Geared motors - I have no idea.

Discussion in 'Hobby Engineering' started by ASIGN_Baz, Feb 3, 2019.

  1. ASIGN_Baz

    ASIGN_Baz "Maker of awesome stuff"

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    G'day OCAUers. I need a bit of help from the experts please.

    RE: Project MkII Colonial Viper.

    I have to drive five aluminium and plywood turbine fans about 600 mm in diameter each, using five separate electric motors, for periods of time. Slow enough that they don't blur beyond visibility and that they don't become dangerous to spectators.

    I'm thinking 60 to 80 rpm would be fast enough to provide the effect, without becoming dangerous.

    I have 240 V mains power plugged into the project, with multiple opportunities to plug in 12V adaptors.

    Problem. A friend has given me some garage door closers. Unfortunately they are all different. Different amps, different shafts, worms and gears. Might just be better off buying a batch of motors all the same.

    Searching Jaycar and the web now. Not really sure what to get though.

    Baz.

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    Last edited: Feb 3, 2019
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  2. OP
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    ASIGN_Baz

    ASIGN_Baz "Maker of awesome stuff"

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    This might do.... shrugs.

    It says high torque, the speed looks good, but will it drive a 600mm aluminium fan without burning out?

    12V Geared motor
     
    Last edited: Feb 3, 2019
  3. psychobunny

    psychobunny Member

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    I'd say 80rpm is probably too fast, 60 might be too.... depends on the effect you are going for. might be a good idea to test the speeds and see how it looks before ordering something
     
  4. OP
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    ASIGN_Baz

    ASIGN_Baz "Maker of awesome stuff"

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    You are probably right. 60RPM might be good enough. Draw a 600mm diameter circle in the air with your finger and keep going every second. That's not too fast I reckon.
     
  5. firey4059

    firey4059 Member

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    Baz, I think you'll find those motors (and maybe gearboxes too) are used as car windscreen wiper motors and also in electric window systems. They would be cheaper and easier to lay your hands on I reckon that garage door components, because you would be able to get them second hand at wreckers, etc.
    Cam.
     
  6. SLATYE

    SLATYE SLATYE, not SLAYTE

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    I think that Jaycar's big geared motors (eg. this) will be fine. At 60RPM the load on the fan will be negligible (it's not going to be moving much air).
     
  7. OP
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    ASIGN_Baz

    ASIGN_Baz "Maker of awesome stuff"

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    Thanks guys. What happens when I cut power on a moving fan on a geared motor? Will it just wind down or will the gears just stop, putting inertial stress on the gearing?

    Baz.
     
  8. mtma

    mtma Member

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    That's referred to as backdriving, and it depends on the type of gearing. Worm gearing isn't back-drivable and damage could arise from the sudden inertial stop. Planetary and parallel shaft gearing is often backdrivable, but at high ratios the slight amounts of drag at the higher speed sections can effectively lock up the high torque low speed shaft.

    Planetary is probably preferable as you won't really know back-drivability of those toy DC contraptions for the most part until you receive them. Alternatively a couple of close fits and some o-rings makes a rudimentary slipper clutch (Ever took apart a 'brain' yoyo?)
     
  9. always.all.in

    always.all.in Member

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    Love ya work.

    When we're limiting rotation to 60 RPM, is that to prevent the fans from distorting/disintegrating, or is it in case a kid pokes a paddle pop stick at it?

    How much kinetic energy would turbines be carrying at design speed?
     
  10. SLATYE

    SLATYE SLATYE, not SLAYTE

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    Lots!

    In a commercial turbine engine the fan blades can be carrying a very large amount of energy. The GE-90 seems to have decent data available. 22 blades in the main fan, each weighing around the 20kg mark, with a fan radius of 1562mm and a rotation speed of 3475RPM (from what I can find).

    If I'm doing the calculations right, this gives an average speed of each blade (ie speed of the middle of the blade) of 284.2m/s. Kinetic energy of the entire fan is then 17.8MJ (assuming the blade is just a flat bit of metal with constant mass distribution - which isn't really valid but is close enough). Disregarding air resistance, this is enough to launch the unfortunate 30kg child who's sticking their fingers in the fan to an altitude of roughly 60km (at an initial speed of about mach 4).

    The HP spools have much smaller blades, but spin at much higher speeds - 10,000+ RPM. They tend to be higher density, as they're built from steel alloys instead of titanium. I would expect they've got somewhat less energy than the big fan.

    The 60RPM proposed here is probably still enough to give someone a cut if the corners aren't chamfered (simply because their hand will slide along the blade as it turns), but I doubt it'd be much more than that.
     
  11. OP
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    ASIGN_Baz

    ASIGN_Baz "Maker of awesome stuff"

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    The fans are 600mm across, with around 30 blades. The aluminium is only 1.6mm thick. I don't want to spin them up too fast in case something goes wrong and they are destroyed. I don't want them to go too fast in the unlikely event that someone puts their hand in there, (very unlikely). But I want them to go fast enough that folks can see the movement, it looks impressive, but doesn't go so fast that the blades become a flat blur and you can't see any movement at all.

    60RPM is about right.

    Baz.
     
  12. mtma

    mtma Member

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    Another way of driving the fan by the way, is to use air coupling to another fan. Obviously your creation will then need to have decent bearings, balancing and relatively low bypass - but then you have some better packaged options like 120mm DC fans that might be beneficial.

    Would probably work well for a 'cooled down' visual. The blades would spin up to a speed relatively slowly, however then stay there unless touched or overwhelmed by external wind.
     
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  13. aXis

    aXis Member

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    You would be amased how much torque a small 60rpm gearmotor has. You could use one about 1/10th of the size of those wiper motors.

    I have personally see these tiny motors here swing some large inertial loads about 50cm diameter: https://www.aliexpress.com/item/30-...-6V-12V-Reduction-Gear-Motor/32901273110.html
    My recommendation is one like this though, I've used them too and they have a better form factor for what you are doing: https://www.aliexpress.com/item/25m...-6v-12v-24v-Low-Speed-RPM-DC/32955601494.html
     
  14. Jazper

    Jazper Member

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    How much weight does it have to drive?

    How much noise?
     
  15. OP
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    ASIGN_Baz

    ASIGN_Baz "Maker of awesome stuff"

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    The larger fans are 1.28kg each. Noise is not an issue as big speakers are providing engine noise. Loud.
     
  16. Jazper

    Jazper Member

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    Thinking arduino speed controlled low speed brushless may be able to drive those at that weight.. direct drive.
     
  17. OP
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    ASIGN_Baz

    ASIGN_Baz "Maker of awesome stuff"

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    Hey Jazper. Do you have an example/link to such?

    Baz.
     
  18. Jazper

    Jazper Member

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    Hey Baz,

    Check this out:
    https://www.instructables.com/id/Control-Brushless-Motor-Using-Arduino/

    Basically you can control any size/shape of brushless with the right ESC and an Arduino, so I would be looking at brushless motors geared (heh) at truggy or off road RC cars for high torque/lower speeds.

    I was looking at doing this for a roomba to convert it to brushless.
     
    Last edited: Feb 6, 2019
  19. Jazper

    Jazper Member

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    Last edited: Feb 6, 2019
  20. psychobunny

    psychobunny Member

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    best option would be to use a one way clutch bearing (sprag clutch) that way when the motor stops the fan can just coast to a stop Looking on fleabay you you are looking at $8 each for some cheapies
     

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