

    JohnMac AK Subscriber Subscriber

    Messages:
    5,403
    Location:
    Front Range, Colorado
    EDIT: Just noticed that twitt1949 posted as I was typing this one. His included link clearly explains the answer. Kinda' cool that one of the responders knows something about turntables too.


    Sometimes you ask for the time and they tell you how to build a watch. All very interesting theories though. Thanks for all the input. :yes:

    I think my question was answered early in this thread. Excluding factors such as angle and pressure and using a single idler between the motor spindle and the platter there is no reason, geometrically speaking, that varying diameters of the idler will change the platter speed.

    In this particular case I'm working on a Dual 1009. After having the idler re-rubbered I noticed that the platter ran slow. My first inclination was that the it must have something to do with the idler size.

    However In this application the angle can't change and do to the way the idler arm pivots I don't think pressure would be increased unless the idler was so greatly over sized that it didn't allow the arm any room to pivot thus causing the idler to be pinched between the spindle and the platter.

    The platter parts have been cleaned and lubed and spins freely as does the idler shaft so drag is not the issue.

    Plug time. Terry Witt @ http://www.terrysrubberrollers.com/ does exceptional work at very reasonable prices and in a timely manner. No affiliation other than as a satisfied customer. :thmbsp:
     
    Last edited: Dec 16, 2011
  6. tewitt1949

    tewitt1949 New Member

    Messages:
    11
    Thereoetically you can eliminate the idler and have the motor shaft run directly on the platter and it should run at the correct speed.
     
    Last edited: Dec 16, 2011
  7. Grainger49

    Grainger49 Old Fart

    Messages:
    6,730
    Location:
    Knoxville, TN
    It interests me that the OP said his question was answered in post #9 and there are 37 more posts after that.

    I really do understand this confusion. I had it once too, not that long ago. It just takes some visual reasoning to get it through a thick skull like mine.
     
  8. JohnMac

    JohnMac AK Subscriber Subscriber

    Messages:
    5,403
    Location:
    Front Range, Colorado
    Well, the continued discussion does keep it interesting. Some of the points made are certainly valid in the applications that they referred to.

    I had to sit down and draw three circles and imagine them turning to reason out my conclusion.:confused:
     
    Last edited: Dec 16, 2011
  9. Doug G.

    Doug G. Addicted Member

    Messages:
    6,755
    Location:
    Rochester, Minnesota
    About the only two considerations which will cause a difference in speed with different idlers is a difference in slippage between the two or different motor bearing loading which causes the motor to run faster or slower thus: changing the platter speed.

    Since the new idler causes a slower speed, it is unlikely that it is the slippage factor as you would expect the new idler to slip less than the old.

    The added friction with the new idler will cause it to "jam" between the motor pulley and platter rim more effectively causing a greater load on the motor shaft/bearings, slowing the motor down.

    A similar thing happens when you replace a belt in a belt-driven turntable, although it is caused more from the belt being shorter, but it still loads the motor bearings more and causes the motor to run slower and thus, the platter. I have always noticed this on my BIC turntables.

    And yes, let's not complain or otherwise denigrate a thread which has to do with honest-to-goodness issues for playing records while some meaningless or trivial threads (What record are you listening to?) extend to a ridiculous number of pages. Sorry Grainger, I'm not singling you out.

    Doug
     
    Last edited: Dec 16, 2011
  10. JohnMac

    JohnMac AK Subscriber Subscriber

    Messages:
    5,403
    Location:
    Front Range, Colorado
    Hey Doug. While you're here and at the risk of going off topic a bit, what may cause my 1009 to a a little slow? I say a little because with the pitch control all the way to the + side it seems to run at the correct speed but that leaves no room for adjustment. My 1019 runs at the correct speed with the pitch control near the center.
     
  11. jancumps

    jancumps iRock

    Messages:
    4,483
    Location:
    Belgium
    But you would hear the satanic messages embedded in the records, 'cause it's turning the wrong way round.
     
  12. Doug G.

    Doug G. Addicted Member

    Messages:
    6,755
    Location:
    Rochester, Minnesota
    John, getting the platter to turn at the correct speed with the pitch control in the center is just a matter of adjusting the height of the idler on the motor pulley since the pulley is tapered to drive the idler at different speeds.

    If it's like the 1019, there is a little locknut underneath the idler you loosen and then turn the slot at the top of the idler shaft which moves the idler up and down.

    Once you have the idler in the correct vertical position (the correct speed with the pitch control at center), tighten the locknut to retain the position.

    Doug
     
  13. JohnMac

    JohnMac AK Subscriber Subscriber

    Messages:
    5,403
    Location:
    Front Range, Colorado
    Thanks Doug. I did see that slot. In fact I once tried to remove an idler by turning that before I realized that there is a retainer that holds it on.
     
  14. Doug G.

    Doug G. Addicted Member

    Messages:
    6,755
    Location:
    Rochester, Minnesota
    I just thought of another reason a new idler may cause a slower platter speed too.

