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Connecting a motor to a fan

  1. Apr 18, 2008 #1
    I have to design a power transmission from a motor to a fan. There are no restrictions on what I design.

    I'm not sure whether I should use a direct (flexible) coupling, a universal joint, or a belt drive. I have designed a belt drive before, and from looking at catalogues, it looks like it's slightly more complicated and time-consuming than the other options since I'll need bearings, pulleys, belts, spacers, two shafts, keys, keyways, etc. and I can simply use a taper lock with coupling. The housing for a belt drive will also be slightly more complicated.

    What are the advantages and disadvantages of the different types? I know with belt drives is there is a jam the belt will slip, so there is a safety element in choosing it.

    What would you recommend? I'm leaning towards either a coupling or a belt drive at the moment. I need to decide on one in a few hours.
  2. jcsd
  3. Apr 18, 2008 #2


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    The easiest things I have ever used are spider type couplings, like those produced by Lovejoy, for this purpose. Of course, you don't say anything about the geometry of the set up you are looking at. These couplers will work very well if you are OK with being close coupled. If you need to separate the two components by a larger distance you can possibly go with a double ended flex pack type of coupler which will get you a little more distance. The nice thing about these is that they are relatively easy to spec out and the only real thing you need is a key way in most cases. Some low torque applications you can get away with just a couple of set screws to mount the coupler.

    Belt drives I only consider if I can not mount the two components on the same centerline and the speeds are low. There are some pretty high speed belt systems out there, but the complexity in your system increases.
  4. Apr 18, 2008 #3


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    How about something different, like hydraulic drive? Maybe an automatic transmission out of an old small car from the junkyard....
  5. Apr 18, 2008 #4
    The geometry of the set up is up to me. I'm going to try to design it using a coupling. They'll be close coupled, and I'll use taper-locks to fix the coupling to the shaft, so there's no need to design keys and keyways. I'll just have to decide on a suitable type of coupling before selecting a suitable size.

    If it doesn't work out, I'll use a belt drive. It's running at less than 600rpm (don't have the exact figure off the top of my head) so it could even run with universal joints.

    Is there any advantage/disadvantage in separating the motor and fan by a larger distance? (Ignoring the set up logistics)

    Using a coupling, is there a need for bearings on both shafts?

    Any ideas on what sort of housing I should use if I proceed with the coupling and how I should go about designing it?
    Last edited: Apr 18, 2008
  6. Apr 18, 2008 #5
    How big is this? Or, how many hp?
  7. Apr 18, 2008 #6
    5.5kW motor
  8. Apr 18, 2008 #7


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    So 7.3 horsepower.... Maybe the car automatic transmission is a bit of overkill, then. What transmission ratio(s) are you considering?
  9. Apr 18, 2008 #8
    More than likely 1:1.

    But, out of interest, could you change transmission ratios using a coupling?

    I'll connect a frequency changer to the motor to vary fan speed.
    Last edited: Apr 18, 2008
  10. Apr 18, 2008 #9


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    Perhaps you ought to share your full project brief, or at least the key objectives?
  11. Apr 19, 2008 #10
    I have to design a connection between a motor and a fan @ 10000 m^3/hr. I also have to design a suitable housing. There are no other restrictions.

    I selected a 5.5kW motor using a chart, but I could also use say a 7.5kW motor on 3/4 power (both are equally efficient).
  12. Apr 19, 2008 #11
    I've had a look at belt drives and couplings and have produced some basic design specs.

    The belt drive is a very simple 1:1 transmission. Both pulleys have the same pitch diameter

    To get a suitable size coupling (bore for the shafts I'm using), I need to use a 11kW motor running at 1/2 power, which as it turns out, is actually slightly more efficient than a 7.5kW at 3/4 power and a 5.5kW at full power.

    I think I'll proceed with the coupling since there's less involved (It'll save me having to draw a pulley, and design keys and seats).
  13. Apr 19, 2008 #12


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    Most fans of this size are belt-driven.
  14. Apr 19, 2008 #13
    I actually ended up going with a belt drive.

    One question: Am I right in thinking you don't need any keys if you use taper lock pulleys?
  15. Apr 19, 2008 #14

    they have to installed right the first time other wise they can come lose and cause damage.
    follow the manufacturers instructions regarding the torqueing and anchoring of the taper lock on the shaft
  16. Apr 20, 2008 #15


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    Just out of curiosity, what led you to do the belt and pulleys?
  17. Apr 21, 2008 #16
    From the catalogue I have, I wasn't happy with the choice of couplings.

    Also, the availability of a certain type of stand. The stands are better suited to belt drives.

    It'll work out cheaper.

    I also thought about using a combination of couplings and belt drive, but it was an overcomplicated, more expensive and unnecessary solution. (It would involve designing two shafts, the need for more bearings, two couplings, and the belt drive, etc)

    I know I came across as a bit all over the place from my above posts. I had a couple of different ideas and was thinking out loud.
  18. Apr 21, 2008 #17


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    Ah. Fair enough. I was just wondering.
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