How to get more torque from relatively low-torque stepper motor?

Discussion in 'The Projects Forum' started by milkisgood, May 4, 2008.

  1. milkisgood

    Thread Starter New Member

    May 4, 2008
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    Hello guys. I'm building a robotic arm as a hobby of mine and so far I am just working on the basic movement operations.

    My question is:

    How do I increase the output torque from a low-torque stepper motor?

    My own thoughts are that I should get/install/create some sort of planetary gear train for the stepper motor and make sure it's at a ratio that would at least give me 3-5 times the output torque.

    But here's my problem, I'm not too good with gears. So I would have to ask one of you smart guys how many gears I would need, what ratio/size do they have to be and how do I go along and build it?

    Again, if anyone knows a website that sells a planetary gear kit or something similar that would increase my stepper motor torque, then by all means just throw me a link. It would make my job much easier!!

    Thanks.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Any gearing that would step down the turn ratio will effectively increase torque. A worm gear setup might be easier.

    This is mechanical engineering, so be patient.

    Meanwhile, you could look at the J.W. Berg catalog online. Using a pair of sprocket gears and a chain connecting them is easier than trying to set up direct gearing.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    You would be better off to select a larger stepper motor that has the torque you require.

    Adding gear reduction units will help multiply torque, but have their own losses due to friction; bearing and teeth. You will also have the "slop" (or takeup) to deal with, and loss of precision due to wear over time. There are ways to reduce the slop but it usually involves pre-loading, which causes more torque-stealing friction and resultant increased wear. As Beenthere mentioned, chain drives are an option - so are toothed-belt drives. The toothed-belt drives and pulleys will be quieter than chain drives and require no lubrication. However, you'll still have the friction losses, and some "slop" to deal with.
     
  4. milkisgood

    Thread Starter New Member

    May 4, 2008
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    Thanks for the input. But I have to ask, do you have any links that might make it a bit easier for me?
     
  5. milkisgood

    Thread Starter New Member

    May 4, 2008
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    This is the stepper motor I'm working with:

    [​IMG]

    4-Phase 12V Unipolar Stepper Motor

    Now what would be a good build in terms of gears, to increase the output torque to at least be able to lift 4-6 pounds with this motor. Or should I buy a different stepper motor altogether?

    Again, thanks for any inputs.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    In order to lift a 5-lb weight using a string on a 2-inch diameter spool, you will need to use a gear reduction of approximately 22 to 1; that is a secondary gear that has 22 times as many teeth on it as the existing gear.

    When your stepper is running at 200pps, it has 0.0265 Newton-meters of torque. This converts to 0.234544763 Inch-lbs, or slightly less than 1/4 lb torque at 1 inch radius.
     
    Last edited: May 4, 2008
  7. Externet

    AAC Fanatic!

    Nov 29, 2005
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    I do not get it. Can someone explain please? :confused:

    I believe a motor with a given torque, (force x radius),
    -if fitted with a smaller radius gear, will have a (larger force x smaller radius) = same torque.

    -If fitted with a larger gear, it will have a (smaller force x larger radius) = same torque.

    Cannot see how the torque could be increased, unless the motor is electrically overdriven.
    Miguel
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    We're talking about using a gear/chain/pulley reduction unit.

    If the motor has 1/4 inch-lbs of torque, and has a 10-tooth gear on it, and you have a 100-tooth gear on a secondary shaft that engages the motor's 10-tooth gear, at a radius of 1 inch the secondary gear will have a torque of 2.5 inch-lbs, or 10 times the torque.
     
  9. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Disagree...:confused:

    In your example of 4 ounces inch torque motor, choosing other gear ratios can provide
    -1/2 ounce on a 8 inch diameter gear/chain/pulley
    or
    - 1 ounce on a 4 inch diameter
    or
    - 2 ounces on a 2 inch diameter
    or
    - 4 ounces on a 1 inch diameter
    or
    - 8 ounces on a 1/2 inch diameter
    or
    - 16 ounces on a 1/4 inch diameter

    Where increasing ounces shown are a FORCE, not a TORQUE.

    The multiplication of force x radius is the same for all cases above; the torque is constant and cannot be increased.:(
    Yes, the force can be increased. Yes, the radius can be increased, but not both which is the torque.

    Miguel
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    On a hypothetical motor's shaft that has 1 inch-lbs of torque, and a 10-tooth gear,
    if you meshed a 100-tooth secondary gear to it,
    what would be your inch-lbs of torque on the secondary gear's shaft? ;)
     
  11. thingmaker3

    Retired Moderator

    May 16, 2005
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    More than one gear at a time, my good Externet, more than one gear at a time. Speed is reduced by the gear ratio and torque is increased by the gear ratio.
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    There are some other things you can do.
    1. Use big fat power resistors to reduce the L/R time constant of the motor windings. 100W Big Browns should do the trick, one per phase. OR
    2. Use a bilevel supply so the coil sees about 40V when a phase turns on and a much lower voltage when the current in the coil approaches say 90% of the holding current.
    3. Use half-stepping instead of full-stepping
    4. Ramp the velocity up and down, carefully avoiding both the mechanical resonance and the electrical resonance of the combined stepper/rotor/load system.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Chopper drives help a great deal with increasing torque and top speed. They basically do what Papabravo mentioned in his 2nd point.

    Unipolar stepper motors are very easy to use, which is why they are so popular with hobbyists. Bipolar stepper motors are more difficult to use, because you have to reverse the direction of current flow in each coil. This requires a full H-bridge for each coil. However, there are fully integrated ICs such as the L293 and L298 that have these H-bridges, and they can be driven using an L297 stepper motor driver. You can actually use a unipolar stepper motor in bipolar mode if you have such IC's; you simply don't use the center tap of the two windings.

    As Papabravo suggested, pay attention to your stepper's rated starting PPM, and realize that this is a no-load number. If you add weight or rotational resistance to the motor's shaft, it will lower the starting PPM. Stepper motors generate their highest torque when they are at 0 RPM. As the RPM increases, the available torque decreases in a somewhat linear fashion. Chopper drives will help a great deal to extend the maximum RPM and torque.
     
    Last edited: May 7, 2008
  14. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Thanks for clearing my misconception. Gearing can increase torque, at expense of angular velocity. OK
    Miguel
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    You are welcome :)

    Now while gearing can increase torque, it cannot increase horsepower. That is because torque is merely an applied force, which can be completely static, rather than a measure of expended energy as in horsepower or Watts.
    1 horsepower = 745.69987158227022 Watts (exact) = 550 ft·lbf/s
    The 550 ft·lbf/s means a force sufficient to raise an object weighing 550 lbs 1 foot in 1 second.

    Gear trains, transmissions, etc are vaguely akin to electrical transformers, where you can exchange voltage for current - but something is always lost in the process.
     
  16. thingmaker3

    Retired Moderator

    May 16, 2005
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    Keep in mind also that gearing will give up some of the energy as heat instead of passing it on as kinetic. Some gear trains will sacrifice as much as 50%.
     
  17. Wigi

    New Member

    Aug 1, 2013
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    Actually, the equation that relates motor power and toque is:
    P = w T
    where P is motor power in Watts
    w is angular speed in rads/sec
    T is torque in Nm

    SInce P is constant for a given motor, varying the angular speed (reducing it) increases the Torque.
     
  18. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    You just revived a 5yr old thread, and your answer is not really relevant to the OP's question anyway. (ie "how many gears do I need") :)
     
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