Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Gears, Pulleys, & Electric Motor Questions

  1. Jan 12, 2012 #1
    Hi all, brand new to the forums. I'm a student of chemistry so I am very unknowledgeable about mechanical engineering and need some help. Not asking for spoon-feeding.

    What I am trying to accomplish is fairly simple(I hope). I would like an electric DC motor to turn a 1in diameter teflon rotor that is housed in a teflon bearing assembly. Ideally I would like for this to spin at a speed of about 60RPM with say a 2kg load. Sounds simple enough.

    Here's where the inquiries come in. Should I be using a a pulley system, a gear system? Chain and sprocket seems like over-kill.

    The way I understand it, the pulley is the way to go with a V type or flat belt to maintain torque(though there is loss) and ensure the belt lasts a bit. Whereas the o-ring belts are designed for low-torque.

    The only caveat with the pulley system is, sourcing the parts. it is incredibly hard to find a decently large(1" diameter) pulley that will mount to a 3mm(3/16") or 4mm electric motor shaft. I was thinking delrin because they are fairly affordable and the project is for fun on my own budget, however the OD's on these tend to be very small. It is also very hard to find a 1" bore pulley with a relatively small OD that has a screw notch(so it can be fixed to the rotor). Then finding a belt to match both of these pulleys is also incredibly pain-staking. Life would be a lot easier if they made pulley pinions.

    edit - so after some research I have found that they use 'prop adapters' to mount things such as propellers directly to motor shafts for RC helicopters/planes. This should be able to fix a pulley directly to a motor. That helps quite a bit.

    If anyone knows a good place to find affordable pulley's please post some links!

    Or should I scrap the pulley idea and try to source gears? Not sure if gears transfer torque very well or not and struggled to find information on this.

    Sorry to make a long-winded first post, but I am pretty stuck here. Thank you for any suggestions.
    Last edited: Jan 12, 2012
  2. jcsd
  3. Jan 13, 2012 #2
    Many of your better local hardware stores have an inventory of pullies. I'd start there. If they don't have the size you need, ask to borrow the catalog that they ordered their stock from to find something that will be useful to you.
  4. Jan 13, 2012 #3
    Your request sounds fairly straight forward, although I have some concerns. I am assuming this is a one off project, if you could provide more detail I'd be happy to help with design. As with sourcing parts you're on your own there, sorry. First what size and rpm is you motor, and how have you calculated the load? What physically is the load? Is it axial? Is this all mounted on a cart? Is 60 a concrete rpm range? What is the most important factor this mechanical system has on your project? And finally and most important, do you have access to a lathe?

    I'm glad to help I'm good with this stuff, I really just need specifics to give you any kind of advice.
  5. Jan 13, 2012 #4
    Wow I am surprised at the helpfulness and how fast the replies have come in. I'll have to stick around this forum.

    I have attached the design I am essentially following. The picture is a top-down view.

    A is the motor
    B is the wooden support with a bearing slot bored and glued into place(ignore the screw coming in from the bottom to the rod, the bearing will be epoxied into place)
    C is the pulley mounted to the motor 2" in outer-diameter
    D is the pulley 3" in outer diameter 1" in inner diameter that will have a set-screw holding it to the teflon rod
    E is the teflon rod
    F is the bearing

    As you can see on the back of the teflon rod is seated another bearing to keep the rotor from sliding. Before the pulley(viewing left-to-right) you will notice another similar device, however I will be using a bearing with a key-slot to directly fix it to the rod instead.

    The entire assembly will be seated at a slight angle pitching the cone shaped end of the rod slightly down-ward by perhaps 15*.

    The teflon rod will support a glass-flask filled partially with various fluids so the load will vary a bit. The design is for a low-tech low-cost rotary evaporator for those familiar with chemistry. My brother is at school to become a watch-maker and I believe he has access to a lathe.

    I did find some v-belt type pulley's that I think will be appropriate for a fair price.
    Which I believe I can use spacers to fix directly to the motor shaft and use a 1" bore pulley directly for the teflon rod. So that is finally solved.

    I also have found a motor I believe will be suitable but would really appreciate some input, though I know the page lacks some specifications.
    It says it has a torque of 90 N cm which is PLENTY. The design I am borrowing says to use 30 oz in which is about 21.2 N cm. However it does not mention stall-torque or stall-amperage? There is another similar 12V motor running 120 RPM with 60 N cm of torque. Not sure which would be better.. It's no help that I am a complete bozo when it comes to electronics :D. I estimate the load it will have to rotate will be around 1kg-4kg and ideally I want the shaft to be able to rotate at around 50RPM's (+/-)10RPMs a little more or less is completely fine. However I have no idea how much the carved teflon shaft will weigh, or the pulley weight. If this motor probably won't work please let me know. Keep in mind teflon has one of the lowest coefficients of friction for known materials and the teflon rod will be seated in a teflon bearing with silicone greese. So friction of the rotor is not an issue in my eyes.

    However at 100rpm with a pulley ratio of ~3in OD to ~2in OD it should run at about 66 RPM's(which hopefully won't be drastically effected by the load). Which hopefully potentiometer will allow for speed adjustment without straining the gears in the motor(knowing china they are plastic).

    Thanks for any input.

    Attached Files:

    Last edited: Jan 13, 2012
  6. Jan 13, 2012 #5
    Last edited: Jan 13, 2012
  7. Jan 13, 2012 #6
    Teflon has excellent chemical resistance(critical), high temperature resistance, and low friction. I got a 1" OD by 1 foot long PTFE rod for 13 dollars. Granted that is expensive, the cost of a rotory evaporator is far larger.

    What do you mean by a gear motor? Isn't that what I have in the post I posted before this one? I'm truly ignorant.

    edit- thank you for the links!
  8. Jan 16, 2012 #7
    Gear motors have mechanical reduction built in so the whole thing mounts as one piece. Good call pantaz. That defiantly simplifies things, in addition to probably lowering your cost and reducing the size and foot print of your project.
  9. Jan 16, 2012 #8
    well I have ordered the gear motor I linked too under the components section of one of my previous posts. 90N cm will hopefully be enough to drive this thing according to an online calculator it should in theory be able to generate a force of about 9N-10N.The teflon, bearings and such are still awaiting shipment.

    I'll be posting back here if it works or I get stuck on the build. Should be straight forward though. Thanks for the help :).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook