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Can someone help a non-ME

  1. Jan 15, 2015 #1
    I've got an electric motor which drives a two-geared system. The first gear is on the motor's shaft. And, the 2nd gear drives a shaft via a a sprague bearing (spins freely in one direction/locks in the other), which is press fit in to an aluminum journal in the center of the larger gear. I'm using a sleeve retainer that's rated to 3000psi shear when pressing the sprague bearing in to its journal. But, it is not holding even after two days curing. The sleeve retainer states full cure in 24 hours, FYI. And, it's a new tube, as I assumed my sleeve retainer may have gone bad when this happened the first time. The entire bearing spins in its journal when the motor is slowly spun up (I'm not applying full torque to the gear train from a stop).

    Unfortunately, I don't know the math to calculate the load my drive system is putting on the sprague bearing's press-fit surface. And, I'm curious is 3000psi shear strength is enough, or if I have another issue.

    I'd like help calculating the maximum force in psi that this system could apply to the bearing journal surface to be able to compare it with the sleeve retainer's rating.

    Here's the details:
    Electric brushless motor with a rated max output of 5200 watts.

    see here if I didn't provide enough info:

    the gear ratio is 11/106 (that's the number of teeth on each gear). the reference diameter of the smaller gear is 11mm, and the reference dia of the larger gear is 106mm.

    the shaft driven by the larger gear that passes through the sprague bearing is 6mm in diameter. and, the journal that the sprague bearing is pressed in to is 15mm in diameter.

    Let me know if you need more info. And, thanks for any help.
  2. jcsd
  3. Jan 15, 2015 #2
    What's your background? This sounds like a difficult enough problem that we might need to know how much we can expect you to know and how much we'll need to provide.
  4. Jan 15, 2015 #3
    I'm a hobbyist. I don't have any background in math aside from what I learned in highschool. I was hoping this would be an interesting and simple enough question that someone might just be able to complete it without too much trouble.

    If it is, and this is not the place for this type of question, I'll be glad to take it elsewhere.

  5. Jan 15, 2015 #4
    It's not too much trouble at all. That info just helps us all know where to start from. Plus, it helps us get to know you too. (Welcome to PF, BTW)

    What voltage are you applying to the motor, or do you already know what the max torque is?
  6. Jan 15, 2015 #5
    50.4v DC max. It's battery operated, so it drops to 45.6vdc eventually. But, I'm concerned with max possible values for this exercise.

    oh, and thanks for the welcome.
  7. Jan 15, 2015 #6
    outer diameter of the sprague bearing is the same as its journal....15mm
  8. Jan 18, 2015 #7

    jim hardy

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    You need the torque, don't you, to figure shear in that press fit joint ?
    Torque = power/(rpm X a constant, i use English units where the constant is 2pi/33,000 , power in hp and torque in ft-lbs
    So let's take a guess at motor power and RPM from its datasheet
    Looks like the motor is capable of 560 RPM/volt, probably a bit less under load
    and the 8 cell LipPo battery mentioned on specsheet is 29.6 volts, probably a bit less under load
    so it could run at a bit less than 29.6 X 560 = 16756 RPM

    Its 15 second power rating is 5600 watts which is 5600/746 = 7.507 hp
    Torque would be approximately 7.5 / (16756 X 2pi/33000) = 2.35 ft-lbs, for a first approximation

    if my arithmetic is right, that is

    i'll leave it to a genuine mechanical type to judge reasonableness of your fit for that torque.
    i'm just an old electrical type

    old jim
  9. Jan 18, 2015 #8

    jim hardy

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    Whoa you're running it at 50 volts not 40 ?

    Not knowing its characteristics, i can't even guess at how much more torque it'll make.
    I'd design for twice rated , ~5 ft-lbs
  10. Jan 18, 2015 #9


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    A 7 horse motor driving a 3/4" shaft? I'd say you're way overpowered for the mechanical components you're driving. Sent up a flare for the expert, however.
  11. Jan 18, 2015 #10


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    Maybe the press fit is not tight enough. You might consider using a paper liner in the journal to reduce the clearance and increase the friction. Metal-metal friction is less than metal-paper-metal friction.
    Is the correct term "sprague" or "sprag"? http://en.wikipedia.org/wiki/Sprag_clutch
  12. Jan 18, 2015 #11

    jim hardy

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  13. Jan 30, 2015 #12
    So the bearing outer race (bearing outside diameter) and the bearing housing that you press the bearing into (bearing journal would be the shaft bearing surface) diameters are the same?
    And the bearing outer race which is "pressed" into the housing ends up spinning?
  14. Jan 31, 2015 #13
    If your bearing dimensions (both 15 mm) is true, that is not an interference fit.
    You need to increase the interference.
    The bearing outer diameter needs to be larger than the housing it fits into.
    Without getting into all the numbers, which is 1 mil (.001") / inch of bore interference up to a certain bore, you need a larger bearing OD
    or smaller housing. You will need to heat the bearing housing or cool the bearing for the fit. once the temps equalize the housing will squeeze the bearing and will not move on you. Now realize as you do this the radial clearance of the bearing will be reduced so you will have to have the correct class of bearing so as not to reduce the rolling element clearance to a value that is to tight.
    With all that said, (LOL) personally, I would use Loctite brand 640 retaining compound. I used it a lot at work when the interference fits were not correct due to wear.

    Also, about the issue of the rotation tightening up on you, do you have proper gear backlash? (Clearance between the gear teeth). You should have "free play" between the teeth. Can you rotate one shaft a little bit with out the other shaft moving? The gear mesh should not be tight for a spur gear.
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