Idler wheel size = platter speed?

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

  1. Nat

    Nat AK Subscriber Subscriber

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    Oh -- I didn't know that you were not talking of the Bogen badged Lencos -- since Bogen used Bogen Presto as their name for a while, or seemed to, I thought you were just refering to a Lenco. But I think the clue to your unusual results are in your unusual set up. I suspect that the narrow rim of the Lenco idler really is too small a contact area for your set up, and as it wears, its behavior changes, not because of diameter, but because of something to do with friction and surface area. In fact, the wearing would suggest exactly that -- I think most idlers suffer from glazing and flat spots, but not from actual wear, so something unusual is going on.
    I don't mean to be contentious -- I'm just guessing on the basis of a very small amount of information.
    But the general argument that idler diameter doesn't matter seems unassailable to me -- for what ever reason, your situation is a different issue.
     
  2. mfrench

    mfrench AK Subscriber Subscriber

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    There is no unusual wear. I've used only enough spring force to cause the wheel to hook. In fact, the spring is in a nearly closed position when it hooks up. Like a Lenco, the wheel is drawn into the platter, and held there by the rotation of the platter. The spring just draws the wheel into the right spot, and it hooks.
    This deck is a torque monster, nearly twisting a finger off when I was up under the platter feeling around for vibrations. The rim caught my finger, drew it into the top plate, twisting it over, and wrenching and bruising the finger at the mid-joint. The skinny knife-edged wheel and motor provides massive torque.
    My wife heard it and my groaning, and couldn't believe I was foolish enough to have had my finger in there in the first place ::chuckles::
    It won't happen again.
     
  3. Pio1980

    Pio1980 AK Member Subscriber

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    I suspect the possibility of a "scrubbing" loss (sort of like the creep scrub loss of a resilient belt drive) with the blunt wheel affecting the speed since it otherwise shouldn't affect the drive ratio.
     
  4. MelodyMaster

    MelodyMaster 78s galore.

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    That's the edge crawl I mentioned, like a car's tire going partly sideways on the road; it still grips but transfer is lost to friciton and heat. There is an actual slight ratio differential between the thread and road..
     
  5. JohnMac

    JohnMac AK Subscriber Subscriber

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    Now we're talking about other variables that I hadn't considered such as angles, infinitely variable speeds and systems with more than one idler.

    Sorry, I should have stated in my original post that I'm referring specifically to Dual tables with one idler. In this case, excluding the variables that don't apply, it should not be debatable. Two newly re-rubbered idlers of different diameters would turn the platter at the same speed. The idler is just along for the ride.

    Like Doug G said: "Didn't make a cotton pickin' bit of difference".
     
    Last edited: Dec 12, 2011
  6. gadget73

    gadget73 junk junkie Subscriber

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    Single wheels won't affect the run speed. Double decker wheels do. The only reason you'd see a speed change is due to slippage or an out of round wheel but that would present more as inconsistant speed than slower or faster.

    and thats the long and short of it.
     
  7. Doug G.

    Doug G. Addicted Member

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    Eggsacally!

    Let's get one thing straight. With a simple idler, it is impossible for a diameter (and hence, circumference) difference to cause a platter speed difference.

    This is because the ratio relationship of the motor pulley circumference and platter rim circumference stays the same regardless of the idler circumference. That's why it's called an idler.

    If there IS a speed change, it is due to other factors.

    :thmbsp:

    Doug
     
  8. mfrench

    mfrench AK Subscriber Subscriber

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    A constant:
    Both wheels operated off the same idler arm, same motor mounting chassis, same axle.
    Some background to that constant:
    Eight idler arms were made ranging from .70" between the pivot and the axle, to .80" as the widest idler arm spread. The pivot and the axle mounting holes were drilled very accurately, with the drill penetrating the X on top of the idler arm, and the X on the underside; squarely aligned.
    An obsessive amount of time spent determining the "perfect" distance between holes (at least as accurate as my garage level gear allows for in fabricating).
    At .72" there is no hook up of the wheel. At .73" there is full hook up of the wheel. At .75" rumble is induced. At .77", the idler hits the platter, but not the capstan. So, we're literally dealing with single point hundredths of an inch between make and break, on the idler arm. Both wheels work the same within these dimensions.
    By eye, I look at the profile of the idler wheel and see that it is square and parallel to the platter. As you move your line of sight up and down, from multiple angles, the wheel reveals itself evenly to the bottom edge of the profile; its "square and plumb" to the platter, and its mounted to the motor mount chassis (meaning square and plumb to the motor chassis).
    So, I'm very confident that its not scrubbing, as the wheel is plumb and squarely mounted. It tracks very straight and true. The wear pattern on the platter is the same width as the wheel, no chatter marks, and no audible scrubbing via speaker playback. In fact, my Lencos need regular cleaning of the platter and show more wheel rubber transfer than this Presto.
    Given the multitude of hours obsessing over these items in the build, I'm quite confident in suggesting that the mounting of the wheel isn't causing scrubbing.
    Why then would there be such a difference in speeds, given the small variance between wheels?
    I wish I could play this out another step, and give the real diameter measures, but I'm readying to move house, and Sarge, the spousal unit, has limited me to one deck while we're trying to sell the house, to reduce clutter for showings. The other decks, and wheels are in storage at present and not real accessible due to that (too deeply buried).
    If you know a Lenco wheel, they're a knife-edge concept. The only difference is one has a rounded knife-edge, and the other has a sharp knife-edge. The differences were fairly small, but there (I might have them listed at LencoHeaven, but haven't looked).
    If the theory of no difference in speed due to wheel diameter holds true, by what some suggest, why would there be a speed difference, given that both wheels have to operate from the same driveline?
    If one is slightly off angle, and scrubbing (its not), then the other is as well, due to the given constants.
    I'm truly curious and not trying to rile the troops. thanks for tolerating these questions.
     