    If the old idler is worn and has a wider tread around the rim (from top to bottom), the lower edge of the idler will be exposed to a part of the tapered pulley which is bigger around and thus, will be driven faster than the new idler which is probably narrower top to bottom and will be exposed to a smaller diameter on the pulley and thus, driven slower.

    Nicht war?

    Doug
     
  15. JohnMac

    JohnMac AK Subscriber Subscriber

    Messages:
    5,403
    Location:
    Front Range, Colorado
    Makes sense. I just adjusted the height of the idler, allowing it to contact a wider part of the spindle, thus turn faster. I took it off and on 5 times to get the correct pitch when the adjustment control is set dead center. It takes VERY little adjustment to make a big difference.
     
  16. Doug G.

    Doug G. Addicted Member

    Messages:
    6,755
    Location:
    Rochester, Minnesota
    Nice to get it back to original though, right?

    I remember a similar sequence when I adjusted my 1019 after putting a new idler on it several years ago now.

    We're only talking about a small percentage of change but it relates to a fairly large perceptual change to the human ear.

    Doug
     
  17. JohnMac

    JohnMac AK Subscriber Subscriber

    Messages:
    5,403
    Location:
    Front Range, Colorado
    Thanks again for the help. I learn something new with each one I work on.

    John
     
  18. ghazzer

    ghazzer Sansui addict Subscriber

    Messages:
    2,552
    Location:
    Sykesville, MD
    My thanks as well! I have two 1019s and will now have to take a closer look at the idler wheels condition and edge profile.

    Good thread!
    .
     
  19. bonejob

    bonejob New Member

    Messages:
    25
    Idler diameter has NO effect on platter speed! Think of the motor and platter combo as a reduction gear, because essentially, that is EXACTLY what it is.

    It is related to the RATIO of the CIRCUMFERENCE of the motor drive pulley to the CIRCUMFERENCE of the inner surface of the platter - the inside circular surface where the idler makes contact.

    EXAMPLE: Common A/C motors - including the kind used in most idler-wheel turntables - typically run at 1800 RPM. ...<brief side rant below>

    I learned to my utter shock that to most people like my nephew (born in 1989) an LP record may as well be a stone tablet or a papyrus scroll for all they know about what they are - or were, let alone what their rotational speed was supposed to be, or even what they looked like up close. Showing him an LP record was like a trip to the Smithsonian, as far as he was concerned. Heck, people like my nephew barely remember what a COMPACT DISC is, because they don't own any. CD's are like Depends - they're both for OLD people; It's all low-fi, low bit-rate compressed files crammed onto key drives and iPods for him and the people he hangs with.

    Everyone else - besides people like my nephew above - remembers that the desired rotational speed of an LP record is 33⅓ RPM - or in more calculator-friendly decimal form - 33.333333333333... until the universe falls in on itself or whatever is out there that passes for God pulls the plug in disgust over His botched creation.

    In order to transfer that torque and RPM from the 1800 RPM motor to the platter, all that is needed is a method of transferring that motion and torque from one spinning thing that is under power - the motor - to the other round thing that is normally static but which we want to move too - like the platter. If speed and torque transfer is all you want, theoretically, all you need to do is mount the motor to the side, attach a platter the same size as the record platter to the motor shaft, then grind gear teeth into both the motor platter and the record platter. Finally, slide the two things together until the teeth mesh, turn it on, and "Voilà!" Your platter is spinning.

    The trouble is, the motor is spinning at 1800 RPM. And so is the record platter! Since both are the same circumference, and they are mechanically linked, there is a 1:1 correspondence between the angular velocities - or, if you will, rotational speed - of each...NOT ACCEPTABLE! So, what do you do? To reduce the spin of the record platter, you must reduce the diameter of the motor-driven gear, so that its circumference covers less linear distance per rotation than the record platter.

    If you want the record platter disc to spin SLOWER, you must reduce the diameter of the motor -driven disc. If the record platter is 12" in diameter, its circumference is therefore 37.7" (actually, it's 37.699111843077518861551720599354", but I rounded up to nearest hundredth - so SHOOT me) - based on the formula for calculating circumference from the diameter:

    CIRCUMFERENCE = Π(or "Pi" or 3.1415926535897932384626433832795) X DIAMETER

    Let's reduce the motor shaft-driven disc to HALF that figure, or 18.85" - again rounded to nearest hundredth. In order to achieve that circumference, we must use an algebraic variant of that same formula to calculate the right diameter:

    DIAMETER = 18.85 ÷ Π or...

    Six inches... So, we put the motor disc on our lathe and cut her down. Then, we cut some more teeth into the diameter - ones that match the platter's teeth, we put it all back together and we turn it on...

    RATS!!.... The platter is still spinning too fast! Using a strobe we measure that it is turning at one-half the speed of the motor - 900 RPM. Well, we are least going in the right direction. For every rotation of the motor the turntable platter goes one-half of a rotation - a ratio of 2:1. This happens because with every rotation the motor shaft gear rim travels 18.85". Since the two rotating bodies are locked mechanically together at their rims, the turntable platter gear travels the exact same distance. But since that is precisely HALF the distance around the platter, that means it travels only HALF of one rotation.

