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Segway, combining stress?

  1. Feb 22, 2014 #1
    Could someone help Please?

    I am trying to mount a driveshaft to an electric motor for a Segway project and determine the required bolt size to prevent failure. The Segway must be able to carry 120kg and is powered by two electric motors.

    The drive shaft has a diameter of ø12mm and is 91.6mm long and a section on the end ø45 by 5mm thick with 4 holes on a 30mm PCD for mounting.

    the electric motor; 500w
    24v (DC)
    1.9n/m (max rated torque)

    I have calculated that with a factor of safety of 2, that I would require at least M4 bolts for the required loading of the person.

    However, I am unsure on how to calculate the stresses caused by the electric motor and then combine them with the 120kg to calculate the required bolt sizes. :confused:

    any help would be greatly appreciated
  2. jcsd
  3. Feb 22, 2014 #2


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    Staff: Mentor

    Welcome to the PF.

    Why are you using a safety factor of just 2? The transient stresses will be much larger than just the weight of the person riding...
  4. Feb 24, 2014 #3

    I was told by my professor to design a drive shaft that will carry an 120kg load, I myself added the F.O.S as a minimum due to concern. However it is not written in stone I can increase if required.

    Once I have established the correct procedure for calculations and obtain some values, I will take cost/budget, weight, materials etc.. into account and alter if required.
  5. Apr 27, 2014 #4
    Hi...Here are the loading scenarios I came up with..
    The shaft will sag under the 120kg payload. That will try to open up the joint between tire and flange...inducing tensile stresses due to sagging...
    Also there will be direct shear due to this 120kg payload....60/(no. of bolts per side)
    There will be a torque transmission again inducing shear stresses in the bolts...you can calculate this by assuming the transient torque as some % overshoot of steady state (S.S) torque(S.S Torque = Power/(2*pi*R.P.M.)
    Also to be on safer side you may include fatigue loading for your bolt design...
    You can then calculate the core diameter of the bolt (that will be the weakest cross section) and then convert it to nominal diameter of the bolt..
    You can assume a factor of safety as you mentioned in OP...
  6. Apr 27, 2014 #5


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    Science Advisor
    Homework Helper

    It's not obvious how the transient stresses in the motor mounting bolts depend on the weight of the rider. The same applies for transient torques in the drive shaft if the speed suddenly changes (e.g. you hit a bump in the road). You can only generate a large transient load if there is something for it to react against. The relevant quantities are probably the mass of the motor itself and the inertia of its rotor.

    If you hit a bump and the whole structure of the segway gets a shock acceleration of say 10g, most of the transient forces are "used up" in decelerating the segway and its 120 kg rider, not the motor with a mass of maybe 1 or 2 kg.
  7. Apr 27, 2014 #6
    Isn't it the case that the transient stresses in the bolts would be mostly due to transient torques...since the tire and flange is bolted to avoid any relative translational motion...so they move as one unit..the transient torque can be maximum in case of vehicle acceleration/deceleration...and as much as the ground friction can support. And frictional force is directly related to the mass of the segway..
    Any comments regarding these situations?
  8. Apr 27, 2014 #7


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    Science Advisor
    Homework Helper

    I don't know much about the design of the Segway specifically, but designing a powered bike so that the full static load path from the rider through to the road goes through the motor bearings and the motor casing into the bike frame seems a dumb idea.

    Shouldn't there be some proper wheel bearings to take that load path, so the motor can then just do what it is meant to do, i.e. apply torque to the wheels?
  9. Jun 29, 2014 #8
    i making a battery operated scooter i need ur help for this actually i,m assuming the load 1963 newton (200kg) and need a speed of 35 km/hr . i want to use 2 brushless dc motor of 12v.I want to know the rpm ,torque and current rating of motor please help me
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