    Last edited: Dec 12, 2011
  9. Pio1980

    Pio1980 AK Member Subscriber

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    The point of interest in this case is of course that there seems to be a difference tho' there should not. So, what possible reason? The only ratio that should matter is that of the platter's rim (or the contact point under in the case of the Lenco drive) and the motor spindle diameter. With a straight Lenco tangential drive the tapered motor spindle is the variance. The idler's size in any case as the intermediary should not influence speed, all other things being equal.
     
  10. Doug G.

    Doug G. Addicted Member

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    Is one loading the motor more than the other?

    Doug
     
  11. pustelniakr

    pustelniakr Silver Miner at Large Super Mod Subscriber

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    No equations are necessary. Lets look practically. If the motor spindle rotates one turn, X distance of idler surface/edge is driven past the spindle. The same exact amount of idler surface/edge is driven past the platter drive edge. This does not matter the diameter of the idler. If the mechanism would allow, for one rotation of motor spindle would drive X distance of platter drive surface, if the motor spindle was able to drive the platter without slippage. The gear ratio is due to the drive spindle and driven drive surface of the platter. The rotational speed of the idler itself will vary with its own diameter, this will have no effect on teh resulting rotational speed of the driven platter.

    Lets drive with gears. For one rotation motor gear, X idler gear teeth will be driven past the motor gear, and the same number of idler gear teeth will be driven past the platter drive gear.

    Visualize what I am showing you. Now if you look into a transmission, you will see that to change gear ratio with an intermediate gear in the chain, the driven gear on the shaft has to have a different diameter than the resulting drive gear on the "SAME" shaft. Otherwise, you only have simple drive transference, no ratiometric change whatsoever.

    Enjoy,
    Rich P
     
    Last edited: Dec 13, 2011
  12. msabol

    msabol Newbie/Just learning

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    It's not a question of where he grips it. It's a simple matter of weight ratios. A five ounce bird cannot carry a one pound coconut. Right...

    Sorry, couldn't resist. I was thinking in Rich's teeth terms and came upon the analogy of a bicycle. You've got a front (drive) gear, rear (driven) gear, and a chain. An idler is the same as a chain. It really doesn't matter if you add or remove links from the chain, it's interaction with either of the gears remains consistent. It only matters when the chain is aligned with a cog of a different size, at which point the drive/driven ratio changes which changes the driven speed.
     
  13. MelodyMaster

    MelodyMaster 78s galore.

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    I gather that the Lenco in question uses a rim drive idler, not their usual underside contact point.

    However, speaking of Lencos, they do not HAVE to have a tapered drive pulley to affect speed variation or pitch control. My S-41 uses fixed diameters on the pulley. Rough speed adjustment is attained with a screw and locknut moving the idler toward and away from the platter spindle or edge, user pitch adjustment is a spring loaded knob that adjusts the TILT of the idler. The speed of the platter depends upon the contact radius of the idler, the rotational velocity is reduced as you get closer to the spindle; reversing that, DRIVING the platter closer to the spindle means a smaller path has to be travelled for each rotation, thus the platter goes faster..

    In straight line torque transfer, where everything is parallel, what you do with the idler does not affect speed, unless it's out of alignment, HOWEVER bisect any pulley or idler, drive a wheel perpendicularly, and rotational velocity can be changed by varying the drive radius, the size of the arc. In this case the idler is a device to allow adjustment of the driven radius, it itself is still transferring speed one to one.
     
  14. mfrench

    mfrench AK Subscriber Subscriber

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    I shouldn't have brought the Lenco name into this discussion.
    Its not a Lenco, nor is it a Presto anymore. Its a hybrid deck from 1955 with a motor in it from 2009. The only thing about it that is Lenco is the idler wheel, as I like the knife-edge design to minimize the contact patch area.
    To straighten this out: Its a 1955 Presto T18 Pirouette that I took the motor out of because it was worn out and noisy with vibration when it heated up. I tried replacing the original motor with another new one (the DC motor). I then found glaring original design issues that caused unforeseen problems due to its three wheel design. To describe those would be a thread highjack of fairly epic proportions.

