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Motor, idler ratio question

  1. Dec 15, 2011 #1
    I'm new to this site so excuse me if I'm posting in the wrong area.
    We are having a discussion about motor, idler pully (yes wheel) ratios and we can't agree on this problem.
    What we have is basically an old record player, The motor shaft is turning a single idler, and this idler is also turning the platten (the part the record sits on). The disagreement is. Will an idler of a different diameter change the platten speed? Some say yes, some say no. We need an expert that can explain the answer so us average minds can understand. Thanks Terry
    Last edited: Dec 16, 2011
  2. jcsd
  3. Dec 16, 2011 #2
    If a pulley is driving something, it's not an idler, it's an output :)

    Pullys work in exactly the same way as gears.

    So in a 2 pully system:
    Input (driving) OD = 1
    Output OD = 2

    The gearing ratio is 1:2
    The output gear will rotate at half the speed of the driving pulley.

    In a 2 Pulley system:
    Input OD = 1
    Output OD = 4

    Ratio = 1:4
    Output turns half the speed of the case above, and quater of the input speed.

    In a three pulley system, with an idler (ie a pully that just spins, and doesn't drive aanything). The idle pulley size is irrelevent to the input and output speeds.
  4. Dec 16, 2011 #3
    I suspect you really meant "idler wheel", not "idler pulley". This is a common setup on low cost phonographs. Often the motor shaft is machined to three different diameters along the exposed length, corresponding to 33-1/3, 45, & 78 rpm. An idler wheel runs against the motor shaft and against the inner diameter of the platter (not platten) rim. By moving the idler wheel up and down (that is, along an axis parallel to the motor and platter spindle axes), the platter speed can be changed.

    You weren't asking about the different motor shaft diameters, though, so ignore that part for the moment. Think about surface speeds. Assuming nothing is slipping, and ignoring such nuances as compression of the rubber of the idler wheel, then the surface speed of the idler will be exactly the same as the surface speed of the motor shaft (at the contact point). Likewise, the surface speed of the platter rim at the contact point with the idler will be exactly the same as the surface speed of the idler (which is also the surface speed of the motor shaft).

    A smaller idler wheel will turn faster (in terms of revolutions per minute) because the path around its perimeter is shorter but its surface speed is constrained to be the same by the lack of slippage. Similarly, a larger idler wheel will experience fewer revolutions per minute because the path around its perimeter is correspondingly longer, but the surface speed is again the same as that of the motor shaft and the platter rim.

    If you do want to change the platter speed without changing the motor speed, you need to introduce a new surface speed into the system. One way to do that is by changing the motor shaft diameter as discussed above. Another way is to couple two surfaces together that have different circumferences. A common way to do this is to have two sections of different diameters on the same piece of material, drive one section and take your output from the other section. Since the two sections have the same rotational speed by definition, and since their circumferences are different, their surface speeds must be different.

    You don't need an expert, you just need to sit back and try thinking about it in different ways.
  5. Dec 16, 2011 #4
    Last edited: Dec 16, 2011
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