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What is 6000 gf.cm (torque) in terms of KG?

  1. Jul 15, 2009 #1
    Hi guys,

    I bought a high-torque DC motor, that says at the back: 6,000 (gf.cm).

    I am intending to use this motor to power a plinth on a turntable. So the motor will be upright, with its shaft pointing up.

    My question is - What is the maximum weight i can put on this turntable, where the motor
    will still be able to turn? (ie. what does gf.cm equate to in terms of kg?)

    Thanks so much for your help!
     
  2. jcsd
  3. Jul 15, 2009 #2

    brewnog

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    The two aren't related. The torque figure tells you how much of a 'twisting' action the motor can apply, not what load its shaft can support.

    Is the turntable directly connected to the motor? If so, you need to find out the axial (thrust) load capability of the motor. Then, you need to look at how your load is distributed on the turntable. If it's right on the centre, the torque needed to accelerate the turntable will be much less than if the load were metres away from the rotational axis. Think about why your doorhandles aren't on the same side of the door as the hinges.
     
  4. Jul 15, 2009 #3
    Hi Brewnog,

    The motor is connected directly to the plate that turns, but is not bearing the load from on top.

    The motor shaft is attached to the middle of the plate. The plate itself is 30 cm in diameter, and weighs about 2 KG on its own.

    I cant seem to find out any information about the axial loads of this motor. However, i know that the torque is 46,000 g.cm at max. efficiency, and a stall torque of 3,000,000 g.cm.

    Would you be able to tell me 'roughly' how much weight i can still place on top of my 2KG plate and still have the motor turn?
     
  5. Jul 15, 2009 #4
    How weird - i got an email saying that 'nvm' posted a reply, but then it isnt here...

    If it helps: the shaft diameter is 6mm.
     
  6. Jul 15, 2009 #5

    nvn

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    I'm thinking the mass you could place on this turntable could be rather large, especially if you somewhat balance the mass. We could probably assume the ball bearing friction acts at a distance of 2.1 times the shaft diameter, with a coefficient of friction of mu = 0.15. And I guess a typical load capacity for small steel ball bearings on steel could be assumed. The first 6-mm-bore ball bearing set I looked up, e.g., had an axial, uniform (thrust) working capacity of 470 N. Of course, you also would not want to exceed the allowable bending stress of the turntable disk material, but we do not know the disk material nor thickness. Is it just a plain, flat disk? Another way to infer whether the disk bending stress is too high might be if you observe excessive deflection of the disk when you start to load it.
     
    Last edited: Jul 16, 2009
  7. Jul 15, 2009 #6
    Hi nvn,

    The plate is completely flat, made of 4 mm thick steel (like the circular base you see supporting outdoor tables. So its quite thick and sturdy.
    And the load / mass on it will be completely balanced.

    When you mean "the mass you could place on this turntable could be rather large",
    could you tell me 'roughly' how heavy? (i need to know how big i can build my model that i am going to put on top of my plinth)
     
  8. Jul 15, 2009 #7

    nvn

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    designchick: I am thinking I would estimate that well-greased thrust bearings for this turntable would provide a long service life under an approximately-balanced load of up to 15 kg (i.e., 13 kg plus the 2 kg steel turntable disk). However, the steel shaft, itself, if nonthreaded, will only handle a load imbalance (difference), from side to side of the turntable, of 2.5 kg (not to mention, this would be significantly more detrimental to the bearings).

    I should qualify what I envisioned for my above answer, in case I am envisioning the parts in your assembly incorrectly. Based somewhat on your above posts, I assumed the steel turntable disk is supported only by a solid shaft; and I assumed the shaft is axially supported by thrust ball bearings, having a center of contact of diameter 2.1 times the shaft diameter. Notice I did not assume ball bearings (or bearing race) are in contact with the turntable disk. Please let us know if I am misinterpreting.
     
    Last edited: Jul 16, 2009
  9. Jul 15, 2009 #8

    FredGarvin

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    Assuming that you are not applying ANY axial loads directly to the motor shaft (which you should NEVER do), you can work back and absolute best case by taking your stall torque and a perfectly balanced plate and mass and use the kinematic relation
    [tex]\tau = I \alpha[/tex]. That will give you the absolute possible mass that you could never exceed. From there, use a factor of, say .5 to allow for friction and other non-niceties in your system. You'll have to assume your own angular acceleration that you would like in the system.
     
  10. Jul 18, 2009 #9
    Figure mentioned at the back of motor is 6000 gf.cm. It is torque with which motor can run.
    It is grams cms unit.
    It looks very small motor. Now to decide max. weight on turn table, you should confirm turn table dia., weight of it and weight you wish to keep on table. Total torque with this should be within 6000 gf.cm.

    Tom
     
    Last edited by a moderator: Aug 8, 2009
  11. Jul 18, 2009 #10

    nvn

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    designchick: (1) I am confused on whether the typical torque rating is 6000 gf*cm (0.5884 N*m) or 46 000 gf*cm (4.511 N*m). Posts 1 and 3 seem to contradict.

    (2) In addition to the torque value(s), what is the power rating data listed on your motor?

    (3) How many revolutions per minute (rpm) do you typically want your turntable to have during operation?

    (4) See the second paragraph of post 7. I am currently thinking the limiting factor in your assembly might be the component stresses, not the motor power. However, we need more information to answer your question. It would be good if you could post a dimensioned, rough sketch of your assembly. Or provide a very detailed description of the parts, how they are assembled, and accurate dimensions in mm. We are particularly interested in your thrust bearings, their dimensions, how load is transmitted to them, and surrounding parts, dimensions, and connections. Close-up photographs might help, too.
     
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