    I then took the idler wheel from another deck, now unnamed, and started redesigning the driveline, because I'd invested in a fairly steeply priced motor and controller board, and the wife was looking at me with that "you spent how much, and now you can't use it?" look. So, I decided to make this work without altering the original deck in any way, so that if it didn't work, I could quickly and simply restore it to its original design specs. The only alterations to the original deck were to remove the motor and its three wheels, which are safely stored together in a box. The only Presto contributions at this point are the platter, platter bearing, and top plate.
    Ultimately it works fine, great. I love the deck, and will keep it in its new configuration - it just took time and was a challenge to get it to where its at.
    But I'm still confused by how a minor diameter change can cause speed alterations.

    thanks for the discussion, guys.
     
  15. mfrench

    mfrench AK Subscriber Subscriber

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    Good question, Doug.
    I can make it load the motor with the idler arm lengths from about .76"->.78" With these lengths, the wheel is getting jammed harder between the platter and capstan. This induced rumble in regular running. It also caused a slamming when the drive engaged that would deflect the suspended motor chassis. Once the initial drive torque necessary to turn the platter hit an equilibrium, the motor chassis would return to its normal position. The loading of the wheel/platter/capstan interface was too much though, and i heard rumble.
    So in an effort to reduce that, I started making incrementally smaller idler arms (less distance between the arm pivot and the axle). Once I got below .75", the rumble was greatly reduced. At .73" the startup hookup was inconsistent, and i had to reach under the platter, and pull the wheel into contact. I finally fitted it with a .74" spanned arm, and get an unassisted hookup every time the deck is turned on, and without the rumble.
    Ultimately, I installed more motor than necessary, and the only real alteration that I'd make at this point would be to re-install a motor with less torque, as what I have now could drive factory equipment.
     
    Last edited: Dec 13, 2011
  16. Nat

    Nat AK Subscriber Subscriber

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    All the reference to gears establishes the general point that an intermediate gear or idler shouldn't have any effect on the final speed. But clearly there is an effect, so what mechanism can explain it? One possibility is the difference between a gear and an idler. Idlers can be distorted much more easily than gears.
    Here's my current idea:
    Given that the motor is powerful enough to mash the OP's finger when it caught it between the idler and rim, I suspect its also powerful enough to pull the idler into such firm contact that it compresses the idler against the small diameter of the pulley, stretching the outer edge around the pulley, functionally increasing the circumference. Stretching doesn't happen (or at least not as much) at the rim of the platter since it's a much gentler curve. So the pulley is travelling a longer distance on the idler than the rim does, which makes for a different speed than if both travel the same distance.
    This is an issue for the OP since he is using Lenco's tapered idlers, rather than the Presto's thicker flat edged idler. The taper allows for more compression and therefore more speed change.
    Now this assumes that the more tapered idler is the one with the greater speed change -- I don't know that the OP specified this, and if I've got it reversed, than I guess my idea is wrong.
     
  17. MelodyMaster

    MelodyMaster 78s galore.

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    I only mentioned Lencos because of the unique way that the platter is driven, which DOES allow pitch control which MIGHT be affected by different ilders.

    Now there's an explanation for the idler affecting speed which makes sense, pressure causing a circumferential difference between idler contact points. Thank You.. :bigok:
     
  18. Pio1980

    Pio1980 AK Member Subscriber

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    Aha! we may have ID'ed the 'X' factor at last. The more pinch force the mechanism applies to the resilient idler, the more deformation and it will be greater on the smaller compressive area than the larger. So there would in theory more "pinch" with a broader angle between the input and output side of the idler than with less angle, and changing the size of the idler, all other things remaining, would change that. A larger and broader idler should be less affected by pinch deformation and perhaps more consistent.
     
    Last edited: Dec 13, 2011
  19. tewitt1949

    tewitt1949 New Member

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    I'm still not satisfied as to the diameter of an idler changing the speed of the platten. I can not think of any reason the idler diameter alone would change the speed of the platten. But there is one possable reason, the more pressure that is put on an idler, the more the "V" will compress and distort which will make it harder to roll, which may in turn may actually slow the platten down. Simular to car tires. A car tire with max pressure in it will roll easier than one 1/2 inflated. It's the flexing of the sidewalls that makes it harder to roll.
    Actually I don't know if you could get enough excess pressure to cause the rubber to flex enough to cause the speed problem either.
     
  20. Bob_in_OKC

    Bob_in_OKC Addicted Member

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    This thread ventured off of the original question and stayed off of it. It's a conceptual question that didn't need to be analyzed down to the finer points of wear, indentation, friction, etc. It's just a matter of principle...literally.
     

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