    Clearly, while we are going in the right direction, we must go much further to achieve our desired rotational speed of 33⅓ RPM. To make a long story short (or at least a little bit short-ER), in order to transfer motor rotation to the platter while simultaneously reducing the rotational speed from the motor's 1800 RPM to the platter's desired 33⅓ RPM, we must cut the circumference of the motor shaft side to 1/54th the size of the platter side. This is because the reduction ratio between 1800 RPM and 33⅓ RPM is 54:1. So, in order for us to make that ratio happen, we must lathe that motor shaft down so that its circumference is 0.698" - or 0.222222222" in diameter. That way, for every 54 rotations of the motor, the platter will spin exactly ONE. At the motor's 1800 RPM, that gets us - at the platter - what we want - 33⅓ RPM.

    But that doesn't finish the project. We have a reduction gear setup that is getting us the right speed, but a direct gear drive around the perimeter of the platter is hardly practical for a record player - gear noise transmission through the record, stylus, up the tonearm to the electronics is the worst for sure. So, instead of a direct, motor-to-platter drive, we need to isolate the two - transferring the motion and torque from the motor to the platter in such a way as to dampen the vibration and noise of the motor before it gets to the platter.

    There are two ways to do it. One is by means of a rubber belt. This works very well - many insist this is the best way to do it. The other way to isolate the motor from the platter is to use a rubber wheel called an "idler" inserted between the spinning motor and the platter. This rubber "idler" transfers the motion and the torque, but it doesn't contribute to the reduction ratio at all. THAT IS WHY IT IS CALLED AN "IDLER!" If the reduction between the motor and platter is 54:1, inserting a 2-inch-wide idler wheel yields a final ratio of precisely 54:1. If you take it out and substitute it in the mechanism with an idler that is 3-inches wide (assuming you can get it to fit in there properly), the resulting final drive ratio is STILL 54:1!

    Why is this? VISUALIZE!!!.... All three of these pieces - motor, shaft, platter - are in direct mechanical connection. So, LINEAR velocity of any hypothetical point on the motor shaft pulley IS EXACTLY THE SAME as the linear velocity of any given point on the outside of the idler, which in turn IS EXACTLY THE SAME as any arbitrary point on the platter. SO... In terms of ANGULAR velocity, the reduction ratio of the outer two rotating elements remains unchanged whether the "idler" is present or not.

    You can - theoretically - add more idlers between - two more, three more, any number more. And the final ratio remains 54:1.

    Instead of the cave-man set-up I illustrated, real-world idler turntables have motors that drive the platter from INSIDE, not the outside. So the motor transfers power and torque though the idler and finally to the inside of a ring cast/pressed/glued/duct-taped to the bottom of the platter.

    Visualizing all this in terms of circumferences transferring motion and power through direct contact results in calculations which are more complicated than necessary, but I think it aids in visualizing the actual motions involved. The first thing you really need to know is the rotation speed of the motor being used to drive the turntable; it is usually 1800 RPM, but there are exceptions. Then, simply divide the motor RPM by 33.33333333. That will give you the final drive reduction ratio. The second thing you need to establish is the inner diameter of that ring on the bottom of the platter. Depending on the size of the platter itself, it will be 8"-10". WHATEVER it is, the outer diameter of the motor drive pulley MUST be of a size so that the ratio of this inner platter drive ring to the motor shaft pulley is EQUAL TO the ratio of the motor speed to 33⅓ RPM.

    If the ratio of motor speed to 33⅓ is calculated to be 54:1, the ratio of the diameters of the driven ring under the platter AND the outer diameter of the motor pulley MUST ALSO be 54:1 - or the speed will NOT be accurate.

    The idler affects speed potentially only in TWO ways. It could get old and start slipping, so that some of the motion and torque of the motor is lost in its transfer to the platter. OR, if the idler is the wrong size and doesn't fit properly within the space between the motor shaft and the platter drive ring, it could be excessively compressed. Depending on the type of motor and its power supply, this could result in either slowing the platter down from friction, OR actually speeding the platter up because the motor's circuitry is overcompensating for the extra friction by making the motor spin faster.

    I think this step-by-step, "the hip-bone's-connected-to-the-thigh-bone's-connected-to-the-knee-bone's-connected-to-the-ankle-bone" method of breaking it down is what many people need to do. At some point, hopefully there will be an "AHA!" moment and there will be true understanding.

    This whole question demonstrates that even mechanical principles that seem fairly simple and fundamental actually involve many logic steps - sometimes many, MANY of them - for all the dots to thoroughly connect. So, actually, they are not really simple at all!

    :thmbsp::thmbsp::thmbsp::thmbsp:
     
  20. JohnMac

    JohnMac AK Subscriber Subscriber

    Messages:
    5,403
    Location:
    Front Range, Colorado
    "AHA!"

    Good (if not wordy) summation bonejob. I like it :yes:

    J
     

Share This Page