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Mechanical Speed Control

  1. Oct 13, 2016 #1
    Hi,

    Here's the problem. So basically I have an input shaft which has non constant Forces acting on it. Thus the input shaft speed varies with time. What I need is an output shaft that is speed controlled i.e. Speed constant with time. To add to it on the output shaft I have a large mass attached to the shaft.


    I tried thinking out the problem and would like to share it with you guys, maybe you guide me in the right direction. So I have drawn a simple sketch showing a way to solve this problem. Please see the attached. Basically as already explained I have an input cross which is subjected to non constant forces causing the input shaft to vary in speed. First of all to counteract the effect of inertia coming from the cylindrical mass I have added a counterweight and a pulley. The direction of rotation is the same as the input shaft. Since the shaft is going to start from stationary I wanted to achieve the constant speed as quickly as possible thus the introduction on the counterweight.

    Secondly, to allow the output shaft to achieve constant speed I included a governor. A pair of disk brakes are mechanically fixed to the governor. As one might expect as speed gets higher the breaks pads are pushed harder in the shaft causing the output shaft to slow. Conversely, like it will happen in the begining low speed will imply no breaking.

    So what do you think. I am in the right directions to achieve the desired outcome?

    Kind Regards,

    Sean.
     

    Attached Files:

  2. jcsd
  3. Oct 13, 2016 #2

    berkeman

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    Welcome to the PF.

    One way of solving this problem that is commonly used would be to have the input shaft connect to a DC generator, and the output shaft driven by a DC motor. Use a storage capacitor on the DC voltage between the generator and motor to help smooth out the variations in input speed/torque. :smile:
     
  4. Oct 13, 2016 #3

    Mech_Engineer

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    I would lean away from a braking solution since it would be less efficient from a power transmission standpoint, and could potentially have higher maintenance requirements. It could be those are less of a concern for you however. Overall I think you'd be better off totally decoupling the input/output shafts to support separate speeds rather than forcefully slowing the output shaft...

    I like berkeman's generator/motor idea, might be expensive but is used in heavy-duty applications all the time (see: Diesel-Electric Transmission). You also might be able to implement a hydraulic coupling solution, maybe a hydraulic system which can vary output speed (see: Speed Control of Hydraulic Motors).

    Can I ask, what is the nature of this application? Are there any examples of this problem being solved elsewhere or in other industries?
     
  5. Oct 13, 2016 #4

    anorlunda

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    I didn't understand the diagram in the OP. It appears that the output and input shafts are coupled so that they would have to have the same speed.

    @berkeman gave you one idea for a speed governor. It is harder for us to do better unless you give more of your requirements.

    • How much power is this apparatus expected to handle?
    • How accurately must the output speed be maintained? Must it maintain an exact speed at all loads, or can there be a slight deviation in the output speed in steady state depending on the load?
    • Your ideas like adding inertia reduce but do not eliminate output speed deviations. Again, what is your requirement?
    • How different can input and output shaft speeds be?
    There have been several continuously variable auto transmissions made that might be close to what you want. Try Google or Wikipedia to search for them.

    A car's cruise control holds constant speed of the wheels as your car goes up and down hills. Is that the kind of thing you are thinking of?
     
  6. Oct 14, 2016 #5

    Chronos

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    The generic term is a governer. It can be mechanical, like a clock spring or balance wheel, hydraulic like a valve, or electronic like a solenoid - or some combination thereof.
     
  7. Oct 14, 2016 #6
    Thanks @berkeman and thanks for the help too.
    I design mechanical ground fireworks as a hobby. The principle behind this is from a driver (this generally is a wheel with fire rockets attached to it - acts like a motor) and how linkages/gears/cams are set up you create different shapes and pattern. The idea here is to use purely mechanical systems. No electricity should be used.
    Thanks once again.
    Sean
     
    Last edited by a moderator: Oct 17, 2016
  8. Oct 14, 2016 #7

    @Mech_Engineer thanks for your reply. As I was discussing with @berkeman I design mechanical ground fireworks as a hobby. The principle behind this is from a driver (this generally is a wheel with fire rockets attached to it - acts like a motor) and how linkages/gears/cams are set up you create different shapes and pattern. The idea here is to use purely mechanical systems. No electricity should be used. This machine will be used maybe 3minutes per year. You can have a look at what I am reffering by following this link: .

    Once again, Thanks.
    Sean
     
  9. Oct 14, 2016 #8

    @anorlunda thanks for your help. As explained above I design mechanical ground fireworks as can be seen in the following link:

    Here are the answer to your questions:

    1. I am calculating that I will have a resistive torque of 500Nm.

    2.I would like as much as possible to have an exact speed. Load will always be the same. The only thing that will vary is the inertia loads.

    3. Essentialy as you said input=output shaft. The only thing is to control is the driving wheel. This wheel is made out of steel. At its end it has rockets attached. These provide the rotational motion of the shaft. We produce the rockets so there is no precision engineering in them. Thus these will vary with time and load.

    Thanks
    Sean
     
  10. Oct 14, 2016 #9
     
  11. Oct 14, 2016 #10
    Hi yes I would like to have a system entirely mechanical

    Sean
     
  12. Oct 14, 2016 #11
  13. Oct 14, 2016 #12

    Mech_Engineer

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    I'm starting to understand your application better, so you don't necessarily have an input/output shaft speed mismatch so much as a need to limit the speed the wheel spins at, correct? In such a case it would seem a mechanical speed governor with brakes would work.
     
  14. Oct 14, 2016 #13
    Exactly you have understood me well. Thanks for your feedback. Now my concern is if by the movement of the balls in the governer and some linkages I would achieve a good breaking force, or else I need to introduce a force multiplier to help in a better breaking.
    Sean
     
  15. Oct 14, 2016 #14

    anorlunda

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    The force multiplier comes from the lever action of the governor.

    Here is a diagram of James Watts flyball governor, invented in 1788 by James Watt. As you can see, moving the pivot pin left/right on that horizontal arm changes the mechanical advantage. Several flyball governor mechanisms are for sale on Ebay.

    Centrifugal_governor.png
     
  16. Oct 17, 2016 #15

    Mech_Engineer

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    I would recommend that you start by figuring out how much force is needed. It seems to me that large forces could be achieved with a relatively large lever arm for the centrifugal balls, but you need to know how much force is needed first.
     
  17. Oct 20, 2016 #16
  18. Oct 20, 2016 #17

    Mech_Engineer

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    Do you have an estimate for how much thrust the rockets which spin the display put out?
     
  19. Nov 4, 2016 #18

    CWatters

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    Do the rockets have to be what makes it rotate? Could you use an electric motor and gearbox to control the speed? Parts from an electric bike perhaps? That might give you the freedom to point the rockets in new directions.
     
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