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using a 180RPM motor, need reducing gears?

Discussion in 'Motor actuators and drivers' started by steveh2112, Oct 6, 2016.

  1. steveh2112

    steveh2112 Member

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    i got these motors for my project because i've seen many people on this forum use then and they seems like good value for money and they shipped cheap(ish) to my location.
    https://www.motiondynamics.com.au/worm-drive-motor-12v-24v-200w-180-rpm-20nm-torque.html
    so they are 180RPM (no load), probaby about 160RPM loaded, so according to this
    http://www.convertunits.com/from/RPM/to/degree/second
    that's 960 degrees/sec, which seems like a lot.

    i don't know for sure, but i think something like 30-90 degrees/sec would be more appropriate? i know a fighters can do about 360degrees/sec http://forum.keypublishing.com/showthread.php?104320-roll-rate-data-of-jets but i'm not a flighter pilot.

    in any case, 960 degrees/sec seems way too fast and gearing down will give me more torque anyhow. just wondering what other people who use this motors, (like marcusb and maybe noorbeast) use for gearing, if any?

    thanks
    steveh2112, Oct 6, 2016
    #1
  2. noorbeast

    noorbeast VR Tassie Devil Staff Member Moderator Race Director

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    There is a speed envelope for good motion, where you want at least 150+mm/s and even more when using VR: https://www.xsimulator.net/community/faq/speed-needed-for-good-motion.218/

    Fast linear speeds really shine for racing sims, where they can convey a lot of the road surface detail, but the same is true for flight, you just generally don't have as much motion detail other than a runway.

    I use lower ratio gearboxes but that has to do with design decisions around my very compact simulator and a personal preference for not being able to back drive the motors, so I can use my sim un-powered as an office chair when not in use for motion simulation.

    Your design should drive decisions about motors and gear ratios.

    It is also easier if you keep all questions in your build thread rather than start a new thread for different aspects of the same project.
    • Like Like x 1
    noorbeast, Oct 6, 2016
    #2
  3. steveh2112

    steveh2112 Member

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    thanks, i started a new thread because when threads get too long and off topic, they become harder to search. personally, as a user of the forum, i prefer short, focus threads, but fine, i'll just create one on the DIY building forum and ask all the questions there.

    i've read that link and seen 150mm/s several times, but have no idea what that means. 150mm is measured at what point in a pivot chair rig? doesn't it make more sense to talk about degrees/sec?
    steveh2112, Oct 6, 2016
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  4. noorbeast

    noorbeast VR Tassie Devil Staff Member Moderator Race Director

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    The linear speed is often used as historically the performance of things like linear actuators are given that way, independent of a particular design, so when making comparisons it is the natural unit to use.

    By all means do continue the topic but on your build thread in reference to an actual planned design.
    noorbeast, Oct 6, 2016
    #4
  5. steveh2112

    steveh2112 Member

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    ok, i was holding off starting a build thread until i had something interesting to show. i'm more in the research phase at the moment. but i have my racing seat mounted on a UJ and i have my motors, so i guess i can start something soon. just waiting for china post with all the other stuff i ordered.

    i don't really understand how linear speed is a useful measurement in a rotating system, i suppose that linear speed is the speed of the linkage arm between the motor and the chair? if so, and if the linkage is say 40cm distance from the UJ, then 150mm/sec translates to 3.58RPM which is 21 degrees/sec.

    so i just went shopping and got a motorbike front chain sprocket, chain and a 14" wheel. that will give me a gear ratio of about 10:1, so 960 degrees/sec will come down to 96 or so, which should give me about 670mm/sec, so i think that should be spot on.

    also, i was thinking about the forces, that motor is rated 20NM, so with a 10:1 reduction gear that will go to 200NM. since the distance from the axle to the push rod arm on my final drive is about 20cm, i think that means i will have about 200x5 newtons of force on the pushrod = 1000N. is that right? maybe i need to draw a picture to explain?

    anyhow, one newton is the force required to accelerate a one kilogram mass at a rate of one meter per second squared. the rig with me in it will be less than 100kg, but lets say 100kg to keep the math easy, so i think that gives me about 10m/ss right? which is basically 1G.

    but that's assuming the motor is lifting my entire weight, but it isn't really because its pivoting on the UJ, so i expect i'll get a bit more than 1G acceleration

    steve
    Last edited: Oct 7, 2016
    steveh2112, Oct 6, 2016
    #5
  6. steveh2112

    steveh2112 Member

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    i did my first test of the gearing system today and the math seems ok. i'm getting about 160 degrees/sec on the final drive wheel, so depending on the distance i put connect the push rod, i can get about 300mm/sec linear speed at the wheel rim which is 180mm radius.

    so 160 degrees/sec from 960 means i'm getting a 6:1 gear reduction, so i think that gives me a 6x torque boost, so i should get 20x6=120NM torque now. so at a radius of 180mm, i think that means my force is 666N (the number of the beast by the way!)

    hopefully that is enough to get me moving?

    as you can see from the pics, my seat is quite high, 70cm, that's so i can get up to +/- 30 degrees of pitch and roll, i want to see how that feels.

    Attached Files:

    steveh2112, Oct 8, 2016
    #6
  7. noorbeast

    noorbeast VR Tassie Devil Staff Member Moderator Race Director

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    The linear velocity is how fast the motor lever moves for a given Center To Center distance.

    If I understand what you have done it is to take a 180 rpm motor and then further reduce that by 10:1, so in effect the rpm is 18.

    While that does increase the torque it kills the speed for a given CTC lever that you may attach to the wheel. It also increases the mechanical loss.

    So the linear speed for a leaver attached to the motor with a 40mm CTC hole spacing would be 750mm/s.

    The linear speed for the same lever and CTC attached to the shaft of your wheel would be 75mm/s, disregarding mechanical loss, which would be significant.

    You don't get something for nothing in physics.
    noorbeast, Oct 8, 2016
    #7
  8. steveh2112

    steveh2112 Member

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    maybe you didn't see my most recent post, its actually 6:1 ratio (30mm radius on primary, 180mm radius on secondary). so the loaded speed on that motor is 160RPM and if i dive by 6 i get 26.6RPM.

    using this
    http://www.calctool.org/CALC/eng/mechanics/linear_angular
    it says i should get 490mm/sec at 180mm (which is the tangential speed i think), the actually linear speed will fall to zero as it approaches the +/- 90 degree point as is a cosine function. this also has the benefit that i should get more torque as it approaches the +/- 90 degree point, which i think will help overcome the top heavy, loading of the me and the chair.

    anyhow, i measured linear speed at about at 300mm/sec over a range of about 140 degrees, and i get a usable linear range of about 200-250mm which i think will work well in my design.
    Last edited: Oct 8, 2016
    steveh2112, Oct 8, 2016
    #8